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Comparative Studies of Postpartum Primiparous and Multiparous Beef Dams and the Effects of Early Weaning in the Subtropics

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PAGE 1

COMPARATIVE STUDIES OF PO STPARTUM PRIMIPAROUS AND MULTIPAROUS BEEF DAMS AND THE EFFECTS OF EARLY WEANING IN THE SUBTROPICS By SEBASTIAN GALINDO GONZALEZ A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2004

PAGE 2

Copyright 2004 by Sebastin Galindo Gonzlez

PAGE 3

To my family, Paula, Sergio, Sebastin, and Ricardo; and to our friends Roger, Karen, Alfredo, and Carlos.

PAGE 4

ACKNOWLEDGMENTS I thank my committee (Dr. J. D. Arthington, Dr. S. W. Coleman, Dr. R. P. Natzke, Dr. A. DeVries, and Dr. Nick. T. Place) for working arduously with me and giving me the guidance needed to complete my study. I would also like to thank the Consejo Nacional de Ciencia y Tecnologa (CONACyT) for supporting my studies. Thanks go to the faculty and staff of the Animal Sciences Department, Agricultural Education and Communication Department, and the Range Cattle Research and Education Center at Ona. Last but not least, thanks go to the administrators at the Universidad Veracruzana for the unconditional support that I have always received from them. iv

PAGE 5

TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES............................................................................................................vii ABSTRACT.....................................................................................................................viii CHAPTER 1 INTRODUCTION........................................................................................................1 2 CATTLE PRODUCTION IN THE TROPICAL ZONES OF THE WORLD.............3 The Tropics...................................................................................................................3 Adaptation and Tolerance to the Environment.............................................................3 Cattle Performance.......................................................................................................7 Reproduction.........................................................................................................8 Nutrition..............................................................................................................11 Management Considerations......................................................................................13 Age at First Calving, and Rebreeding Efficiency................................................13 Early Weaning.....................................................................................................15 Conclusions.................................................................................................................19 3 CASE STUDY: CALVING PRIMIPAROUS BRAFORD HEIFERS AT 2 VERSUS 3 YEARS OF AGE.....................................................................................20 Introduction.................................................................................................................20 Materials and Methods...............................................................................................21 Animal Care and Diet..........................................................................................21 Sample Collection and Analyses.........................................................................22 Results and Discussion...............................................................................................24 Body Weight, Body Condition Score, and Ultrasound Data...............................24 Milk Yield and Constituents................................................................................24 Net Energy...........................................................................................................25 Implications................................................................................................................26 v

PAGE 6

4 EFFECTS OF COW AGE ON RESPONSES TO EARLY CALF WEANING........32 Introduction.................................................................................................................32 Materials and Methods...............................................................................................33 Animal Care and Diet..........................................................................................33 Sample Collection and Analyses.........................................................................34 Results and Discussion...............................................................................................35 Implications................................................................................................................37 5 CONCLUSIONS........................................................................................................43 LIST OF REFERENCES...................................................................................................44 BIOGRAPHICAL SKETCH.............................................................................................53 vi

PAGE 7

LIST OF TABLES Table page 3-1 Daily ration provided to individual pairs (DMI, kg/d).............................................28 3-2 Nutrient composition of stargrass hay and commercial concentrate........................29 3-3 Effect of age at calving BW, body condition score and milk production in Braford cows during mid-lactation..........................................................................30 3-4 Output from energy use simulation..........................................................................31 4-1 Average BW (kg), body condition score (BCS; 1 to 9 scale), and hay DMI by weaning treatment within parity...............................................................................40 4-2 Frequency of pregnancies by parity and weaning treatment....................................41 4-3 Frequency of pregnancies within body condition score...........................................42 vii

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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science COMPARATIVE STUDIES OF POSTPARTUM PRIMIPAROUS AND MULTIPAROUS BEEF DAMS AND THE EFFECTS OF EARLY WEANING IN THE SUBTROPICS By Sebastian Galindo Gonzalez December 2004 Chair: John D. Arthington Major Animal Sciences Our objectives were to determine differences in energy use and performance of primiparous heifers calving at different ages; to investigate the effect of early calf weaning from both primiparous and multiparous cows on the period of postpartum anestrus and fertility, as measured by pregnancy rate to both timed-AI and natural service; and to make the reader aware of management practices that can improve the productivity of beef cattle. Two studies were conducted over 2 consecutive years. In the first study, 4 Braford cows and their calves (n = 12; four pairs/age group) were randomly selected from the Ona REC herd within one of three age groups, consisting of primiparous 2and 3-y-old cows, and multiparous mature cows. Energy use was simulated over 85 d, using three scenarios based on assumptions concerning priority for energy use. Throughout the study, the body weight (BW) of mature cows decreased (P < 0.01), while the BW of the other groups increased (0.17, 0.04, and -0.42, kg/d for 2viii

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y-old, 3-y-old, and mature cows, respectively; SEM = 0.08). The 2-y-old cows had a greater (P < 0.05) body condition score (BCS) when compared with the other groups on D 42; and when compared to the 3-y-old cows (P < 0.05) on D 84. Production of 4% fat corrected milk (FCM) tended (P < 0.09) to be less at D 0 for 2-y-old than for mature cows (4.46 and 6.67 kg/d, respectively; SEM = 1.17). By D 42, the FCM production from the 2-y-old cows was less (P < 0.05) than for either 3-y-old or mature cows (4.19 6.74, and 6.28 kg/d for 2-y-old, 3-y-old, and mature cows, respectively; SEM = 1.17). In the second study, 48 Brahman x British crossbred cows and their calves were stratified by parity and calving date, and were randomly allotted to treatments (n = 12 cows / treatment). The four treatments consisted of cow parity (multiand primiparous) and weaning schedule (earlyor normal-weaned). Calf age at early weaning was approximately 90 d (range, 68 to 107 d). Multiparous cows had greater hay dry matter intake (DMI) (P < 0.001), BW (P < 0.001), and BCS (P < 0.001) than primiparous cows throughout the study. Hay DMI was less (P < 0.001) for early-weaned primiparous and multiparous cows than for their normal-weaned contemporaries. Early-weaned cows had greater (P < 0.03) BW than did normal-weaned cows on Days 30 and 60 of the study. Early-weaned cows had greater (P < 0.001) BCS at Days 30 and 60 than did normal-weaned cows. No difference was found on pregnancy rates between earlyand normal-weaned cows. Data from both studies imply that primiparous Braford heifers calving at 2 years of age produce approximately 25% less milk and direct more energy to gain than do first-calf heifers calving at 3 years of age. Early-weaned cows, both multiparous and primiparous, have a greater increase in BW and BCS, and consume less hay compared to ix

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normal-weaned cows. Further studies are required to investigate the efficiency and priorities of energy use by beef cattle at different ages, and the economic implications of using early-weaning in both primiparous and multiparous cows. x

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CHAPTER 1 INTRODUCTION The worlds cattle inventory in 2003 was almost 1.4 billion head, with more than 95 million in the United States of America (FAOSTAT data, 2004). About 85% of this inventory comprises beef or dual-purpose cattle. However, most of these animals are not producing at optimal rates. Several factors restrain the performance of beef cattle. Because many beef farms are in tropical or subtropical areas, B. indicus genetics are common. These breeds can produce under stressful conditions (heat, parasites, drought, etc.) (Jchle, 1972; Koger, 1963; Turner, 1980), but they tend to reach puberty at older ages than do European cattle (Rodrigues et al., 2002). Because these animals mature late, the heifers are commonly older than 3 years at first calving. These cattle also experience long postpartum anestrus, reduced conception rates, and markedly seasonal calf production (Karikari et al., 1994). Nutrition is another factor negatively influencing the productivity of these animals. During some times of the year forage availability is reduced, and pasture quality is often suboptimal. Some researchers suggest that reducing the age at first calving (Nuez-Dominguez et al., 1991) and alleviating the nutritional burden of lactation for primiparous heifers (Arthington and Kalmbacher, 2003) may increase overall herd productivity. Therefore, our objectives were to determine differences in energy use and performance of primiparous heifers calving at different ages, to investigate the effect of early calf weaning from both primiparous and multiparous cows on the period of 1

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2 postpartum anestrus, and on fertility, as measured by pregnancy rate to both timed-AI and natural service; and to make the reader aware of management practices that can improve the productivity of beef cattle.

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CHAPTER 2 CATTLE PRODUCTION IN THE TROPICAL ZONES OF THE WORLD Although tropical zones concentrate more than one-half of the worlds cattle (80% of the buffalo, 67% of the goats, and 36% of the sheep), these areas contribute less than 20% of the total meat and milk production for these species (McDowell and Hernandez-Urdaneta, 1975). To promote economic and energy efficiency, the animal and environment must be compatible (Green et al., 1991; Syrstad, 1993). Cattle commonly found in the tropics are mainly pure B. indicus breeds and B. indicus x B. taurus crosses, because of their ability and capacity to produce under these environments (Jchle, 1972). The Tropics The tropics (also called tropical zone or torrid zone) include all of the territory (land and water) of the earth that lies between the Tropic of Cancer and the Tropic of Capricorn. The combination of trade winds carrying water from the oceans and creating seasonal rains is what marks the seasons in these areas. Within the tropical belt, several different climatic types can be distinguished, since latitude is only one of the many factors determining climate in the tropics. Terrain elevation, prevailing winds, and distance from the ocean are all elements contributing to climatic conditions (The Columbia Encyclopedia, 2001). Adaptation and Tolerance to the Environment Heat tolerance is only one factor involved in cattles adaptation to the environment, but it may be the most important (Briggs and Briggs, 1980). Hammond et al. (1996) conducted two trials with heifers to determine heat tolerance among temperate B. taurus 3

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4 (Angus, Hereford), B. indicus (Brahman), tropical B. taurus (Senepol, Romosinuano), and reciprocal crosses of Hereford and Senepol in Florida. Differences among breeds in temperament score, blood packed cell volume, and circulating concentrations of cortisol were also investigated. Rectal temperature of Angus was greater than that of Brahman, Senepol, or Romosinuano during the hottest summer date in Trial 1, and rectal temperature and plasma cortisol levels of Senepol were less than in Brahman. Hammond et al. interpreted the differences in rectal temperature between these breeds as being due to differences in stress response, possibly related to differences in temperament. In Trial 1, respiration rates were faster for Angus heifers than for Brahman, Romosinuano, and Senepol. However, on both the hottest and coolest dates of the study, were slower for Brahman heifers than for Romosinuano or Senepol. On the hottest summer date of Trial 2, rectal temperatures were greater for Angus heifers than for Brahman and Senepol heifers. Brahman heifers permanently had the slowest respiration rate and the greatest packed cell volume. Brahmans also had greater plasma cortisol concentrations and temperament scores than Angus. Plasma cortisol and rectal temperatures of Brahman and Senepol were not different during the hottest summer date of the study, supporting the hypothesized relationship between rectal temperature and response to stress. Hammond et al. concluded that their results show the capacity of Senepol and Romosinuano heifers to tolerate heat environments, and also show the Senepols ability to maintain constant body temperature in a hot environment in crosses with a nonadapted breed. Carvalho et al. (1995) characterized some physiological and histological responses to heat stress in 42to 80-mo-old imported B. taurus (Simmental), native B. taurus (Simmental), and native B. indicus cattle in Brazil. After walking 7 km at 37C with 60 to

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5 65% relative humidity during midday, the native B. indicus were environmentally adapted, showing lower temperature and respiration rates than native B. taurus. The imported B. taurus had the greatest body temperatures and respiration rates through all the study. B. indicus breeds also have different skin histology, sweat gland histometry, and number of epithelial strata than B. taurus. Turner (1980) indicated that B. indicus are better adapted than B. taurus to perform on high temperature environments because they have more sweat glands, lesser thermogenesis, greater skin surface to body size ratio, and usually posses smaller frame. Hernandez et al. (2002) conducted a three-year study from 1992-94 to evaluate the physiological responses and grazing behavior of the tropically adapted Senepol (B. taurus), the Holstein (B. taurus) adapted to temperate conditions, and the Brahman (B. indicus). Each year 15 to 17 yearling heifers per breed were evaluated between July and November for weight, rectal temperature, and respiration rate of each heifer. Their study also recorded ambient temperature and relative humidity during this period. To estimate hematocrit and cortisol levels, blood samples were also collected. Grazing behavior was observed 1 day before and 1 day after the physiological responses were recorded, during the third year of the study. Hernandez et al. concluded that these breeds varied in their adaptation to the environmental conditions in Puerto Rico for all the variables evaluated, and that these differences appeared to be due to genetic differences in the adaptation to tropical environments. The authors also concluded that the B. taurus Senepol cattle has the capacity to tolerate and perform well under the hot-humid climate conditions of northern Puerto Rico.

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6 Koger (1963) reported that the tolerance of internal and external parasites, tolerance of high solar energy, ambient temperature and humidity, and the capacity to utilize high fiber forages are some of the adaptive traits of the Brahman and Brahman based breeds that allow their production in subtropical or tropical areas. Burns et al. (1997) researched the ability of four breeds to withstand both tick and intestinal helminthes burdens and high ambient temperatures while growing in a subtropical environment in Australia. The breeds included on this study were Simmental, Hereford, Hereford x Shorthorn crossbred, and Belmont Red. The least heat tolerant and tick resistant breed was the Simmental. In their study, no differences among breeds in helminthes resistance were found. At weaning and at 12 months of age the heaviest among these breeds was the Simmental. However, no differences in weight were found among breeds at 18 months of age. The authors concluded that the Belmont Red breed was the best of these breeds for this subtropical environment due to its demonstrated superior adaptive performance. Koch et al. (2002) studied steers and heifers of purebred B. taurus, B. taurus x B. indicus, B. taurus x B. indicus and purebred B. indicus to determine differences in cortisol concentrations preand post-exogenous ACTH application. The authors did not find differences in pre-ACTH serum cortisol concentrations, post-ACTH serum cortisol concentrations or in the proportional response of cortisol between steers and heifers within breed type. However, in their study the pre-ACTH serum cortisol concentrations were different among breed types. The purebred B. taurus had lesser pre-ACTH serum cortisol concentrations values than the other groups. The authors concluded that animals with different percentages of B. indicus and B. taurus genes respond differently in

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7 production of cortisol after exogenous ACTH. The authors also concluded that an animals genetic conformation might affect its susceptibility and/or resistance to stressing conditions. The authors implied that additional studies might provide a better understanding of how genotype controls these traits that allow an individual to reduce or enhance the negative consequences of stress, which could lead to methods of selection for animals that are stress resistant. Cattle Performance Tropical areas of the world provide hard tests daily for those decided to raise cattle on them, since the animals must overcome stressing conditions on a daily basis. Dual-purpose cattle systems have been implemented in the tropics, attempting to increase production. These systems are characterized by the use of crossbred animals, of B. taurus and B. indicus, where the level of B. taurus genetics in the cross is not well defined (Estrada et al., 2002). Jenkins and Ferrell (2004) collected production data from mature cows produced by mating Angus and Hereford, Brahman and Boran, and Tuli sires by artificial insemination or by natural service to Angus and Hereford cows. These cows were later mated to Charolais bulls. The recorded data included the means for milk yield at peak lactation, total milk yields, birth weight of the calf, weaning weights adjusted to age, preweaning daily gain, and feed efficiency. In their studies, the B. indicus breed crosses manifested greater milk yield at peak lactation and total milk yield. B. indicus also exhibited lesser birth weight, greater daily gain, age-adjusted weaning weight, and greater feed efficiency than B. taurus. The cows that were sired by tropically adapted breeds had greater total milk yield, daily gain, adjusted weaning weight, and feed efficiency; however, these cows also showed lesser milk yield at peak lactation. Angus x Hereford

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8 exhibited greater total milk yield and birth weight than Tuli. The authors concluded that the efficiency of the crossbred Tuli cows was not different from Angus x Hereford F 1 females, but neither of them was similar to the efficiency of crossbred cows that were produced using B. indicus breeds. Reproduction Bos indicus cattle though suitable for hot environments, reach puberty at older ages forcing the heifers to calve for the first time long after 2-y of age. Rodrigues et al. (2002) studied 18 prepubertal heifers in Australia that were 9-mo-old. These heifers belonged to two different genotypes (B. indicus and B. taurus). The heifers were assigned to three groups (6 animals/group) on which different treatments were applied. One group of heifers remained intact (control), the second group was ovariectomized, and the third group was ovariectomized and implanted with estrogen (E2). From 10 to 20 mo-old, the circulating concentrations of progesterone (P4), presence of corpus luteum, and pulsatile pattern of luteinizing hormone (LH) release were determined from each heifer. The authors concluded that B. taurus achieved puberty at younger ages and at lighter weights than B. indicus heifers. Villagomez and Fajardo (1990) reported that the late sexual maturity, long postpartum anestrus, and marked seasonality in calf production characteristic of the B. indicus breeds led to relatively low reproductive indices in the tropics. Hawkins et al. (1999) alleged that the major constraints for economic and biological efficiency in range cowherds were the prolonged postpartum anestrus and the delayed sexual maturity of heifers. In Ghana, several researchers (Karikari et al., 1994; Osei and Effah-Baah, 1989; Tuah and Danso, 1985) have reported poor reproductive performance from native tropical

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9 breeds of cattle, which is characterized by late age at puberty and first calving, reduced conception rates and prolonged postpartum anestrus intervals. Seasonality has been identified as an influencing factor over reproductive performance in the tropical areas. The season of greatest fertility varies depending on geographical location. Research from Cuba (Menndez et al., 1978), and southeast Mexico (Castillo et al., 1983; Romero et al., 1983) reported that the fertility of purebred B. indicus and its crosses is greater during spring and summer, while the less fertility is found during the autumn and winter. The dry season has been found to be the time of the year with greatest fertility rates for Zebu cattle in Guatemala (Sanchez et al., 1969), this response may be due to the better body condition score of these animals after the rainy season. In Thailand, the effects of temperature and humidity on conception rate in small holder crossbred Holstein dairy herds were evaluated in the tropics. Data from primiparous and multiparous dairy cows from 513 herds in the northern part of Thailand showed that an increase of either the minimum temperature or the maximum humidity might produce a decrease in conception occurrence (Punyapornwithaya, et al., 2004). Randel (1984) reported that the reproductive function is mediated by season in the Indian breeds of cattle. The author explained that the reproductive endocrinology of B. indicus is different from that of B. taurus because the estrus is shorter and of less intensity. The author also emphasized the fact that B. indicus cattle have a lesser pre-ovulatory release of luteinizing hormone, and its corpus luteum is smaller containing less progesterone than B. taurus. The author supported the argument of a marked reproductive seasonality in B. indicus, based on the fact that their luteal cells are less responsive to

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10 luteinizing hormone in vitro during the winter, and that the recovery of viable embryos and the survival of these embryos in the recipient cows is greater from July through October. Villagomez et al. (2000) studied 16 heifers and 22 Indubrazil cows on pasture and under controlled feeding and management conditions to determine the effects of season on reproduction in Veracruz, Mexico. In their study all cows and heifers displayed estrus and subsequent ovulation during the spring and winter; however, only 60% of the heifers did it during the winter. The authors concluded that besides nutrition and management, the reproductive functions are affected by season. The authors believe that this influence may explain the documented fact that most Zebu cows become pregnant during the spring (drought season), in the Mexican tropics. Silva-Mena et al. (2002) evaluated the effect of the treatment with progestagens on estrus presentation, estrus behavior and pregnancy rate in 64 Brahman heifers after natural service. In their study, the estrus was synchronized with Norgestomet + estradiol valerate (Crestar, Intervet International, Boxmeer, The Netherlands), and the sexual activity between the heifers and the bull was observed and recorded for 30 h after the treatment. Pregnancies were determined two months later. The authors concluded that an estrus of variable length and intensity can be synchronized up to an average of 90% in Brahman heifers that were treated with Norgestomet + estradiol valerate, and with natural service pregnancy rates comparable to those obtained in B. taurus females treated similarly. Centurin-Castro et al. (1994) compared the pregnancy rates of Zebu cows inseminated once during the morning to that of Zebu cows inseminated twelve hours after

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11 observation of the onset of estrus, in Mexico. In this study the estrus was synchronized in 59 cows, and pregnancies were determined 60 days later by transrectal palpation. The authors concluded that there was no difference in fertility rates between the synchronized cows inseminated during the morning, independently of the moment when estrus was first observed, and those that were inseminated twelve hours after onset of estrus. Nutrition In the tropics, the main source of feed for beef production frequently consists of pastures of poor nutritional value, and which quality and quantity varies enormously over the seasons (Poppi and Mc. Lennan, 1995). It has been stated that B. taurus cattle does not have the same capacity of B. indicus and B. indicus crossbred cattle to utilize low quality forage diets in an efficient way (Karue et al., 1972). The deficiency of nutritional sources required for the growing and finishing of cattle is a disadvantage common to the tropical and subtropical areas of the world (Pate et al., 1984). In these regions, extreme weather conditions and reduced feed availability tend to produce periods of nutritional stress (Hawkins et al. 1999). Mansouri et al. (1992) studied the abilities of B. indicus and B. taurus to digest different feedstuffs in Iran. In their study, three types of roughage diets including alfalfa hay, common reed or wheat straw were fed to three ruminally fistulated Sistani and three Holstein steers in three different periods. The authors concluded that the Holstein steers were better adapted to digest alfalfa, whereas the Sistani steers fermented and digested common reed and wheat straw (low nitrogen feeds) in a more efficient way. The reproductive efficiency of the beef cow is susceptible to improvement through an accurate and improved estimation of her energy requirements (Wiltbank et al, 1962). Estrada et al. (2002) developed a simulation model to estimate the efficiency of use of

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12 metabolic energy in dual-purpose cows, and economic implications. Their model was based on body weight, productive, and reproductive traits that were recorded from a dual-purpose herd. Three genetic groups were simulated on their model, B. taurus, B. taurus B. indicus, B. taurus B. indicus. In their study, the metabolic energy requirements were based on metabolic weight; assuming that the efficiency of energetic conversion towards corporal weigh and milk production did not differ between genotypes. The purebred B. taurus group had the greatest metabolic energy requirements. The three genetic groups produced or contributed with similar amounts of total metabolic energy in the model. The authors conclude that the B. taurus B. indicus exhibited the most efficient use of energy and the best economic efficiency. Energy efficiency should be a criterion to select or evaluate a particular breed or its crosses, particularly the energy for maintenance requirements, because they account approximately for half of the feed consumed in beef production (Prichard and Marshall, 1993). Obese et al. (1999) studied the reproductive performance of 76 Sanga cows in Ghana from February 1995 to July 1996. In their study the interval from calving to the resumption of cyclic ovarian activity was 101 7 days, and from calving to conception was 152 4 days, with a calving interval of 444 16 days. The authors observed that the interval from calving to the resumption of cyclic ovarian activity was shorter during the dry season, and this interval decreased as body condition score was increasing. The authors concluded that long post-partum anestrus may lead to a prolonged calving interval with poor reproductive performance, suggesting also that this performance might be enhanced by feed supplementation and the early weaning of calves to improve body condition scores of the cows.

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13 The body condition score of a cow affects its reproductive performance in a very important way (Kunkle et al., 1994). Rae et al. (1993) studied the interactions between parity and body condition score over pregnancy rate. Over a two-year period they collected and analyzed data from eight herds in Florida. In their study, body condition score, parity, and their interaction had significant effects on pregnancy rate. As both parameters were increased the pregnancy rate was greater. The authors concluded that number of calvings and body condition score are important factors affecting the reproductive performance of beef cows in Florida. Management Considerations Different management strategies have been shown to produce positive changes over productive parameters of cattle. For the purpose of this review, we will focus on the effects of age at first calving and early weaning as alternatives to enhance cow herd productivity. Age at First Calving, and Rebreeding Efficiency If a heifer calves for its first time at 2 years of age, the total proportion of non-productive cows and of energy utilized for maintenance within the herd will decrease, while more of the feed consumed will be used to support pregnancy and lactation (McMillan et al., 1992). A heifer that starts calving as 2-y-old will have the opportunity to wean almost one extra calf through her lifetime than her contemporaries first-calving at 3 years of age (Morris, 1980). Nuez-Dominguez et al. (1991) studied the effect of culling policy in Nebraska, USA and age at first calving over different parameters. These productive parameters included: number of breeding seasons, pregnancies, number of calves born and number of calves alive 72 hours postcalving, calves alive at weaning, cumulative survival, calf

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14 weaning weights, and the input-output efficiency. These data was recorded for each cow up to twelve years of age, and it was obtained from records of Angus, Hereford, Shorthorn, and first-cross cows that were born between 1960 and 1963. Heifers calving for the first time as 2-y-old had greater economic efficiency than those calving first as 3-y-olds, without any effect from culling policy. Also, the economic efficiency reaches its peak when the terminal age of cows was 6 to 9 years and 8 to 9 years for 2-yand 3-y-old cows, respectively. In their study, 3-y-olds had greater repeatability of pregnancy than did 2-y-old cows, but was not enough to make up for the lost calf. Economic efficiency of the herd might be enhanced when heifers calve for their first time as 2-y-olds. Morrison et al. (1992) studied Angus and Angus x Hereford heifer calves that were born either in the spring or fall seasons in Louisiana. These heifers were allotted by weight at weaning to be exposed to bulls for calving either as 2-yor 2 -y-old. In their study, the heifers born during the fall were heavier than the spring-born at weaning. However, at breeding the heifers born during the spring were taller, heavier, and with greater BCS, regardless of age. The authors concluded that heifers calving as 2 -y-old were taller, heavier, their pelvic area was larger, and showed a greater pregnancy rate than primiparous two-year-olds. Werth et al. (1996a) researched the calving intervals and repeatability of calving intervals of beef cows at three different ages (2-, 3-, and 4-y-old) in Nebraska. These parameters were evaluated under breeding seasons immediately initiated after calving. In their study the duration of calving interval from the first to the second and the second to the third parities and the average calving date was recorded from 178 crossbred beef cows. The authors concluded that age and parity interact influencing calving interval, and

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15 this average interval may be less than 365 days when the initiation of the breeding season is not restricted after calving. In another study, Werth et al. (1996b) investigated the relation of the patterns of change in blood progesterone concentration of first-calf heifers to conception rates at the first postpartum estrus. The authors utilized crossbred primiparous 2-y-old cows from which data was collected over 2 years. In their studies, 64% of the cows achieved pregnancy after artificial insemination at the first estrus detected after parturition. Another 32% of the animals became pregnant at the second estrus after parturition, while 4% failed to conceive. The authors observed that conception rates were greater if a transient increase in progesterone preceded the estrus, and in 31.1% of the cows this increase was not detected before the first estrus after calving. The authors concluded that progesterone concentration increases before the first postpartum estrus, and this event is related to enhanced conception rates than those cows without the increase in progesterone preceding their first estrus after parturition. Early Weaning The reproductive cycle of the mammal female is influenced by the suckling stimulus in a very important way (Edgerton, 1980). Different studies have reported that a temporary or permanent restriction of suckling at early stages in the life of the calf will improve the reproductive efficiency of the dam (Laster et al., 1973; Reeves and Gaskins, 1981). Bellows and colleagues, described that a decrease of the postpartum interval can be achieved by the establishment of early-weaning practices (Bellows et al., 1974). Meirelles et al. (1994) conducted several trials to investigate the effects of temporary calf removal and supplementation with phosphorus on the conception rate of Nellore cows in Brazil. The authors reported that the evaluation of the results from the three trials concerning the effects of restricted suckling prior to breeding season produced

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16 inconsistent results on pregnancy rates. The authors concluded that restricted suckling of cows with marginal range plasma phosphorus may improve ovarian function. Hernandez et al. (1999) evaluated the effect of three periods of restricted suckling on milk yield and other productive variables of B. indicus x B. taurus cows in Venezuela. The performance of their calves was also evaluated. Three weaning age treatments were utilized; 8-, 16-, and 24-wk-old (n=12 cows/treatment). In their study, no differences were found among treatments on total milk yield; however, it was noticed that daily milk yield decreased approximately 23% the week after weaning, but the lactation curve trend was recovered 2 to 4 wk later. Their results showed that a greater amount of milk was sold by decreasing the suckling period from 24to 8-wk-old. The authors concluded that the advantages of restricted suckling are maintained but reducing the weaning age to 8-wk increases the quantity of saleable milk. As Arthington stated; the use of early weaning will allow young females to regain their lost body condition, and do so with less forage and supplemental feed. These females will also have a shorter post-partum interval. That is they will become pregnant earlier in the breeding season and therefore produce calves that will be older and heavier at next year's weaning (Arthington, 2002). Story et al. (2000) from the Department of Animal Science, University of Nebraska, Lincoln, USA studied spring-calving cows over a 5-y period to evaluate the effects of calf age at weaning on cow-calf performance and the production economics. The weaning treatments for this study were either early(5-mo-old), traditional(7-mo-old), and late-weaning (9-mo-old). In their study, the cow BW and body condition score were different at the last weaning date among all treatments. However, no differences in

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17 pregnancy rates were found among groups. The authors concluded that the age at weaning affects cow body condition score and weight. Bishop et al. (1994) evaluated the influence of the body energy reserves over the onset of luteal activity and in the concentrations of LH and IGF-I in serum in postpartum anestrous multiparous beef cows after early weaning in Oklahoma. In their study, 100% of the cows with body condition score 5 at the time of weaning initiated luteal activity within 25 d after weaning, while just 43% of the cows with BCS < 5 did the same. The authors concluded that body condition score at weaning influences the frequency of LH pulses, serum IGF-I, and the interval to the onset of ovarian activity after early weaning in anestrous beef cow. Houghton et al. (1990) studied the reproductive performance of mature Charolais x Angus cows adjusted to a moderate body condition at D 190 of gestation. These cows were then randomly blocked to either maintenance or a low-energy diet. Thirty days after parturition, the cows were again randomly blocked to either earlyor normal-weaning. In their study, early weaning reduced the postpartum anestrus interval. However, the first service conception rate was also reduced for the early-weaned group. It is important to mention that the normal-weaned cows in that particular study had an uncommonly elevated first service conception rate (92%). Houghton et al. concluded that a moderate body condition should be achieved before the breeding season, to enhance reproductive performance. Arthington and Kalmbacher (2003) studied the effectiveness of early weaning fall-born calves on heifer and calf performance. In their study, primiparous, three-year-olds Braford and Brahman x Angus heifers were randomly assigned to either earlyor normal

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18 weaning treatments. This study was conducted on two consecutive years. Although the early-weaned calves had a greater average daily gain on the first year, during the second year it was lesser than the normal-weaned group. The early-weaned dams from the study were heavier and with greater body condition score at the time of normal weaning. These same heifers had greater pregnancy rate on each year of the study, and their calving interval was lesser than normal-weaned heifers on Year 2. The authors concluded that early-weaning may improve pregnancy rate and body condition of primiparous heifers. Barker-Neef et al. (2001) studied the effects of early weaning on feedlot performance, carcass characteristics, and economic return to the cow-calf enterprise on Angus steers. These steers were assigned by birth date to either early(100 days) or normal-weaning (200 days). In their study, early-weaned steers had lesser total feed consumption and dry matter intake for the finishing period than normal-weaned steers. Although early-weaned steers had lighter carcass weights, the gain:feed and the cost for gain was improved. The authors concluded that early-weaning may be an economically feasible practice. Myers et al. (1999) conducted a study for two years to determine the effects of three weaning and nutrition treatments on cow-calf performance. The cow-calf pairs were randomly assigned to either early-weaning + a finishing diet, normal weaning + grain supplementation for 55 days on pasture, or normal weaning + 55 days on pasture and then placed on a finishing diet. In their study, early weaning increased the proportion of Average Choice grade steers by 40%. The early-weaned dams had greater average daily gain and improvement in body condition score. The authors concluded that early weaning improved feed efficiency and quality grades of beef steers.

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19 Conclusions Improved nutrition and reproductive management may help to overcome some of the current challenges facing the beef industry. The development of feedstuffs and feeding programs suitable to provide the animals with the required nutrients at the critical times of production is of special relevance in these regions. Breeding heifers at younger ages, and the use of early-weaning and other management practices focused to alleviate the postpartum nutritional pressure may increase the overall productivity of the herd.

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CHAPTER 3 CASE STUDY: CALVING PRIMIPAROUS BRAFORD HEIFERS AT 2 VERSUS 3 YEARS OF AGE Introduction Beef heifers in Florida traditionally are reared to calve first at 3-y-old. However, in other parts of the country, many producers breed heifers as yearlings (13to 15-mo-old) to calve at 2-y of age to improve productivity of young heifers. Several researchers consider this practice a good strategy to begin the recovery of some costs of development, and to increase lifetime productivity (McMillan et al., 1992) by reducing the proportion of non-productive cows in a given herd. Morris (1980) reported that those heifers calving as 2-y-olds have the opportunity to produce almost one extra calf through their productive lifetime. Beef productions efficiency has been linked to breed type (Kress et al., 1990; Marshall et al., 1976), size of the animal (Holloway and Butts, 1983; Kress et al., 1990), and milk yield (Clutter and Nielson, 1987; Neville, 1962; Rutledge et al., 1971). However, milk production and BW tend to be considered the most important components for efficiency evaluation of beef cows (Arango and Van Vleck, 2002; Dickerson, 1970; McMorris and Wilton, 1986; Montao-Bermudez et al., 1990). Some producers have experienced difficulty in re-breeding 2-y-old heifers after first calving. The objective of this study was to determine differences in energy utilization and performance of Braford heifers calving for the first time at 2vs. 3-y of age, at a similar level of nutrition. Mature multiparous Braford cows were included for comparison purposes as a control. 20

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21 Materials and Methods Animal Care and Diet The animals utilized in these experiments were cared for by acceptable practices as outlined in the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (FASS, 1999). The study was conducted over 85 days (d) at the University of Florida, Range Cattle Research and Education Center (REC) Ona, from January 10 to April 4, 2003. One week prior to the start of the trial, four Braford cows and their calves (n = 12; four pairs/age group) were randomly selected from the REC herd meeting one of three age groups, consisting of 2-, 3-y-old and mature cows. Mature cows were multiparous, while 2and 3-y-old cows were primiparous. Cows averaged 85 d (range, 76 to 107 d) in milk at the beginning of the study. Each pair was randomly allocated to one of 12 individual pens in a completely randomized design. During the trial, all cows were fed a daily ration of commercial concentrate (Lakeland Animal Nutrition, Lakeland, FL) and bahiagrass (Paspalum notatum) hay (45:55 respectively) (Table 3-1). Diets were formulated to provide for 0.22 kilograms per day (kg/d) of gain for 2and 3-y-old cows and no gain for mature cows, assuming a daily milk production of 5 kg (NRC, 1984). Forage and concentrate were 54 and 79% TDN, and 10.7 and 12.4% CP (DM basis), respectively. Nutrient analyses of hay and concentrate were conducted by a commercial laboratory (Dairy One, Inc., Ithaca, NY) (Table 3-2). All cows continued nursing their calves during the study. Additionally, to simulate the small amount of forage that would normally be part of their intake, each calf was fed 1 kg of oats (Avena sativa) three times a week.

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22 Sample Collection and Analyses Milk was collected from all cows on d 0, 42 and 84, using an Alfa-Laval single-unit vacuum milking machine. Approximately ten minutes before being milked, each cow received a single i.m. injection of acepromazine maleate (1 mL/100 kg; acepromazine maleate 10 mg/mL; Fort Dodge Animal Health; Fort Dodge, IA). Immediately before milking, each cow received 20 IU of oxytocin i.v. (Pro Labs Ltd., St. Joseph, MO). Two milkings were performed to each cow on the sampling days. The first one started at about 07:00, to empty the mammary glands. The milk collected at that time was discarded. The second milking was performed about 6 h after the first and the collected milk was weighed. Duplicate blind samples of milk were sent to a commercial laboratory (Southeast Milk, Inc., Belleview, FL) for determination of fat and protein concentration, and somatic cell count (SCC). The exact hour of both milkings was recorded for each cow, daily production of milk per cow was estimated from the amount produced in the time elapsed between the first and second milking. This uncorrected milk production was converted to 4% fat corrected milk (FCM). Body weight (BW) and body condition score (BCS; 1 to 9 scale; Kunkle et al., 1999) data were collected from all cows on d 0, 42, and 84 and ADG was calculated for each cow. Backfat thickness was measured ultrasonically between the 12th and 13th ribs, 3/4 the length ventrally over the longissimus muscle. The measurements were taken using an Aloka 500-V ultrasound with a 17.2, 3.5-MHz linear probe (Aloka; Wallingford, CT). Statistical analyses of SCC, FCM, BW, BCS, backfat thickness and ADG were achieved by ANOVA for a factorial experiment with a completely randomized design using the MIXED procedure of the Statistical Analysis System (Littel et al, 1996) to test

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23 the significance of interactions, where cow x treatment was our random statement. Pen (cow) was the experimental unit, and age group the treatment. The model statement contained the effects of age group, day of the trial, and age group x day interaction. The Least-Squares Means procedure was used to separate the differences among treatments at a fixed day of the trial. To estimate the NE utilization and allocation priority into NE m NE l and NE g by each cow, three scenarios were simulated using Microsoft Excel 2002 (Microsoft Corporation 1985-2001). The difference among scenarios consisted in the order on which the three requirements were met: A) NE m NE l NE g ; B) NE m NE g NE l ; C) NE g NE l NE m For each scenario, the first two requirements were calculated (NRC, 1984) based on changes in BW or milk production assuming all functions were constant across time. Residual energy intake was then assumed to be used for the remaining output function (e.g., BW gain) and the level of output was calculated from the available residual energy. The output for the remaining function was then regressed on actual output to determine how well the assumed energy partition agreed with measured output. Body composition was estimated from BCS (Ferrell and Jenkins, 1984) to calculate changes in stored energy and to derive the amount of energy from fat and protein mobilized in animals that were under a negative energy balance. Lipids provided 9.37 Mcal/kg, and protein 5.49 Mcal/kg. The simulation worked under the assumption that the energy mobilized from body tissue was used firstly for milk production. The formulas used to determine NE m NE l and body composition were taken from NRC, 1996; while those used for NE g came from NRC, 1984. According to NRC (1996), the amount of energy that is derived from

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24 BW loss is equivalent to the NEg that was required to achieve that weight initially. We calculated this energy from the live weight gain (LWG) formula presented in NRC 1984. Results and Discussion Body Weight, Body Condition Score, and Ultrasound Data While 2-y-old cows were lighter (P < 0.05) than mature cows at d 0, there were no differences in BW at Days 42 and 84 among the groups. Overall, the BW of mature cows decreased (P < 0.01), while the BW of the other groups increased (0.17, 0.04, and -0.42, kg/d for 2-, 3-y-old, and mature cows, respectively; SEM = 0.08) (Table 3-3). The 2-y-old cows also had a greater (P < 0.05) BCS when compared with the other groups on D 42, and when compared to the 3-y-old cows (P < 0.05) on D 84. Treatment differences were not detected when comparing backfat thickness; however, backfat thickness was positively correlated to BCS (P < 0.05; R 2 = 0.90), and negatively correlated to FCM production (P < 0.05; R 2 = -0.88) among all cows (n = 12). Milk Yield and Constituents Vaccaro and Dillard (1966) proposed that cows experiencing lesser weight gain were probably producing a greater amount of milk. The present trial showed that the production of FCM tended (P < 0.09) to be smaller at D 0 for 2-y-old than mature cows, and by D 42, FCM production of 2-y-old cows was lesser (P < 0.05) than both 3-y-old and mature cows. Averaged over all three collection times, FCM production tended to be 25% lesser for 2-y-old than 3-y-old (P = 0.08) and mature (P = 0.13) cows (4.49, 6.17, and 5.90 kg/d for 2-, 3-y-old, and mature cows, respectively; SEM = 0.62) (Table 3-3). Johnson et al. (2003) reported that multiparous Brangus cows produced 66 and 84% more (P < 0.001) milk than primiparous cows during early and late lactation respectively. It has been commonly accepted as a generalization that mature dairy cows produce about

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25 25% more milk than 2-yr-old heifers, 1/5 due to increase in BW, and 4/5 due to increased udder development; milk yield in dairy cows increases at a decreasing rate until about 8 years of age, and then it decreases at an increasing rate (personal communication, H. H. Head, 2002, Animal Sciences Department, University of Florida, Gainesville, Fl). However, this has not been confirmed in beef cattle. Van Oijten et al. (1993) reported that age of the cow had significant effects on milk production, on 2, 3and 4-y-old dams. However, Diaz et al. (1992) reported that age of the dam had no effect on milk production. Milk protein concentration did not differ (P > 0.10) among the groups. No differences were found in SCC among groups. Sheldrake et al. (1983) reported that age and parity had little effect on SCC. Net Energy Regression of NE g available after NE m and NE l (Scenario A) were calculated had excellent agreement (R 2 = 0.99) with energy in recorded gain (Table 3-4). For the remaining scenarios (B and C), regression of energy available vs. energy actually used were not good (R2 = 0.04 to 0.84) Therefore, our discussion will be limited to the output generated by scenario A. All age groups allocated approximately the same proportion of energy for maintenance (65%). This estimation differs slightly from Ferrell and Jenkins (1985), who reported that approximately 70-75% of the total energy requirement for beef production was used for maintenance. Energy efficiency should be a criterion to select or evaluate a particular breed or line, particularly the energy for maintenance requirements, because they account for nearly half of the feed consumed in beef production (Prichard and Marshall, 1993). Net energy for maintenance is highly related with metabolic BW, but is modified proportional to milk production (Prichard and Marshall, 1993). Montao

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26 Bermudez (1990) reported that almost 23% of the variation in maintenance requirements was due to differences in milk production. Since there were no differences in BW from D 42 within the groups, the variation in energy consumed was likely related to variation in milk production. While 3-y-old and mature cows partitioned almost 35% of the available energy (including energy derived from tissue mobilization) to support milk production, 2-y-old cows used only 25% to satisfy this requirement. The 2-y-old cows directed almost 10% of the total energy toward BW gain, producing an overall increase of 51 Mcal on their energy reserves. However, 3-y-old and mature cows lost 57 and 127 Mcal from their reserves, respectively. The efficiency of energy derived from body tissue loss is approximate 80% for both maintenance and milk production (Flatt et al., 1965; Moe et al., 1970; Russel and Wright, 1983). The 3-y-old and mature cows could apparently afford to mobilize energy from tissue loss to support lactation. For 2-y-old cows, it seems that lactation and gain have a similar importance. When provided similar levels of nutrition these data imply that first-calf Braford heifers calving at 2-years of age tend to produce approximately 25% less milk, and direct more energy for gain than first-calf heifers calving at 3-y of age. Implications Breeding heifers as yearlings may increase overall herd productivity. Therefore, further studies are needed to determine the economic impacts of calving primiparous heifers at 2vs. 3-y of age. An accurate estimation of NE requirements in relation with age and productive stage may allow for a better nutritional management of the cow herd. The identification and selection of cows with reduced maintenance costs when compared to their cohort may result in important savings in energy requirements. The implementation of management practices focused in the alleviation of the nutritional

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27 pressure of lactation, especially on primiparous heifers, may leave more energy available for gain and reproduction.

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28 Table 3-1. Daily ration provided to individual pairs (DMI, kg/d) Age group Ground Hay Concentrate 2-y-old 5.17 4.23 3-y-old 5.17 4.23 Mature 4.23 3.47 Diets were formulated according to NRC (1984) to provide for 0.22 kilograms per day (kg/d) of gain for 2and 3-y-old cows and no gain for mature cows, assuming a daily milk production of 5 kg.

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29 Table 3-2. Nutrient composition of stargrass hay and commercial concentrate Concentrate Hay As Fed DM As Fed DM CP (%) 11.0 12.4 10.2 10.7 TDN (%) 70.0 79.0 51.0 54.0 NE l (Mcal/Kg) 1.65 1.87 0.64 0.66 NE m (Mcal/Kg) 1.74 1.94 0.93 0.97 NE g (Mcal/Kg) 1.17 1.30 0.40 0.42 Commercial concentrate (Lakeland Animal Nutrition, Lakeland, FL). Nutrient analysis was conducted by Dairy One, Inc. (Ithaca, NY).

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30 Table 3-3 Effect of age at calving BW, body condition score and milk production in Braford cows during mid-lactation. Variable 2-yr old 3-yr old Mature Pooled SEM Cow BW, kg a Day 0 378 d 410 d,e 428 e 23.5 Day 42 391 d 410 d 404 d 23.5 Day 84 392 d 413 d 393 d 23.5 ADG 0.17 d 0.04 d -0.42 e 0.08 Cow BCS b Day 0 5.06 d 4.38 d 5.06 d 0.37 Day 42 5.38 d 4.50 e 4.56 e 0.37 Day 84 5.25 d 3.94 e 4.50 e 0.37 Change 0.2 d -0.4 e -0.6 e 0.22 4% FCM production, kg/d c Day 0 4.7 d 6.0 d 6.7 d 1.0 Day 42 4.2 d 6.7 e 6.3 e 0.9 Day 84 4.6 d 5.8 d 4.8 d 0.5 Overall 4.5 d 6.2 d 5.9 d 0.6 Diets were formulated to provide for 0.22 kilograms per day (kg/d) of gain for 2and 3-y-old cows and no gain for mature cows, assuming a daily milk production of 5 kg (NRC, 1984). a Body weight least square means by treatment. b Cow body condition score recorded as an averages score from two technicians at each collection date using a 1 to 9 scale (1 = emaciated and 9 = obese; Kunkle et al., 1999). c Milk was collected from all cows using an Alfa-Laval single-unit vacuum milking machine. d,e Treatment means within a row that do not have a common superscripts differ (P < 0.05).

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31 Table 3-4. Output from energy use simulation. Scenario A 2-yr 3-yr Mature Actual a NEg d42 (Mcal) 45.70 -4.33 -103.52 Simulated b NEg d42 (Mcal) 47.21 -5.37 -120.76 Actual NEg d84 (Mcal) 89.61 -1.27 -164.65 Simulated NEg d84 (Mcal) 90.17 0.44 -187.65 R 2 (actual vs. simulated; P<0.05) 0.99 0.99 0.99 Scenario B 2-yr 3-yr Mature Actual c energy secreted in milk d42 (Mcal) 133.51 132.58 200.18 Simulated d energy secreted in milk d42 (Mcal) 117.74 106.65 204.88 Actual energy secreted in milk d84 (Mcal) 261.90 270.95 367.80 Simulated energy secreted in milk d84 (Mcal) 302.86 236.19 342.92 R 2 (actual vs. simulated; P<0.05) 0.39 0.77 0.84 Scenario C 2-yr 3-yr Mature Calculated e NE m requirement at d42 (Mcal) 7.85 7.80 7.74 Simulated f NE m available at d42 (Mcal) 7.38 6.88 8.13 Calculated NE m requirement at d84 (Mcal) 7.83 7.63 7.54 Simulated NE m available at d84 (Mcal) 9.05 7.71 8.09 R 2 (calculated vs. simulated; P<0.05) 0.29 0.34 0.04 Three scenarios were simulated using Microsoft Excel 2002 (Microsoft Corporation 1985-2001). The difference among scenarios consisted in the order on which the three requirements were met: A) NE m NE l NE g ; B) NE m NE g NE l ; C) NE g NE l NE m a Calculated from the difference between the BW at d0 and the BW at a given day (d 42 and 84), the differential was expressed in Mcal based on the equation LWG=12.21* NE g 0.8936 W -0.6702 or NE g 0.8936 = LWG / (12.21 W -0.6702 ), for a positive or negative energy balance, respectively (NRC, 1984). b Simulated NE g available from DMI or body weight loss, after fulfilling the NE m and NE l requirements. c Obtained from the recorded milk production. The formula used to calculate the energy content of milk (E, Mcal/kg) was E = (0.092 fat percent) + (0.049 solubles non fat) 0.0569 (NRC, 1996). d Simulated NE l available from DMI or body weight loss, after fulfilling the NE m and NE g requirements. e Obtained from the formula NEm = [a1 SBW0.75 (Breed Effect) (Lactation) (Compensation for previous plane of nutrition)] + a2 (NRC, 1996). Where a1 is thermal neutral maintenance requirement and a2 is maintenance adjustment for previous ambient temperature. f Amount of energy available for maintenance, after satisfying NE g and NE l requirements.

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CHAPTER 4 EFFECTS OF COW AGE ON RESPONSES TO EARLY CALF WEANING Introduction The largest limitation for beef cow maintenance is availability of energy (Jenkins and Farrell, 1983). The production efficiency of beef cattle is restrained by reproductive failures (Dickerson, 1970; Dziuk and Bellows, 1983; Koch and Algeo, 1983). Research has shown that body condition score (Richards et al., 1986; Rae et al., 1993), seasonality (Villagomez and Fajardo, 1990), nursing (Moss et al., 1985; Arthington, 2002), and nutrition (Wiltbank et al., 1962; Obese et al., 1999; Villagomez et al., 2003) are all key elements affecting the length of postpartum anestrus. The duration of postpartum anestrus will often dictate the chance of a cow to become pregnant during a limited breeding season (Symington, 1969; Wiltbank, 1970). The search for management alternatives that are focused on decreasing cow dry matter intake without risking productivity is basic for the sustainability of beef cow-calf production systems. Early calf weaning has been traditionally recommended during times of the year when food availability for lactating cows is limited. However, early weaning may also improve the reproductive efficiency of beef cows. Young cows may benefit most by early calf weaning due to their limitations in maintaining optimal body condition through lactation (Arthington and Minton, 2004). The objective of this study was to investigate the effect of early calf weaning from both primiparous and multiparous cows on forage dry matter intake, the period of postpartum anestrus, and fertility as measured by pregnancy rate to both timed-AI and natural service. 32

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33 Materials and Methods Animal Care and Diet The animals utilized in these experiments were cared for by acceptable practices as outlined in the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (FASS, 1999). The study was conducted for 102 days at the University of Florida, Range Cattle Research and Education Center, located in southwest Florida. Using a completely randomized design with a 2 x 2 factorial arrangement of treatments, 48 Brahman x British crossbred cows and their calves were stratified by age and calving date and randomly allotted to treatments (n = 12 cows / treatment). The four treatments consisted of cow parity (multiand primiparous) and weaning schedule (earlyor normal-weaned). Calf age at early weaning was approximately 90-d (range, 68to 107-d). The weaning treatment was applied at D -21 of the study. An estrus synchronization and timed-AI protocol was applied to all cows starting eleven days after weaning treatment (D -10), and finishing at D 0 (Figure 4-1). At D -10, all cows received a single i.m. injection of GnRH (100 mcg of Gonadorelin; OvaCyst; Phoenix Scientific, Inc., St. Joseph, MO), and were implanted with an intravaginal progesterone-releasing insert containing 1.38 g of progesterone (CIDR, Interag, Hamilton, NZ). Seven days later (D -3), the intravaginal insert was removed, and each cow was given a single i.m. injection of PGF 2 (25 mg of PGF 2 ; Lutalyse; Pharmacia and Upjohn Company, Kalamazoo, MI). After 60 hours (D 0), all cows were artificially inseminated with semen obtained from a single collection from an Angus bull, and received a single i.m. injection of GnRH (100 mcg of Gonadorelin). Following timed-AI, all cows were placed into study pastures for 60 days. Cows were maintained on bahiagrass pastures (Paspalum notatum) of equal size (3 pastures / treatment; 4 cows /

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34 pasture). Cows were provided with free-choice access to stored hay and 2.27 kg/hd daily of fortified supplemental molasses (16% CP). At D 60 cow pregnancy status to AI was confirmed by transrectal ultrasonography (5.0-mHz intrarectal transducer; Aloka 500V; Corometrics, Wallingford, CT), and all the cows were moved to a single group and exposed to two mature Angus bulls for 21 days. Three weeks later pregnancy to natural service was confirmed by transrectal ultrasonography. Sample Collection and Analyses The percentage of cows cycling at the time of early weaning was determined by the measurement of blood progesterone concentration in samples collected eleven days apart. Progesterone concentrations were determined by radioimmunoassay (Seals et al., 1998) using DPC kits (Diagnostic Products Corp., Los Angeles, CA) in a single assay with an intra-assay CV of 9%. Sensitivity of the assay was 0.01 ng/tube and 0.1 mL of plasma volume was assayed. Cyclicity was determined when concentrations of progesterone 1.5 ng/ml were found. Cow BW and body condition score (BCS; 1 to 9 scale; Kunkle et al., 1999) data were collected at the time of early weaning (D = -21), upon placement into study pastures (D = 0), D 30, and D 60. For determination of hay consumption, each hay bale was weighed prior to placement in the pasture. At the end of the study, the entire hay residue from each pasture was collected and divided into dry and wet fractions. These fractions were weighed individually, mixed, and samples were collected for DM determination. Statistical analyses of cow BW, BCS, and hay consumption were achieved by ANOVA for a factorial experiment within a completely randomized design using the GLM procedure of the Statistical Analysis System to test the significance of the

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35 interactions. Pasture was the experimental unit. The Least-Squares Means procedure was used to separate the differences among treatments at a fixed day of the trial. Analysis of cow pregnancy rate was achieved by comparing weaning treatment with pregnancy rate using PROC FREQ of SAS. Differences in pregnancy rate were compared using Chi-square analysis. During the course of the experiment, two cows were removed from the study. Both were early-weaned primiparous cows, one was retired due to sickness and the other because she allowed another cows calf to suckle her. Results and Discussion Multiparous cows consumed 46% more (P < 0.001) hay than primiparous cows (8.64 vs. 5.91 kg/d, respectively). Because they did not require energy for milk production, early-weaned primiparous and multiparous cows consumed 20% less (P < 0.001) hay than their normal-weaned contemporaries (5.3 vs. 6.6 kg/d for earlyand normal-weaned cows, respectively). Marston and Lusby, (1995) also noted that lactating cows consume greater amounts of forage than gestating cows. Johnson et al. (2003) reported that each kg increase in milk yield was associated with a 0.33 and 0.37 kg increase in forage DMI for early and late lactation respectively. Considering that total feed costs can account for as much as 70% of the annual costs to keep a cow, this reduction in DMI has economic relevance. The cow-based energy costs associated with raising a calf are important to the efficiency of a cow-calf production system. Further, supplemental energy is widely viewed to be more beneficial for primiparous compared to multiparous cows. Hansen et al. (1982) reported that when primiparous heifers were provided a high-level of nutrition the anestrous period was reduced. However, the authors

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36 did not obtain the same response from the same cows during their second and third lactation. Multiparous cows were heavier (P < 0.001) than primiparous cows over the four data collection days (Table 4-1). There were no differences in initial BW within parity. Early-weaned multiparous cows were heavier than their normal-weaned contemporaries at Days 30 and 60 (P = 0.06 and 0.01, respectively). Overall, early-weaned cows were heavier than normal-weaned cows on Days 30 and 60 of the study (P = 0.03 and 0.01, respectively). From D -21 to 60, the BW of the early-weaned cows reflected a greater (P < 0.001) increase than that from the normal-weaned cows (28.4 vs. 1.8 kg, respectively). Multiparous cows had a greater (P < .0001) BCS than primiparous cows on the first and second data collection dates (Days -21 and 0). However, at Days 30 and 60, there were no differences in BCS between early-weaned multiparous and primiparous cows. Early-weaned cows had a greater (P < 0.001) BCS at Days 30 and 60 than normal-weaned cows. Overall, the BCS of early-weaned cows increased 0.21 points, while the BCS of normal-weaned cows decreased 0.93 points, reflecting a difference (P < 0.001) between treatments. The observed responses on BW and BSC of the early-weaned cows are likely due to the removal of the nutrient requirements associated with lactation. The frequency of AI pregnancy did not differ between earlyand normal-weaned cows. However, overall pregnancy rates (AI + Natural) tended (P = 0.25) to be greater for early-weaned cows (18 of 22 and 15 of 24 pregnant, for earlyand normal-weaned cows, respectively) (Table 4-2). Houghton et al. (1990) reported that early-weaned cows may have a 24-d shorter postpartum anestrous period compared to contemporaries that

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37 remain nursing their calves. In the same study, the authors observed a reduction (P < 0.10) of first-service conception rates in the early-weaned cows. The causes of this reduction were unclear, but it was mentioned that normal-weaned cows on that experiment presented an unusually high first-service conception rate (71.4 11 vs. 91.2 10% first service conception rate, for earlyand normal-weaned cows, respectively). Lucy et al. (2001) reported that the overall response to the CIDR+PGF 2 treatment will depend on the proportion of cattle that are cyclic at the start of the breeding season. Postpartum ovarian activity is suppressed by the suckling stimulus (Williams, 1990). Laster et al. (1973) reported that two-yr-old cows weaned before the breeding season, experienced an overall increase (26%) in pregnancy rate compared to an increase of only 7.9% in four-yr and older cows. In our study, multiparous cows had greater (P = 0.05) pregnancy rates after AI than primiparous (5 of 22 and 12 of 24 pregnant to AI, for primiparous and multiparous cows, respectively). This same difference between the multiparous and primiparous cows was significant (P < 0.001) when comparing overall pregnancy rates. Age and parity have a marked effect on postpartum anestrus (Short et al., 1990). Within BCS ranges, 36% of the cows with BCS 5.0 became pregnant to AI (8 of 22), while the same range achieved an overall pregnancy rate of 85% (11 of 13). The cows with BCS < 5.0 had pregnancy rates of 33 and 63% for AI (8 of 24) and natural service (21 of 33), respectively (Table 4-3). Implications The use of early-weaning as an alternative to reduce energy requirements associated with lactation, may increase cow body weight, body condition score, and reduce postpartum anestrus, in both primiparous and multiparous cows. Due to the

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38 reduction of forage dry matter intake, early-weaning of first-calf heifers may provide savings of 450 kg of hay for every three to four cows, over a 90-d breeding season. Although early-weaning has been traditionally recommended for primiparous heifers, the results from this study showed that multiparous cows were also benefited by its implementation. However, more studies are needed to determine the economic implications of the use of early-weaning on multiand primiparous cows.

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39 TAI e and Weaning treatment a GnRH GnRH b PGF 2 d CIDR c D -21 D -10 D -3 D 0 Days of the study Figure 4-1. Protocol for estrus synchronization and timed artificial insemination (TAI). a Earlyor normal-weaning. b GnRH (2mL of solution, equivalent to 100 mcg of Gonadorelin; OvaCyst; Phoenix Scientific, Inc. St. Joseph, MO). c CIDR (controlled internal drug-releasing device, Interag, Hamilton, NZ), intravaginal progesterone-releasing insert containing 1.38 g of progesterone. d PGF2 (5mL of solution, equivalent to 25 mg of PGF2; Lutalyse; Pharmacia and Upjohn Company, Kalamazoo, MI). e All cows were artificially inseminated with semen obtained from a single collection from an Angus bull.

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40 Table 4-1. Average BW (kg), body condition score (BCS; 1 to 9 scale), and hay DMI by weaning treatment within parity. Weaning treatment Item Early Normal SEM Primiparous BCS a Day -21 4.3 c 4.3 c 0.17 Day 0 4.2 c 4.4 c 0.15 Day 30 4.3 c 3.7 d 0.18 Day 60 4.3 c 3.5 d 0.18 Change 0.1 c -0.09 d 0.1 Multiparous BCS Day -21 4.8 c 5.0 c 0.17 Day 0 4.9 c 5.0 c 0.15 Day 30 5.3 c 4.8 d 0.18 Day 60 5.3 c 4.1 d 0.18 Change 0.3 c -0.9 d 0.1 Primiparous BW, kg Day -21 346 c 334 c 16.8 Day 0 329 c 319 c 15.8 Day 30 374 c 343 c 17.8 Day 60 358 c 324 c 16.2 Change 28 c 5 d 3.9 Multiparous BW, kg Day -21 454 c 437 c 16.8 Day 0 438 c 414 c 15.8 Day 30 490 c 448 d 17.8 Day 60 467 c 413 d 16.2 Change 29 c -1 d 3.9 Hay DMI, kg b Primiparous 322 c 396 c 26.5 Multiparous 499 c 533 c 26.5 a Cow body condition score recorded as an averages score from two technicians at each collection date using a 1 to 9 scale (1 = emaciated and 9 = obese; Kunkle et al., 1999). b For determination of hay consumption, each hay bale was weighed prior to placement in the pasture. At the end of the study, the entire hay residue from each pasture was collected and divided into dry and wet fractions. c,d Treatment means within a row that do not have a common superscripts differ (P < 0.05)

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Table 4-2. Frequency of pregnancies by parity and weaning treatment 41 Parity Weaningtreatment Item Primiparous Multiparous Early Normal AI pregnant, (%) 5 of 22 (23) 12 of 24 (50) 10 of 22 (46) 7 of 24 (29) Overall pregnancy, (%) 10 of 22 (46) 23 of 24 (96) 18 of 22 (81) 15 of 24 (63) Pregnancy status was confirmed by transrectal ultrasonography (5.0-mHz intrarectal transducer; Aloka 500V; Corometrics, Wallingford, CT).

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42 Table 4-3. Frequency of pregnancies within body condition score BCS Reproductive status < 5.0 5.0 AI pregnant, % 8 of 24 (33) 8 of 22 (32) Overall pregnancy, % 21 of 33 (60) 11 of 13 (85) Pregnancy status was confirmed by transrectal ultrasonography (5.0-mHz intrarectal transducer; Aloka 500V; Corometrics, Wallingford, CT). Cow body condition score recorded as an averages score from two technicians at each collection date using a 1 to 9 scale (1 = emaciated and 9 = obese; Kunkle et al., 1999).

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CHAPTER 5 CONCLUSIONS Adequate nutritional and reproductive management may help to overcome some of the current constraints in beef production. This management must include an accurate estimation of the energy requirements at different ages and productive stages. More studies are needed to get a clearer picture of how the energy from feed is partitioned and utilized in the live animal. We still do not know how the energy derived from body weight loss is distributed. It seems that at different ages, the physiological priorities are different. The identification and selection of cows with reduced maintenance costs when compared to their cohort will result in important savings in energy requirements. The use of early-weaning as an alternative to reduce energy requirements associated with lactation, may increase cow body weight, body condition score, and reduce postpartum anestrus, in both primiparous and multiparous cows. Due to the reduction of forage dry matter intake, early-weaning of first-calf heifers may provide savings of 450 kg of hay for every three to four cows, over a 90-d breeding season. Although early-weaning has been traditionally recommended for primiparous heifers, the results from this study showed that multiparous cows were also benefited by its implementation. However, more studies are needed to determine the economic implications of the use of early-weaning on multiand primiparous cows, and to find a way to increase pregnancy rates in younger heifers. 43

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LIST OF REFERENCES Arango, J. A., and L. D. Van Vleck. 2002. Size of beef cows: early ideas, new developments. Genet. Mol. Res. 1:51. Arthington, J. D. 2002. Improving the Productivity of Beef Heifers in Florida. University of Florida, IFAS, Florida Coop. Ext. Serv., Animal Science Dept., EDIS Publication AN132. Arthington, J. D., and R. S. Kalmbacher. 2003. Effect of early weaning on the performance of three-year-old, first-calf beef heifers and calves reared in the subtropics. J. Anim. Sci. 81:1136. Arthington, J. D., and J. E. Minton. 2004. The effect of early calf weaning on feed intake, growth, and postpartum interval in thin, Brahman-crossbred primiparous cows. Prof. Anim. Sci. 20:34. Barker-Neef, J. M., D. D. Buskirk, J. R. Black, M. E. Doumit, and S. R. Rust. 2001 Biological and economic performance of early-weaned Angus steers. J. Anim. Sci. 79:2762. Bellows, R. A., R. E. Short, J. J. Urick, and O. F. Pahnish.1974. Effects of early weaning on postpartum reproduction of the dam and growth of calves born as multiples or singles. J. Anim. Sci. 39:589. Bishop, D. K., R. P. Wettemann, and L. J. Spicer. 1994. Body energy reserves influence the onset of luteal activity after early weaning of beef cows. J. Anim. Sci. 72:2703. Briggs, H.M., and D.M. Briggs. 1980. Modern Breeds of Livestock. Fourth Edition. Macmillan Publishing Ltd. Basingstoke Hampshire, England. Burns, B. M., D. J. Reid, and J. F. Taylor. 1997. An evaluation of growth and adaptive traits of different cattle genotypes in a subtropical environment. Aust. J. Exp. Agric. 37:399. Carvalho, F. A., M. A. Lammoglia, M. J. Simoes, and R. D. Randel. 1995. Breed affects thermoregulation and epithelial morphology in imported and native cattle subjected to heat stress. J. Anim. Sci. 73:3570. Castillo, R. H., R. J. Padilla, M. J. A. Rivera, G. J. Fajardo, and S. J. Prez. 1983. Ciclo anual de las fecundaciones en Bos indicus y Bos taurus por Bos indicus mantenidos en clima tropical. Memoria reunin de investigacin pecuaria en Mxico. 1983:86. 44

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45 Centurin-Castro F., V. Castro-Sandoval, and R. Montes Prez. 1994. Fertilidad de vacas (B. indicus) inseminadas en un periodo fijo del dia. Livestock Res. Rural Dev. 6 (2): October 1994. Clutter, A. C., and M. K. Nielsen. 1987. Effect of level of beef cow milk production on preand postweaning calf growth. J. Anim. Sci. 64:1313. The Columbia Encyclopedia. 2001. Sixth Edition. Columbia University Press. Diaz, C., D. R. Notter, and W. E. Beal. 1992. Relationship between milk expected progeny differences of polled Hereford sires and actual milk production of their crossbred daughters. J. Anim. Sci. 70:396. Dickerson, G. 1970. Efficiency of animal production-molding the biological components. J. Anim. Sci. 30:849. Dziuk, P. J., and R. A. Bellows. 1983. Management of reproduction of beef cattle, sheep and pigs. J. Anim. Sci. 57:355. Eberhart, R.J., H.C. Gilmore, L.J. Hutchinson, and S.B. Spencer. 1979. Somatic cell counts in DHI samples. Page 32 in Proc. 18 th Annu. Meeting, National Mastitis Council, Madison, WI. Edgerton, L. A. 1980. Effect of lactation upon the postpartum interval. J. Anim. Sci. 51:40. Estrada, R. J., M. Parra-Bracamonte, J. G. Magaa, J. Santos, and C. Aguilar. 2002. Estimation of apparent energetic and economic efficiency of cows with different levels of B. taurus x B. indicus blood, using a simulation model, on dual purpose herds in the tropics of Mexico. Departamento de Reproduccin y Mejoramiento Gentico, Facultad de Medicina Veterinaria y Zootecnia. Universidad Autonoma de Yucatan, Mexico. FAOSTAT data. 2004. FAO statistical data bases. Available: http://apps.fao.org/faostat/form?collection=Production.Livestock.Stocks&Domain=Production&servlet=1&hasbulk=0&version=ext&language=EN .Last accessed Nov. 18, 2004. FASS. 1999. Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching. Federation of Animal Science Societies, Savoy, IL. Ferrell, C. L., and T. G. Jenkins. 1984. Relationships among various body components of mature cows. J. Anim. Sci. 58:222. Ferrell, C.L., and T.G. Jenkins. 1985. Cow type and nutritional environment: Nutritional aspects. J. Anim. Sci. 61:725.

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46 Flatt, W. P., L. A. Moore, N. W. Hooven, and R. D. Plowman. 1965. Energy metabolism studies with a high producing lactating dairy cow. J. Dairy Sci. 48:797. Green, R.D., L.V. Cundiff, and G. E. Dickerson. 1991. Life-Cycle biological efficiency of B. indicus x B. taurus crossbred cow-calf production to weaning. J. Anim. Sci. 69: 3544. Hammond, A. C., T. A. Olson, C. C. Chase, Jr., E. J. Bowers, R. D. Randel, C. N. Murphy, D. W. Vogt, and A. Tewolde. 1996. Heat Tolerance in two tropically adapted B. taurus breeds, Senepol and Romosinuano, compared with Brahman, Angus, and Hereford cattle in Florida. J. Anim. Sci. 74:295. Hansen, P. J., D. H. Baik, J. J. Rutledge, and E. R. Hauser. 1982. Genotype x environmental interactions on reproductive traits of bovine females. II. Postpartum reproduction as influenced by genotype, dietary regimen, level of milk production and parity. J. Anim. Sci. 55:1458. Hawkins, D. E., M. K. Petersen, M. G. Thomas, J. E. Sawyer, and R. C. Waterman. 1999. Can beef heifers and young postpartum cows be physiologically and nutritionally manipulated to optimize reproductive efficiency? Proc. Am. Soc. Anim. Sci. Indianapolis, USA, 1999. Available: http://www.asas.org/jas/symposia/proceedings/0928.pdf Last accessed Nov. 18, 2004. Hernandez, A. I., D. Cianzio, and T. A. Olson. 2002. Physiological performance and grazing behavior of Senepol, Brahman and Holstein heifers in Puerto Rico. Senepol Symp., St. Croix, USVI November 8-10, 2002. Available: http://rps.uvi.edu/AES/Senepol/Text/Cianzio1.pdf Last accessed Nov. 18, 2004. Hernandez, M., L. Gabaldon and J. Combellas. 1999. Influence of restricted suckling period on milk yield of B. taurus x B. indicus cows and live weight change of calves. Livestock Res. Rural Dev. 11(2): July 1999. Holloway, J. W. and W. T. Butts, Jr. 1983. Phenotype and nutritional environment interactions in forage intake and efficiency of Angus cows grazing fescue-legume or fescue pastures. J. Anim. Sci. 56:960. Houghton, P. L., R. P. Lemenager, L. A. Horstman, K. S. Hendrix, and G. E. Moss. 1990. Effects of body composition, preand postpartum energy level and early weaning on reproductive performance of beef cows and preweaning calf gain. J. Anim. Sci. 68:1438. Jenkins, T. G., and C. L. Ferrell. 1983. Nutrient requirements to maintain weight of mature, nonlactating, nonpregnant cows of four diverse breed types. J. Anim. Sci. 56:761.

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47 Jenkins, T. G., and C. L. Ferrell. 2004. Preweaning efficiency for mature cows of breed crosses from tropically adapted B. indicus and B. taurus and unadapted B. taurus breeds. J. Anim. Sci. 82:1876. Jchle, W. 1972. Seasonal fluctuations of reproductive functions in Zebu cattle. Int. J. Biometeorol. 16:131. Johnson, C. R., D. L. Lalman, M. A. Brown, L. A. Appeddu, D. S. Buchanan, and R. P. Wettemann. 2003. Influence of milk production potential on forage dry matter intake by multiparous and primiparous Brangus females. J. Anim. Sci. 81:1837. Karikari, P. K., S. A. Osei, P. Gyawu, and K. Asare. 1994. Growth and reproductive performance of N'dama cattle in the forest zone of Ghana. Kumasi J. Univ. Sci. Technol. 4:141. Karue, C. N., J. L. Evans, and A. D. Tillman. 1972. Metabolism of nitrogen in Boran and in Hereford-Boran crossbred steers. J. Anim. Sci. 35:1025. Koch, R. M., and J. W. Algeo. 1983. The beef cattle industry: changes and challenges. J. Anim. Sci. 57:28. Koch, J. W., T. H. Welsh, R. K. Miller, J. O. Sanders, D. G. Riley, D. K. Lunt, J. W. Holloway, T. D. A. Forbes, H. Lippke, F. M. Rouquette, Jr., and R. D. Randel. 2002. Influence of B. taurus and B. indicus breed type on production of cortisol. Beef cattle research in Texas. Available: http://animalscience.tamu.edu/ansc/beef/bcrt/Koch2.pdf Last accessed Nov. 18, 2004. Koger, M. 1963. Breeding for the American tropics. In: Cunha, T.J., Koger, M., Warnick, A.C.(Ed.) Crossbreeding beef cattle. Series I. pp 41-53. University of Florida Press, Gainesville. Kress, D. D., D. E. Doornbos, and D. C. Anderson. 1990. Performance of crosses among Hereford, Angus and Simmental cattle with different levels of Simmental breeding: V. Calf production, milk production and reproduction of threeto eight-year-old dams. J. Anim. Sci. 68:1910. Kunkle, W. E., R. S. Sand, and D. O. Rae. 1994. Effect of body condition on productivity in beef cattle. In: M. J. Fields and R. Sand (Ed.) Factors Affecting Calf Crop. CRC Press, Boca Raton, FL. Kunkle, W. E., R. S. Sand, and D. O. Rae. 1999. Effects of body condition on productivity in beef cattle. Document No. SP 144. University of Florida, Cooperative Extension Service, Gainesville. Laster, D. B., H. A. Glimp, and K. E. Gregory. 1973. Effects of early weaning on postpartum reproduction of cows. J. Anim. Sci. 36:734.

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48 Littel R.C., G. A. Milliken, W. W. Stroup, and R. D. Wolfinger. 1996. SAS system for mixed models. Cary, NC. SAS Inst. Lucy M. C., H. J. Billings, W. R. Butler, L. R. Ehnis, M. J. fields, D. J. Kesler, J. E. Kinders, R. C. Mattos, R. E. Short, W. W. Thatcher, R. P. Wettemann, J. V. Yelich, and H. D. Hafs. 2001. Efficacy of an intravaginal progesterone insert and an injection of PGF 2 for synchronizing estrus and shortening the interval to pregnancy in postpartum beef cows, peripubertal beef heifers, and dairy heifers. J. Anim. Sci. 79:982. Mansouri, H., A. Nikkhah, M. Rezaeian, M. Moradi, and S. A. Mirhadi. 1992. Comparative digestive ability and microbial population of Sistani (B. indicus) and Holstein (B. taurus) cattle fed different roughage diets. Anim.Sci. Res. Inst., Karaj, I. R. Iran. Marshall, D. A., W. R. Parker, and C. A. Dinkel. 1976. Factors affecting efficiency to weaning in Angus, Charolais and reciprocal cross cows. J. Anim. Sci. 43:1176. McDowell, R. E., and A. Hernandez-Urdaneta. 1975. Intensive systems for beef production in the tropics. J. Anim. Sci. 41:1228. McMillan, W.H., C. A. Morris, and D. G. McCall. 1992. Modeling herd efficiency in liveweight-selected and Angus control cattle. Proc. NZ Soc. Anim. Prod. 52: 145. McMorris, M.R., and J. W. Wilton. 1986. Breeding system, cow weight and milk yield effects on various biological variables in beef production. J. Anim. Sci. 63: 1361. Marston, T. T., and K. S. Lusby. 1995. Effects of energy or protein supplements and stage of production on intake and digestibility of hay by beef cows. J. Anim. Sci. 73:651. Meirelles, C. F., G. J. King, D. M. S. S. Vitti, and A. L. Abdalla. 1994. Management and nutrition strategies to reduce the breeding season in beef cows. Livestock Res. Rural Dev. 6(2): October, 1994. Menndez, A., D. Guerra, J. Dora, M. L. Prez, and J. R. Morales. 1978. Comportamiento reproductivo de la vaca Ceb en Cuba. I. Efecto de la poca del ao sobre la gestacin y el parto. Rev. Cubana Reprod. Anim. 4:103. Moe, P. W., H. F. Tyrell, and W. P. Flatt. 1970. Partial efficiency of energy use for maintenance, lactation, body gain and gestation in the dairy cow. Proc. 5 th Int. Symp. Energy Metabolism. EAAP Publ. 13:65. Montao-Bermudez, M., M. K. Nielsen, and G. H. Deutscher. 1990. Energy requirements for maintenance of crossbred beef cattle with different genetic potential for milk. J. Anim. Sci. 68: 2279.

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49 Morris, C.A. 1980. A review of relationships between aspects of reproduction in beef heifers and their lifetime production. Animal Breeding Abstracts. Commonwealth Bureau of Animal Breeding and Genetics, 48, 10: 655. Morrison, D. G., J. I. Feazel, C. P. Bagley, and D. C. Blouin. 1992. Postweaning growth and reproduction of beef heifers exposed to calve at 24 or 30 months of age in spring and fall seasons. J. Anim. Sci. 70:622. Moss, G. E., J. R. Parfet, C. A. Marvin, R. D. Allrich, and M.A. Diekman. 1985. Pituitary concentrations of gonadotrophins and receptors for GnRH in suckled beef cows at various intervals after calving. J. Anim. Sci. 60:285. Myers, S. E., D. B. Faulkner, F. A. Ireland, L. L. Berger and D. F. Parrett. 1999. Production systems comparing early weaning to normal weaning with or without creep feeding for beef steers. J. Anim. Sci. 77:300. Neville, W. E., Jr. 1962. Influence of dams milk production and other factors on 120and 240-day weight of Hereford calves. J. Anim. Sci. 21:315. NRC. 1984. Nutrient requirements of beef cattle. (6 th Rev. Ed.). National Academy Press, Washington. NRC. 1996. Nutrient requirements of beef cattle. (7th Rev. Ed.). National Academy Press, Washington. Nuez-Dominguez, R., L. V. Cundiff, G. E. Dickerson, K. E. Gregory, and R. M. Koch. 1991. Lifetime production of beef heifers calving first at two vs. three years of age. J. Anim. Sci. 69:3467. Obese, F. Y., S. A. Okantah, E. O. K. Oddoye, and P. Gyawu. 1999. Post-partum reproductive performance of Sanga cattle in smallholder peri-urban dairy herds in the Accra plains of Ghana. Trop. Anim. Health. Prod. 31:181. Osei, S.A., and K. Effah-Baah.1989. Reproductive performance of N'dama and West African shorthorn cattle in the humid forest zone of Ghana. Trop. Agric. (Trinidad), 66:256. Pate, F. M., J. Alvarez, J. D. Phillips, and B. R. Eiland. 1984. Sugarcane as a cattle feed: production and utilization. Department of Animal Sciences, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First published June 1984; reviewed March 2002. Bulletin 844. Poppi, P. D., R. S. Mc. Lennan. 1995. Protein and energy utilization by ruminants at pasture. J. Anim. Sci. 73:278. Prichard, D. L., and T. T. Marshall. 1993. Effect of cow size and milk production on nutrient requirements. Proc. 42 nd Annu. Florida Beef Cattle Short Course. Gainesville, Florida.

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50 Punyapornwithaya, V., K. Kreausukon, S. Theepatimakorn, and W. Suriyasathaporn. 2004. Minimum temperature and maximum humidity: Predictors for conception of crossbred Holstein cows in Thailand. J Anim. Sci. 82:257. Rae, D. O., W. E. Kunkle, P. J. Chenoweth, R. S. Sand, and T. Tran. 1993. Relationship of parity and body condition score to pregnancy rates in Florida beef cattle. Theriogenology 39:1143. Randel, R. D. 1984. Seasonal effects on female reproductive functions in the bovine (Indian breeds). Theriogenology 21:170. Reeves, J. J., and C. T. Gaskins. 1981. Effect of once-a-day nursing on rebreeding efficiency of beef cows. J. Anim. Sci. 53:889. Richards, M. W., J. C. Spitzer, and M. B. Warner. 1986. Effect of varying levels of postpartum nutrition and body condition at calving on subsequent reproductive performance in beef cattle. J. Anim. Sci. 62:300. Rodrigues, H. D., J. E. Kinder, and L. A. Fitzpatrick. 2002. Estradiol regulation of luteinizing hormone secretion in heifers of two breed types that reach puberty at different ages. Biol. Reprod. 66:603. Romero, A., E. Hernndez, E. Gonzlez, and C. Vsquez. 1983. Estacionalidad reproductiva de bovinos ubicados al oriente de Yucatn en trpico subhmedo. Memoria reunin de investigacin pecuaria en Mxico. 1983:68. Russel, A. J. F., and I. A. Wright. 1983. Factors affecting maintenance requirements of beef cows. Anim. Prod. 37:329. Rutledge, J. J., O. W. Robison, W. T. Ahlschwede, and J. E. Legates.1971. Milk yield and its influence on 205-day weight of beef calves. J. Anim. Sci. 33:563. Sanchez, F., A. C. Iturbide, and O. H. Colon. 1969. Caracteres reproductivos de un hato Brahman en Guatemala. Rev. Fac. Med. Vet. Zoot. (II):43. Seals, R. C., J. W. Lemaster, F. M. Hopkins, and F. N. Schrick. 1998. Effects of elevated concentrations of prostaglandin F 2 on pregnancy rates in progestogen supplemented cattle. Prostaglandins 56:377. Sheldrake, R. F., R.J.T. Hoare, and G. D. McGregor.1983. Lactation stage, parity, and infection affecting somatic cells, electrical conductivity, and serum albumin in milk. J. Dairy Sci. 66: 542. Short, R. E., R. A. Bellows, R. B. Staigmiller, J. G. Berardinelli, and E. E. Custer. 1990. Physiological mechanisms controlling anestrus and infertility in postpartum beef cattle. J. Anim. Sci. 68:799.

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52 Werth, L. A., J. C. Whittier, S. M. Azzam, G. H. Deutscher, and J. E. Kinder. 1996b. Relationship between circulating progesterone and conception at the first postpartum estrus in young primiparous beef cows. J. Anim. Sci. 74:616. Williams, G. L. 1990. Suckling as a regulator of postpartum rebreeding in cattle: a review. J. Anim. Sci. 68:831. Wiltbank, J. N. 1970. Research needs in beef cattle reproduction. J. Anim. Sci. 31:755. Wiltbank, J. N., W. W. Rowden, J. E. Ingalls, K. E. Gregory, and R. M. Koch. 1962. Effect of Energy level on reproductive phenomena of mature Hereford cows. J. Anim. Sci. 21:219.

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BIOGRAPHICAL SKETCH Sebastin Galindo Gonzlez was born in Mexico City, on October 14 th 1971. He obtained a professional degree in Veterinary Medicine and Animal Sciences from the Universidad Veracruzana, Mexico in 1995. He worked for some years for the veterinary pharmaceutical industry in Mexico, being responsible for sales, marketing, and technical training of the sales force. He switched to academic work in 2000, coordinating international programs for the Facultad de Medicina Veterinaria y Zootecnia of the Universidad Veracruzana. He was assistant professor of the Farm Management course and responsible for the off-campus program on Animal Production. He started his masters degree program on August 2002. Currently, he is an admitted Ph.D. student at the Department of Agricultural Education and Communication at the University of Florida in Gainesville. 53


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COMPARATIVE STUDIES OF POSTPARTUM PRIMIPAROUS AND
MULTIPAROUS BEEF DAMS AND THE EFFECTS OF EARLY WEANING IN THE
SUBTROPICS














By

SEBASTIAN GALINDO GONZALEZ


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2004

































Copyright 2004

by

Sebastian Galindo Gonzalez

































To my family, Paula, Sergio, Sebastian, and Ricardo; and to our friends Roger, Karen,
Alfredo, and Carlos.















ACKNOWLEDGMENTS

I thank my committee (Dr. J. D. Arthington, Dr. S. W. Coleman, Dr. R. P. Natzke,

Dr. A. DeVries, and Dr. Nick. T. Place) for working arduously with me and giving me

the guidance needed to complete my study. I would also like to thank the Consejo

Nacional de Ciencia y Tecnologia (CONACyT) for supporting my studies. Thanks go to

the faculty and staff of the Animal Sciences Department, Agricultural Education and

Communication Department, and the Range Cattle Research and Education Center at

Ona. Last but not least, thanks go to the administrators at the Universidad Veracruzana

for the unconditional support that I have always received from them.
















TABLE OF CONTENTS



A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES ................................................... vii

A B S T R A C T ..................................................................................................................... v iii

CHAPTER

1 IN T R O D U C T IO N ................................................. .............................................. .

2 CATTLE PRODUCTION IN THE TROPICAL ZONES OF THE WORLD ............. 3

T h e T ro p ic s ........................................................... .. ................................ ..................... 3
Adaptation and Tolerance to the Environment ................ ....................................3...
Cattle Performance .............................. .. ........... .............................7
R production .............. ................................................ .................. . .... 8
N nutrition ....................................................................... ..................... ............... 11
M anagem ent C considerations ............................................................ ..... ................ 13
Age at First Calving, and Rebreeding Efficiency........................................... 13
E early W ean in g ..................................................................................................... 15
C o n c lu sio n s............................................................................................................... .. 1 9

3 CASE STUDY: CALVING PRIMIPAROUS BRAFORD HEIFERS AT 2
V ER SU S 3 Y EA R S O F A G E ....................................... ...................... ................ 20

In tro d u ctio n ................................................................................................................ 2 0
M materials and M ethods .. ..................................................................... ................ 2 1
Animal Care and Diet .................................................................................... 21
Sam ple C collection and A nalyses.................................................... ................ 22
R results and D iscu ssion ............................................ ................... ......... .... ........ .... 24
Body Weight, Body Condition Score, and Ultrasound Data...............................24
M ilk Y ield and C onstituents........................................................... ................ 24
N et E n erg y ........................................................................................................... 2 5
Im p lic atio n s ................................................................................................................ 2 6









4 EFFECTS OF COW AGE ON RESPONSES TO EARLY CALF WEANING ........32

In tro d u ctio n ................................................................................................................. 3 2
M materials and M ethods .. ..................................................................... ................ 33
A nim al C are and D iet ............................................ ....................... . ........... 33
Sam ple C collection and A nalyses .................................................... ................ 34
R results and D discussion ............. .. ............... .............................................. 35
Im plications .............. ....................................................................... . .... 37

5 C O N C L U SIO N S .................................................. .............................................. 43

L IST O F R EFE R E N C E S ... ........................................................................ ................ 44

BIOGRAPHICAL SKETCH ............................ ............. 53















LIST OF TABLES


Table page

3-1 Daily ration provided to individual pairs (DMI, kg/d)........................................28

3-2 Nutrient composition of stargrass hay and commercial concentrate.....................29

3-3 Effect of age at calving BW, body condition score and milk production in
B raford cow s during m id-lactation ..................................................... ................ 30

3-4 Output from energy use sim ulation .................................................... ................ 31

4-1 Average BW (kg), body condition score (BCS; 1 to 9 scale), and hay DMI by
w meaning treatm ent w within parity .......................................................... ................ 40

4-2 Frequency of pregnancies by parity and weaning treatment................................41

4-3 Frequency of pregnancies within body condition score......................................42















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science


COMPARATIVE STUDIES OF POSTPARTUM PRIMIPAROUS AND
MULTIPAROUS BEEF DAMS AND THE EFFECTS OF EARLY WEANING IN THE
SUBTROPICS


By

Sebastian Galindo Gonzalez

December 2004

Chair: John D. Arthington
Major Animal Sciences

Our objectives were to determine differences in energy use and performance of

primiparous heifers calving at different ages; to investigate the effect of early calf

weaning from both primiparous and multiparous cows on the period of postpartum

anestrus and fertility, as measured by pregnancy rate to both timed-AI and natural

service; and to make the reader aware of management practices that can improve the

productivity of beef cattle. Two studies were conducted over 2 consecutive years.

In the first study, 4 Braford cows and their calves (n = 12; four pairs/age group)

were randomly selected from the Ona REC herd within one of three age groups,

consisting of primiparous 2- and 3-y-old cows, and multiparous mature cows. Energy use

was simulated over 85 d, using three scenarios based on assumptions concerning priority

for energy use. Throughout the study, the body weight (BW) of mature cows decreased

(P < 0.01), while the BW of the other groups increased (0.17, 0.04, and -0.42, kg/d for 2-









y-old, 3-y-old, and mature cows, respectively; SEM = 0.08). The 2-y-old cows had a

greater (P < 0.05) body condition score (BCS) when compared with the other groups on

D 42; and when compared to the 3-y-old cows (P < 0.05) on D 84. Production of 4% fat

corrected milk (FCM) tended (P < 0.09) to be less at D 0 for 2-y-old than for mature

cows (4.46 and 6.67 kg/d, respectively; SEM = 1.17). By D 42, the FCM production from

the 2-y-old cows was less (P < 0.05) than for either 3-y-old or mature cows (4.19, 6.74,

and 6.28 kg/d for 2-y-old, 3-y-old, and mature cows, respectively; SEM = 1.17).

In the second study, 48 Brahman x British crossbred cows and their calves were

stratified by parity and calving date, and were randomly allotted to treatments (n = 12

cows / treatment). The four treatments consisted of cow parity (multi- and primiparous)

and weaning schedule (early- or normal-weaned). Calf age at early weaning was

approximately 90 d (range, 68 to 107 d). Multiparous cows had greater hay dry matter

intake (DMI) (P < 0.001), BW (P < 0.001), and BCS (P < 0.001) than primiparous cows

throughout the study. Hay DMI was less (P < 0.001) for early-weaned primiparous and

multiparous cows than for their normal-weaned contemporaries. Early-weaned cows had

greater (P < 0.03) BW than did normal-weaned cows on Days 30 and 60 of the study.

Early-weaned cows had greater (P < 0.001) BCS at Days 30 and 60 than did normal-

weaned cows. No difference was found on pregnancy rates between early- and normal-

weaned cows.

Data from both studies imply that primiparous Braford heifers calving at 2 years of

age produce approximately 25% less milk and direct more energy to gain than do first-

calf heifers calving at 3 years of age. Early-weaned cows, both multiparous and

primiparous, have a greater increase in BW and BCS, and consume less hay compared to









normal-weaned cows. Further studies are required to investigate the efficiency and

priorities of energy use by beef cattle at different ages, and the economic implications of

using early-weaning in both primiparous and multiparous cows.














CHAPTER 1
INTRODUCTION

The world's cattle inventory in 2003 was almost 1.4 billion head, with more than

95 million in the United States of America (FAOSTAT data, 2004). About 85% of this

inventory comprises beef or dual-purpose cattle. However, most of these animals are not

producing at optimal rates.

Several factors restrain the performance of beef cattle. Because many beef farms

are in tropical or subtropical areas, B. indicus genetics are common. These breeds can

produce under stressful conditions (heat, parasites, drought, etc.) (Jochle, 1972; Koger,

1963; Turner, 1980), but they tend to reach puberty at older ages than do European cattle

(Rodrigues et al., 2002). Because these animals mature late, the heifers are commonly

older than 3 years at first calving. These cattle also experience long postpartum anestrus,

reduced conception rates, and markedly seasonal calf production (Karikari et al., 1994).

Nutrition is another factor negatively influencing the productivity of these animals.

During some times of the year forage availability is reduced, and pasture quality is often

suboptimal.

Some researchers suggest that reducing the age at first calving (Nufiez-Dominguez

et al., 1991) and alleviating the nutritional burden of lactation for primiparous heifers

(Arthington and Kalmbacher, 2003) may increase overall herd productivity.

Therefore, our objectives were to determine differences in energy use and

performance of primiparous heifers calving at different ages, to investigate the effect of

early calf weaning from both primiparous and multiparous cows on the period of






2


postpartum anestrus, and on fertility, as measured by pregnancy rate to both timed-AI and

natural service; and to make the reader aware of management practices that can improve

the productivity of beef cattle.














CHAPTER 2
CATTLE PRODUCTION IN THE TROPICAL ZONES OF THE WORLD

Although tropical zones concentrate more than one-half of the world's cattle (80%

of the buffalo, 67% of the goats, and 36% of the sheep), these areas contribute less than

20% of the total meat and milk production for these species (McDowell and Hernandez-

Urdaneta, 1975). To promote economic and energy efficiency, the animal and

environment must be compatible (Green et al., 1991; Syrstad, 1993). Cattle commonly

found in the tropics are mainly pure B. indicus breeds and B. indicus x B. taurus crosses,

because of their ability and capacity to produce under these environments (Jochle, 1972).

The Tropics

The tropics (also called tropical zone or torrid zone) include all of the territory

(land and water) of the earth that lies between the Tropic of Cancer and the Tropic of

Capricorn. The combination of trade winds carrying water from the oceans and creating

seasonal rains is what marks the seasons in these areas. Within the tropical belt, several

different climatic types can be distinguished, since latitude is only one of the many

factors determining climate in the tropics. Terrain elevation, prevailing winds, and

distance from the ocean are all elements contributing to climatic conditions (The

Columbia Encyclopedia, 2001).

Adaptation and Tolerance to the Environment

Heat tolerance is only one factor involved in cattle's adaptation to the environment,

but it may be the most important (Briggs and Briggs, 1980). Hammond et al. (1996)

conducted two trials with heifers to determine heat tolerance among temperate B. taurus









(Angus, Hereford), B. indicus (Brahman), tropical B. taurus (Senepol, Romosinuano),

and reciprocal crosses of Hereford and Senepol in Florida. Differences among breeds in

temperament score, blood packed cell volume, and circulating concentrations of cortisol

were also investigated. Rectal temperature of Angus was greater than that of Brahman,

Senepol, or Romosinuano during the hottest summer date in Trial 1, and rectal

temperature and plasma cortisol levels of Senepol were less than in Brahman. Hammond

et al. interpreted the differences in rectal temperature between these breeds as being due

to differences in stress response, possibly related to differences in temperament. In Trial

1, respiration rates were faster for Angus heifers than for Brahman, Romosinuano, and

Senepol. However, on both the hottest and coolest dates of the study, were slower for

Brahman heifers than for Romosinuano or Senepol. On the hottest summer date of Trial

2, rectal temperatures were greater for Angus heifers than for Brahman and Senepol

heifers. Brahman heifers permanently had the slowest respiration rate and the greatest

packed cell volume. Brahmans also had greater plasma cortisol concentrations and

temperament scores than Angus. Plasma cortisol and rectal temperatures of Brahman and

Senepol were not different during the hottest summer date of the study, supporting the

hypothesized relationship between rectal temperature and response to stress. Hammond et

al. concluded that their results show the capacity of Senepol and Romosinuano heifers to

tolerate heat environments, and also show the Senepol's ability to maintain constant body

temperature in a hot environment in crosses with a nonadapted breed.

Carvalho et al. (1995) characterized some physiological and histological responses

to heat stress in 42- to 80-mo-old imported B. taurus (Simmental), native B. taurus

(Simmental), and native B. indicus cattle in Brazil. After walking 7 km at 370C with 60 to









65% relative humidity during midday, the native B. indicus were environmentally

adapted, showing lower temperature and respiration rates than native B. taurus. The

imported B. taurus had the greatest body temperatures and respiration rates through all

the study. B. indicus breeds also have different skin histology, sweat gland histometry,

and number of epithelial strata than B. taurus. Turner (1980) indicated that B. indicus are

better adapted than B. taurus to perform on high temperature environments because they

have more sweat glands, lesser thermogenesis, greater skin surface to body size ratio, and

usually posses smaller frame.

Hernandez et al. (2002) conducted a three-year study from 1992-94 to evaluate the

physiological responses and grazing behavior of the tropically adapted Senepol

(B. taurus), the Holstein (B. taurus) adapted to temperate conditions, and the Brahman

(B. indicus). Each year 15 to 17 yearling heifers per breed were evaluated between July

and November for weight, rectal temperature, and respiration rate of each heifer. Their

study also recorded ambient temperature and relative humidity during this period. To

estimate hematocrit and cortisol levels, blood samples were also collected. Grazing

behavior was observed 1 day before and 1 day after the physiological responses were

recorded, during the third year of the study. Hernandez et al. concluded that these breeds

varied in their adaptation to the environmental conditions in Puerto Rico for all the

variables evaluated, and that these differences appeared to be due to genetic differences

in the adaptation to tropical environments. The authors also concluded that the B. taurus

Senepol cattle has the capacity to tolerate and perform well under the hot-humid climate

conditions of northern Puerto Rico.









Koger (1963) reported that the tolerance of internal and external parasites, tolerance

of high solar energy, ambient temperature and humidity, and the capacity to utilize high

fiber forages are some of the adaptive traits of the Brahman and Brahman based breeds

that allow their production in subtropical or tropical areas.

Burns et al. (1997) researched the ability of four breeds to withstand both tick and

intestinal helminthes burdens and high ambient temperatures while growing in a

subtropical environment in Australia. The breeds included on this study were Simmental,

Hereford, Hereford x Shorthorn crossbred, and Belmont Red. The least heat tolerant and

tick resistant breed was the Simmental. In their study, no differences among breeds in

helminthes resistance were found. At weaning and at 12 months of age the heaviest

among these breeds was the Simmental. However, no differences in weight were found

among breeds at 18 months of age. The authors concluded that the Belmont Red breed

was the best of these breeds for this subtropical environment due to its demonstrated

superior adaptive performance.

Koch et al. (2002) studied steers and heifers of purebred B. taurus, 3% B. taurus x 1/4

B. indicus, 1 B. taurus x 1 B. indicus and purebred B. indicus to determine differences in

cortisol concentrations pre- and post-exogenous ACTH application. The authors did not

find differences in pre-ACTH serum cortisol concentrations, post-ACTH serum cortisol

concentrations or in the proportional response of cortisol between steers and heifers

within breed type. However, in their study the pre-ACTH serum cortisol concentrations

were different among breed types. The purebred B. taurus had lesser pre-ACTH serum

cortisol concentrations values than the other groups. The authors concluded that animals

with different percentages of B. indicus and B. taurus genes respond differently in









production of cortisol after exogenous ACTH. The authors also concluded that an

animal's genetic conformation might affect its susceptibility and/or resistance to stressing

conditions. The authors implied that additional studies might provide a better

understanding of how genotype controls these traits that allow an individual to reduce or

enhance the negative consequences of stress, which could lead to methods of selection for

animals that are stress resistant.

Cattle Performance

Tropical areas of the world provide hard tests daily for those decided to raise cattle

on them, since the animals must overcome stressing conditions on a daily basis. Dual-

purpose cattle systems have been implemented in the tropics, attempting to increase

production. These systems are characterized by the use of crossbred animals, of B. taurus

and B. indicus, where the level of B. taurus genetics in the cross is not well defined

(Estrada et al., 2002).

Jenkins and Ferrell (2004) collected production data from mature cows produced

by mating Angus and Hereford, Brahman and Boran, and Tuli sires by artificial

insemination or by natural service to Angus and Hereford cows. These cows were later

mated to Charolais bulls. The recorded data included the means for milk yield at peak

lactation, total milk yields, birth weight of the calf, weaning weights adjusted to age,

preweaning daily gain, and feed efficiency. In their studies, the B. indicus breed crosses

manifested greater milk yield at peak lactation and total milk yield. B. indicus also

exhibited lesser birth weight, greater daily gain, age-adjusted weaning weight, and greater

feed efficiency than B. taurus. The cows that were sired by tropically adapted breeds had

greater total milk yield, daily gain, adjusted weaning weight, and feed efficiency;

however, these cows also showed lesser milk yield at peak lactation. Angus x Hereford









exhibited greater total milk yield and birth weight than Tuli. The authors concluded that

the efficiency of the crossbred Tuli cows was not different from Angus x Hereford Fi

females, but neither of them was similar to the efficiency of crossbred cows that were

produced using B. indicus breeds.

Reproduction

Bos indicus cattle though suitable for hot environments, reach puberty at older ages

forcing the heifers to calve for the first time long after 2-y of age. Rodrigues et al. (2002)

studied 18 prepubertal heifers in Australia that were 9-mo-old. These heifers belonged to

two different genotypes (B. indicus and B. taurus). The heifers were assigned to three

groups (6 animals/group) on which different treatments were applied. One group of

heifers remained intact (control), the second group was ovariectomized, and the third

group was ovariectomized and implanted with estrogen (E2). From 10 to 20 mo-old, the

circulating concentrations of progesterone (P4), presence of corpus luteum, and pulsatile

pattern of luteinizing hormone (LH) release were determined from each heifer. The

authors concluded that B. taurus achieved puberty at younger ages and at lighter weights

than B. indicus heifers.

Villagomez and Fajardo (1990) reported that the late sexual maturity, long

postpartum anestrus, and marked seasonality in calf production characteristic of the B.

indicus breeds led to relatively low reproductive indices in the tropics. Hawkins et al.

(1999) alleged that the major constraints for economic and biological efficiency in range

cowherds were the prolonged postpartum anestrus and the delayed sexual maturity of

heifers.

In Ghana, several researchers (Karikari et al., 1994; Osei and Effah-Baah, 1989;

Tuah and Danso, 1985) have reported poor reproductive performance from native tropical









breeds of cattle, which is characterized by late age at puberty and first calving, reduced

conception rates and prolonged postpartum anestrus intervals.

Seasonality has been identified as an influencing factor over reproductive

performance in the tropical areas. The season of greatest fertility varies depending on

geographical location. Research from Cuba (Menendez et al., 1978), and southeast

Mexico (Castillo et al., 1983; Romero et al., 1983) reported that the fertility of purebred

B. indicus and its crosses is greater during spring and summer, while the less fertility is

found during the autumn and winter. The dry season has been found to be the time of the

year with greatest fertility rates for Zebu cattle in Guatemala (Sanchez et al., 1969), this

response may be due to the better body condition score of these animals after the rainy

season.

In Thailand, the effects of temperature and humidity on conception rate in small

holder crossbred Holstein dairy herds were evaluated in the tropics. Data from

primiparous and multiparous dairy cows from 513 herds in the northern part of Thailand

showed that an increase of either the minimum temperature or the maximum humidity

might produce a decrease in conception occurrence (Punyapornwithaya, et al., 2004).

Randel (1984) reported that the reproductive function is mediated by season in the

Indian breeds of cattle. The author explained that the reproductive endocrinology of B.

indicus is different from that of B. taurus because the estrus is shorter and of less

intensity. The author also emphasized the fact that B. indicus cattle have a lesser pre-

ovulatory release of luteinizing hormone, and its corpus luteum is smaller containing less

progesterone than B. taurus. The author supported the argument of a marked reproductive

seasonality in B. indicus, based on the fact that their luteal cells are less responsive to









luteinizing hormone in vitro during the winter, and that the recovery of viable embryos

and the survival of these embryos in the recipient cows is greater from July through

October.

Villagomez et al. (2000) studied 16 heifers and 22 Indubrazil cows on pasture and

under controlled feeding and management conditions to determine the effects of season

on reproduction in Veracruz, Mexico. In their study all cows and heifers displayed estrus

and subsequent ovulation during the spring and winter; however, only 60% of the heifers

did it during the winter. The authors concluded that besides nutrition and management,

the reproductive functions are affected by season. The authors believe that this influence

may explain the documented fact that most Zebu cows become pregnant during the

spring (drought season), in the Mexican tropics.

Silva-Mena et al. (2002) evaluated the effect of the treatment with progestagens on

estrus presentation, estrus behavior and pregnancy rate in 64 Brahman heifers after

natural service. In their study, the estrus was synchronized with Norgestomet + estradiol

valerate (Crestar, Intervet International, Boxmeer, The Netherlands), and the sexual

activity between the heifers and the bull was observed and recorded for 30 h after the

treatment. Pregnancies were determined two months later. The authors concluded that an

estrus of variable length and intensity can be synchronized up to an average of 90% in

Brahman heifers that were treated with Norgestomet + estradiol valerate, and with natural

service pregnancy rates comparable to those obtained in B. taurus females treated

similarly.

Centuri6n-Castro et al. (1994) compared the pregnancy rates of Zebu cows

inseminated once during the morning to that of Zebu cows inseminated twelve hours after









observation of the onset of estrus, in Mexico. In this study the estrus was synchronized in

59 cows, and pregnancies were determined 60 days later by transrectal palpation. The

authors concluded that there was no difference in fertility rates between the synchronized

cows inseminated during the morning, independently of the moment when estrus was

first observed, and those that were inseminated twelve hours after onset of estrus.

Nutrition

In the tropics, the main source of feed for beef production frequently consists of

pastures of poor nutritional value, and which quality and quantity varies enormously over

the seasons (Poppi and Mc. Lennan, 1995). It has been stated that B. taurus cattle does

not have the same capacity of B. indicus and B. indicus crossbred cattle to utilize low

quality forage diets in an efficient way (Karue et al., 1972). The deficiency of nutritional

sources required for the growing and finishing of cattle is a disadvantage common to the

tropical and subtropical areas of the world (Pate et al., 1984). In these regions, extreme

weather conditions and reduced feed availability tend to produce periods of nutritional

stress (Hawkins et al. 1999).

Mansouri et al. (1992) studied the abilities of B. indicus and B. taurus to digest

different feedstuffs in Iran. In their study, three types of roughage diets including alfalfa

hay, common reed or wheat straw were fed to three ruminally fistulated Sistani and three

Holstein steers in three different periods. The authors concluded that the Holstein steers

were better adapted to digest alfalfa, whereas the Sistani steers fermented and digested

common reed and wheat straw (low nitrogen feeds) in a more efficient way.

The reproductive efficiency of the beef cow is susceptible to improvement through

an accurate and improved estimation of her energy requirements (Wiltbank et al, 1962).

Estrada et al. (2002) developed a simulation model to estimate the efficiency of use of









metabolic energy in dual-purpose cows, and economic implications. Their model was

based on body weight, productive, and reproductive traits that were recorded from a dual-

purpose herd. Three genetic groups were simulated on their model, B. taurus, 3% B. taurus

11 B. indicus, 1 B. taurus 1 B. indicus. In their study, the metabolic energy requirements

were based on metabolic weight; assuming that the efficiency of energetic conversion

towards corporal weigh and milk production did not differ between genotypes. The

purebred B. taurus group had the greatest metabolic energy requirements. The three

genetic groups produced or contributed with similar amounts of total metabolic energy in

the model. The authors conclude that the 12 B. taurus 12 B. indicus exhibited the most

efficient use of energy and the best economic efficiency. Energy efficiency should be a

criterion to select or evaluate a particular breed or its crosses, particularly the energy for

maintenance requirements, because they account approximately for half of the feed

consumed in beef production (Prichard and Marshall, 1993).

Obese et al. (1999) studied the reproductive performance of 76 Sanga cows in

Ghana from February 1995 to July 1996. In their study the interval from calving to the

resumption of cyclic ovarian activity was 101 7 days, and from calving to conception

was 152 4 days, with a calving interval of 444 + 16 days. The authors observed that the

interval from calving to the resumption of cyclic ovarian activity was shorter during the

dry season, and this interval decreased as body condition score was increasing. The

authors concluded that long post-partum anestrus may lead to a prolonged calving

interval with poor reproductive performance, suggesting also that this performance might

be enhanced by feed supplementation and the early weaning of calves to improve body

condition scores of the cows.









The body condition score of a cow affects its reproductive performance in a very

important way (Kunkle et al., 1994). Rae et al. (1993) studied the interactions between

parity and body condition score over pregnancy rate. Over a two-year period they

collected and analyzed data from eight herds in Florida. In their study, body condition

score, parity, and their interaction had significant effects on pregnancy rate. As both

parameters were increased the pregnancy rate was greater. The authors concluded that

number of calvings and body condition score are important factors affecting the

reproductive performance of beef cows in Florida.

Management Considerations

Different management strategies have been shown to produce positive changes over

productive parameters of cattle. For the purpose of this review, we will focus on the

effects of age at first calving and early weaning as alternatives to enhance cow herd

productivity.

Age at First Calving, and Rebreeding Efficiency

If a heifer calves for its first time at 2 years of age, the total proportion of non-

productive cows and of energy utilized for maintenance within the herd will decrease,

while more of the feed consumed will be used to support pregnancy and lactation

(McMillan et al., 1992). A heifer that starts calving as 2-y-old will have the opportunity

to wean almost one extra calf through her lifetime than her contemporaries first-calving

at 3 years of age (Morris, 1980).

Nufiez-Dominguez et al. (1991) studied the effect of culling policy in Nebraska,

USA and age at first calving over different parameters. These productive parameters

included: number of breeding seasons, pregnancies, number of calves born and number of

calves alive 72 hours postcalving, calves alive at weaning, cumulative survival, calf









weaning weights, and the input-output efficiency. These data was recorded for each cow

up to twelve years of age, and it was obtained from records of Angus, Hereford,

Shorthorn, and first-cross cows that were born between 1960 and 1963. Heifers calving

for the first time as 2-y-old had greater economic efficiency than those calving first as 3-

y-olds, without any effect from culling policy. Also, the economic efficiency reaches its

peak when the terminal age of cows was 6 to 9 years and 8 to 9 years for 2-y- and 3-y-old

cows, respectively. In their study, 3-y-olds had greater repeatability of pregnancy than

did 2-y-old cows, but was not enough to make up for the lost calf. Economic efficiency of

the herd might be enhanced when heifers calve for their first time as 2-y-olds.

Morrison et al. (1992) studied Angus and Angus x Hereford heifer calves that were

born either in the spring or fall seasons in Louisiana. These heifers were allotted by

weight at weaning to be exposed to bulls for calving either as 2-y- or 2 12 -y-old. In their

study, the heifers born during the fall were heavier than the spring-born at weaning.

However, at breeding the heifers born during the spring were taller, heavier, and with

greater BCS, regardless of age. The authors concluded that heifers calving as 2 1/2 -y-old

were taller, heavier, their pelvic area was larger, and showed a greater pregnancy rate

than primiparous two-year-olds.

Werth et al. (1996a) researched the calving intervals and repeatability of calving

intervals of beef cows at three different ages (2-, 3-, and 4-y-old) in Nebraska. These

parameters were evaluated under breeding seasons immediately initiated after calving. In

their study the duration of calving interval from the first to the second and the second to

the third parities and the average calving date was recorded from 178 crossbred beef

cows. The authors concluded that age and parity interact influencing calving interval, and









this average interval may be less than 365 days when the initiation of the breeding season

is not restricted after calving. In another study, Werth et al. (1996b) investigated the

relation of the patterns of change in blood progesterone concentration of first-calf heifers

to conception rates at the first postpartum estrus. The authors utilized crossbred

primiparous 2-y-old cows from which data was collected over 2 years. In their studies,

64% of the cows achieved pregnancy after artificial insemination at the first estrus

detected after parturition. Another 32% of the animals became pregnant at the second

estrus after parturition, while 4% failed to conceive. The authors observed that

conception rates were greater if a transient increase in progesterone preceded the estrus,

and in 31.1% of the cows this increase was not detected before the first estrus after

calving. The authors concluded that progesterone concentration increases before the first

postpartum estrus, and this event is related to enhanced conception rates than those cows

without the increase in progesterone preceding their first estrus after parturition.

Early Weaning

The reproductive cycle of the mammal female is influenced by the suckling

stimulus in a very important way (Edgerton, 1980). Different studies have reported that a

temporary or permanent restriction of suckling at early stages in the life of the calf will

improve the reproductive efficiency of the dam (Laster et al., 1973; Reeves and Gaskins,

1981). Bellows and colleagues, described that a decrease of the postpartum interval can

be achieved by the establishment of early-weaning practices (Bellows et al., 1974).

Meirelles et al. (1994) conducted several trials to investigate the effects of

temporary calf removal and supplementation with phosphorus on the conception rate of

Nellore cows in Brazil. The authors reported that the evaluation of the results from the

three trials concerning the effects of restricted suckling prior to breeding season produced









inconsistent results on pregnancy rates. The authors concluded that restricted suckling of

cows with marginal range plasma phosphorus may improve ovarian function.

Hernandez et al. (1999) evaluated the effect of three periods of restricted suckling

on milk yield and other productive variables of B. indicus x B. taurus cows in Venezuela.

The performance of their calves was also evaluated. Three weaning age treatments were

utilized; 8-, 16-, and 24-wk-old (n=12 cows/treatment). In their study, no differences

were found among treatments on total milk yield; however, it was noticed that daily milk

yield decreased approximately 23% the week after weaning, but the lactation curve trend

was recovered 2 to 4 wk later. Their results showed that a greater amount of milk was

sold by decreasing the suckling period from 24- to 8-wk-old. The authors concluded that

the advantages of restricted suckling are maintained but reducing the weaning age to 8-

wk increases the quantity of saleable milk.

As Arthington stated; "the use of early weaning will allow young females to regain

their lost body condition, and do so with less forage and supplemental feed. These

females will also have a shorter post-partum interval. That is they will become pregnant

earlier in the breeding season and therefore produce calves that will be older and heavier

at next year's weaning" (Arthington, 2002).

Story et al. (2000) from the Department of Animal Science, University of

Nebraska, Lincoln, USA studied spring-calving cows over a 5-y period to evaluate the

effects of calf age at weaning on cow-calf performance and the production economics.

The weaning treatments for this study were either early- (5-mo-old), traditional- (7-mo-

old), and late-weaning (9-mo-old). In their study, the cow BW and body condition score

were different at the last weaning date among all treatments. However, no differences in









pregnancy rates were found among groups. The authors concluded that the age at

weaning affects cow body condition score and weight.

Bishop et al. (1994) evaluated the influence of the body energy reserves over the

onset of luteal activity and in the concentrations of LH and IGF-I in serum in postpartum

anestrous multiparous beef cows after early weaning in Oklahoma. In their study, 100%

of the cows with body condition score > 5 at the time of weaning initiated luteal activity

within 25 d after weaning, while just 43% of the cows with BCS < 5 did the same. The

authors concluded that body condition score at weaning influences the frequency of LH

pulses, serum IGF-I, and the interval to the onset of ovarian activity after early weaning

in anestrous beef cow.

Houghton et al. (1990) studied the reproductive performance of mature Charolais x

Angus cows adjusted to a moderate body condition at D 190 of gestation. These cows

were then randomly blocked to either maintenance or a low-energy diet. Thirty days after

parturition, the cows were again randomly blocked to either early- or normal-weaning. In

their study, early weaning reduced the postpartum anestrus interval. However, the first

service conception rate was also reduced for the early-weaned group. It is important to

mention that the normal-weaned cows in that particular study had an uncommonly

elevated first service conception rate (92%). Houghton et al. concluded that a moderate

body condition should be achieved before the breeding season, to enhance reproductive

performance.

Arthington and Kalmbacher (2003) studied the effectiveness of early weaning fall-

born calves on heifer and calf performance. In their study, primiparous, three-year-olds

Braford and Brahman x Angus heifers were randomly assigned to either early- or normal-









weaning treatments. This study was conducted on two consecutive years. Although the

early-weaned calves had a greater average daily gain on the first year, during the second

year it was lesser than the normal-weaned group. The early-weaned dams from the study

were heavier and with greater body condition score at the time of normal weaning. These

same heifers had greater pregnancy rate on each year of the study, and their calving

interval was lesser than normal-weaned heifers on Year 2. The authors concluded that

early-weaning may improve pregnancy rate and body condition of primiparous heifers.

Barker-Neef et al. (2001) studied the effects of early weaning on feedlot

performance, carcass characteristics, and economic return to the cow-calf enterprise on

Angus steers. These steers were assigned by birth date to either early- (100 days) or

normal-weaning (200 days). In their study, early-weaned steers had lesser total feed

consumption and dry matter intake for the finishing period than normal-weaned steers.

Although early-weaned steers had lighter carcass weights, the gain:feed and the cost for

gain was improved. The authors concluded that early-weaning may be an economically

feasible practice.

Myers et al. (1999) conducted a study for two years to determine the effects of

three weaning and nutrition treatments on cow-calf performance. The cow-calf pairs were

randomly assigned to either early-weaning + a finishing diet, normal weaning + grain

supplementation for 55 days on pasture, or normal weaning + 55 days on pasture and then

placed on a finishing diet. In their study, early weaning increased the proportion of

Average Choice grade steers by 40%. The early-weaned dams had greater average daily

gain and improvement in body condition score. The authors concluded that early weaning

improved feed efficiency and quality grades of beef steers.






19


Conclusions

Improved nutrition and reproductive management may help to overcome some of

the current challenges facing the beef industry. The development of feedstuffs and

feeding programs suitable to provide the animals with the required nutrients at the critical

times of production is of special relevance in these regions. Breeding heifers at younger

ages, and the use of early-weaning and other management practices focused to alleviate

the postpartum nutritional pressure may increase the overall productivity of the herd.














CHAPTER 3
CASE STUDY: CALVING PRIMIPAROUS BRAFORD HEIFERS AT 2 VERSUS 3
YEARS OF AGE

Introduction

Beef heifers in Florida traditionally are reared to calve first at 3-y-old. However, in

other parts of the country, many producers breed heifers as yearlings (13- to 15-mo-old)

to calve at 2-y of age to improve productivity of young heifers. Several researchers

consider this practice a good strategy to begin the recovery of some costs of

development, and to increase lifetime productivity (McMillan et al., 1992) by reducing

the proportion of non-productive cows in a given herd. Morris (1980) reported that those

heifers calving as 2-y-olds have the opportunity to produce almost one extra calf through

their productive lifetime.

Beef production's efficiency has been linked to breed type (Kress et al., 1990;

Marshall et al., 1976), size of the animal (Holloway and Butts, 1983; Kress et al., 1990),

and milk yield (Clutter and Nielson, 1987; Neville, 1962; Rutledge et al., 1971).

However, milk production and BW tend to be considered the most important components

for efficiency evaluation of beef cows (Arango and Van Vleck, 2002; Dickerson, 1970;

McMorris and Wilton, 1986; Montafio-Bermudez et al., 1990). Some producers have

experienced difficulty in re-breeding 2-y-old heifers after first calving. The objective of

this study was to determine differences in energy utilization and performance of Braford

heifers calving for the first time at 2- vs. 3-y of age, at a similar level of nutrition. Mature

multiparous Braford cows were included for comparison purposes as a control.









Materials and Methods

Animal Care and Diet

The animals utilized in these experiments were cared for by acceptable practices as

outlined in the Guide for the Care and Use of Agricultural Animals in Agricultural

Research and Teaching (FASS, 1999). The study was conducted over 85 days (d) at the

University of Florida, Range Cattle Research and Education Center (REC) Ona, from

January 10 to April 4, 2003. One week prior to the start of the trial, four Braford cows

and their calves (n = 12; four pairs/age group) were randomly selected from the REC

herd meeting one of three age groups, consisting of 2-, 3-y-old and mature cows. Mature

cows were multiparous, while 2- and 3-y-old cows were primiparous. Cows averaged 85

d (range, 76 to 107 d) in milk at the beginning of the study. Each pair was randomly

allocated to one of 12 individual pens in a completely randomized design.

During the trial, all cows were fed a daily ration of commercial concentrate

(Lakeland Animal Nutrition, Lakeland, FL) and bahiagrass (Paspalum notatum) hay

(45:55 respectively) (Table 3-1). Diets were formulated to provide for 0.22 kilograms per

day (kg/d) of gain for 2- and 3-y-old cows and no gain for mature cows, assuming a daily

milk production of 5 kg (NRC, 1984). Forage and concentrate were 54 and 79% TDN,

and 10.7 and 12.4% CP (DM basis), respectively. Nutrient analyses of hay and

concentrate were conducted by a commercial laboratory (Dairy One, Inc., Ithaca, NY)

(Table 3-2). All cows continued nursing their calves during the study. Additionally, to

simulate the small amount of forage that would normally be part of their intake, each calf

was fed 1 kg of oats (Avena sativa) three times a week.









Sample Collection and Analyses

Milk was collected from all cows on d 0, 42 and 84, using an Alfa-Laval single-unit

vacuum milking machine. Approximately ten minutes before being milked, each cow

received a single i.m. injection of acepromazine maleate (1 mL/100 kg; acepromazine

maleate 10 mg/mL; Fort Dodge Animal Health; Fort Dodge, IA). Immediately before

milking, each cow received 20 IU of oxytocin i.v. (Pro Labs Ltd., St. Joseph, MO). Two

milkings were performed to each cow on the sampling days. The first one started at about

07:00, to empty the mammary glands. The milk collected at that time was discarded. The

second milking was performed about 6 h after the first and the collected milk was

weighed. Duplicate blind samples of milk were sent to a commercial laboratory

(Southeast Milk, Inc., Belleview, FL) for determination of fat and protein concentration,

and somatic cell count (SCC). The exact hour of both milkings was recorded for each

cow, daily production of milk per cow was estimated from the amount produced in the

time elapsed between the first and second milking. This uncorrected milk production was

converted to 4% fat corrected milk (FCM).

Body weight (BW) and body condition score (BCS; 1 to 9 scale; Kunkle et al.,

1999) data were collected from all cows on d 0, 42, and 84 and ADG was calculated for

each cow. Backfat thickness was measured ultrasonically between the 12th and 13th ribs,

3/4 the length ventrally over the longissimus muscle. The measurements were taken

using an Aloka 500-V ultrasound with a 17.2, 3.5-MHz linear probe (Aloka; Wallingford,

CT).

Statistical analyses of SCC, FCM, BW, BCS, backfat thickness and ADG were

achieved by ANOVA for a factorial experiment with a completely randomized design

using the MIXED procedure of the Statistical Analysis System (Littel et al, 1996) to test









the significance of interactions, where cow x treatment was our random statement. Pen

(cow) was the experimental unit, and age group the treatment. The model statement

contained the effects of age group, day of the trial, and age group x day interaction. The

Least-Squares Means procedure was used to separate the differences among treatments at

a fixed day of the trial.

To estimate the NE utilization and allocation priority into NEm, NEi, and NEg by

each cow, three scenarios were simulated using Microsoft Excel 2002 (Microsoft

Corporation 1985-2001). The difference among scenarios consisted in the order on which

the three requirements were met: A) NEm NE1 NEg; B) NEm NEg NEi; C) NEg -

NEI NEm.

For each scenario, the first two requirements were calculated (NRC, 1984) based on

changes in BW or milk production assuming all functions were constant across time.

Residual energy intake was then assumed to be used for the remaining output function

(e.g., BW gain) and the level of output was calculated from the available residual energy.

The output for the remaining function was then regressed on actual output to determine

how well the assumed energy partition agreed with measured output. Body composition

was estimated from BCS (Ferrell and Jenkins, 1984) to calculate changes in stored energy

and to derive the amount of energy from fat and protein mobilized in animals that were

under a negative energy balance. Lipids provided 9.37 Mcal/kg, and protein 5.49

Mcal/kg. The simulation worked under the assumption that the energy mobilized from

body tissue was used firstly for milk production. The formulas used to determine NEm,

NEi, and body composition were taken from NRC, 1996; while those used for NEg came

from NRC, 1984. According to NRC (1996), the amount of energy that is derived from









BW loss is equivalent to the NEg that was required to achieve that weight initially. We

calculated this energy from the live weight gain (LWG) formula presented in NRC 1984.

Results and Discussion

Body Weight, Body Condition Score, and Ultrasound Data

While 2-y-old cows were lighter (P < 0.05) than mature cows at d 0, there were no

differences in BW at Days 42 and 84 among the groups. Overall, the BW of mature cows

decreased (P < 0.01), while the BW of the other groups increased (0.17, 0.04, and -0.42,

kg/d for 2-, 3-y-old, and mature cows, respectively; SEM = 0.08) (Table 3-3). The 2-y-

old cows also had a greater (P < 0.05) BCS when compared with the other groups on D

42, and when compared to the 3-y-old cows (P < 0.05) on D 84. Treatment differences

were not detected when comparing backfat thickness; however, backfat thickness was

positively correlated to BCS (P < 0.05; R2 = 0.90), and negatively correlated to FCM

production (P < 0.05; R2 = -0.88) among all cows (n = 12).

Milk Yield and Constituents

Vaccaro and Dillard (1966) proposed that cows experiencing lesser weight gain

were probably producing a greater amount of milk. The present trial showed that the

production of FCM tended (P < 0.09) to be smaller at D 0 for 2-y-old than mature cows,

and by D 42, FCM production of 2-y-old cows was lesser (P < 0.05) than both 3-y-old

and mature cows. Averaged over all three collection times, FCM production tended to be

25% lesser for 2-y-old than 3-y-old (P = 0.08) and mature (P = 0.13) cows (4.49, 6.17,

and 5.90 kg/d for 2-, 3-y-old, and mature cows, respectively; SEM = 0.62) (Table 3-3).

Johnson et al. (2003) reported that multiparous Brangus cows produced 66 and 84% more

(P < 0.001) milk than primiparous cows during early and late lactation respectively. It

has been commonly accepted as a generalization that mature dairy cows produce about









25% more milk than 2-yr-old heifers, 1/5 due to increase in BW, and 4/5 due to increased

udder development; milk yield in dairy cows increases at a decreasing rate until about 8

years of age, and then it decreases at an increasing rate (personal communication, H. H.

Head, 2002, Animal Sciences Department, University of Florida, Gainesville, Fl).

However, this has not been confirmed in beef cattle. Van Oijten et al. (1993) reported that

age of the cow had significant effects on milk production, on 2-, 3- and 4-y-old dams.

However, Diaz et al. (1992) reported that age of the dam had no effect on milk

production.

Milk protein concentration did not differ (P > 0.10) among the groups. No

differences were found in SCC among groups. Sheldrake et al. (1983) reported that age

and parity had little effect on SCC.

Net Energy

Regression of NEg available after NEm and NE1 (Scenario A) were calculated had

excellent agreement (R2 = 0.99) with energy in recorded gain (Table 3-4). For the

remaining scenarios (B and C), regression of energy available vs. energy actually used

were not good (R2 = 0.04 to 0.84) Therefore, our discussion will be limited to the output

generated by scenario A. All age groups allocated approximately the same proportion of

energy for maintenance (65%). This estimation differs slightly from Ferrell and Jenkins

(1985), who reported that approximately 70-75% of the total energy requirement for beef

production was used for maintenance. Energy efficiency should be a criterion to select or

evaluate a particular breed or line, particularly the energy for maintenance requirements,

because they account for nearly half of the feed consumed in beef production (Prichard

and Marshall, 1993). Net energy for maintenance is highly related with metabolic BW,

but is modified proportional to milk production (Prichard and Marshall, 1993). Montafio-









Bermudez (1990) reported that almost 23% of the variation in maintenance requirements

was due to differences in milk production. Since there were no differences in BW from D

42 within the groups, the variation in energy consumed was likely related to variation in

milk production. While 3-y-old and mature cows partitioned almost 35% of the available

energy (including energy derived from tissue mobilization) to support milk production, 2-

y-old cows used only 25% to satisfy this requirement. The 2-y-old cows directed almost

10% of the total energy toward BW gain, producing an overall increase of 51 Mcal on

their energy reserves. However, 3-y-old and mature cows lost 57 and 127 Mcal from their

reserves, respectively. The efficiency of energy derived from body tissue loss is

approximate 80% for both maintenance and milk production (Flatt et al., 1965; Moe et

al., 1970; Russel and Wright, 1983). The 3-y-old and mature cows could apparently

afford to mobilize energy from tissue loss to support lactation. For 2-y-old cows, it seems

that lactation and gain have a similar importance. When provided similar levels of

nutrition these data imply that first-calf Braford heifers calving at 2-years of age tend to

produce approximately 25% less milk, and direct more energy for gain than first-calf

heifers calving at 3-y of age.

Implications

Breeding heifers as yearlings may increase overall herd productivity. Therefore,

further studies are needed to determine the economic impacts of calving primiparous

heifers at 2- vs. 3-y of age. An accurate estimation of NE requirements in relation with

age and productive stage may allow for a better nutritional management of the cow herd.

The identification and selection of cows with reduced maintenance costs when compared

to their cohort may result in important savings in energy requirements. The

implementation of management practices focused in the alleviation of the nutritional






27


pressure of lactation, especially on primiparous heifers, may leave more energy available

for gain and reproduction.









Table 3-1. Daily ration provided to individual pairs (DMI, kg/d)
Age group Ground Hay Concentrate
2-y-old 5.17 4.23


3-y-old 5.17 4.23
Mature 4.23 3.47
Diets were formulated according to NRC (1984) to provide for 0.22 kilograms per day (kg/d) of gain for 2-
and 3-y-old cows and no gain for mature cows, assuming a daily milk production of 5 kg.











Table 3-2. Nutrient composition of stargrass hay and commercial concentrate
Concentrate Hay
As Fed DM As Fed DM
CP (%) 11.0 12.4 10.2 10.7
TDN(%) 70.0 79.0 51.0 54.0
NE1 (Mcal/Kg) 1.65 1.87 0.64 0.66
NEm (Mcal/Kg) 1.74 1.94 0.93 0.97
NE, (Mcal/Kg) 1.17 1.30 0.40 0.42
Commercial concentrate (Lakeland Animal Nutrition, Lakeland, FL). Nutrient analysis was conducted by
Dairy One, Inc. (Ithaca, NY).










Table 3-3 Effect of age at calving BW, body condition score and milk production in
Braford cows during mid-lactation.
Variable 2-yr old 3-yr old Mature Pooled SEM
Cow BW, kga
Day 0 378 d 410d,e 428e 23.5
Day 42 391d 410d 404d 23.5
Day 84 392d 413d 393d 23.5
ADG 0.17d 0.04d -0.42e 0.08
Cow BCSb
Day 0 5.06d 4.38d 5.06d 0.37
Day 42 5.38d 4.50e 4.56e 0.37
Day 84 5.25d 3.94e 4.50e 0.37
Change 0.2d -0.4e -0.6e 0.22
4% FCM production, kg/dc
Day 0 4.7d 6.0d 6.7d 1.0
Day 42 4.2d 6.7e 6.3e 0.9
Day 84 4.6d 5.8d 4.8d 0.5
Overall 4.5d 6.2d 5.9d 0.6
Diets were formulated to provide for 0.22 kilograms per day (kg/d) of gain for 2- and 3-y-old cows and no
gain for mature cows, assuming a daily milk production of 5 kg (NRC, 1984).
a Body weight least square means by treatment.
b Cow body condition score recorded as an averages score from two technicians at each collection date
using a 1 to 9 scale (1 = emaciated and 9 = obese; Kunkle et al., 1999).
Milk was collected from all cows using an Alfa-Laval single-unit vacuum milking machine.
d,e Treatment means within a row that do not have a common superscripts differ (P < 0.05).









Table 3-4. Output from energy use simulation.
Scenario A 2-yr 3-yr Mature
Actual a NEg d42 (Mcal) 45.70 -4.33 -103.52
Simulated b NEg d42 (Mcal) 47.21 -5.37 -120.76
Actual NEg d84 (Mcal) 89.61 -1.27 -164.65
Simulated NEg d84 (Mcal) 90.17 0.44 -187.65
R2 (actual vs. simulated; P<0.05) 0.99 0.99 0.99

Scenario B 2-yr 3-yr Mature
Actual c energy secreted in milk d42 (Mcal) 133.51 132.58 200.18
Simulated d energy secreted in milk d42 (Mcal) 117.74 106.65 204.88
Actual energy secreted in milk d84 (Mcal) 261.90 270.95 367.80
Simulated energy secreted in milk d84 (Mcal) 302.86 236.19 342.92
R2 (actual vs. simulated; P<0.05) 0.39 0.77 0.84

Scenario C 2-yr 3-yr Mature
Calculated e NEm requirement at d42 (Mcal) 7.85 7.80 7.74
Simulated fNEm available at d42 (Mcal) 7.38 6.88 8.13
Calculated NEm requirement at d84 (Mcal) 7.83 7.63 7.54
Simulated NEm available at d84 (Mcal) 9.05 7.71 8.09
R2 (calculated vs. simulated; P<0.05) 0.29 0.34 0.04
Three scenarios were simulated using Microsoft Excel 2002 (Microsoft Corporation 1985-2001). The
difference among scenarios consisted in the order on which the three requirements were met: A) NEm NE1
- NEg; B) NEm NEg NEi; C) NEg NEI NEm.
a Calculated from the difference between the BW at dO and the BW at a given day (d 42 and 84), the
differential was expressed in Mcal based on the equation LWG= 12.21* NEg0 8936 \ *- -* or NEg 8936
LWG / (12.21 N\ '), for a positive or negative energy balance, respectively (NRC, 1984).
b Simulated NEg available from DMI or body weight loss, after fulfilling the NEm and NE1 requirements.
Obtained from the recorded milk production. The formula used to calculate the energy content of milk (E,
Mcal/kg) was E = (0.092 fat percent) + (0.049 solubles non fat) 0.0569 (NRC, 1996).
d Simulated NE1 available from DMI or body weight loss, after fulfilling the NEm and NEg requirements.
e Obtained from the formula NEm = [al SBW0.75 (Breed Effect) (Lactation) (Compensation for previous
plane of nutrition)] + a2 (NRC, 1996). Where al is thermal neutral maintenance requirement and a2 is
maintenance adjustment for previous ambient temperature.
f Amount of energy available for maintenance, after satisfying NEg and NEi requirements.














CHAPTER 4
EFFECTS OF COW AGE ON RESPONSES TO EARLY CALF WEANING

Introduction

The largest limitation for beef cow maintenance is availability of energy (Jenkins

and Farrell, 1983). The production efficiency of beef cattle is restrained by reproductive

failures (Dickerson, 1970; Dziuk and Bellows, 1983; Koch and Algeo, 1983). Research

has shown that body condition score (Richards et al., 1986; Rae et al., 1993), seasonality

(Villagomez and Fajardo, 1990), nursing (Moss et al., 1985; Arthington, 2002), and

nutrition (Wiltbank et al., 1962; Obese et al., 1999; Villagomez et al., 2003) are all key

elements affecting the length of postpartum anestrus. The duration of postpartum anestrus

will often dictate the chance of a cow to become pregnant during a limited breeding

season (Symington, 1969; Wiltbank, 1970). The search for management alternatives that

are focused on decreasing cow dry matter intake without risking productivity is basic for

the sustainability of beef cow-calf production systems. Early calf weaning has been

traditionally recommended during times of the year when food availability for lactating

cows is limited. However, early weaning may also improve the reproductive efficiency of

beef cows. Young cows may benefit most by early calf weaning due to their limitations in

maintaining optimal body condition through lactation (Arthington and Minton, 2004).

The objective of this study was to investigate the effect of early calf weaning from both

primiparous and multiparous cows on forage dry matter intake, the period of postpartum

anestrus, and fertility as measured by pregnancy rate to both timed-AI and natural

service.









Materials and Methods


Animal Care and Diet

The animals utilized in these experiments were cared for by acceptable practices as

outlined in the Guide for the Care and Use of Agricultural Animals in Agricultural

Research and Teaching (FASS, 1999). The study was conducted for 102 days at the

University of Florida, Range Cattle Research and Education Center, located in southwest

Florida. Using a completely randomized design with a 2 x 2 factorial arrangement of

treatments, 48 Brahman x British crossbred cows and their calves were stratified by age

and calving date and randomly allotted to treatments (n = 12 cows / treatment). The four

treatments consisted of cow parity (multi- and primiparous) and weaning schedule (early-

or normal-weaned). Calf age at early weaning was approximately 90-d (range, 68- to

107-d). The weaning treatment was applied at D -21 of the study.

An estrus synchronization and timed-AI protocol was applied to all cows starting

eleven days after weaning treatment (D -10), and finishing at D 0 (Figure 4-1). At D -10,

all cows received a single i.m. injection of GnRH (100 mcg of Gonadorelin; OvaCyst;

Phoenix Scientific, Inc., St. Joseph, MO), and were implanted with an intravaginal

progesterone-releasing insert containing 1.38 g of progesterone (CIDR, Interag,

Hamilton, NZ). Seven days later (D -3), the intravaginal insert was removed, and each

cow was given a single i.m. injection of PGF2a (25 mg of PGF2a; Lutalyse; Pharmacia

and Upjohn Company, Kalamazoo, MI). After 60 hours (D 0), all cows were artificially

inseminated with semen obtained from a single collection from an Angus bull, and

received a single i.m. injection of GnRH (100 mcg of Gonadorelin). Following timed-AI,

all cows were placed into study pastures for 60 days. Cows were maintained on

bahiagrass pastures (Paspalum notatum) of equal size (3 pastures / treatment; 4 cows /









pasture). Cows were provided with free-choice access to stored hay and 2.27 kg/hd daily

of fortified supplemental molasses (16% CP).

At D 60 cow pregnancy status to AI was confirmed by transrectal ultrasonography

(5.0-mHz intrarectal transducer; Aloka 500V; Corometrics, Wallingford, CT), and all

the cows were moved to a single group and exposed to two mature Angus bulls for 21

days. Three weeks later pregnancy to natural service was confirmed by transrectal

ultrasonography.

Sample Collection and Analyses

The percentage of cows cycling at the time of early weaning was determined by the

measurement of blood progesterone concentration in samples collected eleven days apart.

Progesterone concentrations were determined by radioimmunoassay (Seals et al., 1998)

using DPC kits (Diagnostic Products Corp., Los Angeles, CA) in a single assay with an

intra-assay CV of 9%. Sensitivity of the assay was 0.01 ng/tube and 0.1 mL of plasma

volume was assayed. Cyclicity was determined when concentrations of progesterone >1.5

ng/ml were found. Cow BW and body condition score (BCS; 1 to 9 scale; Kunkle et al.,

1999) data were collected at the time of early weaning (D = -21), upon placement into

study pastures (D = 0), D 30, and D 60. For determination of hay consumption, each hay

bale was weighed prior to placement in the pasture. At the end of the study, the entire

hay residue from each pasture was collected and divided into dry and wet fractions.

These fractions were weighed individually, mixed, and samples were collected for DM

determination.

Statistical analyses of cow BW, BCS, and hay consumption were achieved by

ANOVA for a factorial experiment within a completely randomized design using the

GLM procedure of the Statistical Analysis System to test the significance of the









interactions. Pasture was the experimental unit. The Least-Squares Means procedure was

used to separate the differences among treatments at a fixed day of the trial. Analysis of

cow pregnancy rate was achieved by comparing weaning treatment with pregnancy rate

using PROC FREQ of SAS. Differences in pregnancy rate were compared using Chi-

square analysis.

During the course of the experiment, two cows were removed from the study. Both

were early-weaned primiparous cows, one was retired due to sickness and the other

because she allowed another cow's calf to suckle her.

Results and Discussion

Multiparous cows consumed 46% more (P < 0.001) hay than primiparous cows

(8.64 vs. 5.91 kg/d, respectively). Because they did not require energy for milk

production, early-weaned primiparous and multiparous cows consumed 20% less

(P < 0.001) hay than their normal-weaned contemporaries (5.3 vs. 6.6 kg/d for early- and

normal-weaned cows, respectively). Marston and Lusby, (1995) also noted that lactating

cows consume greater amounts of forage than gestating cows. Johnson et al. (2003)

reported that each kg increase in milk yield was associated with a 0.33 and 0.37 kg

increase in forage DMI for early and late lactation respectively. Considering that total

feed costs can account for as much as 70% of the annual costs to keep a cow, this

reduction in DMI has economic relevance. The cow-based energy costs associated with

raising a calf are important to the efficiency of a cow-calf production system. Further,

supplemental energy is widely viewed to be more beneficial for primiparous compared to

multiparous cows. Hansen et al. (1982) reported that when primiparous heifers were

provided a high-level of nutrition the anestrous period was reduced. However, the authors









did not obtain the same response from the same cows during their second and third

lactation.

Multiparous cows were heavier (P < 0.001) than primiparous cows over the four

data collection days (Table 4-1). There were no differences in initial BW within parity.

Early-weaned multiparous cows were heavier than their normal-weaned contemporaries

at Days 30 and 60 (P = 0.06 and 0.01, respectively). Overall, early-weaned cows were

heavier than normal-weaned cows on Days 30 and 60 of the study (P = 0.03 and 0.01,

respectively). From D -21 to 60, the BW of the early-weaned cows reflected a greater

(P < 0.001) increase than that from the normal-weaned cows (28.4 vs. 1.8 kg,

respectively).

Multiparous cows had a greater (P <.0001) BCS than primiparous cows on the

first and second data collection dates (Days -21 and 0). However, at Days 30 and 60,

there were no differences in BCS between early-weaned multiparous and primiparous

cows. Early-weaned cows had a greater (P < 0.001) BCS at Days 30 and 60 than normal-

weaned cows. Overall, the BCS of early-weaned cows increased 0.21 points, while the

BCS of normal-weaned cows decreased 0.93 points, reflecting a difference (P < 0.001)

between treatments. The observed responses on BW and BSC of the early-weaned cows

are likely due to the removal of the nutrient requirements associated with lactation.

The frequency of Al pregnancy did not differ between early- and normal-weaned

cows. However, overall pregnancy rates (AI + Natural) tended (P = 0.25) to be greater

for early-weaned cows (18 of 22 and 15 of 24 pregnant, for early- and normal-weaned

cows, respectively) (Table 4-2). Houghton et al. (1990) reported that early-weaned cows

may have a 24-d shorter postpartum anestrous period compared to contemporaries that









remain nursing their calves. In the same study, the authors observed a reduction

(P < 0.10) of first-service conception rates in the early-weaned cows. The causes of this

reduction were unclear, but it was mentioned that normal-weaned cows on that

experiment presented an unusually high first-service conception rate (71.4 11 vs. 91.2 +

10% first service conception rate, for early- and normal-weaned cows, respectively).

Lucy et al. (2001) reported that the overall response to the CIDR+PGF2u treatment will

depend on the proportion of cattle that are cyclic at the start of the breeding season.

Postpartum ovarian activity is suppressed by the suckling stimulus (Williams, 1990).

Laster et al. (1973) reported that two-yr-old cows weaned before the breeding season,

experienced an overall increase (26%) in pregnancy rate compared to an increase of only

7.9% in four-yr and older cows. In our study, multiparous cows had greater (P = 0.05)

pregnancy rates after Al than primiparous (5 of 22 and 12 of 24 pregnant to Al, for

primiparous and multiparous cows, respectively). This same difference between the

multiparous and primiparous cows was significant (P < 0.001) when comparing overall

pregnancy rates.

Age and parity have a marked effect on postpartum anestrus (Short et al., 1990).

Within BCS ranges, 36% of the cows with BCS > 5.0 became pregnant to Al (8 of 22),

while the same range achieved an overall pregnancy rate of 85% (11 of 13). The cows

with BCS < 5.0 had pregnancy rates of 33 and 63% for Al (8 of 24) and natural service

(21 of 33), respectively (Table 4-3).

Implications

The use of early-weaning as an alternative to reduce energy requirements

associated with lactation, may increase cow body weight, body condition score, and

reduce postpartum anestrus, in both primiparous and multiparous cows. Due to the






38


reduction of forage dry matter intake, early-weaning of first-calf heifers may provide

savings of 450 kg of hay for every three to four cows, over a 90-d breeding season.

Although early-weaning has been traditionally recommended for primiparous heifers, the

results from this study showed that multiparous cows were also benefited by its

implementation. However, more studies are needed to determine the economic

implications of the use of early-weaning on multi- and primiparous cows.










Weaning
treatment a


D-21


GnRH b


PGF 2a d


D-10 D-3


Days of the study

Figure 4-1. Protocol for estrus synchronization and timed artificial insemination (TAI).
a Early- or normal-weaning.
b GnRH (2mL of solution, equivalent to 100 mcg of Gonadorelin; OvaCyst; Phoenix Scientific, Inc. St.
Joseph, MO).
CIDR (controlled internal drug-releasing device, Interag, Hamilton, NZ), intravaginal progesterone-
releasing insert containing 1.38 g of progesterone.
dPGF2a (5mL of solution, equivalent to 25 mg of PGF2a; Lutalyse; Pharmacia and Upjohn Company,
Kalamazoo, MI).
e All cows were artificially inseminated with semen obtained from a single collection from an Angus bull.


TAI e and
GnRH


I


DO










Table 4-1. Average BW (kg), body condition score (BCS; 1 to 9 scale), and hay DMI by
weaning treatment within parity.
Weaning treatment
Item Early Normal SEM
Primiparous BCSa
Day -21 4.3c 4.3c 0.17
Day 0 4.2c 4.4c 0.15
Day 30 4.3c 3.7d 0.18
Day 60 4.3c 3.5d 0.18
Change 0.1O -0.09d 0.1
Multiparous BCS
Day -21 4.8c 5.0S 0.17
Day 0 4.9c 5.0S 0.15
Day 30 5.3c 4.8d 0.18
Day 60 5.3c 4.1d 0.18
Change 0.3c -0.9d 0.1
Primiparous BW, kg
Day -21 346c 334c 16.8
Day 0 329c 319c 15.8
Day 30 374c 343c 17.8
Day 60 358c 324c 16.2
Change 28c 5d 3.9
Multiparous BW, kg
Day -21 454c 437c 16.8
Day 0 438c 414c 15.8
Day 30 490c 448d 17.8
Day 60 467c 413d 16.2
Change 29c -1d 3.9
Hay DMI, kg b
Primiparous 322c 396c 26.5
Multiparous 499c 533c 26.5
a Cow body condition score recorded as an averages score from two technicians at each collection date
using a 1 to 9 scale (1 = emaciated and 9 = obese; Kunkle et al., 1999).
b For determination of hay consumption, each hay bale was weighed prior to placement in the pasture. At
the end of the study, the entire hay residue from each pasture was collected and divided into dry and wet
fractions.
,,d Treatment means within a row that do not have a common superscripts differ (P < 0.05)












Table 4-2. Frequency of pregnancies by parity and weaning treatment
Parity Weaning treatment
Item Primiparous Multiparous Early Normal
AI pregnant, (%) 5 of 22 (23) 12 of 24 (50) 10 of 22 (46) 7 of 24 (29)
Overall pregnancy, (%) 10 of 22 (46) 23 of 24 (96) 18 of 22 (81) 15 of 24 (63)
Pregnancy status was confirmed by transrectal ultrasonography (5.0-mHz intrarectal transducer; Aloka 500V; Corometrics, Wallingford, CT).






42


Table 4-3. Frequency of pregnancies within body condition score
BCS


Reproductive status < 5.0 > 5.0
AI pregnant, % 8 of 24 (33) 8 of 22 (32)
Overall pregnancy, % 21 of 33 (60) 11 of 13 (85)
Pregnancy status was confirmed by transrectal ultrasonography (5.0-mHz intrarectal transducer; Aloka
500V; Corometrics, Wallingford, CT). Cow body condition score recorded as an averages score from two
technicians at each collection date using a 1 to 9 scale (1 = emaciated and 9 = obese; Kunkle et al., 1999).














CHAPTER 5
CONCLUSIONS

Adequate nutritional and reproductive management may help to overcome some of

the current constraints in beef production. This management must include an accurate

estimation of the energy requirements at different ages and productive stages.

More studies are needed to get a clearer picture of how the energy from feed is

partitioned and utilized in the live animal. We still do not know how the energy derived

from body weight loss is distributed. It seems that at different ages, the physiological

priorities are different. The identification and selection of cows with reduced

maintenance costs when compared to their cohort will result in important savings in

energy requirements.

The use of early-weaning as an alternative to reduce energy requirements

associated with lactation, may increase cow body weight, body condition score, and

reduce postpartum anestrus, in both primiparous and multiparous cows. Due to the

reduction of forage dry matter intake, early-weaning of first-calf heifers may provide

savings of 450 kg of hay for every three to four cows, over a 90-d breeding season.

Although early-weaning has been traditionally recommended for primiparous heifers, the

results from this study showed that multiparous cows were also benefited by its

implementation. However, more studies are needed to determine the economic

implications of the use of early-weaning on multi- and primiparous cows, and to find a

way to increase pregnancy rates in younger heifers.















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BIOGRAPHICAL SKETCH

Sebastian Galindo Gonzalez was born in Mexico City, on October 14th, 1971. He

obtained a professional degree in Veterinary Medicine and Animal Sciences from the

Universidad Veracruzana, Mexico in 1995. He worked for some years for the veterinary

pharmaceutical industry in Mexico, being responsible for sales, marketing, and technical

training of the sales force. He switched to academic work in 2000, coordinating

international programs for the Facultad de Medicina Veterinaria y Zootecnia of the

Universidad Veracruzana. He was assistant professor of the Farm Management course

and responsible for the off-campus program on Animal Production. He started his

master's degree program on August 2002. Currently, he is an admitted Ph.D. student at

the Department of Agricultural Education and Communication at the University of

Florida in Gainesville.