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Strategies for Enhanced Growth and Reproductive Performance of Yearling Bos Taurus and Bos Taurus X Bos Indicus Beef Heifers

Permanent Link: http://ufdc.ufl.edu/UFE0024929/00001

Material Information

Title: Strategies for Enhanced Growth and Reproductive Performance of Yearling Bos Taurus and Bos Taurus X Bos Indicus Beef Heifers
Physical Description: 1 online resource (127 p.)
Language: english
Creator: Austin, Bradley
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: beef, heifer, interval, nutrition, programmed, reproduction
Animal Sciences -- Dissertations, Academic -- UF
Genre: Animal Sciences thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The development of replacement heifers is one of the major economic considerations in a cow-calf operation. Two experiments were conducted to evaluate alternative heifer development strategies effects on physiological, growth, and reproductive responses of growing Angus and Brangus heifers. Experiment 1 utilized Angus (n=30) and Brangus (n=30) heifers supplemented with distillers dried grains (DDG) either daily (SPD) or three times a week (SP3). Heifers were provided bermudagrass round bale silage at all times. No differences were observed in BW gain, BCS, or HH between treatments. Greater fluctuations in both PUN and plasma glucose concentrations were observed in the SP3 heifers. Reproductive responses were similar between the SPD and SP3 treatments. In experiment 2, growth, body composition, reproductive, and physiological responses of Angus (n=30) and Brangus (n=30) heifers were supplemented with DDG for the entire trial (168 d, CON) or only for the last 79 d prior to breeding (L-H) were compared. Heifers were provided bermudagrass round bale silage at all times. CON heifers had greater BW gain during the first 89 d and for the entire trial (d 0-168) compared to L-H heifers. Mean BW gain for L-H heifers during the last 79 d tended to be greater compared to CON heifers. Ultrasound measurements of rump fat, fat over the 13th rib, and longismus dorsi area were decreased for L-H heifers during the first 89 d, but similar by d 174. Blood PUN and NEFA concentrations fluctuated greatly during the trial for both treatments, but plasma glucose concentrations were not affected by treatment. Heifers in the L-H treatment had significantly reduced puberty and pregnancy rates compared to CON heifers. The use of DDG is a viable supplement to developing Angus and Brangus heifers consuming round bale silage. Supplementation of heifers three times a week as opposed to daily is a viable alternative to decrease costs in a heifer development program. Delaying supplementation of heifers to decrease costs is not a viable management decision if forage is not of a high quality.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Bradley Austin.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Yelich, Joel V.
Local: Co-adviser: Hersom, Matthew J.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024929:00001

Permanent Link: http://ufdc.ufl.edu/UFE0024929/00001

Material Information

Title: Strategies for Enhanced Growth and Reproductive Performance of Yearling Bos Taurus and Bos Taurus X Bos Indicus Beef Heifers
Physical Description: 1 online resource (127 p.)
Language: english
Creator: Austin, Bradley
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: beef, heifer, interval, nutrition, programmed, reproduction
Animal Sciences -- Dissertations, Academic -- UF
Genre: Animal Sciences thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The development of replacement heifers is one of the major economic considerations in a cow-calf operation. Two experiments were conducted to evaluate alternative heifer development strategies effects on physiological, growth, and reproductive responses of growing Angus and Brangus heifers. Experiment 1 utilized Angus (n=30) and Brangus (n=30) heifers supplemented with distillers dried grains (DDG) either daily (SPD) or three times a week (SP3). Heifers were provided bermudagrass round bale silage at all times. No differences were observed in BW gain, BCS, or HH between treatments. Greater fluctuations in both PUN and plasma glucose concentrations were observed in the SP3 heifers. Reproductive responses were similar between the SPD and SP3 treatments. In experiment 2, growth, body composition, reproductive, and physiological responses of Angus (n=30) and Brangus (n=30) heifers were supplemented with DDG for the entire trial (168 d, CON) or only for the last 79 d prior to breeding (L-H) were compared. Heifers were provided bermudagrass round bale silage at all times. CON heifers had greater BW gain during the first 89 d and for the entire trial (d 0-168) compared to L-H heifers. Mean BW gain for L-H heifers during the last 79 d tended to be greater compared to CON heifers. Ultrasound measurements of rump fat, fat over the 13th rib, and longismus dorsi area were decreased for L-H heifers during the first 89 d, but similar by d 174. Blood PUN and NEFA concentrations fluctuated greatly during the trial for both treatments, but plasma glucose concentrations were not affected by treatment. Heifers in the L-H treatment had significantly reduced puberty and pregnancy rates compared to CON heifers. The use of DDG is a viable supplement to developing Angus and Brangus heifers consuming round bale silage. Supplementation of heifers three times a week as opposed to daily is a viable alternative to decrease costs in a heifer development program. Delaying supplementation of heifers to decrease costs is not a viable management decision if forage is not of a high quality.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Bradley Austin.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Yelich, Joel V.
Local: Co-adviser: Hersom, Matthew J.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024929:00001


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1 STRATEGIES FOR ENHAN CED GROWTH AND REPRO DUCTIVE PERFORMANCE OF YEARLING BOS TAURUS AND BOS INDICUS BOS TAURUS BEEF HEIFERS By BRADLEY RYAN AUSTIN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORI DA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2009

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2 2009 Bradley Ryan Austin

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3 To my loving wife, Meghan, and my family for providing endless su pport and encouragement

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4 ACKNOWLEDGMENTS I would like to thank my co chairs of my supervisory committee Dr. Matt Hersom and Dr. Joel Yelich for their guidance, patience, support, and dedication throughout my doctoral program. I would also like to than k Drs. John Arthington and Owen Rae for their contributions and insight as members of my committee. A thank you to Dr Thrift for many long discussions, during both my master and doctoral programs, on science and life in general. Special thanks are exten ded to the current and former members of my lab Regina Esterman, Erin McKinnis, Aline Monari, Reyna Speckmann, and Steaven Woodall Their time, patience, and assistance with these and other projects made this work possible. I would also like to thank the staff of the Santa Fe Beef Research Unit, Bert Faircloth, Steve Chandler, and Jamie, for their infinite cooperation and assistance during my trials. Completion of my laboratory work would not have been possible without the help and support of many other technicians in the department: Nancy Wilkinson, Jan Kivipelto, Sergi Sennikov, Joyce Hayden, and Idania Alverez. I would also like to thank my friends, co workers, and fellow graduate students for their input, assistance and good times shared during my te nure. I would like to thank my family for their endless support and love throughout my educational process. Finally, I would like to thank my wife, Meghan, for her help, encouragement, and support during this long and sometimes trying process.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ ............... 4 LIST OF TABLES ................................ ................................ ................................ ........................... 7 LIST OF FIGURES ................................ ................................ ................................ ......................... 8 ABSTRACT ................................ ................................ ................................ ................................ ..... 9 CHAPTERS 1 I NTRODUCTION ................................ ................................ ................................ .................. 11 2 R EVIEW OF LIT ERATURE ................................ ................................ ................................ 13 Puberty ................................ ................................ ................................ ................................ .... 13 Endocrine ................................ ................................ ................................ ......................... 13 Predicting the Onset of Puber ty ................................ ................................ ....................... 15 Factors Affecting Onset of Puberty ................................ ................................ ........................ 17 Age ................................ ................................ ................................ ................................ .. 17 Body Weight ................................ ................................ ................................ .................... 18 Season ................................ ................................ ................................ .............................. 19 Breed ................................ ................................ ................................ ................................ 20 Level of Nutrition ................................ ................................ ................................ ............ 22 Management ................................ ................................ ................................ .................... 24 Heifer Development Systems ................................ ................................ ................................ 26 Supplementation Programs ................................ ................................ ................................ ..... 32 Energy Supplementation ................................ ................................ ................................ 33 Animal performance ................................ ................................ ................................ ........ 34 Protein Supplementation ................................ ................................ ................................ 38 Animal response to degradable intake protein ................................ ......................... 40 Animal response to non protein nitrogen ................................ ................................ 42 Animal response to undegradable intake protein ................................ ..................... 44 Supplement Interval ................................ ................................ ................................ ......... 46 3 E FFECT OF FEEDING INTERVAL O N GROWTH, REPRODUCTIVE PERFORMANCE, AND BLOOD METABOLITES IN YEARLING BOS TAURUS AND BOS INDICUS BOS TAURUS BEEF HEIFERS ................................ ....................... 51 Introduction ................................ ................................ ................................ ............................. 51 Methods and Materials ................................ ................................ ................................ ........... 52 Animals ................................ ................................ ................................ ............................ 52 Pastures ................................ ................................ ................................ ............................ 52 Di ets ................................ ................................ ................................ ................................ 53 Breeding ................................ ................................ ................................ .......................... 54

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6 Sampling ................................ ................................ ................................ .......................... 55 Blood Analysis ................................ ................................ ................................ ................ 56 Statistical Analysis ................................ ................................ ................................ .......... 56 Results and Discussion ................................ ................................ ................................ ........... 57 Performance ................................ ................................ ................................ ..................... 57 Blood Metabolites ................................ ................................ ................................ ........... 58 Reproductive Performance ................................ ................................ .............................. 62 Implications ................................ ................................ ................................ ............................ 64 4 EFFECT OF PROGRAMMED FEEDING ON GROWTH, REPRODUCTIVE PERFORMANCE, AND BLOOD METABOLITES IN YEARLING BOS TAURUS AND BOS INDICUS BOS TAURUS BEEF HEIFERS ................................ ....................... 77 Introduction ................................ ................................ ................................ ............................. 77 Materials and Methods ................................ ................................ ................................ ........... 78 Animals ................................ ................................ ................................ ............................ 78 Pastures ................................ ................................ ................................ ............................ 78 Diets ................................ ................................ ................................ ................................ 79 Breeding ................................ ................................ ................................ .......................... 80 Sampling ................................ ................................ ................................ .......................... 81 Blood Analysis ................................ ................................ ................................ ................ 82 Statistical Analysis ................................ ................................ ................................ .......... 82 Results and Discussion ................................ ................................ ................................ ........... 83 Performance ................................ ................................ ................................ ..................... 83 Blood Metabolites ................................ ................................ ................................ ........... 87 Reproductive Measures ................................ ................................ ................................ ... 91 Implications ................................ ................................ ................................ ............................ 96 LIST OF REFERENCES ................................ ................................ ................................ ............. 107 BIOGRAPHICAL SKETCH ................................ ................................ ................................ ....... 127

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7 LIST OF TABLES Table page 3 1 Amount of dried distillers grains (DDG) supplement offered by day of experiment and total amount of round bale silage (RBS) and DDG offered to yearling Angus and Brangus heifers during the experiment. ................................ ................................ ............. 65 3 2 Nutritive value of forage and supplement ingredients offered to developing beef heifers. ................................ ................................ ................................ ................................ 66 3 3 Growth characteristics of heifers meal consuming round bale silage and supplemented with dried distillers grains and soybean ................................ ...................... 67 3 4 Breed x time(da y) effect of plasma urea nitrogen (mg/dL) of Angus (AN) and Brangus (BN) heifers fed round bale silage and dried distillers grains based supplement. ................................ ................................ ................................ ........................ 68 3 5 Treatment, and breed effects for pub erty, estrous response, conception rate, synchronized pregnancy rate, and 30 d AI pregnancy rate of Angus and Brangus heifers supplemented with DDG either daily (SPD ) or three times weekly (SP3). ........... 76 4 1 Amount of dried distillers grains supplement offered to yearling beef heifers consuming round bale silage. ................................ ................................ ............................. 97 4 2 Nutritive value of forage and supplement ingredients offered to dev eloping beef heifers. ................................ ................................ ................................ ................................ 98 4 3 Growth characteristics of heifers offered round bale silage and supplemented with dried distillers grains to gain at a constant or programmed rate of BW gain. ................... 99 4 4 Effect of treatment and breed on BW gain of heifers offered round bale silage and supplemented with dried distillers grains. ................................ ................................ ....... 100 4 5 Body ultrasound measurements of heifers offered round bale silage and supplemented with dried distillers grains to gain at a constant or programmed rate of gain. ................................ ................................ ................................ ................................ .. 101 4 6 Treatment, bre ed, and treatment breed effects for pelvic area, reproductive tract score, heifers pubertal at d 89, and heifers pubertal at breeding for Angus and Brangus heifers consuming round bale silage and supplemented with dried distillers grains to gain at a con stant (CON) or programmed (L H) rate of gain. .......................... 105 4 7 Treatment, breed, and treatment breed effects for estrous response, conception rate, timed AI conception rate, synchronized pregnancy ra te, and 28 d AI pregnancy rate of Angus and Brangus heifers consuming round bale silage and supplemented with dried distillers grains to gain at a constant (CON) o r programmed (L H) rate of gain ... 106

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8 LIST OF FIGURES Figure page 3 1 Plasma urea nitrogen (PUN) concentrations by treatment and day of experiment for heifers consuming round bale silage and supplemented with dried distillers grains based suppl ements daily (SPD) or three times/wk (SP3). ................................ ................. 69 3 2 Plasma urea nitrogen (PUN) concentrations by hour after supplementation for heifers consuming round bale silage and supplemented with dried d istillers grains based supplements either daily (SPD) or three times/wk (SP3). ................................ ................. 70 3 3 Plasma glucose concentration by treatment and day of experiment for heifers consuming round bale silage an d supplemented with dried distillers grains based supplements daily (SPD) or three times/wk (SP3). ................................ ........................... 71 3 4 Plasma glucose concentrations by hour after supplement administration for heifers con suming round bale silage and supplemented with dried distillers grains based supplements daily (SPD) or three times/wk (SP3). ................................ ........................... 72 3 5 Plasma glucose concentration breed and day of experiment fo r heifers consuming round bale silage and supplemented with dried distillers grains based supplements dail y (SPD) or three times/wk (SP3) ................................ ................................ ................. 73 3 6 Plasma NEFA concentrations day of experiment fo r heifers consuming round bale silage and supplemented with dried distillers grains based supplements daily (SPD) or three times/wk (SP3). ................................ ................................ ................................ .... 74 3 7 Plasma NEFA concentrations by breed and day of experiment for heifers consuming round bale silage and supplemented with dried distillers grains based supplements daily (SPD) or three times/wk (SP3). ................................ ................................ ................ 75 4 1 Plasma urea nitrogen (PUN) conce ntrations by treatment and day of experiment for heifers consuming round bale silage and supplemented with either dried distillers grains to gain at a constant rate (CON) or programmed (L H) rate of gain ................... 102 4 2 Plasma glucose concentrations by day for heifers consuming round bale silage and supplemented with dried distillers grains. ................................ ................................ ....... 103 4 3 Plasma NEFA concentrations by treatment and day of experiment for heifers consuming round bale silage and supplemented with dried distillers grains to gain at a constant rate (CON) or programmed (L H) rate of gain ................................ ............... 104

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9 Abstract of Dissertation Presented to the Gr aduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy STRATEGIES FOR ENHAN CED GROWTH AND REPRO DUCTIVE PERFORMANCE OF YEARLING BOS TAURUS AND BOS INDICUS BOS TAURUS BEEF HEIFERS By Br adley Ryan Austin August 2009 Chai r : Joel Yelich Co Chair: Matt Hersom Major: Animal Sciences The development of replacement heifers is one of the major economic considerations in a cow calf operation. Two experiments were conducted to evaluate altern ative heifer development strategies effects on physiological, growth, and reproductive responses of growing Angus and Brangus heifers. Experiment 1 utilized Angus (n=30) and Brangus (n=30) heifers supplemented with distillers dried grains (DDG) either dai ly (SPD) or three times a week (SP3 ) Heifers were provided bermudagrass round bale silage at all times. No differences were observed in BW gain, BCS, or HH between treatments. Greater fluctuations in both PUN and plasma glucose concentrations were obse rved in the SP3 heifers. Reproductive responses were similar between the SPD and SP3 treatments. In experiment 2, growth, body composition, reproductive, and physiological responses of Angus (n=30) and Brangus (n=30) heifers were supplemented with DDG fo r the entire trial (168 d, CON) or only for the last 79 d prior to breeding (L H) were compared. Heifers were provided bermudagrass round bale silage at all times. CON heifers had greater BW gain during the first 89 d and for the entire trial (d 0 168) c ompared to L H heifers. Mean BW gain for L H heifers during the last 79 d tended to be greater compared to CON heifers. Ultrasound measurements of rump fat, fat over the 13 th rib, and longismus dorsi area

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10 were decreased for L H heifers during the first 8 9 d, but similar by d 174. Blood PUN and NEFA concentrations fluctuated greatly during the trial for both treatments, but plasma glucose concentrations were not affected by treatment. Heifers in the L H treatment had significantly reduced puberty and pre gnancy rates compared to CON heifers. The use of DDG is a viable supplement to developing Angus and Brangus heifers consuming round bale silage. Supplementation of heifers three times a week as opposed to daily is a viable alternative to decrease costs i n a heifer development program. Delaying supplementation of heifers to decrease costs is not a viable management decision if forage is not of a high quality.

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11 CHAPTER 1 INTRODUCTION The growth and development of replacement heifers from weaning until the y enter the cow herd is a primary factor affecting the overall efficiency of a cow calf operation (Bagley, 1993). The cost of developing replacement heifers is one of the major economic factors associated with a cow calf operation. Productivity for beef cattle herds increases when a greater percentage of heifers become pregnant early in the first breeding season (Lesmeister et al., 1973). Consequently, selection and management of replacement heifers involves decisions that affect the future productivity of the cowherd, and considerable attention should be given to this segment of the cow calf enterprise. Replacement heifers were traditionally bred at two years of age to calve at three years of age, but as management of beef cattle has become more intensiv e most producers now breed heifers as yearlings or 12 15 months of age. Calving heifers at two years of age has been shown to increase the economic inputs of a cow calf enterprise. Nunez Dominguez et al (1991) observed a 6 8% decrease in costs per unit of output in heifers calving at two compared to three years of age suggesting it is more beneficial to breed heifers at a year of age. F rom an animal productivity standpoint, heifers that calve by two years of age have greater lifetime productivity than heifers that calve at an older age (Lesmeister et al., 1973; Wiltbank et al., 1985). Morris (1980) concluded, from a review of worldwide literature, that lifetime production would be increased by approximately 0.7 calves by breeding heifers as yearlings a s opposed to two year olds. Heifers that attain puberty prior to the start of the first breeding season calve earlier in their first calving season and conceive earlier in subsequent breeding seasons (Wiltbank et al., 1985). For heifers to be bred succes sfully as yearlings to calve at two years of age, they should reach puberty by 12 to 14 months of age. This early age at puberty ensures that a high percentage of

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12 heifers are going through estrous cycles and that the effects of lowered potential fertility at the pubertal estrus are minimized (Byerley et al., 1987). The decision to breed heifers as yearlings involves careful consideration of the economics of production, reproductive status of the heifers, and breed type of the heifers (Short et al., 1990). Short et al. (1990) offered both the advantages and disadvantages of breeding heifers as yearlings. Advantages included, shorter interval to a return on investment, increased lifetime productivity of each cow, increased output per year on a herd or aver age cow basis, and less demand for pasture space for yearling heifers to separate them from the cow herd during the breeding season. In contrast the disadvantages included increased costs associated with breeding heifers at younger ages, increased calf l oss due to dystocia and related problems including the necessary management inputs for dealing with dystocia, lower rebreeding rates for first calf heifers as compared to older ages, and fewer and smaller calves weaned from 2 year old dams. There are many factors and possible interactions that can affect the economic outcome of age at first breeding. In some cases, the economic advantage of early breeding and calving can be offset by biological limitations of the animal such as when it attains puberty and management constraints of the environment such as availability of grazeable forage (Short et al., 1990). For these reasons age at puberty is an important factor influencing production efficiency of the cowherd. Geographical (and) or regional difference s in the age at which heifers are first exposed for breeding depend on management systems, forage quality and availability, and adaptation of respective breed types to specific environmental conditions (Short et al., 1990).

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13 CHAPTER 2 REVIEW OF LITERATURE Puberty Puberty is often defined as the time at which an animal is capable of reproduction itself (Robinson, 1977). The physiological onset of puberty is the state or stage of development in which the female first expresses estrus, ovulates, and develops a functional corpus luteum for a period characteristic of that particular species which is approximately 19 to 21 days in the bovine (Kinder et al, 1987). The attainment of puberty is a gradual process. It is initiated before birth and continues through out the prepubertal and peripubertal periods of the developing female (Kinder et al., 1990). During this process there are many physiological and endocrine changes occurring. Factors such as age, weight, nutrition, season, breed, and management all play major roles in the timing of the onset of puberty. Endocrine The hypothalamus is the site of gonadatropin releasing hormone (GnRH) synthesis and secretion GnRH is released into the hypothalamic hypophyseal portal vessels that carry it to the anterior pituitary. Upon entering the anterior pituitary GnRH stimulates the synthesis and secretion of LH and FSH These pituitary derived gonadotropins act on ovarian cells to signal changes in ovarian function and secretion of hormones. Luteinizing hormone i s released from the anterior pituitary in a pulsatile fashion (Rahe et al., 1980) and the frequency and amplitude of its secretion is dependent on the age and pubertal status of the heifer (Kinder et al., 1987). Pulses of LH have been identified in heifer s as early as 1 month of age (Schams et al., 1981) but the pulses of LH in the prepubertal heifer are infrequent and occur approximately once every 4 to 24 hours (Day et al., 1987). Day et al. (1984) reported that the decreased frequency of LH puls es prio r to puberty is due to negative feedback of

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14 estradiol on the hypothalamic pituitary axis. It was further observed that LH pulse frequency increased with heifer age as the negative feedback effect of estradiol deceased. Additionally, a s heifers approach p uberty the number of estradiol receptors in the hypothalamus decline, this decline in receptors is believed to lead to the decrease in the negative feedback (Day et al., 1987). As a result, approximately 50 days before the onset of puberty LH pulses incr ease in frequen cy to approximately 1 pulse every hour in the days prior to puberty (Day et al., 1987). In summary, t his continual increase in the frequency of LH secretion along with the removal of the estradiol negative feedback becomes the primary endoc rine factor regulating the onset of puberty (Kinder et al., 1987). Previous research has addressed modifying LH release based on nutritional status. Restrict ing nutrient intake causes a decrease in pulsatile LH release which prevents the onset of pubert y (Schillo, 1992). Heifers maintained on a low energy diet exhibit ed a decreased LH pulse frequency compared to heifers fed an adequate energy that exhibited increased LH pulse and attained puberty (Day et al., 1986). Consequently, reduced nutrient intak e during the prepubertal period decrease s LH pulse frequency and can lead to a delayed onset of puberty in heifers (Day et al., 1986: Hall et al., 1994). Changes in metabolites and ( or ) metabolic hormones reflecti ve of nutritional status may influence LH secretion (Steiner et al., 1983). D uring periods of reduced nutrient intake circulating concentrations of nonesterified fatty acids (NEFAs) and growth hormone (GH) increase (Gill and Hart, 1981; Peters, 1986; Canfeild and Butler, 1991), whereas insulin and insulin like growth factor I (IGF I) decrease (Bassett et al., 1971; Breir et al., 1986; Rutter et al., 1989). It is possible that NEFAs and GH are inhibitory whereas IGF I and insulin are stimulatory to LH release (Schillo et al., 1992). During peri ods of increased nutrient intake,

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15 circulating concentrations of insulin decline (Richards et al., 1989), glucose increases, and IGF I increase (Granger et al., 1989; Yelich et al., 1996). Feeding a concentrate diet has been shown to increase LH concentrat ions in heifers which hastens the onset of puberty (Gasser, et al., 2006). The growth and development of the reproductive tract, including the ovaries and uterus, and any role it may have in the onset of puberty, is less clear. The ovaries are responsive to gonadatropins and capable of ovulation shortly after birth (Howe et al., 1962; Seidel et al., 1971) suggesting that maturation of the ovaries is probably not a limiting factor in the onset of puberty F ollicle diameters increase from 2 to 34 weeks of a ge, with the greatest increase occurring between 2 and 8 weeks of age (Evans et al., 1994). As puberty approaches ovarian weights do not change (Day et al., 1987). The number of small (<3 mm), medium (3 to 6 mm), or large follicles (7 to 12 mm) also do n ot change as puberty approaches. However, numbers of follicles >12 mm increases as heifers approach the onset of puberty (Day et al., 1987). In contrast, t here are dramatic changes in uterine development during the peripubertal period. Day et al. (1987) observed significant increases in uterine weight during the 50 days prior to puberty which is coincident with the significant changes in the pulsatile secretion of LH It is most likely that development of the reproductive tract is dependent on the matu ration of the hypothalamic pituitary axis, which allows for the proper hormone milieu for the development of the uterus. However, it should be noted that this all has to occur in the presence of adequate nutrition to allow for adequate growth of the anim al. Predicting the Onset of Puberty Since the attainment of puberty is a gradual process it would be beneficial for producers to have an inexpensive and simple method to estimate the onset of puberty. One such tool is the reproductive tract scoring (RTS ) system that was developed at Colorado State University (Anderson, et al., 1991) and is used to evaluate pubertal status by rectal palpation of the uterus

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16 and ovaries. The RTS is based on the size and tone of the uterus and size and palpable structures o n the ovaries and has a scale of 1 to 5. A RTS of 1 is assigned to heifers with infantile tracts, as indicated by immature uterine horns (<20 mm diameter and no tone) and small ovaries (15 x 10 x 8 mm) with no palpable structures. A RTS of 1is indicative of a heifer that is likely the furthest from puberty at the time of examination. Heifers assigned a RTS of 2 are closer to puberty than those scoring 1 ; they have larger uterine horns (20 25 mm diameter and no tone) and ovaries (18 x 12 x 10 mm) with pal pable ovarian structures (8 mm follicles). Heifers assigned a RTS of 3 are thought to be on the verge of estrous cycles based on uterine size and tone (20 25 mm diameter and slight tone) and larger ovaries (22 x 15 x 10 mm ) with palpable ovarian structur es (8 10 mm follicles). Heifers assigned a score of 4 are considered to be pubertal, as indicated by uterine size and tone (30m diameter and good tone), coiling of the uterine horns, and large ovaries (30 x 16 x 12 mm) with large palpable follicles (>10 mm follicle). However, heifers assigned a score of 4 do not have an easily distinguished corpus luteum (CL). Heifers with RTS of 5 are similar to those scoring 4, except for the presence of a palpable CL. The RTS can be used as a culling tool to remove he ifers from the herd that are least likely to conceive early in a defined breeding season. By doing this, a producer can place a some selection pressure on age at puberty granted the heifers were provided adequate nutrition to express their growth potenti greater likelihood of being pregnant during the first thirty days of the breeding season compared liklihood compared to heifers with a RTS 3. Anderson et al. (1991) observed an increase in breeding season pregnancy rates of 47% for a RTS 1 (37%) compared to a RTS 5 (85%). Patterson and Bullock (1995) reported a significant increase in

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17 synchronized pregnancy rates from RTS 1(34%) and 2 (8%) heifers compa red to heifers with a RTS of 3 (60%), 4 (65%), and 5 (66%) following synchronization with a melengestrol acetate + PG protocol. In summary reproductive tract scoring of heifers offers producers another tool to use in a heifer selection program and to aid in determining puberty status Factors Affecting Onset of Puberty Puberty in heifers is influenced by several factors including, age, body weight, breed, and nutrition level (Patterson, et al., 1992). Heifers must be bred by 15 months of age to calve at two years of age. Ideally heifers should reach puberty at least one month prior to the beginning of the breeding season. Heifers are less likely to conceive on the puberal estrus as compared to the third estrus (Byerley et al., 1987). Taking this int o account along with the fact that heifers are often bred one month prior to the cow herd, heifers must attain puberty by 11 to 13 months of age (Larson, 2007). To meet these requirements the aforementioned factors must be taken into consideration. Age Ag e at puberty is most important as a production trait when heifers are bred to calve as 2 year olds and in management systems that impose restricted breeding periods (Ferrell, 1982) Age at puberty is a commonly used term that can be difficult to relate to puberty. Age is a measure of time which is a continuous variable that can not be altered. However, the importance of puberty is in conjunction with age, for it is not until puberty is placed in an age context (time dimension) and the impact of decision variables on age at puberty are known that management practices can be evaluated (Greer et al. 1983). Mosely et al. (1982) suggested that the onset of puberty may be limited by age in heavyweight heifers and limited by weight in lightweight contemporarie s. Short et al. (1990) concluded that age at puberty is primarily a function of the

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18 genetic makeup of the heifer and the level of nutrition or rate of gain that occurred from weaning to the start of the breeding period. Body Weight The effect of weight on puberty has been well documented. A negative relationship exists between age and weight at puberty as faster growing heifers are younger but heavier at puberty (Short and Bellows, 1971; Arije and Wiltbank, 1974; Laster et al., 1979; Ferrell, 1982; Gre er et al., 1983, Yelich et al., 1995). From a biological point of view we must be aware that age at puberty is not determined by weight alone but by some set of physiological conditions that also result in a given weight (Greer et al., 1983). Although fr om a management perspective, body weight is a good tool for monitoring heifer performance and predicting when they will reach puberty. originally proposed in the rat by Kennedy and Mitra (1963). Frische et al (1970) observed that body fat was a good predictor of puberty in humans. However, the effect of body composition on age at puberty is not as well documented or understood in cattle. Grass et al. (1982) and Broo ks et al. (1985) measured fat concentrations (using gamma ray activity of natural 40 K to determine fat free body composition) and reported that body fat percentages were not constant at the onset of puberty. Direct measures of body fat have also shown bod y fat percentages to be inconsistent at puberty (Yelich et al., 1995; Hall et al., 1995). The amount of energy stored in adipose tissue is an important component of total body energy balance, which is important in puberty attainment (Bronson and Manning, 1991) but it is unclear how important the total amount of body fat is in the attainment of puberty. Yelich et al. (1995) observed that total lean and bone mass were similar at puberty for heifers fed at different rates of gain regardless of fact

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19 that the amount of adipose tissue varied significantly. Yelich et al. (1995) concluded that the percentage of body fat is not the sole regulator in determining the onset of puberty in the heifer. The target weight principle states that puberty can be expected to o ccur at a genetically predetermined size among individual animals, and only when heifers reach their predetermined target BW can high pregnancy rates be obtained (Patterson et al., 1992). This will be further discussed later in the review. Season Cattle a re not generally thought of as seasonal breeders like sheep and horses where the incidence of estrous cycles is determined by day length. Early research focused on the season of birth and its effect on the onset puberty but with inconsistent results. Spr ing born heifers reached puberty at a younger age than fall born heifers (Menge et al., 1960; Roy et al., 1980; Grass et al., 1982; Hansen et al., 1983), fall born heifers reached puberty at younger ages than spring born heifers, (Schillo et al., 1982; Sch illo et al., 1992), and season of birth had no effect on when heifers reached puberty (Greer et al., 1983). It should be mentioned, that t he effects of climactic and nutritional stress were not taken into account in any of the aforementioned studies Oth er studies evaluated what effect supplementation lighting could have on the onset of puberty. Supplemental lighting of spring born heifers during the subsequent winter has been shown to decrease age at puberty (Peters et al., 1978; Hansen et al., 1983). It is inconclusive whether the decease in age at puberty was due to increased growth during the lighting phase (Peters et al., 1978) or if it was strictly a photoperiod effect (Hansen et al., 1983). It should be noted that season has been shown to influen ce LH secretion in cows. Increased LH secretion occurs during the spring; whereas, LH secretion is decreased in the fall months in mature ovariectomized cows (Day et al., 1986; Stumpf et al., 1988) suggesting that the bovine has the ability to receive and integrate seasonal cues that modulate gonadatropin release.

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20 Photoperiodic cues are thought to have subtle effects to induce an earlier age at puberty in fall born heifers by stimulation of LH release, which results in increased gonadal function and initi ation of estrous cycles (Kinder et al. 1990). Breed The genetic makeup of the heifer has a significant role in the age at puberty. Breed differences, sire and dam effects within a breed, and heterosis all contribute to the genetic control of age at puber ty (Wiltbank et al., 1966; Cundiff et al., 1986, Short et al., 1990). In general, faster gaining breeds of larger mature size reach puberty at later ages than do slower gaining breeds of smaller mature size. Laster et al. (1979) compared Hereford and Ang us cows bred to smaller mature sized Hereford and Angus sires, or larger mature sized Limousin, Charolais, Simmental, Gelbvieh, Maine Anjou, and Chianina sires. Age at puberty was later for all large sire breeds except Gelbvieh compared to the smaller mat ure size breeds. Breeds that have been selected for milk production like Holstein, Jersey, and Brown Swiss, tend to be younger and weigh less at puberty compared to breeds with similar genetic potential for growth and mature size that are not selected fo r milk production (Marten et al., 1992). The majority of the cattle raised in the Southeastern United States have some degree of Bos indicus breeding H eifers that are Bos indicus or Bos indicus cross breeds tend to mature slower and reach puberty at an older age (Plasse et al., 1968; Warnick et al., 1956 ). However, age at puberty in Brahman cattle can be reduced when crossed with Bos taurus cattle, which is a direct response of heterosis. The role of heterosis on the onset of puberty will be addressed later in this discussion. Reynolds et al. (1963) observed 209 Angus, Brahman, Brangus, and Angus x Brahman heifers for puberty over a four year period in Louisiana, and observed Angus to be the youngest at puberty (433 d), followed by the Angus Brahman (460 d), Brangus (531 d), and Brahman (816 d). Stewart et al. (1980) compared age at puberty in Angus, Brahman, Hereford,

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21 Holstein, and Jersey heifers. Brahman heifers were older at puberty compared to all other s with Holsteins being the youngest at pu berty. Mukasa Mugerwa (1989) reviewed data from various locations around the world giving a range of ages at puberty of B os indicus females from 15.6 to 40 months of age. Additionally, Peacock et al. (1976) reported that the percentage of Brahman breedin g in Bos indicus Bos taurus heifers was negatively correlated with the proportion of heifers calving as 2 year olds. In summary, Brahman heifers reach puberty at older ages compared to Bos taurus heifers and the inclusion of Bos indicus breeding into a breeding program can result in heifers that reach puberty at older ages. By definition, heterosis is the difference between the mean for reciprocal F1 crosses (e.g., Brahman Hereford or Hereford Brahman) and the mean for parental purebreeds (e.g., Bra hman and Hereford) contributing to the cross. It is caused by nonadditive gene effects, such as dominance or epistasis (Martin et al., 1992). In general, the age at puberty is younger for the crossbreds compared to the straightbreds that the cross came f rom. Dow et al., (1982) reported that 18% more crossbred heifers (Red Poll Hereford and Angus Hereford), were cycling at 11.5 months, 30% more at 15 months, and 12% more at 19.5 months, as compared to their straightbred Hereford contemporaries. By cr ossing Bos indicus with Bos taurus cattle age at puberty will be older than the Bos taurus but younger than the Bos indicus breeds used in the cross The range in age at puberty is approximately 14 to 24 mo nths for Bos indicus and 15 to 20 mo nths for Bos indicus Bos taurus crossbred heifers (Plasse et al., 1968), and 9 to 15 mo nths for Bos taurus (Wiltbank et al., 1966). This heterotic effect of age at puberty, can allow Bos indicus cattle to fit into many of the breeding programs in use today. From a b reeding management perspective several things can be done to select for an early age at puberty. First, a ge at puberty can be decreased by selecting a breed with a younger age at

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22 puberty; second, selecting within a breed for younger age at puberty; or thi rd, crossbreeding between breeds or with another breed that has a similar or younger age at puberty to take advantage of heterosis. Level of Nutrition Nutritional management has a significant influence on age at puberty in beef heifers and both energy and protein play major roles in the growth and development of heifers. Energy is usually the limiting factor in achieving satisfactory ADG and target body weights in growing heifers (Rice, 1991). One of the primary reasons energy is a limiting factor is bec ause forages are not high enough in energy to meet the nutrient requirements of the developing heifer. Energy requirements vary based on BW, frame size, breed, and environment. Of particular interests to producers in the Southeastern United States is tha t growing cattle of Bos indicus breeding require approximately 10% less energy compared to Bos taurus breeds for maintenance (NRC, 1996). However, when supplementing with energy, it is very important to understand the consequences of feeding too much or t oo little energy as it relates to the development of the growing heifer. Underfeeding energy can result in increased age at puberty, reduced conception rates, and underdeveloped udders (Sorenson et al., 1959; Wiltbank et al., 1966; Short and Bellows, 1971) In contrast, overfeeding in creases cost and can result in weak estrus, reduced conception rates, high embryonic mortality, decreased mammary gland development, and a decrease in subsequent milk production (Sorenson et al., 1959; Wiltbank et al., 1966; Short and Bellows, 1971; Gardner et al., 1977). It is well documented that growth rate influences age at puberty in beef heifers (Warnick et al., 1956; Menge et al., 1960; Reynolds et al., 1963). Arije and Wiltbank (1971) reported that high preweaning gr owth rates and heavy weaning weights were associated with early onset of puberty and heavier weights at puberty. Bellows et al. (1965) fed crossbred heifers at low,

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23 medium, and high levels of dietary energy to attain ADG of 0.23, 0.5, and 0.67 kg/d, respe ctively during the winter after weaning. Heifers fed the lower levels of energy achieved puberty at 433 d ays of age, while the medium and high heifers achieved puberty at 411 and 388 d ays of age, respectively. Pregnancy rates were also affected by the en ergy level fed with low energy heifers having pregnancy rates of 63% and medium and high heifers having pregnancy rates of 90%. Greer et al. (1983) analyzed data on 556 heifers in Montana and reported that heifers were heavier at weaning and fed at highe r levels of nutrition reached puberty at a heavier weight and earlier ages. Patterson et al. (1989) reported that Brahman Hereford heifers fed to reach 65% of mature BW had an increased first service conception rate of approximately 30% compared to heif ers to reach 55% mature BW. In summary, as postweaning nutrient intake is increased resulting in greater ADG, age at puberty is decreased (Arije and Wiltbank, 1974; Bellows et al., 1965; Grass et al., 1982; Greer et al., 1983). Producers often think that protein is the most important nutrient for growth of developing heifers and commonly overfeed protein to heifers (Rice, 1991). Oyedipe et al. (1982) fed Zebu heifers isocaloric diets with two levels of protein, and reported that the high protein group had decreased age at puberty and increased pregnancy rates compared to the lower protein group, which agrees with several earlier studies (Bellows et al., 1965; Wiltbank et al, 1966; McCartor et al., 1979). Clanton and Zimmerman (1970) fed heifers four diffe rent diets including high protein and energy, low protein and energy, high protein low energy, and low protein high energy. High protein and energy heifers had the greatest ADG (0.37 kg/d) followed by the high protein low energy (0.20 kg/d) and low prote in high energy heifers (0.20 kg/d) and finally the low protein and energy heifers (0.10 kg/d). The average age of the first estrus was young est for high protein and energy heifers (384 d) with the other treatments being similar (>450 d). For

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24 diets adequa te in energy, higher protein can support faster growth that results in an earlier age at puberty and greater pregnancy rates compared to diets with restricted protein. Energy and protein are both vital components for developing heifers. If one is deficien t the optimal growth will not be realized. Although, energy is the most limiting nutrient, supplementation with protein is necessary when feeding low quality forages. O ver feeding of either energy or protein is inefficient as it increases costs but it ca n also cause short and long term problems associated wi th productivity of the cattle. Management Management of replacement heifers should focus on factors that enhance physiological processes that promote puberty (Patterson et al., 1992). Several manage ment strategies and tools are available to producers that can be used either alone or together in decreasing age at puberty in heifers. Sorting heifers into groups based on weight has received some attention as a management tool in heifer development. Di viding heifers based on weaning weight should allow nutritional requirements to be met more accurately. Staigmiller and Moseley (1981) reported that separating heifers into heavy and light groups allowed heifers to more closely attain their target weights as compared to a group that was not separated. In addition, pregnancy rate was greater for the groups fed separately (85%) compared to the non separated group (70%). Varner et al. (1977) also reported a decreased age at puberty and increased pregnancy r ates for heifers separated by weight compared unsorted groups of heifers. Wiltbank et al. (1985) separated heifers into two groups, by weight. Weight groups were then divided and fed to a light or heavy target weight. The separation allowed the groups t o reach the target weights and no differences in age at puberty or pregnancy rates were found between the two groups. Separation of heifers

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25 into groups based on BW for feeding can increase performance of small heifers and possibly decrease age at puberty and improve reproductive performance. Creep feeding is a process of feeding calves prior to weaning as a means to improve calf gains. Although, creep feeding can increase weaning weights, it can alter postweaning growth and development and subsequent prod uctivity. Holloway and Totusek (1973) reported that creep feeding high energy feeds to heifers can have detrimental effects on milk production. Martin et al (1981) reported that creep fed heifers had heavier BW at weaning compared to controls but had eq ual BW at one year of age. However, animal longetivity, number of calves weaned, and average weaning weight of calves from creep fed dams was reduced. Similar decreases in performance were reported by Mangus and Brinks (1971), Beltran (1978), and Johnson and Obst (1984). Creep feeding can increase weaning weights, which can aid in puberty attainment, but the long term effects, future milk productivity, future calf production, and the overall longetivity of the heifers productive life. In animals fed on p astures, natural infections with gastrointestinal nematodes usually occur. The presence of a subclinical parasite burden will affect nutrient utilization and possibly alter animal metabolism. Consequently, i t can have a major impact on heifer productivity and her ability to efficiently gain weight during that critical period leading up to the onset of puberty. In beef herds, internal parasites reduce growth rate (Ryan et al., 1997) and retard sexual maturation (Zajac et al., 1991). Purvis and Whittier (1 996) reported that 9 month old heifers that received one treatment of Ivomec had decreased fecal egg counts, increased feed:gain (0.132 vs 0.125), decreased age at puberty (424 vs 434 d), and decreased BW at puberty (349 vs 362 kg) compared to untreated h eifers. However, no differences in first service conception or overall pregnancy rates between treatments were observed. Meja et al. (1999) reported growing

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26 Holstein heifers treated with ivermectin every 14 d from birth to 150 kg followed by ivermectin ruminal bolus administration had decreased fecal egg counts, increased BW gain (0.12 kg/d), increased pelvic are (11% greater), and decreased age a puberty (3 wk earlier) compared to untreated heifers. Treatment with ivermectin has been shown to decrease age at puberty by up to three weeks compared to untreated heifers (Larson et al., 1995) and to increase conception rates (Rickard et al., 1992). Use of anthelmentics is a simple management strategy that can assist in the development of growing heifers. Ionophores like monensin, lasalocid, and laidlomycin are antimicrobial compounds that are commonly fed to cattle to improve feed efficiency. These antibiotics alter the rumen microflora resulting in increased efficiency and decreased acetate:propionate ra tio in the rumen Propionate is an important precursor for glucose in gluconeogenesis. Propionate accounts for up to 76% of the glucose synthesized in the liver (Reynolds et al., 1994). By increasing glucose concentrations, IGF I concentrations will als o be increased. High IGF I concentrations have been associated with onset of puberty (Yelich et al., 1995, 1996; Cooke et al., 2007). Inclusion of ionophores in heifer diets has been shown to decrease the age at puberty and increase gain: feed ratios (Pur vis and Whittier, 1996, Mosely et al., 1977 Staigmiler and Mosely, 1981; Purvis et al., 1993 ) McCartor et al. (1979 ) reported that heifers fed a 50:50 roughage:concentrate diet with monesnsin had equivalent age and weight at puberty as heifers fed a 20: 80 roughage:concentrate diet. Inclusion of an ionophore to the diet can assist in decreasing age at puberty and increasing BW gains for heifers consuming diets high in roughage. Heifer Development Systems There are many programs utilized by producers in t he development of replacement heifers. As previously stated the goal of any heifer development program should be to grow heifers to a point where the majority of heifers are pubertal prior to the start of the breeding season. Heifers

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27 should be managed se parate from the rest of the cowherd for several reasons. First, heifers have different nutrient requirements than both growing and mature cows, which is primarily related to their increased growth requirements. Second heifers do not have the body size t o compete with larger cows for feed resources. Competition from older and larger cattle often results in decreased nutrient intake for heifers resulting in decreased growth rates and a delayed onset of puberty. The target BW principle is commonly used by beef cattle producers to determine the necessary BW gains for heifers to reach puberty. Studies indicated that puberty can be expected to occur at a genetically predetermined BW among individual heifers, and only when heifers reach their predetermined ta rget BW can high pregnancy rates be obtained (Lamond, 1970; Taylor and Fitzhugh, 1971). It is generally recommended that beef heifers attain between 53 to 66% of mature BW, depending on frame size and breed type (Patterson et al., 1992; Funston and Deutsc her, 2004). Heifers with the genetic potential to reach a heavier mature BW must attain a heavier BW before the start of the breeding season. Patterson et al. (1989) fed Bos taurus and Bos indicus heifers to reach a target BW of 55 or 65% of mature BW at breeding. Heifers fed to reach 65% of mature BW had greater 45 d pregnancy rates compared to heifers fed to reach 55% of mature BW. Bos taurus heifers fed to the lower target BW also had increased dystocia compared to the higher target BW Bos taurus and Bos indicus groups. Dale et al. (1959) reported that Brahman heifers reached puberty at 60% of mature BW and 95% of mature height. The target BW principle allows producers to calculate expected gains and determine the required nutrient intake to reach t hese gains. The most common method of feeding heifers to reach their target BW is to feed heifers to gain at a constant rate throughout the feeding phase prior to the breeding season. The ADG most

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28 commonly targeted is approximately 0.75 kg/d. Programme d feeding of heifers is another viable alternative for the development of heifers. Programmed feeding usually involves a period during the beginning of the development phase when heifers are fed to gain at a slower rate, often less than 0.5 kg/d. This ph ase is followed by a phase where heifers are fed to gain at a rate usually > 1 kg/d to reach their target BW prior to breeding. The theory behind the program feeding method is that takes advantage of the potential for compensatory growth as the heifers ad just from a low to high rate of gain. Compensatory growth is an accelerated rate of growth and an increased efficiency of feed utilization experienced after a period of restricted growth. Additionally, programmed feeding could be used to reduce the total amount utilized and eventually decrease the total cost in developing replacement heifers. Weekley (1991) evaluated the traditional continuous gain regiment to a programmed feeding regiment in yearling heifers that were drylotted and fed a silage based di et. Heifers were fed either to gain continuously (ADG = 0.61 kg/d) from weaning to breeding (5 mo) or to gain at a slower rate (0.53 kg/d)for the first three months followed by higher gains (0.74 kg/d) the last two months. At the end of the first three m onths, the heifers gaining at a continuous rate were 18 kg heavier compared to slower gaining heifers. However, at the end of the trial, BW was similar between treatments. Heifers on the continuous gain treatment attained puberty 30 days earlier compared to the programmed gain, but breeding season pregnancy rates were similar between treatments. Lynch et al. (1997) developed heifers over two consecutive years using corn and silage to gain at a continuous rate of BW gain (0.45 kg/d) or at a decreased rate (0.11 kg/d) for the first four months followed by an increased rate of BW gain (1.0 kg/d) for the last two months. Age at puberty did not differ in the first year, but was delayed in the late gaining heifers in the second year by an average of three week s. At the start of the breeding season, BW, BCS, and frame scores were

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29 not different between the two groups. Furthermore, breeding season pregnancy rates were similar between the groups. It should be noted that the low to high gaining heifers consumed l ess feed on both years of the trial (year 1 = 12%, year 2 = 2.5%). Clanton et al. (1983) fed heifers a corn silage based diet to gain at two rates: continuous rate of BW gain (0.5 kg/d) or no BW gain for the first half of the trial followed by 1.0 kg/d ga in for the last half of the trial. Body weights, wither heights, and heart girths were greater for heifers fed at a continuous rate after the end of the first period of the trial. Body weight gains for the restricted heifers were greater than predicted d uring the second period of the trial, which the authors attributed to compensatory growth. By the end of the trial, BW, heart girth, and wither height were similar between treatments. Age and BW at puberty were not different between treatments. Granger et al. (1990) conducted an experiment evaluating the effects of wintering diet on subsequent heifer growth and reproductive performance. Heifers were wintered (107 d) on a diet of poor quality hay ( H ; 4.7% CP, 77% NDF, and 41% ADF), ammoniated hay ( AH ), h ay plus cottonseed meal ( HC ), or hay plus cottonseed meal and corn ( HCC ). During the wintering period, H heifers lost 0.20 kg/d, AH heifers lost 0.10 kg/d while HC heifers gained 0.17 kg/d, and HCC heifers gained 0.29 kg/d. Following this wintering perio d, heifers were commingled on ryegrass pasture for early spring grazing (70 d) followed by summer grazing (147 d) with strategic supplementation. During the ryegrass grazing period, H and AH (0.90 and 0.71 kg/d, respectively) heifers experienced compensat ory gains, which were greater than HC and HCC heifers (0.63 and 0.68 kg/d, respectively). Age at puberty was greater for H, AH, and HC heifers (560, 530, 502 d, respectively) compared to HCC heifers (476 d). Both H and AH heifers tended to have decreased pregnancy rates (mean = 76% vs 90%) and calving rates (63 vs 83%) compared to HC and HCC heifers, respectively. In summary, utilization of programmed feeding of yearling

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30 heifers has been shown to increase feed efficiency through compensatory growth and h ave minimal effect on reproductive performance of growing heifers. The use of compensatory gain in the development of heifers can provide a viable alternative system for the development of heifers. Forages are the base of almost every heifer development p rogram used by producers today. Forages can be grazed forages or stored forages, including as hay and silage. There are many factors that a ffect the selection of the type of forages used by producers including season, availability, price, and quality. I ncreasing the quality of the forages fed to the heifers should decrease the cost of supplementation. High quality forages can be procured through proper pasture management techniques, utilization of winter or summer annuals, or proper hay field and harves t management. Bahiagrass ( Paspalum notatum ) is the primary pasture forage available in Florida, covering an area of approximately 1 million ha (Chambliss and Sollenberger, 1991). Limpograss ( Hermathria altissima ), bermudagrass ( Cynodon dactylon ), and star grass ( Cynodon spp.) are also commonly used as forage source for cattle (Arthington and Brown, 2005). The aforementioned, tropical and sub tropical perennial grasses often do not have either adequate nutrient composition to meet the requirements of cattle or are of such low quality that cattle cannot consume enough to meet their nutritional needs (Moore et al., 1991). Moore et al. (1991) compiled the nutritional analysis of 637 samples of forages commonly grown in Florida (bahiagrass, bermudagrass, digitg rass, stargrass, and limpograss) and reported that most of these grasses contained between 5 to 7% of CP (DM basis), and 48 to 51% of TDN (DM basis) on a yearly basis In comparison, d evelop ment of growing heifer of average size requir e s a diet containing approximately 55% TDN and 8% CP of diet DM (NRC, 1996). Consequently, to meet the nutritional requirements

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31 of the growing heifer supplementation must be considered. Protein supplementation can increase intake of low quality forages (Kster et al., 1996; Moore et al., 1999; Kunkle et al., 2000) which can sometimes aid in meeting the energy requirements of the heifer, but often does not (Bodine and Purvis, 2003) In contrast high energy supplements may substitute voluntary forage intake, resulting in dec reased forage DMI and digestibility (Caton and Dhuyvetter, 1997; Bodine and Purvis, 2003). Moore et al. (1999) reviewed 66 p ublications and c oncluded that supplements decreased intake when the : 1) TDN:CP ratio was < 7 (adequate N) except for ammoniated st raws, 2) forage intake fed alone was > 1.75% of BW, 3) supplemental TDN intake was > 0 .7% of BW. Because of these findings supplementation programs need to be designed to closely meet the nutritional requirements of the animal to be supplemented based o n the inadequacies of the consumed forage and economic viability of feeding the supplement (DelCurto et al., 2000; Kunkle et al., 2000). As mentioned previously, m ost perennial forages found in Florida are warm season grasses which are limited in quantity during the cool months of the year. Th e cool months include November through April for north Florida and January through March for south Florida depending on species of forage (Sollenberger and Chambliss, 1991). The amount of forage available is compound ed by decreased rainfall typical during the period from April to early June (Chambliss et al., 1998; Sollenberger and Chambliss, 1991). The amount of forage available for grazing is limited by geographical region, season, and rainfall availability. As a result, m ethods to conserve surplus summer forage in the forms of silage, baleage, and hay, can allow producers to provide forage to cattle throughout the year (Sollenberger and Chambliss, 1991). Timely harvest of forages for hay can make a dramatic diff erence in forage quality. The utilization of round bale silage (RBS) is one method of increasing forage quality through timely

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32 harvest. Hersom et al. (2007) examined the stored forage quality and forage yield of two different forage management systems. A hay harvest only system, which allowed for hay production when weather conditions allowed, and a RBS system, which harvested the forage every four weeks, were compared. The RBS system resulted in two more cuttings of forage harvested, 55 tons more forag e dry matter, and a greater TDN (57.1%) and CP (12.9%) as compared to the hay only system (TDN = 53.8%, CP = 10.1%) The method of forage preservation (silage, baleage, or hay) and its possible effects on animal performance must also be considered. Peters on et al. (1974) reported that sheep offered alfalfa hay or alfalfa haylage had no differences in DMI, meals/d, or time spent at the feeder. Luqinbuhl et al. (2000) reported that steers fed switchgrass preserved as hay or ensiled had altered patterns of i ntake. Steers consuming the ensiled forage had greater DMI, total ruminating time. However, preservation type had no effect on time spent eating. Helene et al. (1992) reported that steers fed timothy silage had decreased DMI (0.8 kg/d) and ADG (0.49 g/d ) compared to steers consuming timothy hay. However, supplementation with fish meal improved DMI and ADG for steers consuming timothy silage so that they were similar to hay fed steers. Ensiling of grass hay, as haylage or baleage, offers an opportunity to increase nutritive quality and total tonnage of forage harvested. Regardless of the forage source used, knowledge of the nutritive value of that forage is essential in determining if supplementation is necessary and meeting the needs of the animal. S upplementation Programs According to Burns (2006), the beef cattle industry in the Southeast is nearly 100% dependent on grasslands for production. However, the forages grown in Florida usually do not have adequate nutrient density to meet the nutrient re quirements of cattle consuming typical amounts of DM (Moore et. al., 1991), particularly cattle with increased nutrient demands such as

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33 growing yearling heifers and pregnan t or lactati ng cattle ( NRC 1996) Supplementation of growing heifers with energy a nd protein is often necessary to meet the nutrient demands for growth in order for the heifer to reach puberty. Energy S upplementation Energy supplements can be separated into supplements high in non structural carbohydrates (NSC) and those with a low NSC that tend to be high in NDF. The NRC (2001) defines the NSC fraction of a feedstuff as being found inside the plant cells, and is composed of sugars, starches, organic acids and other carbohydrates that serve as the major source of energy for cattle. T he NSC fraction is highly soluble and rapidly degraded in the rumen. By increasing the NSC content in a diet, ruminal VFA production is elevated resulting in decrease d ruminal pH which may be detrimental to fibrolytic bacteria resulting in impaired forag e digestibility (Aldrich et al., 1993; Knowlton et al., 1998). Rum e n pH 6.2 has been proposed as the level at which fiber digestion is reduced in the rumen (Miller and Muntifering, 1985; Hoover, 1986). In e nergy containing feed resources that are low in NSC ruminal microorganisms utilize NDF or pectin as the ir source of ene rgy Neutral detergent fiber is the measurement that best differentiates structural from nonstructural carbohydrates and constitutes most of the structural components in plant cells including cellulose, hemicellulose and lignin. Generally, NDF is less di gestible than NSC and its concentration in feedstuffs is negatively associated with energy content of the feedstuff (NRC, 2001). The constituents of NDF also affect fiber digestibility. Cellulose and hemicellulose are digestible by the microbial populati on in the rumen. In contrast, lignin is indigestible and decreases the total digestibility of the other feed components. Therefore, feedstuffs with similar NDF concentrations will not necessarily have similar energy concentrations because of the lignin c oncentrations. Depending on the lignin concentration, feeds with greater NDF may have more available energy than feeds with lesser NDF content

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34 (NRC, 2001). The concentration of NDF in the diet is positively associated with ruminal pH since the microbes g enerally ferment the NDF fraction slower. Therefore, less ruminal VFA production/accumulation and a greater acetate:propionate molar ratio in the rumen may result. Animal performance Energy supplementation of cattle is often required to meet energy requir ements and maintain acceptable animal performance. Often available grazing is inadequate to meet the energy requirements of cattle (Moore et al., 1999). Depending on the quality of the forage, protein supplementation may increase forage DMI enough to mee t the energy requirements of the animal, but not in all cases (Bodine and Purvis, 2003). In cases of forage energy deficiency the supplementation of high energy containing feedstuffs can potentially improv e animal performance. Horn and McCollum (1987) re viewed energy supplementation of grazing cattle and concluded that concentrates can be fed at 0.5% BW before forage intake was decreased. Bowman and Sanson (1996) determined supplementing grain over 0.25% of BW had negative effects on forage utilization. Sanson et al. (1990) demonstrated that increasing levels of corn starch supplementation resulted in decreased forage intake. Crossbred steers consuming low quality hay were utilized to evaluate the effects of supplements with increasing levels of starch. Supplements provided 0, 2, or 4 g / kg of BW of starch from corn. Forage DMI decreased by 5% for the supplement containing 2 g starch/kg of BW and 22% for the supplement containing 4 g starch/kg of BW DelCurto et al. ( 1990) used steers to determine the effects of high and low protein supplements (0.66 and 1.32g CP per kg BW) in combination with high and low energy supp lements (9.2 and 18.4 kcal ME/ kg BW) on low quality forage intake and digestion.

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35 Increasing energy supplementation tended to depress for age intake with no effect on total dry matter intake, suggesting a substitution effect caused by energy supplementation. The use of feedstuffs high in TDN but low in NSC can prevent the decrease in DMI (Sunvold et al., 1991; Horn et al., 1995) By product feeds, such as soybean hulls, wheat high levels of fermentable fiber for energy. These high fiber by product feeds are usually considerably cheaper than h igh NSC grain products. Sunvold et al. (1991) evaluated the efficacy of wheat middlings at two levels (low = 0.39%/BW, high = 0.78%/BW) compared to sorghum as a supplement for beef cattle consuming low quality forage. Forage dry matter intake was increas ed for the high level of wheat middlings. Dry matter digestibility was increased with supplementation, but there was no difference between supplements. Similar results were reported by Horn et al. (1995 ) that compared the effects of high starch corn base d supplement to high fiber soybean hulls ( SBH ) and wheat middling based supplement. Calves grazed wheat pasture and were supplemented daily at 0.65 % of BW Daily gain was not influenced by the type of supplementation in this study. These studies indica te that high starch supplements, that are a good supply of energy could be replaced by highly digestible fiber supplements which may compliment the forage base and still provide adequate energy Bowman and Sanson (2000) reviewed the literature and conclu ded that low NSC energy supplements increased low quality forage intake and digestibility. High starch diets appear to influence the age or BW of puberty in heifers. Gasser et al. (2006) reported that heifers fed a high starch diet (60% corn; NEm = 2.02 M cal/kg, NEg = 1.37 Mcal/kg) were younger but had lighter BW at puberty compared to heifers fed a lower energy control diet (30% corn; NEm = 1.70 Mcal/kg, NEg = 1.09 Mcal/kg) Marston et al. (1995)

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36 reported that heifers fed a high concentrate diet reached puberty at the same BW but a younger age than heifers on a lower energy diet. While still others have reported that high concentrate diets decrease age at puberty, but increase BW at puberty (Yelich et al., 1995). Supplementation of energy to cattle con suming poor quality forages can boost BW gains and effect age at puberty. However, high levels of starch can have negative effects on DMI. The use of feeds high in TDN but low in NSC can overcome the negative effects of high starch diets with little or n o loss of performance. The supplementation of energy based feeds can effect the production of metabolites and hormones in the growing heifer. Changes in the diet that alter the rumen fermentation pattern towards propionate production have been shown to de crease age and BW at puberty (McCartor et al., 1979). High energy, high quality supplements can effectively alter rumen fermentation patterns toward propionate production (Bagley, 1993). Propionate is the primary substrate for gluconeogenesis in the rumi nant (Bergman, 1973). Forage fed cattle depend significantly on liver gluconeogenesis to meet their metabolic glucose requirements (Huntington, 1997). Randel (1990) reviewed several studies and reported that diets that increased the propionate production in the rumen have been shown to promote gluconeogenic activity, which hastens the onset of puberty in beef heifers. Glucose is essential for maintenance and reproductive function in ruminants (Huntington, 1997; Reynolds, 2005). Research has indicated t hat blood glucose concentrations in beef cattle are positively associated with feed intake and rates of BW gain (Vizcarra et al., 1998; Hersom et al., 2004; Cooke et al 2007). Cooke et al. (2007) reported no positive associative effects of blood glucose on puberty in Brahman crossbred heifers. G ondatropin releasing hormone secretion has been shown to be impa ired by low glucose concentrations but return to normal

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37 when glucose concentrations are adequate (Hess et al., 2005). Reynolds et al. (1989), using nutritionally anestrus cows, reported that a lack of available glucose may not allow adequate secretion of LH to stimulate ovarian function. In contrast, McCaughey et al. (1988) reported that infusion of glucose did not affect LH secretory patterns in co ws on adequate plane of nutrition. Wettemann and Bossis (2000) reported that glucose concentrations in anestrous heifers were similar to those observed in estrous cycling heifers at least two follicular waves before the wave that resulted in the ovulatory follicle The positive effects of increased glucose through enhanced gluconeogenesis on reproduction of cattle may be associated with improvements in overall energy status and concentrations of other blood metabolites and hormones instead of the increase in glucose availability (Hess et al., 2005). Increases in nutrient intake generally lead to increases in blood glucose concentrations resulting in increases in insulin and insulin like growth factor I (IGF I). Both insulin and IGF I are associated with i ncrease d LH secretion (Rutter et al., 1983; Schillo et al., 1992) but it is not clear if they have a direct effect on the secretory capacity of the hypothalamic hypophyseal axis to enhance LH secretion Similar to glucose, insulin concentrations have been positively associated with feed intake and rate of BW gain of cattle (Bossis et al., 2000; Lapierre et al., 2000; Cooke et al 2007). Cooke et al. (2007) reported no correlation between blood insulin concentration s and attainment of puberty or pregnancy r ate in heifers. In contrast, Sinclair et al. (2002) reported that postpartum anestrous beef cows with low plasma insulin concentrations (< 5 mIU/L) failed to ovulate the first dominant follicle in response to restricted calf access compared to cows with m oderate plasma insulin (5 to 8 mIU/L). This result was attributed to an inadequate

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38 responsiveness of the dominant follicle to the increase in LH secretion that frequently accompanies restricted calf access. In beef cattle, feed intake and BW gain have be en positively associated with circulating IGF I concentrations ( Bossis et al., 2000; Armstrong et al., 2001). Recent studies indicated that IGF I is a major metabolic signal regulating reproduction in cattle (Wettemann and Bossis, 2000). The direct effec ts of IGF I on LH secretion and steriodogenisis are integral to reproduction. Research with heifers reported that IGF I was positively correlated with ADG and reproductive performance (Cooke et al. 2007; Cooke et al., 2008). Although, Granger et al. (19 89) reported that plasma IGF I concentrations were negatively associated with age at puberty. Meeting the energy demands of the heifer is essential for growth and reproductive function. Increasing nutrient intake has positive effects on growth and repr oduction which are mediated through the alteration of metabolites. Protein S upplement ation Protein is often the most limiting nutrient in forages in the fall and winter months, especially in Florida (Hughes and Hersom, 2009a; Hughes and Hersom, 2009b) A great deal of research has been conducted in the area of protein supplementation to forage based cattle diets. Protein quality in ruminants is based on the availability of amino acids leaving the rumen rather than the availability of amino acids in the di et. The amino acids supplied to the small intestine include those from rumen bacteria, also called microbial protein, sloughed gastro intestinal tract cells, digestive enzymes and those from the feed. This mix of sources creates the complexity of rumina requirement has been the understanding of the separate requirements of the rumen microbes and the ruminant. Microbial protein rarely provides adequate protein to meet the requirements of growing ruminants (NRC, 1996).

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39 Dietary protein can be divided into two types. The first type is protein that is broken down by rumen microbes and converted to either ammonia or amino acids, which is used by rumen microbes and refereed to as rumen degradable protein (RDP) or degradable intake protein (DIP). A portion of the RDP is enzymatically digested in the small intestine along with the microbial protein passing from the rumen to yield the amino acids absorbed for tissue nitrogen m etabolism. The remaining portion of dietary CP that is not metabolized by rumen microbes is referred to by several names such as bypass protein, escape protein, rumen undegradable protein (RUP) or undegradable intake protein (UIP). Feedstuffs vary greatly in their rumen degradability and, therefore, contain different fractions of RUP and RDP, regardless of their total CP content. Based on differences in microbial protein requirements and animal protein requirements, improving only dietary CP may not impro ve animal performance. For this reason, the metabolizable protein (MP) system, which The metabolizable protein system (NRC, 1996) covers the requirements of microbial an d animal N. Intake protein (IP) refers to the N in the feed consumed and can be utilized several ways by the ruminant. If the levels of DIP and UIP in feedstuffs and forages are quantified, supplements can be designed to fill the forage deficiency and be tter meet the requirements of the animal. Supplements should be formulated based on forage quality and quantity, animal requirements, and desired level of performance. Protein can be further broken down into natural and non protein nitrogen (NPN) sources Natural proteins are characterized as sources of protein that contain naturally occurring essential and nonessential amino acids. A variety of by product feedstuffs of both plant and animal origin, such as oilseed meals, feather meal, and blood meal, f all into the category of natural

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40 proteins. N on protein nitrogen is nitrogen which comes from sources other than proteins, but which may be metabolized by rumina l microbes in the building of proteins. N on protein nitrogen feed sources consist of urea, biu ret, and anhydrous ammonia. Animal response to degradable intake protein Moore et al. (1991) compiled the nutritional analysis of 637 samples of commonly grown Florida grasses and reported that the grasses contained 5 7% CP (DM basis). Developing heif ers require at least 8.5% CP of diet DM to sustain growth rates > 0.5kg/d (NRC, 1996) Degradable intake protein is often considered to be the first limiting nutrient in poor quality forages. Kster et al. (1996) determined that mature cows require 11% D IP as a percentage of digestible organic matter ( OM ) Supplementing with DIP promotes increased forage intake and flow of nutrients to the small intestine (Hannah et al., 1991). Mathis et al. (2000) suggests that 7% CP (range of 6 8% CP) in the basal for age was considered to be the threshold for a response and when CP is greater than approximately 7%, protein supplementation has little benefit. However, when cattle are fed forages with CP below this threshold, the response to supplemental protein can be tremendously variable (Heldt et al., 1999). Other research has focused on the balance between DIP and energy or TDN. Supplements high in DIP are most effective when the N content of pasture is less than the requirement for optimal microbial growth in the rumen and when digestible energy is readily available (Sierbert and Hunter, 1982). Microbial nitrogen requirements are generally met when CP is 9 to 13% of dietary TDN (NRC, 1996), which would require 5.8 to 7.8% DIP, to satisfy microbial requirements (N RC, 1996). Moore and Kunkle (1995) suggested that requirements for DIP are generally met when the TDN:CP ratio is 7 or less It is often difficult to determine the TDN:CP ratio of forage consumed by grazing cattle due to selective grazing nature of cattl e (Kunkle et al., 1994). Plasma urea nitrogen ( P UN) concentrations have been used as indicators of adequate DIP relative to TDN. Hammond et al.

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41 (1994) reviewed several experiments and concluded that P UN concentrations above 10 mg/dL indicated a acceptabl e balance between dietary DIP and TDN, but BUN concentrations below 8 mg/dL indicate that dietary DIP may be deficient. Supplementi ng growing cattle consuming low quality forage with different DIP sources has yielded mixed results as measured by animal p erformance. Kster et al. (1996) examined the effects of supplemental DIP on DMI and digestion of low quality hay by beef cows. Cows were supplemented with five levels (0, 180, 360, 540, and 720 g/d) of DIP (sodium caseinate) intraruminally administered twice daily. Forage OM intake increased quadratically with the increasing supplemental DIP, peaking at 540 g/d. Ruminal OM and NDF digestion exhibited variable responses to increased supplementation. The authors concluded that increasing supplemental DI P generally improves forage utilization Mathis et al. (2000) also examined the effects of supplemental DIP on the utilization of low to medium quality forages by beef steers in three experiments. These forages were offered ad libitum and DIP (sodium ca seinate) was supplemented intraruminally once daily at one of the three levels of supplementation, 0, 0.041, 0.082, or 0.124% BW. In the first two experiments, forage intake and NDF digestibility were not affected by increasing DIP levels. Ruminal ammoni a concentrations increased linearly with increasing DIP supplementation, but the response for total volatile fatty acid concentration was quadratic with increasing DIP supplementation. In the third experiment forage intake and NDF digestibility were incre ased linearly as DIP supplementation increased. Ruminal ammonia and volatile fatty acid concentrations also increased linearly as DIP supplementation increased. The authors concluded that improvements in intake and digestion of low quality forages were a chieved by DIP supplementation. However, the response to supplemental DIP was not consistently predicted. Mathis et al. (1999) supplemented DIP in the form of soybean meal

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42 (SBM) to steers consuming low quality forage and examined the effects on intake an d digestibility. The experiment had five treatments consisting of a no SBM (control), 0.08, 0.16, 0.33, or 0.50% BW per d of supplemental SBM on a DM basis. Forage OM intake increased cubically as the level of SBM increased. Organic matter digestibility increased quadratically as the level of SBM increased in the diet. Ruminal pH tended to decline with increasing level of supplemental SBM (Control = 7.02, 0.5% BW = 6.91), whereas ruminal ammonia nitrogen concentration increased dramatically in response to supplementation (Control = 0.62; 0.5% BW = 6.20). Supplementation with DIP can improve the performance of animal consuming low quality forages. Animal response to non protein nitrogen Non protein nitrogen (NPN), such as urea or biuret, can be fed to ca ttle to provide the N necessary for rumen microbes to produce protein provided there is enough fermentable substrates present to produce energy. Because fibrolytic bacteria use ammonia as a chief N source (Russell et al., 1992), one should be able to sub stitute NPN for a portion of DIP. N on protein nitrogen sources are metabolized into ammonia in the rumen and t h e ammonia can be utilized by rumen microbes or absorbed across the rumen wall into the bloodstream Circulating ammonia is metabolized by liver into urea. Excessive ammonia concentrations and rapid absorption into the circulation can lead to potential urea toxicity. It has been suggested that urea should supply no more than 15% of total dietary DIP when supplemented to cattle consuming low qual ity forage (Farmer et al., 2004). Kster et al. (1997) examined the effect of increasing the proportion of NPN in the diet on the utilization of low quality forage. Fistulated steers were fed tallgrass prairie forage ad libitum and supplemented with 380 g /d of DIP from sodium caseinate and/or urea 0, 25, 50, 75, and 100%. The supplements were administered intraruminally twice daily. Intake of forage was not

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43 affected by urea level. Ruminal and total tract digestibilities of OM and NDF responded in a quad ratic manner to increasing urea inclusion. Clanton et al. (1978) compared the effects of urea and biuret to natural protein sources (s oybean meal and alfalfa hay) fed to growing calves on native range. Across all experiments, gains were either less or no t different as level of NPN increased. However, ADG were greater (0.04 kg) for the natural protein supplements compared to NPN supplements. Stateler et al. (1995 ) examined the effects of protein supplementation in molasses slurries on the performance of growing cattle fed bermudagrass hay. Cattle were supplemented with varying protein levels and sources including molasses, molasses+urea, molasses+soybean meal, and molasses+blood meal+feather meal. Supplementation increased ADG over the unsupplemented ca ttle. Supplementation with molasses+urea demonstrated no advantage over molasses supplementation alone. Supplementation with natural protein sources increased ADG by 0.10 kg/d in year 1 and 0.06 kg/d in year 2 compared with molasses+urea. The authors co ncluded that the use of NPN in growing cattle is not recommended. The performance of animals supplemented with NPN sources has been attributed to the inefficient utilization of NPN as a result of asynchronous availability of energy and nitrogen in the rum en (McCollum and Horn, 1990). Growing cattle and high producing dairy cows usually need greater MP than they can obtain from microbial protein and forage UIP (NRC, 1996). When microbial protein production is limited, or animal AA requirements are high, ruminally produced microbial protein may not meet the AA needs of the ruminant (Merchan and Tigmeyer, 1992). Under these conditions an than optimal, unless AA from a UIP source are provided. Many authors agree that UIP supp lementation is only beneficial if DIP levels are adequate relative to requirement (Klopfenstein, 1996; Paterson et al., 1996).

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44 Degradable intake protein supplementation in cattle allows for optimal microbial growth in cattle fed or grazed on forages that a re low in CP relative to TDN. Currently, DIP requirements are assumed to be met when dietary CP is 9 13% of dietary TDN, when TDN:CP is 7 or lower, or when BUN concentrations are greater than or equal to 10 mg/dL. Reports indicate that cattle have increa sed gains when supplemented with natural DIP sources or NPN; however considerable variations in animal performance are evident. Animal response to undegradable intake protein Stateler (1993) reported that supplementation with 140 g/d of UIP from a blood me al+feather meal mix in molasses; increased ADG of steers 0.11 kg compared to steers fed only molasses Kunkle et al. (1994) also reported increased ADG (0.17 kg) in steers consuming low quality stargrass hay, when supplemented with 0.15 kg/d of feather me al and molasses+urea compared to steers consuming the molasses+urea product. Anderson et al. (1988) fed steers molasses based supplements providing 110, 230, or 340 g/d UIP from corn gluten meal and blood meal. Average daily gains were increased by 0.03 to 0.15 kg compared to control steers supplemented with molasses. Hafley et al. (1993) treated yearling steers grazing native range with no supplement an energy control supplement, energy control supplement plus 100 g/day of UIP and an energy control su pplement plus 200 g/day of UIP. A verage daily gain was increased for steers supplemented with UIP. Steers supplemented with 200 g/d of UIP had 0.13 kg greater ADG compared to unsupplemented steers. Karges et al. (1992) also examined the effects of UIP s upplementation in growing cattle graz ing native summer pasture in Nebraska for 83 d. Treatments included no supplement, three levels of DIP (0.15, 0.27, and 0.37 kg/d), and three levels of UIP (0.07, 0.14, and 0.21 kg/d). No response to the DIP supplemen t was observed. However, supplementation with UIP caused a linear increase in cattle gains. Gutierrez Ornelas and Klopfenstein (1991) examined the effects of supplemental UIP on the performance of

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45 growing steers grazing low quality corn crop residue fora ge. In the first trial, steers were fed supplements containing 60, 88, 116, 144, 172, or 200 g/d UIP from a corn gluten meal and blood meal mix. Each gram of supplemental UIP increased the ADG of the steers 1.31 g. In summary, UIP supplementation can i ncrease ADG and feed efficiency in growing cattle fed forage based diets. However, considerable variation in animal performance to UIP supplementation is evident. Cattle fed the same basal diets but supplemented with higher levels of UIP did not always h ave the greatest ADG or feed efficiency. Results from these trials suggest that additional properties of supplemental UIP, such as amino acid composition of feed, influences the performance of growing cattle. Bedrack et al (1964) and Oyedipe et al. (1982) reported that fertility in yearling heifers was positively associated with protein content of diets that were equivalent in energy. Further research by Fajjerson and Barradas (1991) observed no difference in age at puberty in Zebu heifers fed a diet cont aining either low or high protein concentrations. Patterson et al. (1992) reviewed the literature and concluded that diets high in protein support faster growth rates that result in earlier onset of puberty and greater pregnancy rates compared to diets wi th restricted protein concentrations. Kane et al. (2004) reported that ovarian and pituitary function in heifers may be influenced by UIP level. Diets high in UIP (321 g/d) caused decreases in anterior pituitary LH and FSH concentrations compared to medi um (216 g/d) and low (115 g/d) UIP diets. Wiley et al. (1991) reported a greater percentage of first calf heifers pregnant during the first estrous cycle of the breeding season for heifers receiving UIP compared to DIP supplementation. Elrod and Butler (1993) d airy heifers fed diets containing 21.8% CP with 82.5% of CP as DIP had decreased conception rate compared with h eifers fed diets with 15.5% CP with 73% of CP as DIP due to decreased uterine pH seven d ays after estrus. Ferguson et al

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46 (1988) repor ted that dairy cows with a PUN greater than 20 mg/dL were three times less likely to conceive than cows with lower PUN concentrations. Protein is an essential nutrient to the growth and development of the heifer. The under feeding of protein can have ne gative results on animal performance, while the overfeeding can have negative results on the economic performance. Differences in DIP and UIP concentrations can effect growth and possible reproduction in the developing heifer. Supplementation with DIP or UIP depends on the forages being consumed. Supplement Interval Supplementation of cattle on a daily basis can result in a significant investment in time and labor resources. Decreasing the frequency with which supplements are delivered to cattle is a com monly utilized method used by beef cattle producers to reduce cost. Melton and Riggs (1964) suggested that supplementing twice a week resulted in savings of 60% in labor and travel expenses compared to daily supplementation. Research has shown that prote in supplements can be offered less frequently to cattle while resulting in similar animal performance compared to daily supplemented cattle (Melton and Riggs, 1964; Hunt et al., 1989; Huston et al., 1999). One of the reasons that less frequent supplementa tion can be used is because recycling of ammonia supports the rumen environment and function between times of supplementation (Nolan and Leng, 1972). However, reducing the supplementation frequency of energy feeds to cattle consuming low quality forages c an be detrimental to their performance (Kunkle et al., 2000). Kunkle et al. (2000) compiled results from several experiments comparing different supplementation frequencies of protein based feeds for cattle grazing low quality forages, and reported that su pplementing protein as infrequently as once a week instead of daily did not alter cattle performance. Wettemann and Lusby (1994) reported similar changes in BW and BCS, and conception rates between range cows supplemented six times/week or three times/wee k with a

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47 protein based supplement. In a four year study, Huston et al. (1999) examined the effects of supplementing a protein source to range cattle daily, three times/week, or once/week. Cattle supplemented less frequently had similar los s es in BCS and BW compared to cattle supplemented daily. All supplemented cattle regardless of frequency of supplementation lost less BW and BCS compared to unsupplemented cattle. Variation in supplement intake was 33% less for three times weekly and 31% less for once weekly compared with daily supplemented cows. Melton and Riggs (1964) supplemented cows daily, twice weekly, or three times weekly. They found that cows supplemented twice and three times weekly gained 101 and 95% of the weight of daily supplemented cont rols, respectively. Hunt et al. (1989) supplemented growing steers consuming low quality hay with a protein supplement every 12, 24, 48 hours or no supplement. Average daily gain (0.20 kg/d) and digestible DMI (0.18% of BW) was increased for supplemented steers regardless of interval of supplementation. No differences in ADG or DMI were noted among treatments. As previously stated, decreasing the supplementation frequency of energy based supplements to cattle consuming low quality forages has been show n to be detrimental to animal performance. Chase and Hibberd (1989) reported that cattle consuming low quality forages and offered corn based supplements daily experienced slight improvements in DM digestibility and OM intake compared to every other day s upplements Beaty et al. (1994) reported that mature cows grazing dormant tallgrass prairie and offered a sorghum based supplement daily as opposed to three times/week had decreased loss of BW and BCS during the winter. Kunkle et al. (2000) summarized re sults of several trials indicating that cattle offered energy based supplements feed daily instead of less frequent feeding had improved performance. The improvement in

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48 performance of the daily supplemented cattle was attributed to improved ruminal functi on and forage intake. The effects of supplementation interval on heifer performance are not well researched. Wallace (1988) compared supplementing developing heifers on native range with a low protein grain supplement fed daily or twice weekly. The BW gains were similar between the groups, but conception rate was numerically less for the twice weekly fed group compared to the daily fed group. Arthington et al. (2004) reported that pregnancy rate was increased in heifers supplemented with a molasses+cot tonseed meal slurry (76.3%) compared to heifers supplemented with a wheat midd based range cube (49.2 %). Both treatments were fed to supply 1.47 and 0.34 kg of TDN and CP (respectively) on a daily basis, but both were offered three times a week. Observ ed differences were partly attributed to differences in feeding behavior. Heifers receiving range cubes consumed the supplement in less than one hour, while molasses supplemented heifers took almost 48 hours to consume the supplement. Loy et al. (2007) r eported that forage fed heifers offered supplements based on distillers gra ins daily or on alternate days had similar mean forage intake (1.69 vs. 1.66% of BW, respectively), similar rumen pH (6.12 vs. 6.17, respectively), and in situ rate of NDF disappear ance (4.09 vs. 4.01% per h our respectively ). Cooke et al. (2008) supplemented heifers consuming low quality forage either daily or three times/week with a high fiber wheat midd based energy supplement. Daily supplementation of heifers decreased age at p uberty, increased ADG, and pregnancy rate compared to heifers supplemented three times/week. The authors reported less variation in circulating BUN, glucose, IGF I, and insulin concentrations in daily supplemented heifers compared to three time/week suppl emented heifers. They concluded that the increase in ADG, heifers pubertal, and pregnancy rates was due to the reduction in variation of blood metabolites.

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49 Infrequent supplementation of heifers is common management practice in the beef industry today. This practice has been shown to reduce costs by decreasing labor. However, research shows that there may be large negative impacts on reproduction due to this practice. Supplementation of developing heifers is often required in Florida to compensate fo r the reduced nutrient density of forages, and to meet the nutritional requirements of the growing heifer. Supplements are usually offered three times or once a week in order to reduce labor costs. Most research indicates that nutritional status of cattl e is improved when energy based supplements are offered daily instead of three times weekly, but nutritional status is unaffected by offering protein supplements less frequently than daily supplementation. Dried distiller grain is a byproduct feed of the corn derived ethanol industry that is becoming less expensive and more readily available. Due to the increase in ethanol production during the last decade, the supply of DDG has increased (Loy et al., 2005). Distillers grains are a novel feed in the fact that it is high in UIP high in fat, and digestible fiber, which provides energy. The price of DDG is less expensive compared to most protein feeds, and can be competitively priced as a source of energy (Schroeder, 2003). Corrigan et al. (2007) reported that in steers fed alfalfa hay and sorghum silage based diets increasing DDG supplementation from 0.25 to 1.00% BW caused a linear increase in BW gain (20 kg) and a linear decrease in forage intake (1.27 kg/d). Klopfensteint et al. (2007) summarized eigh t grazing experiments using yearling cattle grazing smooth brome grass pasture or Sandhills range Distillers grains were supplemented at either 0.5 and 1.0% BW. Cattle supplemented at 0.5% BW had an increased daily gain by 0.97 kg/d, while supplementati on at 1.0% BW increased daily gains by 1.13 kg/d compared to unsupplemented cattle. The increases in ADG were attributed to the CP and energy supplied by the DDG. The

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50 nutritional composition of DDG may allow it to be supplemented to developing heifers wi th minimal negative effects that are often encountered when supplementing energy feeds. Another method of heifer development that could reduce the cost of heifer development is programmed feeding of heifers. This is a common practice outside of the Southe ast. Heifers are often drylotted and fed to gain at a reduced rate of a short period to take advantage of compensatory gain. Little data is available on the use of high forage diets and supplementation as an effective method of programmed feeding. Takin g advantage of the improved efficiencies from compensatory gain may provide producers another method of reducing the cost of heifer development.

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51 CHAPTER 3 EFFECT OF FEEDING IN TERVAL ON GROWTH, RE PRODUCTIVE PERFORMAN CE, AND BLOOD METABOLITE S IN YEARLING B OS TAURUS AND BOS INDICUS BOS TAURUS BEEF HEIFERS Introduction The nutritional quality of grazed sub tropical forages in Florida is often meet the nutri ent requirements of growing yearling beef heifers ( Moore et al., 1991 ) Therefore, supplemen tation is necessary to meet the nutri ent requirements of developing heifers so that they achieve p roper BW to breed at a year of age Daily s upplementation of cattle also requires a significant investment in labor resources. Decreasing the frequency of s upplement delivery to cattle is a common management strategy used to decrease the cost associated with supplementation programs Research has shown that decreasing the frequency that protein supplements are offered to cattle results in similar cattle perf ormance compared to daily supplement ation (Melton and Riggs, 1964; Hunt et al., 1989; Huston et al., 1999). However, decreas ing the supplementation frequency of energy feeds to cattle consuming low quality forages can be detrimental to cattle performance (Kunkle et al., 2000). Forage alone often ca n not provide adequate energy and ( or ) protein to meet the nutrient requirements of growing yearling beef heifers so supplements are frequently added to the ments. grain (DDG) is a product of the dry corn milling industry (Stock et al., 2000). Although high in CP ( 31.6% ), DDG are relatively low in degradable intake protein (DIP; 27.9% DIP, as a % CP ) Additionally, DDG is low in starch, hi gh in digestible fiber, and contains 11 to 12% fat (Lodge et al., 1997) Consequently, DDG may represent a viable source of both supplemental energy and protein to forage based diets of cattle Loy et al. (2007) demonstrat ed that heifers consuming low qu alit y grass hay and supplement ed with DDG on alternate days compared to daily supplementation had

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52 similar DMI rumen pH, and in situ NDF disappearance. Based on these observations we hypothesized that yearling Brangus and Angus heifers consuming round bal e silage (RBS) and supplemented with DDG three d/wk would experience similar growth and reproductive performance compared to heifers supplemented daily Methods and Materials The experiment was conducted from October 2006 until May 2007 at the University o f Florida Santa Fe Beef Research Unit, north of Alachua, FL. The experiment was divided into a sampling phase (d 0 140) and a breeding phase (d 140 1 82 ) with d 0 being the start of the experiment Heifers were cared for in accordance with acceptable prac tices as outlined in the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (FASS, 1999). Animals Sixty heifers (Angus n = 30, Brangus n = 30 initial BW = 265 22 kg ; initial age = 265 22 d ) were utilized in th e ex periment. Heifers were stratified by breed, BW, age, and sire and allocated to 12 pens with five heifers/pen on d 14. Pens were randomly assigned to receive a DDG based supplement either daily ( SPD ) or three d/wk (S P 3) Heifers were adapted to assigned supplement treatments from d 14 to 1. Heifers remained in assigned pens through out the sampling and breeding phase s of the experiment. P ens Pens consisted of 1.2 ha of established bahiagrass ( Paspalum notatum ) and common bermudagrass ( Cynodon dactylon ). Established forage received no fertilization during the previous summer. The forage was managed by mowing to result in similar available forage DM in all pens at the initiation of the experiment. Mean forage availability at the initiation of the expe riment w as 1 543 DM kg/ha with 10.3 % and 46.9 % CP and TDN respectively

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53 Diets S upplements w ere offered either daily or three d/wk in amounts to allow 0.68 kg/d gain as predicted by the Level 1 model of the Beef Cattle NRC ( 2001 ) for growing yearling bee f heifers The amount of supplement offered was altered on a monthly basis based on animal performance to meet the targeted level of BW gain (Table 3 1). Soybean meal was included at a fixed amount 1 d 1 ) to meet predicted DIP requirements of the heifers based on Beef Cattle NRC (2000) All heifers had access to Tifton 85 bermudagrass ( Cynodon dactylon ) RBS throughout the experimen t in amounts to ensure ad libitum consumption A complete commercial mineral/vitamin mix (14% Ca, 9% P, 24% NaCl 0.20% K, 0.30% Mg, 0.20% S, 0.005% Co, 0.15% Cu, 0.02% I, 0.05% Mn, 0.004% Se, 0.3% Zn, 0.08% F, and 82 IU/g of vitamin A) and water were offered for ad libitum consumption throughout the experiment. Samples of DDG were collected every two wk and ever y bale of RBS was core sampled. Pasture samples were collected month ly from each pen to estimate DM availability and forage quality by hand clipping three different 0.25m 2 areas and compositing the samples. All pasture in a forced air oven for approximately 72 h. Dried samples were ground to pass through a 1 mm screen in a Wiley mill (Arthur H. Thomas Company, Philadelphia, PA, USA). Dry matter for all samples was determined by drying samples for two h at 135C (proce dure 930.15; AOAC, 2000). The RBS and DDG were pooled by 28 d periods for laboratory analysis of CP, OM, in vitro digestible organic matter (IVDOM) and in vitro dry matter digestibility (IVDMD). Nutrient values for DDG, pasture, and RBS are presented in Table 3 2 Total nitrogen was determined by rapid combustion using a macro elemental N analyzer (Elementar, vario MAX CN, Elementar Americas, Mount Laurel, NJ, USA) and used to calculate CP (CP = N x 6.25). In vitro dry matter and organic matter digesti bility of samples was determined by the procedure of Tilley and Terry (1963), as modified by Marten and Barnes

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54 (1980) with filtration on filter paper. Inoculum for IVDMD was obtained from a ruminally fistulated cow fed ad libitum coastal bermudagrass ( Cyn odon dactylon ) hay and 900 g/d of soybean meal, and strained through four layers of cheesecloth. Organic matter concentration of of TDN for DDG were determined by a commercial laboratory (Dairy One Forage Laboratory, Ithica, NY). Concentrations of TDN for hay and pasture samples were determined using the equation (Fike et al., 2002): %TDN = [(% IVOMD 0.59) + 32.2] OM concentration. Breeding On d 145 of the exp eriment, heifers received a progesterone insert (EAZI BREED CIDR ; GnRH (Cystorelin, Merial, Inc., Duluth, GA). Seven d later, CIDR were removed and heifers received 25 mg (i.m.) prostaglandin F (PG; Lutalyse Sterile Solution, Pfizer Animal Health, New York, NY). Estrus was detected using radiotelemeric estrous detection devices (HeatWatch Cow Chips, Denver, CO; Dransfield et al., 1998), which were fitted to all heifers beginning on d 152. Heifers were artificially inseminated (AI) by one AI technic ian with frozen thawed semen 8 to 12 h after the onset of the PG induced estrus. Seventy two h after PG administration and CIDR removal all heifers not exhibiting estrus were AI and administered 100 GnRH. Estrus was detected for the subsequent 30 d and heifers were AI 8 to 12 h after the onset of estrus. After the 30 d AI period, heifers were grouped by breed and each breed group was pasture exposed for an additional 30 d to a bull simila r to its respective breed type that had passed a breeding soundness evaluation Pregnancy was determined approximately 30 60, and 90 d after the initial AI by transrectal ultrasonography, using a real time, B mode ultrasound (Aloka 500v, Corometrics Medi cal Systems, Wallingford, CT) equipped with a 5.0 MHz transducer. In retrospect analysis,

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55 it was determined that the Brangus bull became infertile sometime during the breeding period as no Brangus heifers became pregnant during the 30 d natural service br eeding period. Consequently, only the AI pregnancy data for the first 30 d of the breeding season will be presented. Sampling Heifers were weighed after 16 h of feed and water restriction to determine shrunk BW one wk before the initiation of the samplin g phase (d 7) and one wk after the end of the sampling phase (d 147). Heifers were weighed weekly, and hip heights (HH) and BCS (1 =severely emaciated; 5 = moderate; 9 = very obese; Wagner et al., 1988) were recorded every 28 d (d 0, 28, 56, 84, 112, and 140) Blood samples were collected prior to supplement feeding on Wednesday mornings via jugular venipuncture into 7.5 mL polypropylene syringes containing 1.6 mg potassium EDTA as an anticoagulant (Monovette, Sarstedt Inc., Newton, NC). Samples were im mediately placed on ice and transported to the lab for further processing. Blood samples were centrifuged at 855 g 30 C for subsequent analysis. Weekly blood samples were used to determine NEFA concentrations (every 28 d) and the onset of puberty using plasma progesterone concentrations. Heifers were consider ed pubertal when progesterone concentrations were 5 ng/mL for two consecutive wk. In addition to the weekly blood sample collections, 20 heifers (Angus n = 10, Brangus n = 10) were utilized to collect a series of intensive blood samples over a 48 h per iod (0, 2, 4, 8, 12, 24, 26, 28, 32, 36, and 48 h) beginning on d 0 28, 56, 84, 112, and 140 of the experiment. The intensive blood collection periods started on Wednesday and ended on Friday to allow for a full 48 h sampling period and to coincide with the time when the SPD and SP3 treatments received their supplements. The time 0 sample was taken prior to delivery of supplement followed by supplement administration. The 24 and 48 h blood samples were taken prior to supplement

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56 administration for both t he SPD and SP3 treatments. The intensive blood samples were analyzed for plasma urea nitrogen (PUN) and glucose concentrations. Blood A naly s is A Technicon Autoanalyzer (Technicon Instruments Corp., Chauncey, NY) was used to determine both plasma glucose (Coulombe and Favreau, 1963; modified and described by Bran + Luebbe Industrial Method #339 01) and PUN (Gochman and Schmitz, 1972; modified and described by Bran + Luebbe Industrial Method #339 19). Concentrations of plasma NEFA were determined using Wa ko HR series NEFA HR kit (Wako Diagnostics, Richmond, VA). Concentrations of plasma progesterone were determined using Coat A Count Kit (DPC Diagnostic Products Inc., Los Angeles, CA) solid phase 125 I RIA (Seals et al., 1998) The intra and interassay CV were 10 and 9%, respectively. Sensitivity of the assay was 0.10 ng/mL and 0.1 mL of plasma was assayed. Statistical Analysis Performance and physiological data were analyzed using the MIXED procedure of SAS (SAS Inst., Inc., Cary, NC) The model sta tement used for ADG HH, and BCS contained the effect of treatment breed and treatment x breed Data were analyzed using pen(treatment) as the random variable. The model statement used for metabolite analysis contained the effects of treatment, breed day time, time ( day), and all appropriate interactions. Data were analyzed using heifer(pen) and pen(treatment) as random variables. Results are reported as least square means Reproductive data were analyzed using GENMOD procedure of SAS. The model st atement used for all reproductive data contained the effects of treatment, breed, and treatment x breed. For all analysis, significance was set at P tendencies were determined if P > 0.05

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57 Results and Discussion Performance At the initi ation of the experiment (d 0), heifer BW, shrunk BW, BCS, and HH were similar ( P > 0.05) between treatments. Body weights and BCS were similar ( P > 0.05) between Angus and Brangus at the start of the experiment ; h owever, Brangus heifers tended ( P = 0.08) to be heavier and were taller ( P = 0.03) compared to Angus heifers at the start of the experiment (Table 3 4). At the end of the experiment (d 140) BW and shrunk BW were similar ( P > 0.05) between treatments and breeds. Hip heights and BCS were similar b etween treatments, but Brangus heifers were taller ( P = 0.02) and tended to have greater BCS ( P = 0.08) compared to Angus heifers. The percentage change in HH ( P = 0.80) and BCS ( P = 0.80) from the initiation to the end of the sampling phase were not diff erent between treatments. The percentage change in HH was not different between breeds ( P = 0.61); however, Brangus heifers tended ( P = 0.06) to gain more BCS compared to Angus heifers. Heifer ADG exceeded the projected gain based on the NRC (2001) mode l. Average daily gain and shrunk ADG for the entire trial did not differ between treatments ( P = 0.83 and 0.76 respectively, Table 3 4) or breeds ( P = 0.40 and 0.42, respectively). The similar growth rates observed between treatments should be expected a s heifers were fed to reach a target ADG (Table 3 3). In a review of numerous studies Kunkle et al. (2000) concluded that ADG were similar between daily and alternate day feedings for protein based supplements. In contrast, Loy et al. (2008) reported a 10% decrease in ADG for heifers fed ground native prairie grass hay (CP = 7.8%, IVMDM = 52%) supplemented with DDG on alternate days compared to daily supplemented heifers. Loy et al. (2008) attributed the decrease in ADG to a 12% decrease in DMI for heif ers supplemented on alternate days compared to daily. Although DMI was not evaluated in the present experiment, the amount of supplement provided was not different ( P >

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58 0 10) between treatments at any period during the trial (Table 3 1). The amount of DD G supplied decreased ( P d 1 during the first d 1 during the last period of the trial, which was similar between treatments. Mean total supplement offered (SPD = 1,463 kg/pen, SP3 = 1,501 kg/pen) and RBS offered (SPD = 16,184 kg/pen, SP3 = 16,094 kg/pen) w ere similar ( P > 0.10) between treatments and breeds (Table 3 2). Farmer et al (2001) reported that forage OM intake and NDF digestion decreased linearly as supplementation f requency declined, from 7 d/wk to 2 d/wk, for steers that were fed low quality (CP = 4.8%, NDF = 73.5%) tallgrass prairie hay and supplemented with a high protein (CP = 43%) supplement. Krehbiel et al. (1998) supplemented ewes consuming bromegrass hay (CP = 7.5%, NDF = 72.6%) with soybean meal every 24 or 72 h and reported no difference in forage or total DM intake. It is possible that the amount of supplement offered was not a large enough amount to negatively affect intake or the effect was to small to b e measured without more intensive measurements. Blood Metabolites There was a day ( P < 0.05) and treatment day interaction ( P < 0.01) on PUN concentrations, which are presented in Figure 3 1. Heifers had similar PUN concentrations on d 0 at the start of the experiment and declined throughout the sampling phase to a low point on d 84. However, on d 56 of the sampling phase the SP3 heifers had greater ( P < 0.05) PUN concentrations compared to SPD heifers. From d 84 until d 140 of the sampling phase, PU N concentrations increased in both treatments Mean PUN concentrations throughout the sampling phase ranged between 11 to 15 mg/dL, which has been recommended to ensure maximum rates of gain by Byers and Moxon (1980). Hammond et al. (1993) indicated that PUN concentrations of cattle maintained on subtropical forages should be between 8 12 mg/dL. Concentrations below 8 mg/dL can indicate that protein is deficient while concentrations above 12 mg/dL can

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59 indicate that more dietary energy is needed. To fi t these recommendations supplements should be correctly balanced for energy and protein (Hammond, 1997) or supplements that should be synchronous in the rates of ruminally degraded energy and protein (Spicer, et al., 1986, Matras et al., 1991). Even thoug h h eifers had PUN concentrations greater than 12 mg/dL at the majority of the plasma sampling periods, the actual ADG exceeded the predicted ADG based on Beef Cattle NRC (2001) Level 1 model, which suggests that energy was not limiting. The use of DDG SBM supplement in combination with the RBS was probably supplying excess amount of protein to the heifers which caused the elevated PUN levels. For the intensive blood sampling period, there was a treatment time effect ( P < 0.01) on PUN concentrations (Fi gure 3 2). An eight h lag time was observed in both treatments before PUN concentrations increased. The SP3 heifers had a greater increase in PUN concentration from 12 to 24 h compared to SPD heifers. The PUN concentrations remained elevated for the SP3 heifers for the following 12 h at which time they decreased to concentrations similar to what was observed at the start of the sampling period. These concentrations of PUN are positively associated with intake of protein, levels of ruminal ammonia, and r uminal protein:energy ratio (Hammond, 1997). The SPD heifers did not receive as large of a bolus of protein as the SP3 heifers The sustained and elevated concentrations of PUN for SP3 heifers the day after supplementation is due to the large bolus intak e of protein from the DDG supplement and the retention time in the rumen This biphasic pattern of PUN concentrations detected in SP3 heifers is similar to reports by others (Bohnert et al., 2002; Cooke et al. 2008) in animals fed a bolus dose of supplem ent and it illustrates the greater daily variation of energy and protein intake of SP 3 heifers compared to SPD heifers The increase in PUN concentrations in SP3 heifers from 0

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60 h to 24 h was similar to the increase in PUN observed in wethers supplemented with high UIP supplements every third day reported by Bohnert et al. (2002). A breed x time(day) effect ( P = 0.02) was observed for PUN (Table 3 4 ). Differences between the breeds occurred mainly on d 0 and 28. Brangus heifers had greater ( P < 0.05) PUN concentrations on d 0 at 24, 26, 28, and 32 h than Angus heifers. On d 28, Brangus heifers had greater (P < 0.05) PUN concentrations at 4, 26, 32, and 48 h and tended to have greater concentrations at 0, 2, 8, 24, and 28h ( P = 0.07, 0.07, 0.06, 0.08, 0.07, and 0.09, respectively ) The differences in PUN concentrations between breeds were almost nonexistent for the remainder of the trial. Alvarez et al. (2000) reported that lactating Brahman cows had greater PUN concentrations than lactating Angus co ws consuming the same perenniel peanut bahigrass hay. It has been shown that Brahman cattle can digest more protein and consume more DM from a low protein diet than Hereford cattle (Warwick and Cobb, 1976) which may result in the increase in PUN concentra tions A treatment x day effect was observed ( P < 0.01) for plasma glucose (Figure 3 3). On d 0, SPD heifers had a greater ( P < 0.05) plasma glucose concentration compared to SP3 heifers. Plasma glucose concentrations declined across the remaining days of the trial and were not different between treatments. A day effect was observed ( P < 0.01) for plasma glucose. Heifers started the trial with a plasma glucose concentration of 120 ng/mL and steadily declined during the study to end at 95 mg/dL. The d ecrease in plasma glucose throughout the trial could be explained by the decrease in DDG supplement offered which would decrease the available gluconeogenic substrates available to the heifer A similar decline in plasma glucose during a 70 d trial was ob served by Lopez et al. (2006) in growing Angus, Brangus, and Brahman heifers developed using a corn alfalfa diet (CP = 14.9%, TDN = 75%)

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61 A treatment time interaction ( P = 0.03) was observed for plasma glucose concentrations (Figure 3 4). Plasma gluco se concentrations were greater ( P < 0.05) for SPD heifers at 4, 8 and 12 h after supplementation compared to SP3 heifers. Glucose concentrations increased and were similar for both treatments at 24 h and remained stable for the following 24 h. The decli ne in plasma glucose concentrations after supplementation agrees with other studies in dairy cattle that involved intensive blood collections (Sutton et al., 1988; Nikk h ah et al., 2008). The post supplementation decline in plasma glucose may be attributed to an increase in plasma insulin due to an increase in propionate availability and an insulin driven increase in glucose utilization by the periphery (Sutton et al., 1988; Blum et al., 2000; Nikk h ah et al., 2008) The large bolus intake of supplement by t he SP3 heifers could be eliciting a greater insulin response to feeding than that experienced by SPD heifers due to the more consistent nutrient supply. A breed day effect ( P < 0.01) was observed for plasma glucose. Brangus heifers had greater ( P < 0. 05) plasma glucose concentrations compared to Angus heifers on d 0, 28, 84, 112, and 140 (Figure 3 5). The differences in plasma glucose between the two breeds had previously been noted by other researchers, mainly in adult cows (Alvarez et al., 2000; Obe idat et al., 2002). the sampling phase, plasma glucose concentrations declined more than 16 ng/dL for the Brangus heifers There were no ( P > 0.05) treatment or i nteractions of treatment with any dependent variables for NEFA concentrations but there was a day effect ( P = 0.01) observed for NEFA concentrations (Figure 3 6). The NEFA concentrations on d 28 were decreased ( P < 0.05) compared to the other five samplin g days. Concentrations of NEFAs increased from d 28 until the end of the sampling phase. Elevated NEFA concentrations are usually indicative of a

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62 negative energy balance (Brier et al., 1986; Peters, 1986; Richards et al., 1989), which wa s certainly not t he case in the present study as heifers were gaining both BCS and BW as the trial progressed. A breed x day interaction ( P < 0.05) was observed for NEFA concentrations (Figure 3 7). Brangus heifers had greater ( P < 0.05) NEFA concentrations on d 28 and 1 40 compared to Angus heifers. Concentrations of NEFA were not different ( P > 0.10) between treatments for the other four sampling days. Reproductive Performance The number of heifers pubertal at the first AI was not affected ( P > 0.05) by supplement fre quency or breed (Table 3 5 ). One of the primary determinants of age at puberty has been identified as adequate BW gain (Short and Bellows, 1971) and there were no BW gain differences between SPD and SP3 heifers. Arthington et al. (2004) reported younger age at puberty in Brangus and Braford heifers supplemented with a molasses cottonseed meal slurry compared to heifers supplemented with a wheat middling based range cube. The difference in age at puberty was attributed to feeding behavior. The range cube supplemented heifers consumed their supplement in one hr, while the molasses slurry supplemented heifers took 48 hr to consume the supplement. This bolus intake of supplement by the range cube fed group may have led to an alteration of the rumen environm ent and the metabolic hormone profile of the heifers. Cooke et al. (2008) reported that daily supplemented heifers consuming a wheat middling based diet had a greater incidence of puberty prior to the breeding season compared to heifers supplemented three times a w ee k ly The pubertal response was attributed to reduced daily variation in concentrations of PUN, glucose, and insulin. These metabolic effects lead to increased BW gain and earlier puberty for daily supplemented heifers. Estrus response tended ( P = 0.08) to be greater in SPD heifers compared to SP3 heifers. Estrus response for SP D heifers was greater than previously reported Florida studies by 9 to 36%

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63 (Esterman, 2006; McKinniss, 2008). Angus and Brangus cattle had a similar ( P < 0.05) estrus response both of which were also greater than previous studies with Bos taurus x Bos indicus cattle from our lab synchronized with a similar synchronization program (Esterman, 2006; McKinniss, 2008). Martin et al. (2007) reported that heifers supplement ed with DDG and synchronized with a 14 d split dose of prostaglandin had an estrus response of 76%. Conception rate was not different ( P > 0.05) between treatment s or breed s The mean conception rate (55%) for the heifers in this trial were less than thos e reported by Lamb et al. (2006), of 63%, and Lucy et al. (2001), of 59% synchronized with a similar protocol. Martin et al. (2007) reported a conception rate of 75% for heifers fed DDG and synchronized with a split dose of prostaglandin, but 76% of the h eifers were cycling at synchronization. The synchronized pregnancy rate, and 28 d pregnancy rate were not different ( P > 0.05) between treatment or breed (Table 3 5 ). Arthington et al. (2004) reported increased pregnancy rate (76.3 vs. 49.2 %) in heifer s supplemented with a molasses cottonseed meal slurry compared to heifers supplemented with a range cube. Martin et al. (2007) reported a synchronized pregnancy rate of 57% and an overall pregnancy rate of 91% for heifers fed DDG. In summary, supplement ing growing heifers with DDG and SBM three times a week had no negative effects on heifer growth or reproductive performance compared to heifers supplemented on a daily basis. Heifers supplemented three times a week had similar ADG, shrunk ADG, change in HH, change in BCS amount of supplement offered and amount of RBS offered compared to heifers supplemented daily Although there were significant variations in BUN and plasma glucose concentrations over time this was due to large bolus intakes of supple ment by heifers supplemented three times weekly compared to daily supplementation ; h owever, the variation in metabolites did not have any affects on growth or reproduction. The

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64 observed ADG and BW for the heifers in both treatments allowed a large number of heifers to reach puberty prior to the breeding season. The large number of heifers puberal at the start of the breeding season in both treatments led to good reproductive performance of the heifers regardless of treatment. Implications Offering a DDG b ased supplement to Angus and Brangus heifers three times weekly compared to daily resulted in no differences in growth response or reproductive performance. The high quality of the RBS offered to the heifers was a major factor in this experiment. The RBS was of such a high quality that the amount of supplement provided was able to be decreased as the experiment progressed and heifers continue d to achieve the targeted rates of BW gain Forage quality is a key component of any heifer development program. In this study, d ried distillers grains and SBM was a very good supplement to compliment the RBS fed to the heifers. Supplementation of growing Angus and Brangus heifers with DDG and SBM three times weekly can offer beef cattle producers a means of decreas ing labor costs involved with developing heifers without negatively affecting performance

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65 Table 3 1. Amount of dried distillers grains supplement offered by day of experiment and total amount of round bale silage and dried distillers grains offered to y earling Angus and Brangus heifers during the experiment. Treatment 1 Day of experiment SPD SP3 k g hd 1 d 1 0 28 2.00 0.16 2.03 0.15 29 56 1.96 0.18 1.99 0.13 57 84 1.87 0.19 1.91 0.15 85 112 1.81 0.18 1.86 0.21 113 140 1.74 0.19 1.85 0.18 k g hd 1 0 140 DDG 2 293 300 0 140 RBS 3 3,237 3,219 1 SPD: supplement provided daily; SP3: supplement provided three times per wk. 2 Dried distillers grains. 3 Round bale silage.

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66 Table 3 2 Nutritive value of forage and supplement ing redients offered to developing beef heifers Item RBS 1 DDG 2 SBM 3 Pasture 4 DM % 51.83 87.41 90.71 66.39 DM basis OM 92.27 93.43 92.43 94.96 CP 12.85 31.83 49.94 12.40 IVDMD 5 55.17 59.38 87.22 30.85 TDN 6 59.73 84.52 77.33 46.90 1 Round bal e silage of Tifton 85 bermudagrass. 2 Dried distillers grains. 3 Soybean meal 4 Dormant bahiagrass forage. 5 In vitro dry matter digestibility. 6 Estimated using Fike et al. (2002) for RBS and Pasture

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67 Table 3 3 Growth characteristics of heifers cons uming round bale silage and supplemented with dried distillers grains based supplement Treatment 1 Breed Item SPD SP3 Pooled SE P value Angus Brangus Pooled SE P value Shrunk BW, kg d 7 251 248 8. 2 0.78 239 260 8. 2 0.08 d 147 379 373 12.3 0.75 369 384 12.3 0.39 Shrunk ADG, kg/d 0.82 0.81 0.03 1 0.76 0.84 0.80 0.03 1 0.42 BW, kg d 0 266 265 8. 7 0.88 257 275 8. 7 0.14 d 140 385 381 12.6 0.85 377 389 12.6 0.51 ADG, kg/d d 0 140 0. 84 0.83 0.04 2 0.83 0.86 0.81 0.04 2 0.40 BCS 2 d 0 5.3 5.2 0.7 5 0.19 5.3 5.2 0.7 5 0.57 d 140 6.2 6.0 0.7 7 0.20 6.0 6.2 0.7 7 0.08 d 0 140 change 0.9 0.8 0.1 2 0.88 0.7 1.0 0.1 2 0.06 HH, cm d 0 113. 7 113. 9 1.12 0.90 112.09 115.48 1.12 0.03 d 140 122.9 123.3 0.91 0.76 121.62 124.64 0.91 0.02 d 0 140 change 9.23 9.44 0.53 2 0.80 9.53 9.15 0.53 2 0.61 No treatment breed ( P > 0.10) for all reported values. 1 SPD: h eifers supplemented daily ; SP3: heifers supplem ented three times a week. 2 BCS : 1 = severely emaciated, 5 = moderate, 9 = very obese.

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68 Table 3 4 Breed x time(day) effect of plasma urea nitrogen (mg/dL) of Angus (AN) and Brangus (BN) heifers fed round bale silage and dried distillers grains based su pplement Hour s after supplementation SEM 0 2 4 8 12 24 26 28 32 36 48 0 d AN 16.79 15.18 14.73 13.09 13.88 18.36 17.19 15.91 13.41 14.58 14.31 1. 095 BN 16.14 15.44 14.52 12.71 12.97 21.12 23.02 22.52 21.52 21.77 16.51 0.96 2 P 1 0.46 0 .85 0.88 0.79 0.52 0.05 <0.01 <0.01 <0.01 0.14 0.12 28 d AN 11.25 10.42 9.71 8.91 10.59 13.23 12.95 13.27 12.16 14.62 10.89 1.06 1 BN 13.72 12.96 12.68 11.50 13.00 15.72 16.02 15.67 15.57 16.66 14.52 0.9 57 P 1 0.07 0.07 0.03 0.06 0.08 0.07 0 .03 0.09 0.01 0.14 0.01 56 d AN 12.72 12.14 11.74 10.42 11.79 14.56 14.93 15.55 14.85 15.13 10.94 1.06 1 BN 14.32 14.03 13.46 12.52 13.96 16.83 17.28 17.69 17.56 17.18 12.18 0.9 57 P 1 0.25 0.18 0.21 0.13 0.12 0.11 0.09 0.12 0.05 0.14 0.37 84 d AN 10.34 9.70 9.57 9.09 11.06 12.23 12.23 12.24 11.43 13.45 10.98 1.06 1 BN 10.78 10.25 9.92 9.05 10.63 13.94 13.74 13.39 13.06 14.29 11.44 0.9 57 P 1 0.74 0.44 0.80 0.97 0.76 0.22 0.28 0.40 0.24 0.54 0.74 112 d AN 11.09 10. 38 10.04 9.38 11.82 13.74 13.74 14.19 12.73 13.37 10.87 1.06 1 BN 12.27 11.73 11.11 10.68 12.07 15.28 15.22 15.31 14.05 15.08 12.36 0.9 57 P 1 0.40 0.33 0.44 0.35 0.86 0.27 0.29 0.42 0.34 0.22 0.28 140 d AN 14.66 13.68 13.07 11.05 12.59 16.19 16.26 16.23 14.93 15.24 14.79 1.06 1 BN 15.82 15.35 15.06 12.81 13.69 18.07 18.13 18.04 21.53 17.16 15.41 0.9 57 P 1 0.41 0.23 0.15 0.21 0.43 0.18 0.18 0.19 <0.01 0.17 0.66 1 Breed comparisons within day and hour

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69 Figure 3 1. Plasma urea nitrogen ( PUN) concentrations by treatment and day of experiment for heifers consuming round bale silage and supplemented with dried distillers grains based supplements daily (SPD) or three times/wk (SP3). (Treatment x day, P = 0.01; a,b Means within a day differ P < 0.05)

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70 Figure 3 2. Plasma urea nitrogen (PUN) concentrations by hour after supplementation for heifers consuming round bale silage and supplemented with dried distillers grains based supplements either daily (SPD) or three times/wk (SP3). Supplement provided to both SPD and SP3 at 0 h, and at 24 h for SPD. (Treatment x time, P < 0.0001; Pooled SE = 0.71; a,b Means within a time differ P < 0.05)

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71 Figure 3 3. Plasma glucose concentration by treatment and day of experiment for heifers consuming roun d bale silage and supplemented with dried distillers grains based supplements daily (SPD) or three times/wk (SP3). (Treatment day, P < 0.0001; a,b Means within a day differ P < 0.05)

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72 Figure 3 4. Plasma glucose concentrations by hour after supplemen t administration for heifers consuming round bale silage and supplemented with dried distillers grains based supplements daily (SPD) or three times/wk (SP3). Supplement provided to both SPD and SP3 at 0 h and at 24 h for SPD. (Treatment time, P = 0.03; Pooled SE = 1.02; a,b Means within a time differ P < 0.05)

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73 Figure 3 5. Plasma glucose concentration breed and day of experiment for heifers consuming round bale silage and supplemented with dried distillers grains based supplements daily (SPD) or three times/wk (SP3). (Breed day, P < 0.0001; a,b Means within a day differ P < 0.05)

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74 Figure 3 6. Plasma NEFA concentrations day of experiment for heifers consuming round bale silage and supplemented with dried distillers grains based supplements daily ( SPD) or three times/wk (SP3). (Day, P = 0.02; a, b, c Means differ ( P < 0.05)

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75 Figure 3 7. Plasma NEFA concentrations by breed and day of experiment for heifers consuming round bale silage and supplemented with dried distillers grains based supplements daily (SPD) or three times/wk (SP3). (Breed day, P = 0.002; a,b Means within a day differ P < 0.05)

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76 Table 3 5 Treatment, and breed effects for puberty, estrous response, conception rate, synchronized pregnancy rate, and 30 d AI pregnancy rate of A ngus and Brangus heifers supplemented with DDG either daily ( SPD ) or three times weekly ( SP3 ). Number in parenthesis is the number of heifers in each category. 1 Treatment Breed Item SPD SP3 P value Angus Brangus P value Pubertal at breeding (%) 2 6 0 (18/30) 40 (12/30) 0.13 53 (16/30) 47 (14/30) 0.83 Estrous response (%) 3 77 (23/30) 57 (17/30) 0.08 70 (21/30) 63 (19/30) 0.65 Conception rate (%) 4 43 (10/23) 70 (12/17) 0.12 48 (10/21) 63 (12/19) 0.51 Timed AI pregnancy rate (%) 5 43 (3/7) 38 (5/13) 0.92 56 (5/9) 27 (3/11) 0.18 Synchronized pregnancy rate (%) 6 43 (13/30) 57 (17/30) 0.27 50 (15/30) 50 (15/30) 0.52 30 d AI pregnancy rate (%) 7 63 (19/30) 70 (21/30) 0.56 63 (19/30) 70 (21/30) 0.91 No tr eatment x breed ( P > 0.10) for all reported values. 1 ap proximately 8 to 12 h later. Heifers which had not displayed estrus were timed AI at 72 76 h and received GnRH. 2 Percentage of heifers with two consecutive wk of P4 > 1.5 ng. 3 Percentage of heifers displaying estrus three d after PGF of total treated 4 Percentage of heifers pregnant to AI of the total the exhibited estrus and were AI. 5 Percentage of heifers pregnant to the TAI. 6 Percentage of heifers pregnant during the synchronized breeding of the total treated. 7 Percentage of heifers pregnant du ring the first 30 d of the breeding season to AI.

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77 CHAPTER 4 EFFECT OF PROGRAMMED FEEDING ON GROWTH, R EPRODUCTIVE PERFORMANCE, AND BLO OD METABOLITES IN YE ARLING BOS TAURUS AND BOS INDICUS BOS TAURUS BEEF HEIFERS Introduction Management strategies for developing yearling replacement beef heifers should focus on factors that enhance physiological processes that promote puberty (Patterson et al., 1992) while trying to keep the economic cost at a minimum. The most common management strategy is to feed hei fers to reach a target BW prior to the start of the breeding season. The target BW of a heifer is a genetically predetermined size among individual heifers when puberty is expected to occur (Lamond, 1970). Recommended target BW guidelines range from 53 t o 66% of the expected mature BW in beef heifers, depending on frame size and breed type (Patterson et al., 1992; Funston and Deutscher, 2004). There are many systems for growing heifers to this target BW Programmed feeding of heifers can be viable system for the development of heifers. Programmed feeding usually involves a period at the beginning of the development phase when heifers gain at a slower rate, usually < 0.5 kg/d for a defined time period. The initial phase is followed by a phase where heife rs are fed to gain at an increased rate, often > 1 .0 kg/d to reach a target BW P rogrammed feeding takes advantage of compensatory growth in the heifer which lead s to increased feed efficiency (Ferrell et al., 1986; Hersom et al., 2004) and decreased supp lement costs (Lynch et al., 1997) The majority of programmed feeding research has been conducted in yearling heifers of Bos taurus breeding (Clanton et al., 1983; Lynch et al., 1997; Park et al., 1987) In contrast, limited information is available on t he effects of rate and timing of gain on the BW gains and reproductive performance in Bos indicus Bos taurus heifers (Weekley, 1991) consuming low quality forages Therefore, the objectives of this research were

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78 to evaluate the effects of programmed fee ding on the growth and reproductive performance of Angus and Brangus heifers fed low quality forage and offered dried distillers grains. Materials and Methods The experiment was conducted from October 2007 to May 2008 at the University of Florida Santa Fe Beef Research Unit, north of Alachua, FL. The experiment was divided into a sampling phase (d 0 174) and a breeding phase (d 175 23 8 ) with d 0 being the start of the experiment. The heifers utilized were cared for in accordance with acceptable practices as outlined in the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (FASS, 1999) Animals Sixty heifers (Angus n = 30, Brangus n = 30; i nitial BW = 257 17.5 kg ; initial age = 2 60 15 d ) were utilized in this expe riment. Heifers were stratified by breed, BW, age, and sire and allocated to 12 pens of five heifers/pen on d 14 of the experiment Pens were randomly assigned to one of two treatments: 1) round bale silage (RBS) and dried distillers grains (DDG) supple mented three d/wk for duration of experiment (174 d, CON) or 2) ad libitum RBS for the first 88 d and RBS ad libitum plus DDG supplemented three d/wk from d 89 174 (L H). Heifers were adapted to assigned treatments from d 14 to 1 of the experiment. Hei fers remained in the assigned pens through the sampling phase (d 0 to 174) and first 30 d of the breeding phase (d175 to 205). For the final 30 d of the breeding phase (d 205 to 235) heifers were sorted by breed into two breeding groups and exposed to a s ingle satisfactory potential breeder bull per group. P en s Pens consisted of 1.2 ha of established bahiagrass ( Paspalum notatum ) and common bermudagrass ( Cynodon dactylon ). Established forage received no fertilization during the previous summer. The forag e was managed by mowing to result in similar available forage DM

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79 in all pens at the initiation of the experiment. Mean forage availability at the initiation of the experiment were 722 DM kg/ha, 9.7% and 52.7% CP and TDN respectively. Diets The DDG suppl ement was offered three d/wk to CON heifers in amounts to allow 0.75 kg/d gain as predicted by the Level 1 Model of the Beef Cattle NRC (2001) for growing yearling beef heifers. Heifers in the L H group upon supplementation were offered DDG supplement thr ee d/wk in amounts to allow 1.36 kg/d as predicted by the Level 1 Model of the Beef Cattle NRC (2001) for growing yearling beef heifers. The amount of DDG supplement offered was altered on a monthly basis based on heifer BW gain to meet the targeted level of performance (Table 4 1). All pens had ad libitum access to Tifton 85 bermudagrass ( Cynodon dactylon ) RBS throughout the trial. A complete commercial mineral/vitamin mix (14% Ca, 9% P, 24% NaCl, 0.20% K, 0.30% Mg, 0.20% S, 0.005% Co, 0.15% Cu, 0.02% I 0.05% Mn, 0.004% Se, 0.3% Zn, 0.08% F, and 82 IU/g of vitamin A) and water were offered for ad libitum consumption throughout the experiment. Samples of DDG were collected every two wk and every bale of RBS was core sampled. Pasture samples were collect ed month ly from each pen to estimate DM yield and forage quality by hand clipping three different 0.25m 2 area and compositing the samples. All pasture and RBS ground to pass through a 1 mm screen in a Wiley mill (Arthur H. Thomas Company, Philadelphia, PA, USA). Dr y matter for all samples was determined by drying samples for two h at 135C (procedure 930.15; AOAC, 2000). The RBS and DDG were pooled by 28 d periods for laboratory analysis of CP, OM, IVDOM and IVDMD. Nutrient values for DDG, pasture, and RBS are pr esented in Table 4 2. Total nitrogen was determined by rapid combustion using a macro elemental N analyzer (Elementar, vario MAX CN, Elementar Americas, Mount Laurel,

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80 NJ, USA) and used to calculate CP (CP = N x 6.25). In vitro dry and organic matter dige stibility of samples was determined by the procedure of Tilley and Terry (1963), as modified by Marten and Barnes (1980) with filtration on filter paper. Inoculum for IVDMD was obtained from a ruminally fistulated cow fed ad libitum coastal bermudagrass ( Cynodon dactylon ) hay and 900 g/d of soybean meal, and strained through four layers of cheesecloth. Organic matter Concentrations of TDN for DDG were determined by a commercial l aboratory (Dairy One Forage Laboratory, Ithica, NY). Concentrations of TDN for hay and pasture samples were determined using the equation (Fike et al., 2002): %TDN = [(% IVDOM 0.59) + 32.2] OM concentration. Breeding On d 171 of the experiment, heifer s received a progesterone insert (EAZI BREED CIDR ; GnRH (Cystorelin, Merial, Inc., Duluth, GA). Seven d later, CIDR were removed and heifers received 25 mg (i.m.) prostaglandin F (PG; Lutalyse Sterile Solution, Pfizer Animal Health, New York, NY). Estrus was detected using radiotelemeric estrous detection devices (HeatWatch Cow Chips, Denver, CO; Dransfield et al., 1998), which were fitted to all heifers, beginning on day 175. Heifers were artificially inseminated (AI) by one AI tech nician with frozen thawed semen 8 to 12 h after the onset of the PG induced estrus. Seventy two h after PG administration and CIDR removal all heifers not exhibiting estrus were AI and administered for the subsequent 30 d and heifers were artificially inseminated 8 12 h after the onset of estrus. After 30 d AI period heifers were grouped by breed and each breed group was pasture exposed for an additional 30 d to a fertile bull similar to its respective breed type. Pregnancy was determined approximately 30, 60, and 90 d after the initial AI by transrectal ultrasonography,

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81 using a real time, B mode ultrasound (Aloka 500v, Corometrics Medical Systems, Wallingfor d, CT) equipped with a 5.0 MHz transducer for determination of pregnancy status. Sampling Heifers were weighed after 16 h of feed and water restriction to determine shrunk BW one wk before the initiation of the sampling phase (d 7) and 6 d after the end of the sampling phase ( d 174 ). Heifers were weighed every two wk from d 0 88 of the experiment and weekly from d 89 174 of the experiment. H ip heights (HH) body condition score (BCS: 1 = severely emaciated, 5 = moderate; 9 = very obese; Wagner et al., 1988), body length (BL; Gilbert et al., 1993), and heart girth (HG; Gilbert et al., 1993) were recorded on d 0, 91, and 168 of the experiment. Ultrasound measurements of longismus dorsi area (LM area) at the 13 th rib, 13 th rib fat thickness (FT), rump fa t thickness (RF), and intramuscular fat of the LM (IMF) were taken on d 19, 89, and 168. The IMF measurement for the LM will only be reported for d 168 due to inconsistencies in evaluating IMF with ultrasonography in young cattle (Brethour, 2000; Crews an d Kemp 2001 ). The initial ultrasound measurement was not taken until d 19 of the experiment due to scheduling conflicts with the ultrasound technician. On d 171 of the experiment, Pelvic areas measurements were taken and RTS (Anderson et al., 1991) were recorded. Blood samples were collected on d 0, 28, 56, 89, 91, 105, 119, 147, and 168 to be analyzed for plasma NEFA, urea nitrogen (PUN), and glucose concentrations. Blood samples were collected via jugular venipuncture into 7.5 mL polypropylene syring es containing 1.6 mg potassium EDTA as an anticoagulant (Monovette, Sarstedt Inc., Newton, NC). Samples were immediately placed on ice and transported to the lab for further processing. Blood samples were centrifuged at 855 x g 30 C for subsequent analysis. Blood samples were also collected on d 2 and 8, d 79 and 89, and d 158

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82 and 168 for plasma progesterone concentrations to determine if heifers were pubertal at t he beginning, middle, and end of the sampling phase. Heifers were considered pubertal when progesterone concentrations were 5 ng/mL at either sample within a time period. Blood Analysis Plasma PUN concentrations were determined using BioAssay Systems QuantiChrom Urea Assay Kit series DIUR 500 kit (BioAssays Systems, Hayward, CA). Plasma glucose concentrations were determined using Cayman Chemical Co. Glucose Analysis Kit (Cayman Chemical Co., Ann Arbor, MI). Plasma NEFA concentrations were determin ed using Wako HR series NEFA HR kit (Wako Diagnostics, Richmond, VA). Plasma progesterone c oncentrations were determined using Coat A Count Kit (DPC Diagnostic Products Inc., Los Angeles, CA) solid phase 125 I RIA (Seals et al., 1998) The intra and int erassay CV were 10 and 9%, respectively. Sensitivity of the assay was 0.10 ng/mL and 0.1 mL of plasma was assayed. Statistical Analysis Heifer p erformance and metabolite data were analyzed using the MIXED procedure of SAS (SAS Inst., Inc., Cary, NC) Th e model statement used for BW, Shrunk BW, ADG HH, BCS, HG, BL, LM area, FT, RF, LM area/unit BW, change in LM area, RTS, and pelvic area contained the effect of treatment breed and treatment x breed Data were analyzed using pen(treatment) as the random variable. The model statement used for metabolite analysis contained the effects of treatment, breed day and all appropriate interactions. Data were analyzed using heifer(pen) and pen(treatment) as random variables. Results are reported as least squa re means Reproductive data were analyzed using GENMOD procedure of SAS. The model statement used for all reproductive data contained the effects of treatment, breed, and treatment x breed. For all analysis, significance was set at P were determined if P > 0.05

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83 Results and Discussion Performance At the initiation of the experiment (d 0) heifer BW, shrunk BW, BCS, HH, HG, and BL were similar ( P > 0.05) between treatments (Table 4 3). Angus and Brangus heifers had similar ( P > 0.05) BW, shrunk BW and HG (Table 4 3) while the Brangus heifers had greater BCS ( P = 0.01) and HH ( P = 0.006) compared to Angus heifers. In contrast, Angus heifers tended ( P = 0.08) to have greater BL compared to Brangus heifers. At the mid point of the sampling phase (d 89) CON heifers were 50 kg heavier ( P = 0.0002) compared to L H heifers. Furthermore, CON heifers had greater BCS ( P < 0.0001), HH ( P = 0.002), HG ( P = 0.0001), and BL ( P = 0.01) compared to L H heifers. Although BW was not differen t ( P = 0.67) between Angus and Brangus heifers, the Brangus heifers had greater BCS ( P < 0.0001) and continued to have greater HH ( P = 0.002) compared to Angus heifers. There was a treatment breed effect ( P = 0.04) on BCS at the midpoint of the sampling phase. There were no breed effects ( P > 0.05) on HG and BL. During the first half of the sampling phase CON heifers (0.47 kg/h) had a greater ( P < 0.0001) ADG compa red to L H heifers ( 0.1 kg/h; Table 4 4). The ADG of CON heifers was considerably less than the 0.75 kg/h predicted by the Level 1 Model of the Beef Cattle NRC (2001) for growing yearling beef heifers. Additionally, there was treatment by breed interaction ( P < 0.0 1 ) for ADG during the first 89 d of the trial (Table 4 4). The ADG for the A ngus L H heifers was less ( P < 0.05) compared to Brangus L H heifers and the ADG of the Angus and Brangus L H were less ( P < 0.05) compared to ADG of CON Angus and Brangus heifers, which were similar to each other (Table 4 4). Because the L H heifers were not supplemented during the first half of the sampling phase they were expected to gain less; however, the rate of gain of the L H heifers was considerably less than expected. The overall decreased performance of the

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84 heifers on both treatments can be att ributed to the quality of the RBS (Table 4 2) being lower than what was expected which considerably hindered the ability of the L H heifers to meet their nutrient requirements for growth during the first 89 d of the experiment. The fact that Brangus L H heifers had an improved ADG compared to the Angus L H heifers is interesting This observation could be attributed to an increased DM digestibility of forage due to possible differences in retention time and fermentation rates between cattle of Bos indicu s compared to Bos taurus breeding (Warwick and Cobb, 1976; Ikhatua et al., 1986). At the conclusion of sampling phase of the expe riment (d168) CON heifers were heavier (40 kg; P < 0.0 1 ) compared to L H heifers (Table 4 3). Shrunk BW for CON heifers was 38 kg heavier ( P = 0.006) compared to L H heifers. Control heifers also had greater BCS ( P = 0.03), HH ( P < 0.01 ), HG ( P < 0.01 ), and BL ( P = 0.01) compared to L H heifers. The Brangus heifers continued to be taller ( P = 0.01) compared to Angus heifers. H owever, BW, BCS, HH, HG, and BL were not different ( P > 0.05) between Angus and Brangus heifers. Relative to the ADG during the second half of the sampling phase of the experiment, there tended ( P = 0.06) to be both treatment and treatment breed effects (Table 4 4). The ADG for the L H and CON heifers were 0.74 kg and 0.61 kg, respectively. The ADG of L H Angus heifers was the greatest at 0.81 kg/d followed by L H Brangus heifers (0.67 kg/d), CON Brangus heifers (0.65 kg /d), and finally CON Angus heife rs (0.57 kg/d). This increase in ADG for the L H heifers is commonly observed in cattle that have had restricted growth (Carstens et al., 1989; Yambayamba et al., 1996). These faster growth rates are attributed to increased efficiency of protein and ener y utilization (Fox et al., 1972), increased feed intake, and decreased maintainence requirements (Carstens et al., 1989). Granger et al. (1990)reported similar results in Angus and Brangus heifers wintered on either hay alone; hay and cottonseed meal; hay cottonseed meal, and corn;

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85 or hay, cottonseed meal, corn, and monensin. They reported that heifers fed hay alone lost 0 .2 kg/d during the 107 d winter feeding phase, while all supplemented heifers gained weight (mean = 0.28 kg/d). During the following 70 d, heifers were grazed together on ryegrass pasture. During this time hay only heifers had increased ADG (0.9 kg/d) compared to all supplemented heifers (0.63 kg/d), but still had BW that were 33 kg less than supplemented heifers. The amount of DDG de livered to L H heifers (3.91 kg/d) during the second half of the sampling phase was greater ( P < 0.05) compared to CON heifers (1.89 kg/d). For the entire sampling phase, the CON heifers consumed less ( P < 0.05) DDG supplement (304 kg/hd as fed) compared to the L H heifers (DDG = 336 kg/hd). Round bale silage offered to CON heifers (1,593 kg/hd as fed) was greater ( P < 0.05) compared to RBS offered to L H heifers (1,321 kg/hd as fed) during the sampling phase. The decreased amount of RBS offered to the L H heifers was due to a substitution effect because of the high supplementation rate of the DDG during the second half of the sampling phase. As previously mentioned, L H heifers lost BW during the first half of the sampling phase, which was attributed to the quality of the RBS. Supplementation during the first half of the sampling phase of the trial would ideally help meet the nutrient requirements of the L H heifers. Supplementation as little as once a week may have improved performance. Huston et al. (1999) reported that mature cows supplemented with sorghum and cottonseed meal as little as once a week helped reduce BW loss in mature cows grazing Texas range. The significant loss in BW and BCS from the initiation to the mid point of the sampling i ndicates that the heifers were forced to mobilize body fat and muscle protein to meet their maintenance requirements. This is supported by the decreases in LM area and fat covering at the 13th rib and rump as determined by the ultrasonography examinations (Table 4 5).

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86 Ultrasound measurements taken to evaluate body composition at three points during the experiment are presented in Table 4 5. The value LM are/unit BW was calculated by dividing the LM area by the full BW of the heifer. This was done to tr y to account for differences in LM area between heifers due to differences in BW. The LM area, LM area/unit BW, FT, and RF did not differ (P > 0.05) between treatments on d 19 of the sampling phase. Brangus heifers had greater LM area/unit BW, and RF com pared to Angus heifers (P < 0.05) at d 19 At the end of the first half of the sampling phase (d 89) CON heifers had greater (P < 0.0 1 ) LM area compared to the L H heifers and tended (P = 0.07) to have greater LM area/unit of BW compared to L H heifers. The CON heifers also had more FT (P = 0.02), and RF (P = 0.03) compared to L H heifers. The percentage change in LM area was also greater (P < 0.01 ) for CON heifers (10.22 %) compared to L H heifers ( 14.46%). Brangus heifers tended (P = 0.08) to have la rger LM area compared to Angus heifers but when expressed a s area/ unit of BW, LM area was not different (P = 0.12) between Angus and Brangus heifers. Thirteenth rib fat (P = 0.28) was not different between Angus and Brangus heifers, but Brangus heifers h ad more (P = 0.01) RF compared to Angus heifers. The decrease in LM area, FT, and RF all indicate the mobilization of body reserves of muscle and fat during the first half of the sampling phase for the L H heifers. This matches the loss of BW experienced by these animals at that time. At the conclusion of the sampling phase of the experiment (d 168), LM area/unit BW, FT, RF, and IMF did not differ ( P > 0.05) between treatments (Table 4 5). Although, CON heifers had a greater ( P = 0.01) LM area compared to L H heifers. The LM area ( P = 0.11), LM are/unit BW ( P = 0.24), FT ( P = 0.63) and IMF ( P = 0.63) were not different between Brangus and Angus heifers. However, Brangus heifers had more ( P = 0.04) RF compared to Angus heifers. From the middle to the end of the sampling phase, L H heifers had a greater ( P < 0.01 )

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87 percentage change in LM area compared to CON heifers ( 32.0 vs. 12.14% respectively ). The difference in percentage change in LM area between the L H and CON heifers could be due to the increa sed supplement supplied to the L H heifers during the second period. The increased supplement supplied to the L H heifers provided more protein and energy for growth. The difference could also be attributed to compensatory gain being exhibited by the L H group. The greater BW gains, greater change in LM area, and increase in both FT and RF for the L H heifers fit the accelerated growth patterns often displayed by heifers that had previously had restricted growth. Fox et al. (1972 reported that steers un der going compensatory gain deposited more protein than fat early in the compensatory period, but relatively more fat than protein later. Blood Metabolites A treatment day effect ( P < 0.01) was observed (Figure 4 1) for PUN. The PUN concentrations of th e CON heifers were greater ( P < 0.05) compared to the L H heifers on d 89. However, PUN concentrations for the L H heifers were greater ( P < 0.05) on d 105 and at the end of the trial on d 147 and 168 compare d to CON heifers. The difference in PUN concen trations between CON and L H heifers at the end of the trial was likely a result of the increased amount of DDG supplement being offered to the L H heifers compared to the CON heifers. During the second half of the sampling phase, L H heifers received an average of 3.91 1 1 of DDG which equated to 1. 04 1 1 from the DDG compared to the CON heifers which received an average of 1.89 1 1 of DDG, which equated to 1 1 from the DDG. Heifers in bot h the CON and L H groups were probably over supplied protein during the end of the sampling phase of the trial, as the PUN levels exceed those suggested by Byers and Moxon (1980) of 11 to 15 mg/dL and Hammond et al (1993) of 8 12 mg/dL to be sufficient f or animal performance.

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88 Plasma urea nitrogen concentrations are indicative of protein balance (Ellenberger, 1989). Elevated PUN concentrations can be observed in feed restricted steers compared to ad libitum fed steers and PUN concentrations decline once steers are realimentated ( Hayden et al. 1993) Ellenberger et al. (1989) and Yambayamba (1996) did not observe greater P UN concentrations in restricted steers and heifers. During realimentation, both studies reported an initial decease in PUN which was attributed to the increased efficiency of protein and N use in the restricted heifers and steers. Surprisingly, L H heifers PUN decreased during the first 89 d of the trial even though LM area decreased 14% during this time period. The decrease in LM ar ea indicates a maintenance requirement This depletion of muscle usually causes an increase in PUN levels (Schrick et al., 1990). However, based on the values suggested by Byers and Moxon (1980) of 11 to 15 mg/ dL and Hammond et al (1993) of 8 12 mg/dL the L H heifers were only deficient in protein at d 89. The depletion of muscle may have increased the PUN levels to keep them above these levels A breed ( P < 0.01) effect was also observed for the mean PUN c oncentration as Brangus (15.55 mg/dL) heifers had greater PUN compared to Angus heifers (13.41 mg/dL). These results are similar to those reported in the previous experiment (Chapter 3) where Brangus heifers had greater PUN concentrations compared to Angu s heifers. Obeidat et al. (2002) reported similar breed effects between lactating Brahman and Angus cattle grazing dessert in the Southwest. It was suggested that during the months reported forage quality was low and BW was declining so some protein cata bolism occurred. They reported that based on their data and others (Simpson et al., 2004; Alverez et al., 2000) that there are possibly differing mechanisms between Angus and Brahman cattle for their use of protein tissues as an energy source for producti on and maintenance. This may explain why Brangus heifers lost less weight and less LM are during the

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89 first 89 d. It has also been shown that Brahman cattle can digest more protein and consume more DM from a low protein diet than Hereford cattle (Warwick and Cobb, 1976). Granger et al. (1990) reported that Brangus heifers fed mixed grass hay (4.77% CP) or ammoniated mixed grass hay receiving cottonseed meal and corn supplementation had greater gain:DMI ratios than Angus heifers receiving the same treatmen ts suggesting a more efficient conversion of digestible DM to live weight gain. The Brangus cattle may be better able to digest protein from the RBS that was fed. This could explain the tendency for the Brangus heifers to have greater LM area at d 89 of the sampling phase. Plasma glucose did not differ ( P = 0.76) between CON (63.3 mg/dL) and L H (62.5 mg/DL) treatments or between ( P = 0.69) Angus (62.44 mg/dL) and Brangus (63.43 mg/dL) heifers. Yelich et al. (1996) and Yambayamba et al. (1996) both rep orted decreased plasma glucose concentrations in heifers fed restricted diets, which was attributed to a decrease in gluconeogenic substrates. The L H heifers experienced significant BW loss during the first 89 d of the trial. This weight loss coupled wi th a lack of supplementation should result in low plasma glucose concentrations, but plasma glucose concentrations remained similar to those of CON heifers. Yambayamba et al. (1996) reported plasma glucose concentrations of restricted heifers were equival ent to control heifers within ten d of realimentation. The reason for the lack of a difference in plasma glucose concentrations between CON and L H heifers is not readily discernable The timing of the sample could be the cause of the lack of a differenc e between the two treatments. The CON heifers had been supplemented 48 h previous, this length of time could allow the glucose concentration to fluctuate and settle to a level similar to that of the L H heifers that were only consuming forage

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90 Plasma gluc ose concentrations were different ( P < 0.01) across days of the experiment (Figure 4 1). Plasma glucose concentrations decreased from d 0 to d 89 thereafter increasing on d 91 and 105. There was another decrease on d 119 followed by increased concentrati ons on d 147 and 168. The decline in glucose concentrations during the first phase of the trial (d 0 to 89) is similar to the previous experiment and data from Lopez et al. (2006) involving growing Angus, Brangus, and Brahman heifers fed a corn alfalfa ba sed diet The overall pattern of plasma glucose concentrations during the trial is difficult to explain. Plasma glucose concentrations are positively associated with increased nutrient intake (Yelich et al., 1996), which explains the increase in plasma g lucose concentrations from d 119 to 168 during the realimenation period as both treatments were receiving increased supplement However, the decline in glucose concentrations at d 119 is difficult to explain. The results that we observed could also be du e to the infrequent sample collection. Also, animal variation in supplement or forage intake on a daily basis could cause sample differences. Mean plasma NEFA concentrations did not differ by treatment ( P = 0.81) for CON (0.25 mE q /L) and L H (0.24 mE/L ) heifers or breed ( P = 0.28) between for Angus (0.23 mE/L) and Brangus (0.25 mE/L) heifers. However a treatment day effect ( P < 0.0001) was observed (Figure 4 3). Plasma NEFA concentrations of CON heifers were greater ( P <0.05) compared to L H heifers on d 105. Increased concentrations of NEFA are generally indicative of negative energy balance ( Brier et al., 1986; Richards et al., 1989 ) and reduced feed intake (Yelich et al. 1996). Yambayamba et al. (1996) also reported elevated NEFA concentrations in heifers on a restricted feed intake compared to ad libitum intake heifers. This difference in NEFA concentrations between treatments was diminished within ten d after restriction fed heifers were realimentated.

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91 As cattle lose BW, body fat is mobilized as an energy source. Metabolism of fat results in increased plasma NEFA concentrations (DiMarco et al., 1981). The L H heifers lost BW during the first phase of the trial, but NEFA concentrations decreased from d 0 to 56 before increasing slightly on d 89. No differences ( P > 0.05) in NEFA concentrations were observed between the treatments at d 0, 28, and 56. On d 89, the CON heifers tended ( P = 0.06) to have greater NEFA concentrations compared to L H heifers. The CON heifers, which were in a positi ve energy balance, had an increase ( P = 0.01) in NEFA concentrations from d 56 to 89. The reason for this increase in NEFA concentrations is unclear. The fat in the DDG could be responsible for the increase in NEFA. The plasma NEFA concentrations of the CON heifers were similar to those reported in heifers consuming DDG in Chapter 3. But this still does not explain the decline in NEFA concentrations for the L H heifers during this period. The increase in NEFA concentrations at the end of the trial (d 1 68) may be due to the concentration of fat in DDG. The high level of supplementation may cause an effect on plasma NEFA levels. Bindel et al (2000) reported plasma NEFA levels were increased by feeding tallow to finishing heifers at a rate of 2 or 4 % or the diet. Khorasani et al (1992) also reported increases in NEFA concentrations of early lactation Holstein cows fed canola seed. Reproductive Measures Treatment had a n effect ( P = 0.03) on the percentage of heifers that attained puberty at the midpoint of the sampling phase (Table 4 6) as more CON heifers attained puberty compared to L H heifers. B y the end of sampling phase (d 174), more ( P = 0.03) CON heifers attained puberty compared to L H heifers. Result s of the present study are similar to other studies that used programmed feeding to develop yearling beef heifers and reported decreased number of heifers reaching puberty at the end of the initial restriction phase and after a realimentation stage (Yelich et al., 1996; Lynch et al., 1997). The dec reased onset of puberty in L H heifers was due

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92 primarily to the fact that the heifers did not attain the adequate BW at the start of the breeding season, which is important since the onset of puberty in highly correlated to heifer BW and BW gain (Warnick e t al., 1956; Schillo et al., 1992). Day et al. (1986) reported that heifers maintained on a low energy diet had decreased LH pulse frequency which can lead to a delayed onset of puberty. The nutritional stress that the L H heifers underwent during the fi rst half of the sampling phase should have been severe enough to have a negative effect on the LH production, which would lead to the decreased number of L H heifers reaching puberty before breeding. Reproductive tract scores are often used by producers to predict the reproductive potential of yearling heifers based on the matur ity of the reproductive tract, thusly RTS scores are highly correlated with puberty. The L H heifers (2.9) had decreased ( P = 0.02) RTS compared to CON heifers (3.8; Table 4 6). Brangus heifers (3.6) tended ( P = 0.07) to have greater RTS compared to Angus heifers (3.2). The decreased RTS of L H heifers is supported by the decreased number of L H heifers that had attained puberty compared to CON heifers. Based on the RTS system devised by Anderson, et al. (1991), heifers with a RTS of 1 and 2 are considered pre pubertal, while heifers with a RTS of 3 are approximately 1 mo from initiating estrous cycles, and heifers with a RTS of 4 or 5 are considered to be estrous cycling Simi larly, pelvic area was less ( P < 0. 01) for L H heifers (192.9 cm 2 ) compared to CON heifers (206.7 cm 2 ) and Brangus heifers had larger ( P < 0. 01; 219.6 cm 2 ) pelvic areas compared to Angus heifers (180.1 cm 2 ) at the start of the breeding phase. The smalle r pelvic area of the L H heifers is associated with their decreased HH and BW compared to CON heifers. As observed with the significant decrease in LM area of LH heifers, nutrient restriction also had a negative effect on bone development as reflected by the smaller pelvic area of L H heifers compared to CON. Patterson et al. (1991) also reported decreases in pelvic area of heifers fed to reach 55% of their mature BW at breeding compared to

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93 heifers fed to reach 65% of mature BW at breeding. If the L H he ifers were to continue to receive adequate nutrition and continue to grow, differences in pelvic areas should become less by the start of the calving season (Neville et al., 1978). Laster (1974) reported that cow BW was the largest source of variation in pelvic area. If growth patterns for L H heifers follow those traditionally observed by heifers undergoing compensatory gain L H heifers will have accelerated growth and achieve similar BW to CON heifers (Lynch et al. 1997). The means for both treatments exceed the recommended minimum values for pelvic areas for heifers at one year of age of 140 180 cm 2 (Deutscher, 1987; Patterson and Bullock, 1995 ). The last phase of the experiment was the breeding phase that consisted of a synchronized breeding followed by a 25 d AI period and an additional 30 d natural service period. The synchronized e strous response was greater ( P = 0.008) for CON compared to L H heifers (Table 4 7 ). However, estrous response was not different ( P = 0.62) between Angus and Brangus hei fers. The primary reason for the increased estrous response for the CON compared to the L H was due to the increased number of CON heifers that had attained puberty at the start of the breeding phase. This is also supported by the increased RTS of the CO N heifers compared to the L H heifers Patterson et al. (2000) reported significant increases in synchronized estrous response in Bos taurus heifers as RTS increased from a 1 to 3 The estrous response for CON heifers was numerically similar to Angus and Brangus heifers in Chapter 3 and in Angus and Bos indicus Bos taurus heifers synchronized with a similar synchronization program reported by McKinniss (2008). Artificial in semination c onception rate was not different ( P = 0.77) between CON and L H heifers and the mean conception rate for both treatments was 50%. Conception rates for Brangus heifers (63%) tended ( P = 0.09) to be greater compared to Angus heifers (44%) The mean

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94 conception rate of the present study was less compared to the trial in Chapter 3 (57%) and studies reported by Lamb et al. (2006; 63%), and Lucy et al. (2001; 59%) for heifers synchronized using the same protocol. Martin et al. (2007) reported a conc eption rate of 75% for heifers fed DDG and synchronized with a two doses of prostaglandin. Synchronized pregnancy rates were not different between treatments ( P = 0.29) or breeds ( P = 1.00) and the mean synchronized pregnancy rate was 41.7%. Synchronized pregnancy rates were less than the previous experiment (Chapter 3; 50%) and those reported by Martin et al. (2007) of 57%. The decreased synchronized pregnancy rates is most likely a result of the decreased number of heifers that attained puberty at the start of the breeding phase for both the CON (33%) and L H (7%) heifers. This was considerably less than what was observed in the previous study (Chapter 3) where 50% of the heifers had attained puberty at the start of the breeding period resulting in a s ynchronized pregnancy rate of 50%. Thirty d pregnancy rates ( P = 0.01) and overall pregnancy rates ( P = 0.003) were greater for CON heifers compared to L H heifers. However, 30 d pregnancy rates ( P = 0.13) and overall pregnancy rates ( P = 0.20) for Angus and Brangus heifers were not different. There tended ( P = 0.06) to be a treatment breed effect on overall pregnancy rates. The Angus CON heifers had greater overall pregnancy rates compared to Brangus CON heifers and the opposite was true for the L H h eifers. There was a significant carryover effect of the L H treatment on the future reproductive potential of the heifers in this study as reflected in the significantly decreased 30 d and overall pregnancy rates of the L H treatment compared to the CON. Consequently, a larger number of L H heifers failed to reach puberty early in the breeding period and become pregnant by the end of the trial. The present trial emphasizes one of the limitations of using programmed feeding when developing yearling replac ement heifers. When heifers have significant reductions

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95 in BW gain it can have negative effects on the future reproductive potential of heifers and the heifers may not able to overcome this by the end of a 60 d breeding period. Other researchers (Clanto n et al., 1983; Lynch et al., 1997) have shown that programmed feeding can work without a negative effect on reproduction but the heifers cannot have their nutrient intake restricted to the extent that they were in the current experiment In summary, CON heifers had improved growth and reproductive performance compared to L H heifers. The quality of the RBS was lower than expected and had negative consequences on the performance of both treatments. However, the effect on the L H heifers was much greater L H heifers were unable to meet their nutrient requirements during the first 89 d of the sampling phase as was demonstrated by loss of BW, BCS, HG, FT, RF and LM area, indicating the utilization of subcutaneous fat and protein from muscle to meet nutrie nt requirements. Body weight gains were improved during the second half of the sampling phase along with f at deposits and LM area measured by ultrasound The increased efficiency of feed utilization commonly seen in cattle undergoing compensatory gain wa s not necessarily observed in this trial but it can not be definitively stated because of methods of recording supplement and RBS offered The previously reported metabolite responses of cattle undergoing restriction and compensatory growth were not obse rved in this study; however, this could be attributed to differences in supplements. The loss of BW during the first 89 d of the sampling phase was too great for the L H heifers to recover from in the final 79 d and had a significant negative effect on su bsequent reproductive performance. As a result, t he L H heifers had a decreased percentage of heifers reaching puberty at breeding and decreased RTS and pelvic area measurements compared to CON heifers Although, synchronized pregnancy rates were similar between the CON and L H heifers, L H had decreased 30 d and overall pregnancy rates.

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96 Implications The performance of the heifers in this trial, regardless of treatment, was affected by the quality of the RBS. The heifers on the L H treatment were not a ble to meet their nutrient requirements while consuming the RBS during the first 89 d. This led to metabolization of body fat and muscle. The loss of BW was severe enough to affect the performance of the heifers for longer than normally observed during r ealimentation. Reproductive performance of the L H heifers was greatly affected by the alteration of their growth pattern. For this to be an effective method of heifer development forage of greater quality must be offered, heifers must be strategically s upplemented to prevent excessive loss of BW and body condition or the un supplemented phase must be decreased in length with a corresponding increase in the supplemented phase The knowledge of the quality of the forage being fed to the heifers is key in helping predict the performance and possible supplementation necessary to develop heifers in an efficient manner. Further research is needed to determine the optimal nutrient intake and duration, of both the restricted and supplemented period, to allow t he most efficient economic and biological growth of heifers with out negatively affecting reproduction.

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97 Table 4 1. Amount of dried distillers grains supplement offered to yearling beef heifers consuming round bale silage Day of experiment Treatment 0 88 89 174 1 1 CON 1 1.45 1.89 L H 2 0.00 3.91 1 CON: c ontrol treatment, heifers offered dried distillers grains and round bale silage to gain 0.75 kg/d from d 0 174. 2 L H: l ow high treatment, heifers offered round bale silage only durin g d 0 88 and round bale silage and dried distillers grains fed to gain 1.36 kg/d during d 89 174.

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98 Table 4 2 Nutritive value of forage and supplement ingredients offered to developing beef heifers Feedstuff Item Pasture 1 RBS 2 DDG 3 DM % 70.40 5 3.90 88.60 % of DM OM 95.13 91.33 84.19 CP 11.56 7.96 30.10 IVDMD 4 32.67 45. 49 61.31 TDN 5 49.05 53. 64 86.33 1 Dormant bahiagrass forage. 2 Round bale silage of Tifton 85 bermudagrass. 3 Dried distillers grains. 4 In vitro dry matter di gestibility. 5 TDN estimated by Fike et al. ( 2002 ) for pasture and RBS. DDG TDN from Forage One Laboratory.

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99 Table 4 3. Growth characteristics of heifers offered round bale silage and supplemented with dried distillers grains to gain at a constant or p rogrammed rate of BW gain. Treatment 1 Breed Item CON L H Pooled SE P value Angus Brangus Pooled SE P value Day 0 S hrunk BW (kg) 257 257 6.9 0.95 250 246 6.8 0.66 BW (kg) 248 248 6.9 0.98 259 255 6.9 0.74 BCS 2 5.0 5.1 0.7 0.64 4.9 5 .2 0.7 0.01 H ip height (cm) 114.5 114.1 0.6 0.67 113.1 115.5 0.6 <0.01 H eart girth (cm) 150.2 148.3 1.2 0.32 149.9 148.6 1.2 0.46 B ody length (cm) 66.5 67.0 0.9 0.69 67.8 65.7 0.9 0.08 Day 89 BW (kg) 299 248 5.7 <0.01 271 275 5 .7 0.67 BCS 5.4 4.2 0.1 <0.01 4.6 5.1 0.09 <0.01 H ip height (cm) 120.0 116.5 0.5 <0.01 117.0 119.5 0.5 <0.01 H eart girth (cm) 158.7 147.3 1.1 <0.01 153.5 152.4 1.1 0.51 B ody length (cm) 101.0 97.0 0.9 0.01 99.2 98.7 0.9 0.69 Day 168 S hrunk BW (kg) 329 291 7.1 <0.01 309 311 7.1 0.80 BW (kg) 351 311 7.8 <0.01 331 331 7.8 0.95 BCS 5.6 5.2 0.1 0.03 5.3 5.5 0.1 0.33 H ip height (cm) 122.8 119.6 0.6 <0.01 120.0 122.3 0.6 0.01 H eart girth (cm) 166.2 158.8 1.4 <0.01 163.0 162.0 1.4 0.61 B ody length (cm) 108.0 103.0 1.1 0.01 105.3 105.7 1.1 0.82 No treatment breed ( P > 0.10) for all reported values except treatment breed ( P = 0. 04 ) at d 89 for BCS 1 CON : heifers supplemented three times a week for the duration of the tria l; L H : no supplementation for d 0 88 and supplemented three times a week from d 89 168. 2 BCS: 1 = severely emaciated, 9 = very obese.

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100 Table 4 4 Effect of treatment and breed on BW gain of heifers offered round bale silage and supplemented with dried distillers grains TRT 1 CON L H P Value Item Angus Brangus Angus Brangus Pooled SE TRT BRD TRT BRD ADG (kg/d) d 0 89 0.50 a 0.45 a 0.20 b 0.004 c 0.04 <0.01 0.09 <0.01 d 89 168 0.57 0.65 0.81 0.67 0.06 0.06 0.62 0.06 1 CON: heif ers supplemented three times a week for the duration of the trial; L H: no supplementation for d 0 88 and supplemented three times a week from d 89 168. a, b, c Means within rows with differing superscripts differ (P< 0.05).

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101 Table 4 5. Body ultrasound measurements of heifers offered round bale silage and supplemented with dried distillers grains to gain at a constant or programmed rate of gain. Treatment 1 Breed Item CON L H Pooled SE P value Angus Brangus Pooled SE P value Initial ( d 19 ) LM area 2 (cm 2 ) 42.65 41.31 1.24 1 0.47 40.82 43.14 1.24 1 0.19 LM area /unit BW 0.16 2 0.1 58 0.003 0.33 0.1 55 0.1 65 0.003 0.02 FT 3 (cm) 0.44 0.45 0.02 2 0.82 0.43 0.45 0.02 2 0.52 RF 4 (cm) 0.43 0.42 0.02 4 0.59 0.38 0.46 0.02 4 <0.01 Mid point ( d 89 ) LM area (cm 2 ) 46.80 35.10 1.36 7 <0.01 39.25 42.65 1.36 0.08 LM area /unit BW 0.1 57 0.1 42 0.005 0.07 0.1 4 4 0.1 5 5 0.005 0.12 FT (cm) 0.41 0.34 0.02 5 0.02 0.36 0.39 0.02 0.28 RF (cm) 0.42 0.31 0.02 3 <0.01 0.33 0.40 0.02 0.01 End ( d 168 ) LM area (cm 2 ) 51.18 45.31 1.99 1 0.01 47.08 49.42 1.99 1 0.11 LM area /unit BW 0.1 46 0.1 45 0.004 0.90 0.14 3 0.1 49 0.004 0.24 FT (cm) 0.43 0.42 0.02 6 0.68 0.42 0.44 0.02 6 0.63 RF (cm) 0.43 0.38 0.02 8 0.19 0.37 0.44 0.02 8 0.04 IMF (%) 5 2.71 2.68 0 .11 7 0.86 2.66 2.73 0.11 7 0.63 % change in LM area d 19 89 10.22 14.46 3.29 4 <0.01 3.89 0.46 3.29 4 0.46 d 89 168 12.14 31.97 3.70 9 <0.01 22.33 21.78 3.70 9 0.91 d 19 168 22.93 10.14 2.5 1 1 <0.01 15.11 17.96 2.51 1 0.43 No treatment bree d ( P > 0.10) for all reported values. 1 CON : heifers supplemented three times a week for the duration of the trial; L H : no supplementation for d 0 88 and supplemented three times a week from d 89 168. 2 LM: Longismus dorsi area 3 FT: 13 th rib subcutaneou s fat thickness 4 RF: rump subcutaneous fat thickness 5 IMF: intramuscular fat in Longismus dorsi muscle.

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102 Figure 4 1. Plasma urea nitrogen (PUN) concentrations by treatment and day of experiment for heifers consuming round bale silage and suppleme nted with either dried distillers grains to gain at a constant rate (CON) or programmed (L H) rate of gain (Treatment day, P < 0.0001; a, b Means within a day differ P < 0.05)

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103 Figure 4 2. Plasma glucose concentrations by day for heifers consuming rou nd bale silage and supplemented with dried distillers grains. (Day, P < 0.0001; a, b, c d Means with different letters across days differ P < 0.05)

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104 Figure 4 3. Plasma NEFA c oncentrations by treatment and day of experiment for heifers consuming round bale silage and supplemented with dried distillers grains to gain at a constant rate (CON) or programmed (L H) rate of gain (Treatment day, P < 0.0001; a, b Means with different letters within a day differ P < 0.05)

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105 Table 4 6. Treatment, breed, and tr eatment breed effects for pelvic area, reproductive tract score, heifers pubertal at d 89, and heifers pubertal at breeding for Angus and Brangus heifers consuming round bale silage and supplemented with dried distillers grains to gain at a constant (CON ) or programmed (L H) rate of gain. Treatments N Pelvic area (cm 2 ) R eproductive tract score 1 Pubertal by d 89 (%) Pubertal at breeding (%) CON 30 206.7 3.8 13 33 Angus 15 184.3 3.8 20 33 Brangus 15 229.2 3.9 7 33 L H 30 192.9 2.9 0 7 Angus 15 175.9 2.5 0 7 Brangus 15 209.9 3.3 0 7 P value TRT 0.02 <0.01 0.03 <0.01 Breed <0.01 0.07 1.00 1.00 TRT Breed 0.35 0.13 1.00 1.00 1 Value of 1 5 as described by Anderson et al., 1991.

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106 Table 4 7. Treatment, breed, and treatment breed effe cts for estrous response, conception rate, timed AI conception rate, synchronized pregnancy rate, and 28 d AI pregnancy rate of Angus and Brangus heifers consuming round bale silage and supplemented with dried distillers grains to gain at a constant (CON) or programmed (L H) rate of gain Number in parenthesis is the number of heifers in each category. 1 Treatments Estrous response (%) 2 AI conception rate (%) 3 T ime AI conception rate (%) 4 Synchronized pregnancy rate (%) 5 30 d pregnancy rate (%) 6 Overall pr egnancy rate (%) 7 CON 73 (22/30) 50 (11/22) 38 (3/8) 47 (15/30) 83 (25/30) 93 (28/30) Angus 73 (11/15) 45 (5/11) 50 (2/4) 47 (7/15) 93 (14/15) 100 (15/15) Brangus 73 (11/15) 55 (6/11) 25 (1/4) 47 (7/15) 73 (11/15) 87 (13/15) L H 40 (12/30) 50 (6/12) 22 (4/18) 33 (10/30) 57 (17/30) 67 (20/30) Angus 47 (7/15) 29 (2/7) 38 (3/8) 33 (5/15) 53 (8/15) 60 (9/15) Brangus 33 (5/15) 80 (4/5) 10 (1/10) 33 (5/15) 60 (9/15) 73 (11/15) P value TRT <0.01 0.77 0.46 0.29 0.01 <0.01 Breed 0.62 0.09 0.17 1.00 0. 31 0.20 TRT Breed 0.62 0.22 0.81 1.00 0.15 0.06 1 removal. Estrus was detected for 3 d, and heifers that exhibited estrus were AI ap proximately 8 to 12 h later. Heifers which had not displayed estrus were timed AI at 72 76 h and received GnRH. 2 Percentage of heifers displaying estrus three d after PGF of total treated. 3 Percentage of heifers pregnant to AI of the total the exhibi ted estrus and were AI. 4 Percentage of heifers pregnant to the TAI. 5 Percentage of heifers pregnant during the synchronized breeding of the total treated. 6 Percentage of heifers pregnant during the first 30 d of the breeding season to AI. 7 Percentage o f heifers pregnant during the entire 60 d breeding season.

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127 BIOGRAPHICAL SKETCH Bradley Ryan Austin was born in Fort Lauderdale, Florida to F. Dean Austin and Laureen K. Austin. After graduating from high school in 1997, he attended the University of Florida where he received his Bachelor of Science degree in a nimal s ciences in December of 2000. He then began his Master of Science program under Bill Kunkle, which he completed in August of 2003. After taking a year off and marrying his wife, Meghan, Brad returned to the University of Florida to pursue his doctorate wh ile working for Dr Yelich as a Biological Scientist. During his tenure at the University of Florida Brad was involved with many projects focusing on reproductive physiology and the nutritional management of the cow and heifer herd.