THE RELATIONSHIP BETWEEN FEED EFFICIENCY AND FERTILITY IN REPLACEMENT BEEF HEIFERS By LUARA BERTELLI CANAL 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 2018
2018 Luara Bertelli Canal
To my family
4 A CKNOWLEDGMENTS First, I would like to t hank God for giving me strength, hope, and putting amazing people i n my path who has pushed me since I was young to believe in m y self and taught me the value of hard work. My mom, Sandra Bertelli, who made me feel loved and cared for, even from a distance and has always been my example of a mother and strength. I thank my siblings Maialu Canal and Raoni Canal, you are my inspiration to be someone better and to be honest and loving to people around me. Your sisterhood/ brotherhood was what it took for me to get here and enjoy the opportunities I received, for that I will always love and support you. In addition, I thank Mariel Adan and Mar ina Frontana, my sisters by heart, my best friends, all this distance showed us that nothing can get in the way of the three Musketeers. Thank you for being part of my life. I kindly thank Dr. Cliff Lamb who granted me the opportunity of a lifetime, and no t only was a great teacher, but he was always there to give guidance and support. Because of his mentorship, I developed skills and gain ed knowledge that I never believed I could. That has opened doors for a greater future for which I will always be gratef ul. I also thank his family: Margo, Jack and Dante, who made every student feel safe and at home. I thank Nicola Oosthuizen and Pedro Levy. Even th ough they had their own busy programs they always made time for me to call and ask for help. Without your la te nights, patience and knowledge I would not have completed my thesis Thank you for being so kind. I would like to express my deepest gratitude for Dr. Nicolas DiLorenzo who was willing to have an open door and listened and supported me during my diffic ult times You have taught me how to be a great professional and student. Thank you for being a friend, having faith i n my capacities and for helping me deserve the opportunities I was granted. I thank Dr. John Bromfield for being such a supportive part of my committee and for guiding me through my first
5 steps of what it would take to be a researcher and for giving me the opportunity to work with him. In addition, I thank Dr. Jose Dubeux, for helping me in everything I needed I hope I can return all the su pport. I thank Tessa Schulmeister, Carla Sanford and Francine Henry who stayed until the end with me in Marianna. We became true friends, honest enough to point out weaknesses yet always with the intention of helping me develop and grow. They were patie nt, helped me during late hours, reminded me to read my emails, were there for tough times and good times you were the big part of my joy during this experience. In addition, I thank Kalyn Waters for inspiring my future career, for showing me how to be a strong, successful and a driven woman, for giving me a house every time I needed it and for expecting the best of me I look forward to working together in the future I wish to thank the NFREC beef crew: Olivia, Mark, Cole, Chad, Don, Butch, and Pete for all their assistance and laughs during with my experiments A special thank you to David Thomas who was not only a good boss but made me feel like part of the family and letting me know I always had someone I could count on. In addition, I am grateful to Tina and Gina for keeping the grad uate students and all the others from the NFREC family in line and on task I thank the Ladner family who received me with open arms, kindness and love. Thank you for opening your home to me, my ho me and my heart will always be open to you too. Finally, I thank Ty Ladner for believing in me, holding m e together, and teaching me how to be brave. I thank God for putting you in my life, you are a real example of love and support and I will carry this wit h me forever.
6 TABLE OF CONTENTS page ACKNOWLEDGM ENTS ................................ ................................ ................................ ............... 4 LIST OF TABLES ................................ ................................ ................................ ........................... 8 LIST OF FIGURE S ................................ ................................ ................................ ......................... 9 LIST OF ABBREVIATIONS ................................ ................................ ................................ ........ 10 ABSTRACT ................................ ................................ ................................ ................................ ... 11 CHAPTER 1 I NTRODUCTION ................................ ................................ ................................ .................. 13 2 L ITERATURE REVIEW ................................ ................................ ................................ ....... 15 Follicle Dynamics ................................ ................................ ................................ ................... 15 Corpus Luteum Formation ................................ ................................ ............................. 15 Luteolysis ................................ ................................ ................................ ....................... 16 Hypothalamic Pitui tary Ovarian Axis ................................ ................................ .................. 17 Puberty ................................ ................................ ................................ ............................ 18 Factors that Affect Puberty ................................ ................................ ............................. 19 Prepubertal Gains ................................ ................................ ................................ ........... 23 Relationship Between Feed Efficiency and Reproduction ................................ ..................... 24 Feed Efficiency ................................ ................................ ................................ ............... 25 Feed Conversion Ratio ................................ ................................ ................................ ... 26 Residual Feed Intake ................................ ................................ ................................ ...... 27 3 T HE RELATIONSHIP BETWEEN FEED EFFICIENCY AND FERTILITY IN REPLACEMENT BEEF HEIFERS ................................ ................................ ....................... 30 Materials and Methods ................................ ................................ ................................ ........... 31 Animals and Management ................................ ................................ .............................. 31 Feed and Sample Collection ................................ ................................ ........................... 32 Bl ood Collection ................................ ................................ ................................ ............. 32 Assessment of Puberty ................................ ................................ ................................ ... 33 Ultrasonography and Reproductive Management ................................ .......................... 33 Statistical Analysis ................................ ................................ ................................ ......... 34 Results and Discussion ................................ ................................ ................................ ........... 36 Fertility and Performance ................................ ................................ ............................... 36 Feed Intake ................................ ................................ ................................ ..................... 40 4 C ONCLUSION ................................ ................................ ................................ ....................... 51
7 LIST OF REFERENCES ................................ ................................ ................................ ............... 52 BIOGRAPHIC AL SKETCH ................................ ................................ ................................ ......... 60
8 LIST OF TABLES Table P age 3 1 Fertility and performance data based on Angus, Brangus and Braford heifers ................. 47 3 2 Fertility and performance data based on heifer rankings consid ered as Low, Medium and High feed efficiency categories ................................ ................................ ................... 48 3 3 Pearson correlation coefficients (top number) and P value (botto m number) among feed efficiency and fertility traits for replacement beef heifer, N = 179, Prob > |r| under H0: Rho = 0 ................................ ................................ ................................ ............. 49 3 4 Nu tritional values of feeds offed during the two years of project ................................ .... 50
9 L IST OF FIGURES Figure P age 3 1 Yearly schematic for data collection of heifers ................................ ................................ 42 3 2 Residual feed intake of replacement heifers associated with breed ................................ ... 43 3 3 Percentage of pubertal heifers during the 120 days of development and breeding phase for Angus, Brangus and Braford heifers ................................ ................................ .. 44 3 4 Percentage of pubertal heifers during the 120 days of development and breeding phase for Angus, Brangus and Braford heifers. ................................ ................................ 45 3 5 Mean age at puberty onset of replacement heifers selected by High RFI, Medium RFI and Low RFI ................................ ................................ ................................ ............... 46
10 LIST OF ABBREVIATIONS ADG Average daily gain BW Body weight DMI Dry matter intake F TG Feed to Gain RFI Residual feed intake ACTH Adrenocorticotropic hormone AI Artificial Insemination BCS Body condition score on a 1 to 9 scale CL Corpus luteum CP Crude protein D Day DMI Dry matter intake FSH Follicle stimulating hormone GnRH Gonadotropin releasing hormone Hr Hour IGF I Insulin like growth factor I LH Luteinizing hormone NPY Neuropeptide Y POMC Proopiomelanocortin Prostaglandin
11 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for t he Degree of Master of Science THE RELATIONSHIP BETWEEN FEED EFFICIENCY AND FERTILITY IN REPLACEMENT BEEF HEIFERS By Luara Bertelli Canal December 2018 Chair: Nicolas DiLorenzo Major: Animal Science s The objective of our study was to investigate the relationship between feed efficiency and fertility in growing heifers One hundred and seventy nine replacement beef heifers were enrolled. Heifers from 10 to 12 months of age entered the University of Flor ida Feed Efficiency Facility (FEF) and received a diet high in forage to reflect a pasture based diet. While in the FEF, heifers were exposed to a 14 d adaptation period, followed by a 70 d data collection period that included data for residual feed intak e (RFI) and feed to gain ratio. Weekly BW, BCS, and blood samples were collected. Attainment of puberty was determined by concentrations of progesterone. Antral follicle count was assessed in all heifers on the first day of the breeding season using transr ectal ultrasonography. Conception and pregnancy rates were determined after transrectal ultrasonography evaluating the presence of a viable conceptus. Our data revealed no differences in the mean age at puberty, pregnancy rates to AI and overall pregnancy ADG, FTG, RFI among breeds. An effect was detected between breed and DMI, BCS and BW, where Braford heifers had a lower DMI, BW and BCS R esults indicated an effect of RFI group on ADG, FTG and DMI R anking heifers in to RFI groups did not affect pregnanc y rates, and initial or final BW and BCS However, heifers in the Low RFI group attained puberty at a younger age
12 than heifers in the high RFI group. Therefore, more efficient heifers attained puberty earlier compared to less efficient heifers.
13 C HAPTER 1 INTRODUCTION B eef cattle productivity is influenced by reproductive maturity, ability to sustain pregnancy and age at first calving (Lesmeister et al., 1973) S trategies to improve replacement heifer development by de creas ing age at the onset of puberty will be very beneficial to overall productivity of the cattle industry. Pubertal development involves physical and metabolic events such as the activation of the hypothalamic adenohypophyseal gonadal axis (Sisk and Foster, 2004 ), and these events are largely controlled by genetics and nutrient availability (Amstalden et al., 2011) ; therefore, correct nutritional management of heifers, allowing adequate average daily gain (ADG) to support the attainment of puberty, is critical to optimize heifer development (Bischoff, 2011). However, feed costs are responsible for two thirds of the annual cost of production within the beef industry, serving as a stimulus as to why producers are continuously searching for methods to reduce this inp ut cost without compromising overall production quality (Arthur et al., 2001a). The selection of animals with superior feed efficiency traits may result in an improvement in the efficiency of production and reduction of feed cost (Loyd, 2011). There are se veral biological processes responsible for changing feed efficiency in cattle, however many of those processes have not yet been identified and it is important to acknowledge that th is selection may be undesirable (Moore et al ., 2009). Studies indicate tha t selecting cattle with a lower RFI (i.e., more feed efficient) are also selecting for leaner cattle (Fox et al., 2004 ; Basarab et al., 2003 ) ; therefore, these results indicate that the selection for improved RFI (i.e., more feed efficient) may potentially impact carcass quality traits (i.e., m arbling ) in a contradicting manner
14 Leptin, a regulator of feed intake, body weight, and energy expenditure, has been positively associated with body fat deposits in beef cattle (Minton et al., 1998) and leptin concen trations are positively correlated with phenotypic RFI (Richardson et al., 2004) Studies have elucidated t he involvement of hypothalamic neuropeptides o n the mechanism in which nutrition advances puberty onset (Amstalden et al., 2014; Ryan and Foster, 198 0) the relationship between RFI, metabolic variables and feed behavior (Kelly et al., 2009) However, th ere is no reported data demonstrating the relationship between feed efficiency and time at puberty onset on replacement beef heifers T herefore, under standing the impact of selection for feed efficiency of growing cattle is essential to understand the impact s that it can bring on the reproductive performance of cattle
15 CHAPTER 2 LITERATURE REVIEW Follicle D ynamics The bovine estrous cycle is typically characterized by 2 to 3 follicular waves where the dominant follicle of the last wave will ovulate (De Rensis and Peters 1999). In every follicular wave, several antral follicles develop under the stimulus of follicle stimulating hormone (FSH) and luteinizing hormone (LH). Follicle stimulating hormone provides proliferation of the antral follicles and prevents atresia and degeneration of the early antral follicles. When follicles reach a 5 mm diameter it acquires the ca pacity to suppress FSH secretion (Gibbons et al. 1999). During the growth wave, the recruited follicles will suffer a selection process and one follicle will be further developed and will acquire dominance over the others, and those remaining follicles wi ll undergo atresia (Ginther et al. 2000). The dominant follicle may also either suffer atresia or be ovulated. The dominant follicle is the primary follicle responsible for inhibition of FSH and transitions its gonadotropin dependence from FSH to LH, allow ing its growth under low FSH concentrations. Increased concentrations of E2 provided by the dominant follicle will be responsible for changing LH pulsatility and result on a LH surge. Subsequently, in the absence of a functional corpus luteum (CL), the dom inant follicle will go through ovulation. When ovulation occurs, the oocyte along with the cumulus cells that surround the oocytes are released from the follicle into the oviduct and the remaining cells will form the CL ( Aerts and Bols, 2010) Corpus L uteum F ormation Luteinization is the morphological and functional changes that the granulosa and thecal cells undergo after ovulation. Luteinizing hormone is the primary luteotropic hormone and is
16 responsible for inducing luteinization, which is characterized by rapid cellular proliferation, differentiation and extensive angiogenesis (Silva et al., 2000) Luteal development starts with formation of small and large luteal cells. The large luteal theca interna cells (Alila an d Hansel, 1984) After the follicular cells have been transformed into luteal cells they acquire the capacity to synthesize steroid hormones, such as E2 and P4 with large lutea l cells being responsible for a majority of steroidogenesis whereas the small luteal cells have little steroidogenic capacity. The luteal tissue develops fast in its early formation and its growth rate can be compared to the growth of a tumor and at an ear ly stage is insensitive to the luteolytic stimulus of PGF (Silva et al., 2000). Luteolysis Prostaglandin F is a hormone produced by the endometrium of the non pregnant uterus and is responsible for inducing luteolysis in the mid cycle CL. The non pregnan t uterus is stimulated by E2 that will up regulate the expression of oxytocin receptors on the endometrium wall. Oxytocin, mainly produced by the hypothalamus and secreted by the posterior pituitary, binds to its receptors in the uterus endometrium resulti ng in PGF production and secretion. Prostaglandin acts on the CL after it acquires luteolytic capacity causing an increase in blood flow, followed by an immediate decrease, resulting in luteolysis by a deprived supply of nutrients and substrates for steroi dogenesis (Silva et al., 2000). With the regressing and non functional CL, concentrations of P4 in the blood decrease, resulting in a reduction in the negative feedback on the hypothalamus, enabling the E2 produced by the dominant follicle to induce GnRH s ecretion, resulting in an LH surge and ovulation. When a viable embryo is present and recognized by the uterus, luteolysis fails to occur and the CL lifespan is prolonged with maintained production of P4 and will support the
17 maintenance of the pregnancy. T he protein responsible for luteal rescue in ruminants was first the estrous cycle, which was associated with the maternal recognition of pregnancy. The protein was later identified as Interferon tau (IFNT) and is responsible for the maternal recognition of the pregnancy by interacting with the endometrium, down regulating the expression of oxytocin receptors and therefore inhibiting PGF production. The endometrium and conceptus produce PGE2, a luteotropic factor that is proposed to be a secondary mechanism participating on luteal rescue (Ealy and Yang, 2009) The role of IFNT however is not limited to the uterus, once the expression of interferon stimulated genes (ISGs) can be found in cells of peripheral blood leukocytes, which may play a role in suppressing the uterine local immune system to tolerate the presence of the fetus. To support this mechanism of action, previous studies have reported that neutrophils with an increased expression of ISGs had decreased phagocytic activity (Sheikh et al., 2018) Hypothalamic P ituitary O varian A xis Cattle estrous cycle s are controlled by several hormones and re gulated in a dynamic manner through hormone feedback. So different concentrations of one hormone in the blood may induce or inhibit the production and secretion of other hormones. There are two important neurons (Kiss 1 and GnRH) present in the hypothalamu s that are responsible for regulating the estrous cycle. Kiss 1 is responsible for producing and secreting the neuropeptide kisspeptin. Kisspeptin binds and activates the GPR54 receptors expressed on GnRH neurons, resulting in production and secretion of GnRH (Atkins et al., 2008) Neurosecretory neurons in the hypothalamus is responsible for the production of GnRH, which is secreted from the median eminence into a primary capillary plexus and travel by the portal hypophysial vein until it reaches a second capillary plexus present in the anterior p ituitary
18 gland. When GnRH reaches the anterior pituitary, it will stimulate the synthesis and secretion of the two gonadotropins, FSH and LH. Both gonadotropins will be released into the systemic blood stream and reach the ovaries, resulting in follicular growth. Follicular growth occurs in a wave like pattern, where several follicles will be stimulated to grow and release E2, and only one dominant follicle will continue to develop. This dominant follicle will produce greater amounts of E2 and inhibin, whic h will be secreted into the bloodstream and exert a negative feedback with FSH, resulting in atresia of the remaining follicles. In the absence of P4 (i.e., presence of a CL) increased concentrations of E2 will induce the surge of LH and ovulation of the d ominant follicle. Puberty Puberty in beef heifers can be defined as the first fertile ovulation, accompanied by visual signs of estrus, resulting in the development of a functional corpus luteum followed by normal estrous cycles. It is a complex series of events that requires the m aturation of the hypothalamic pituitary ovarian axis. A negative feedback of E2 on LH release has to be reversed to a stimulatory feedback to induce an LH surge and ovulation. Although ovulation can occur by inducing a surge of LH during the prepubertal st age, a return to anestrous occurs and normal estrous cyclicity is not sustained. Although nutrition, age, and genetics are well known regulators of age at puberty, their role in advancement in the age at puberty is mainly as regulators of the endocrine mat uration that must occur for sustained cyclicity activity to be initiated, and onset of puberty is characterized by an increase of GnRH pulse, that will generate and stimulate an increase in the pulsatile release of LH. When Angus heifers were specifically selected to reach puberty at younger ages, they were able to increase pregnancy correlating age at puberty with reproductive performance (Morris et al., 1999). Many studies have reported that heifers that became pregnant during the
19 first 21 days of their first breeding season had greater reproductive longevity compared to heifers that became pregnant in the second 21 day s period or later of their first breeding season (Cushman 2012). Factors that A ffect P uberty Nutritio n Animals that receive low energy d iets were associated with reduced concentrations of pituitary hormones and poor reproductive performance (Perry et al., 1991). The energy available in nutrition comes from the denaturation of protein carbohydrates, and lipids. The energy supplies in the an imals are first used to meet maintenance requirements, and after by partitioning for production (NRC, 2000). The physiological mechanisms responsible for the effect of nutrition on fertility and puberty are not fully elucidated, however, there is strong e vidence indicating that nutrition plays an important role in puberty onset of fertility. The reproductive axis was reported to be more sensitive to nutrient availability th a n the growth axis (Hileman et al., 1991), therefore specific metabolic status by ca ttle is necessary to the onset of puberty. This metabolic status is characterized by metabolites and hormones such as glucose, insulin, and insulin like growth factor I (IGF I; Steiner et al., 1987) Feed intake and diet composition has a high influence on metabolic factors as heifers approach puberty, increases in body weight may lead to increased intake, resulting in differing concentrations in plasma of glucose. Changes in rumen fermentation and volatile fatty acid profiles have been reported to alter LH concentrations. Increases in propionate, the key component of gluconeogenesis in the ruminant, are reported to increase LH pulse frequency and amplitude (Rhodes et al., 1978; McCartor et al., 1979).
20 Furthermore, the role of leptin was reported to be a r egulator of BW, feed intake and reproduction (Garcia et al., 2002), and its concentrations in the plasma are related to the amount of body fat (Milton et al., 1998). Leptin is a hormone secreted by adipocytes and among other functions, appears to be the h ormone largely responsible for conveying information regarding with the brain. Neurons present in the hypothalamus, responsible for GnRH secretion, were found to have leptin receptors and its pulsatile secretion can be stimulated by leptin (Blher and Mantzoros, 2007) It has been reported that leptin has greater stimulatory effects in peripubertal rats compared to prepubertal rats, which indicates that GnRH sensitivity to leptin increases at the onset of puberty (Reynoso et al., 2003). In cattle, administration of leptin resulted in increased GnRH secretion and LH peak and great concentrations of leptin were found in the blood of heifers at the onset of puberty (Zieba et al., 2005) In addition, it has been reported that the effects of lep tin on the hypothalamic pituitary axis is dose dependent (Zieba et al., 2004). Recombinant leptin injected intravenously caused an inverse, dose related increase in basal plasma concentrations of LH in ovariectomized beef cows fasted for 60 hr. This strongly supports the necessity of energy reserves for cyclicity, thu s the amount of body fat may affect the attainment of puberty. Body composition One of the most important factors influencing pregnancy rate in beef cattle is body energy reserves at calving ( Ciccioli et al., 2003), and is a very important factor determin ing when beef heifers and cows resume postpartum estrous cycles after calving. Body condition score (BCS) is an easy and common way to access nutrient reserve in beef cattle. Studies analyzed the fertility of beef females from 8 different herds and found t hat pregnancy success throughout a breeding season wa s reduced for low BCS when compared to greater BCS
21 females (BCS rate; P < 0.05; Rae et al., 19 93) In addition, cows with moderate BCS had greater follicle diameter at AI compared to thin cows and an increase of one unit of BCS yielde d 11.5% and 22.9% improvement of pregnancy rate to a TAI protocol, respectively (Larson et al., 2006; Lamb et al., 2001). Many studies indicate that the interaction between breed and dietary energy can change body composition. Increased rate of gain resul ts in a greater BCS, carcass weight, fat thickness, and total separable fat at puberty (Hopper et al., 1993; Yelich et al., 1995). Undernutrition can be detrimental and delay age at puberty. There have been studies focusing on nutritional strategies and ta rget weight from weaning to the start of the breeding season and the effects on the onset of puberty, indicating that the proportion of heifers attaining puberty at 14 mo of age tended to be lower ( P =0.1) in heifers that had their diet restricted compared to control fed heifers (Roberts et al., 2007). In addition, metabolic changes can decrease the responsiveness of the hypothalamic axis to estrogen negative feedback that will result in an increase in LH secretion, a key component hormone responsible for th e attainment of puberty (Schillo, 1992). Thus, body composition is not the only characteristic that regulates the attainment of puberty but is directly associated with many hormones and metabolites that can control puberty onset (Yelich et al., 1995). Gene tics. Cattle temperament is a behavioral response to human handling. Studies have indicated that excitable temperament has a detrimental effect on production traits, such as hot carcass weight and marbling scores (Francisco et al., 2012) growth rate (Grandin, 1998) and reproduction (Cooke, 2014) When cattle behavior is not well managed it becomes stressful and
22 stressed cattle have increased concentrations of circulating cortisol when compared to calmer animals (Cooke, 201 4). The effects of temperament and stress on fertility have been noted. When temperament in Bos taurus cows was evaluated at the beginning of the breeding season, followed by FTAI and 50 days of natural sire exposure excitable cows had greater blood conce ntrations of cortisol and decreased pregnancy, calving and weaning rates when compared to calm cows (Cooke et al., 2011) The genetic influence on excitable phenotype was observed where heifer calves born from sires with low docility EPDs exhibited excitable temperament at weaning, lowered estrus expression and lowered fertility later at br eeding when compared to heifers sired by high docility EPDs (Kasimanickam et al., 2018) The adoption of semen from sires providing high docility EPDs may be an interesting strategy for producers to overcome the detrimental effect of temperament on fertility. The selection of sires with high traits for docility may be a solution to decrease the proportion of excitable animals in the herd, since temperament has a low to moderate genetic correlation of temperament traits and fertility (Valente et al., 2017) Studies have also demonstrated that when replacement heifers were exposed to a 28 day period of human acclimation within 45 days after weaning, there was an improvement in temperament traits, reduced plasma cortisol concentrations and greater distribution of pubertal heifers following the end of acclimation period when compared to non acclimated heifers (Cooke et al., 2012) Besides behavior, difference s in age and body weight at puberty onset have been e s tablished across breeds (Cundiff et all, 1986), and those differences are related to several genetic effects. Bos indicus influenced breeds tend to be older and heavier, thus presenting an
23 increased fram e size at puberty, when compared to Bos taurus heifers (Warnick et al., 1956; Temple et al., 1961; Baker et al.,1989). However, larger, heavier heifers also reach puberty at older ages (Martin et al., 1992). A significant difference in age at puberty among different breed types has been reported, indicating that Bos indicus influenced heifers were older at puberty onset, compared with Bos taurus heifers (Dow et al. 1982). Pubertal traits were estimated for a wide variety of cattle at one location and conclu ded, based on breed differences, that selection among breeds, and perhaps within breeds, should be successful in changing age at puberty (Nelsen et al. 1982). Age at puberty is a moderately heritable trait and is associated with weaning weight and yearling weight (Brinks 1994). However, age at puberty can also be influenced by nutrition and prepubertal gains (Laster et al., 1972; Hansel. 1959). The strength of the genetic association between growth and reproductive traits measured in young animals will help to determine if selection for a growth character will cause any genetic changes in heifer fertility (Vargas, 1998). Prepubertal G ains Body weight gain can impact attainment of puberty on beef heifers. Studies where heifers were fed for rapid gain for 80 to 90 days either early post weaning or immediately prior to the breeding season have reported similar age at puberty and pregnancy rates to constant gain heifers (Clan ton et al. 1983; Hall et al. 1995; Lynch et al. 1997). The reproductive axis is sensiti ve to the availability of nutrients, and since onset of puberty is influenced by several metabolites and hormones, such as insulin like growth factor I and glucose (Steiner et al., 1987), metabolic status can be critical to puberty onset. Metabolic factors are dependent of diet composition and feed intake. As heifers approach puberty, an increase in feed intake and body weight may result in an increase in glucose. Therefore heifers should be fed a high energy diet for a minimum of 80
24 days prior to reaching the target body weight, independent of whether this occurs at post weaning or immediately prior to start of the breeding season (Lancaster et al., 2009). Increases in propionate, an important component of gluconeogenesis in ruminants, increases LH pulse f requency and amplitude (Moseley et al., 1977; Rhodes et al., 1978; McCartor et al., 1979). However, greater concentrations of glucose were reported to be present in post pubertal heifers, when compared to prepubertal heifers (Verde and Trenkle, 1987). In a ddition, a decrease in serum concentrations of insulin from 40 to 17 days before puberty onset was observed in heifers that had no change in concentrations of glucose, indicating possible increases in peripheral tissue sensitivity to insulin in heifers at onset of puberty, as well as an increase in LH pulsatility (Jones et al., 1991). Hammond et al. (1988) reported that a prepubertal increase of IGF I may increase hypothalamic hypophyseal ovarian axis activity, with high concentrations of IGF I in follicula r fluid. Relationship B etween F eed E fficiency and R eproduction The majority of feed inputs are used to feed females from the breeding herd, to support in feed inputs to improve the efficiency of production system s that there is a correlation between inputs and outputs. Therefore, live weight gain and daily DMI are usually used to measure feed efficiency. Concentrations of IGF I, a hormone associated with growth and cellular proliferation (Kelly at al., 2010) is positively correlated to feed efficiency in a genetic and phenotypic manner (Moore et al., 2005; Brown et al., 2004). Nutritional status has been suggested to be an import ant mediator of reproductive events (Day et al., 1986). The differences in feed intake can affect the age at attainment of puberty in heifers as well as the anestrous length period for cows (Shaffer et al. 2010). When h eifers were grouped in more feed effi cient and less feed efficient, the average
25 age at attainment of puberty was similar between the two groups; however, less feed efficient heifers ten d ed (P = 0.09) to reach puberty earlier compared to more efficient heifers (Basarad et al., 2009). These rep orts indicate that puberty may be delayed in more efficient heifers. Maternal nutrient restriction also has an impact on the reproductive performance of the female offspring. The process by which changes in nutrition during the early stages of pregnancy Studies reported a 19 d increase in age at attainment of puber ty in the heifers born from prepartum energy restricted cows (Corah et al., 1975), as well as a change in the adrenal steroid production in ewes that were restricted at late portions of the gestation (Bloomfield et al., 2003). Feed E fficiency Determinati on of feed efficiency in cattle is difficult and costly; thus, the dataset to estimate the correlation between feed efficiency and genetic are small and it is difficult to achieve precise genetic correlation estimates. When females are mobilizing body tiss ue, they may have greater availability of energy, but that can also be detrimental to their health and reproduction (Roche et al., 2009). To relate feed intake to production system efficiency, several measurements tool were developed over t ime such as: r esidual feed intake (RFI), feed conversion ratio ( F:G ), partial efficiency of growth (PEG) and relative growth rate (RGR). When selecting other feed efficiency measurements, such as PEG, it is required that individual animal feed intake data is collected. For PEG, the calculation divides average daily gain (ADG) by the difference between average daily feed intake and expected dry matter intake (Arthur et al., 2001a). As for RGR, this is an indirect measure of efficiency that requires only
26 measurements of growth. A correlation between PEG and ADG in hybrid steers and bulls was detected, indicating that this selection may increase growth rate and mature size similar to F :G (Nkrumah et al., 2004). In addition, this indirect measurement of feed efficiency has shown to have a positive relationship with ADG, which may result in faster growing cattle (Nkrumah et al., 2004). However, the selection of F :G PEG and RGR, have resu lted in an increase of the maintenance requirement, thus an increase in production costs. For that reason, the concept of residual feed intake (RFI) was proposed as a feed efficiency trait that can identify individual differences in intake existing for gro wing cattle at the same level of production ( Koch et al. 1963) This way, feed efficiency was determined to be a function of gain, feed consumption and average weight. Feed C onversion R atio Traditionally, the most common measure of feed efficiency in beef enterprises has been feed conversion ratio, also referred to as feed:gain (F:G). It is the ratio of feed consumed to the amount of weight gained over a specific period of time such as daily DMI:ADG (Brody, 1945). An animal with a low F:G consumes less feed per unit of gain compared to its counterparts with a high F:G and is classified as more feed efficient. Although commonly used and easy to calculate, F:G is influenced by the growth rate and composition gain, resulting in highly negative correlations bet ween F:G and growth rate (Koots et al., 1994). Increases in mature cow size resulting from F:G selection was reported, suggesting 1990) Therefore, selection for F :G can result in different traits that may affect production (Nkrumah et al., 2004).
27 Residual F eed I ntake Residual feed intake (RFI) is defined as the difference between energy intake and energy demand, which represents the amount of feed consumed net of maintenance of body weight and level of production (Arthur et al., 2014). When animals eat more than predicted based on their performance they have a positive RFI, therefore deemed to be inefficient. Conversely, cattle that e at less than predicted based on their performance have a negative RFI and are considered efficient animals, relative to the average of the population. The predicted daily feed intake value is obtained by regressing daily dry matter intake (DMI) on ADG and mid metabolic body weight (MBW; [body weight at test midpoint] 0.75 ) or midtest BW. Residual feed intake can be calculated as phenotypic RFI (RFIp) or genetic RFI (RFIg). The RFIg trait would require the incorporation of genetic covariances of feed intake with both weight and production traits (Crews, 2005); thus the genetic approach of RFI would exclude any potential changes in growth rate and increased maintenance requirements. Therefore, the selection of animals with a lower RFI does not result in heavi er body weights (BW) because it is independent of ADG (Arthur et al., 2001a; Nkrumah et al., 2004). In addition, it has been reported that divergent selection for Low RFI and High RFI lines of cattle across 3 mating seasons, resulted in High RFI cows with significantly greater rib fat depths. Therefore, a genetic component for fat deposition based on energetic efficiency may be important to consider when selecting cattle based on RFI. (Arthur et al. 2005). Thus, selecting cattle with a lower RFI (i.e., more efficient) also select s for leaner cattle (Fox et al., 2004). Individual variation in feed intake is due to differences in the maintenance requirements of cattle (Herd and Arthur, 2009), and several factors h ave shown to affect feed efficiency, creating numerous sources of variation that need to be considered during the evaluation of cattle for this cha ra cteristic. During different physiological states in beef cattle (maintenance, growth,
28 gestation, or lactati on) there are different energy requirements, due to the energetic constraints associated with that particular physiological state. In both growing and mature cattle, requirements for maintenance and productivity level are positively correlated (NRC, 2000) Deposition of lean tissue and adipose tissue comes at different energetic costs due to their diverse chemical compositions (NRC, 2000). For an animal to gain weight, the net energy required is dictated by the proportion of protein and fat deposited wi thin the empty body tissue (NRC, 2000). The fact that growing and finishing phases are characterized by protein deposition and fat deposition, energetic requirements from these two phases is different. Therefore, efficiency should always be compared amon g animals at the same growth stage. A correlation (r = 0.43) between whole body chemical c o mposition and genetic variation in RFI has been reported, where steers born from high RFI parents had an increase in body fat (Richardson et al., 2001). In addition a different study indicated a trend for low RFI steers to have lower carcass fat and intramuscular fat (respectively, P = 0.08 and P = 0.06) compared to medium RFI steers (Basarab et al., 2003). Thus, body composition has a potential role in quantifying energetic efficiency. These results indicate that the selection for improved RFI may potentially impact carcass quality traits in a contradicting manner, such as marbling and have an impact on age of attainment of puberty. No relationship between overall c oncentrations of plasma IGF 1, a hormone that regulates growth and cellular proliferation, and RFI in growing beef heifers was reported (Kelly et al., 2010a), and selection for concentrations of serum IGF 1 had little effect on RFI (Lancaster et al., 2008) Leptin, a regulator of feed intake, body weight, and energy expenditure, has been positively associated with body fat deposits in beef cattle (Minton et al., 1998). It has been
29 reported that leptin concentrations are positively correlated with phenotyp ic RFI (Richardson et al., 2004), whereas other studies have observed no significant relationship between leptin and RFI (Brown et al., 2004; Kelly et al., 2010a). These results indicate that lept i n may not be an accurate indicator for RFI selection. Nutr itional status has shown to be an important mediator of reproductive events, and differences in feed intake may affect the age of puberty for beef heifers. In addition, reports indicate that heifers with a positive RFI reached puberty at a younger age, whe n compared to heifer with an negative RFI (Shaffer et al., 2010) Furthermore, a one unit increase in RFI resulted in 7.5 days more to the attainment of puberty. T here was a phenotypic correlation between RFI and subcutaneous rib and rump fat, indicating t hat more efficient heifers may have delayed attainment of puberty due to a decresed body fat. By sorting crossbred beef heifers into positive and negative RFI groups, the average age at puberty were similar for both RFI groups of heifer s; however, there wa s a tendency ( P = 0.09) for more heifers from the positive RFI group to reach puberty earlier when compared to the negative RFI group of heifers (Basarad et al., 2009).
30 CHAPTER 3 THE RELATIONSHIP BETWEEN FEED EFFICIENCY AND FERTILITY IN REPLACEMENT BEEF HEIFERS Feed costs are responsible for two thirds of the annual cost of production in the beef industry (Arthur et al., 2001a) P roducers are continuously searching for method s to reduce this input cost without compromising overall production quality. The relationship of fertility and feed efficiency has barely been researched. To beef producers, fertility often is overlooked as one of the most important traits to ensuring the economic viability of their operations. The successful development of replac ement heifers is important to minimize the time that a heifer enter a herd, optimizing their lifetime productivity and future profitability. It is well known that attainment of puberty in heifers prior to the initiation of the breeding season is likely th e single most important factor impacting when heifers will become pregnant during the breeding season, and heifers that calve during the first 21 days of calving season have greater reproductive longevity, when compared to heifers that calve on the followi ng 21 days or later (Cushman, 2012). Residual feed intake (RFI) is a feed efficiency trait that can identify individual differences in intake existing for growing cattle at the same level of production. It is defined as the difference between energy intake and energy demand and represents the amount of production (Arthur et al., 2014). Therefore, the objective of this study was to investigate feed efficiency in repla cement beef heifers that are known to be adapted to subtropical/tropical climates, and the relationship between feed efficiency and fertility in replacement heifers. It is known that selecti on of cattle with low RFI may also result in selecti on of leaner c attle. F at is correlated to the release of hormones and metabolites that are necessary for the attainment of puberty ; t herefore, i t was
31 hypothesized that more efficient heifers (low RFI ) would attain puberty a t older age than less efficient heifers Materials and Methods All heifers were handled in accordance with procedures approved by the University of Florida's Animal Care and Use Committee. Animals and M anagement O ne hundred and seventy nine heifers, housed at the North Florida Research and Educa tion Center (NFREC) were enrolled in a two year study The heifers were of Angus, Brangus and Braford genetics during the 2016 2017 and 2017 2018 calf crop. At 5 to 10 months of age, heifers were weaned and back grounded on pasture for three months. At 1 0 to 12 months of age, heifers were place d in the University of Florida Feed Efficiency Facility in Marianna, FL (FEF) Each year, u pon arrival to the FEF, heifers were fitted with e lectronic i dentification (EID) tags (Allflex USA Inc., Dallas Fort Worth, TX), weighed for two consecutive days and randomly assigned to pens (108 m 2 / pen) equipped with 2 GrowSafe feed bunks each ( GrowSafe Systems Ltd., Alberta, Canada, 2011). There was a mean number of 11.5 heifers (range of 1 0 to 13 heifers per pen) per pen per year, thus allowing 5.75 calves to feed per GrowSafe bunk (2 bunks / pen / year ). A 14 d acclimation period to the diet and facilities preceded a 70 d feeding trial, where the GrowSafe System recorded daily feed intak e for each individual. This period included the collection of data for residual feed intake, feed to gain ratio, various measurements of temperament, and stress responsiveness. Upon initiation of the 70 d trial (d 0), heifers were weighed and body conditi on scored (1 to 9 scale; 1 being emaciated; 9 being extremely obese) weekly (Figure 3 1)
32 Feed and S ample C ollection Heifers had ad libitum access to water and a diet consisting of 22.5% corn gluten, 22.5% soyhulls, 5% supplement pellets and 50% fiber pellets (Table 3 5 ) Bimonthly samples of the diet were taken from each feed bunk. All feed samples analyzed for nutritive values were dried at 55C for 72 hr in a forced air oven. At the end of the drying period all samples were ground in a Wiley mill (Arthur H. Thomas Company, Philadelphia, PA, USA) using a 2 .0 mm screen. After grinding, samples were composited for analysis on an equal weight basis. All nutritive value samples (round bale cores, CSBM, PPH, and ryegrass) were analyzed for DM, CP, TDN, ADF, NDF, Ca, and P in duplicate by a commercial laboratory using NIR procedures (Dairy One Forage Laboratory, Ithaca, NY). B lood C ollection Blood samples were collected weekly from January to May for analysis of concentrations of progesterone The collections were made via jugular or coccygeal venipuncture using 10 mL glass vials containing 143 IU units of Na heparin (BD Vacutainer Franklin Lakes, NJ). All blood samples were placed on ice following collection, and then centrifuged for 18 min at 4,000 g at 4 C. After centrifugation a pipette was used to siphon plasma into polypropylene vials (12mm 75mm; Fisherbrand; Thermo Fish er Scientific Inc., Waltham, MA) which were stored at 20 C until analyses. Concentrations of plasma progesterone were determined by a chemoluminescent assay ( IMMULITE 2000 XPi Immunoassay System; Siemens Healthcare Diagnostics, Malvern, PA ) The Immulit e method allows for ease of assaying serial blood samples while being safe, fast, accurate and repeatable (Boland et al., 2004)
33 Assessment of P uberty To determine attainment of puberty, the first increase in progesterone was considered a s evidence of fir st pubertal ovulation when followed by a progesterone pattern consistent with normal estrous cycle (Perry et al., 1991). Age of puberty (AOP) was defined as the age of the heifer at the first rise in progesterone. H eifers t hat had not attained puberty by the conclusion of the blood collection period were assigned an onset of puberty age to coincide with th e final blood collection date Ultrasonography and R eproductive M anagement To further assess potential fertility factors, an antral follicle count (AFC) was assessed on all heifers in April on the first day of the breeding season using transrectal ultrasonography (Ibex portable ultrasound 5.0 MHz linear multi frequency transducer, Ibex, E.I. Medical Imaging, Loveland, CO) T his evaluation allows for the predict ion of ovarian reserves (Ireland et al., 2008) and reports have indicated that high AFC heifers are more fertile (Cushman et al, 2009; Mossa et al., 2012). At this time, all heifer s also received an injection of 25 g of Prostaglandin ) which was followed by a second injection of PGF 12 days later After the second heat detection patches ( Estrotect Rockway Inc, Spring Valley, WI) were place d on the tail head of each heifer to assist wit h detection of estrus Heat detection was conducted twice a day at 0600 and 1800 hr for 45 min during each session for 29 days Each heifer detected in estrus was inseminated artificially by an experienced technician using the AM/PM rule (Larson et al., 20 09). Pregnancy w as determined by transrectal ultrasonography (Ibex portable ultrasound 5.0 MHz linear multi frequency transducer, Ibex, E.I. Medical Imaging, Loveland, CO) at 28 d after the last day of AI to confirm pregnancy to AI.
34 After completi on of the feeding trial period, the heifers were removed from the FEF and natural service sires were introduced to the herd for 60 d. Three yearling sires were used each year, and all sires passed a breeding soundness exam prior to introduction into the he rd. Final pregnancy rates were determined by transrectal ultrasonography (Ibex portable ultrasound 5.0 MHz linear multi frequency transducer, Ibex, E.I. Medical Imaging, Loveland, CO) at 3 7 d after bulls were removed from the herd. Statistical Analysis Average daily gain (ADG) for heifers was calculated by the mean BW from d 1 and d 0 minus the mean BW from d 69 and d 70, divided by the 70 days feeding period The RFI for heifers was calculated by subtracting expected DMI from the predicted DMI value (K och et al., 1963; Archer et al., 1997; Arthur et al., 2001 a, b). Residual feed intake was computed for each heifer and assumed to represent the residuals from a multiple regression model regressing DMI on ADG and BW. The base model was (3 1) w here : = standardized DMI of the j animal = regression intercept = re gression coefficient om BW animal.
35 Heifers were sorted and placed into Low (< 0.5 SD; n = 24), Medium (< 0.5 SD >; n = 24), and High (> 0.5 SD; n = 26) feed efficiency groups based on their RFI values, with more negative values being efficient and positive values inefficient. Breed differ ences for RFI were analyzed using the MIXED procedure of SAS, where breed was the predictor variable. The MIXED procedure of SAS was used to determine whether differences existed between Angus Brangus and Braford heifers for age at puberty, initial and final BW, BCS, ADG, FTG and DMI of heifers. The PROC CORR procedure was used to detect correlations existing among RFI, age at puberty onset, ADG, FTG and DMI between heifers. In order to assess the effect of heifer RFI rank (Low, Medium and High) on heifer performance, the MIXED procedure of SAS was used to identify differences in initial and final BW, BCS, ADG pregnancy to AI and final pregnancy rates. The procedure LIFETEST was used fo r survival analyses on the age at puberty with the resulting statistical model: S (t)= Pr(T ij >t) (3 2) w here: S = the survival function T= random response variable of the i th heifer from the j th pen t = Time (d) until T is achieved Pr = probability that time of T is later then time t For all analysis, statistical differences were reported at P 0.05, tendencies were identified 0.0 5 P 0.1, with means being reported as LS means SE.
36 Results and D iscussion Fertility and P erformance Reproductive maturation in heifers is a process that involves physical and behavioral changes and is also associated with the activation in development of the hypothalamic pituitary ovarian axis (Sisk and Foster, 2004). Several studies have reported favorable genetic relationships between traits in females, such as age at puberty (Martin et al., 1992; Brinks et al., 1978 ; Berg and Walters, 1983 ) In a ddition, it has been reported that Bos indicus influenced heifers reach puberty at an older a ge compared to heifers of Bos taurus breeds (Martin et al., 1992) thus, heifer fertility was evaluated based on heifer breed (Angus, Brangus and Braford; Table 3 2 ) O ur findings did not reveal a difference of breed on mean age at puberty (P = 0. 103 ) for Angus (393.4 42 d), Brangus (407.9 42 d) and Bradford (402.4 43 d) heifers This agreed with survival analysis which indicated no differences in age of puberty (P = 0.502; Figure 3 3 ); however, it is necessary t o increase the number of animals per g roup of breeds to decrease the chance of a type II statistical error. Breed may affect both growth and RFI (Nkrumah et al., 2004; Schenkel et al., 2004), but no reports have demonstrated difference between calf sex and RFI (Archer et al., 1997; Arthur et al., 2001a). No differences among Bos taurus crossbred cattle sired by Angus and Charolais bulls were found (Nkrumah et al., 2004). The same study reported bulls to be more efficient than steers in a group of crossbred cattle composed of various Bos taurus breeds including Angus, Charolais, Galloway, Hereford and Holstein. In agreement to these reports, our studies revealed no interaction (P > 0.05) between breed and RFI groups for age at onset of puberty, RFI, ADG, FTG, DMI, Initial BW, Final BW, initial B CS, final BCS, estrus expression, pregnancy to AI and overall pregnancy rates. Estimates of genetic parameters have confirmed that genetic variability
37 existed, however, an increased number of feed efficiency data need to continue to be collected to compute accurate populational genetic parameters for multiple traits (Elzo et al., 2009). Increased rates of BW gain during early development of heifers mediate the nutritional programming of puberty by functional alteration of the NPY, POMC and kisspeptin neuro nal pathway, and these alterations lead to decreased inhibition and enhanced excitation of GnRH neurons stimulating an early increase in GnRH pulsatility, critical for the maturation of the reproductive neuroendocrine axis (Amstalden et al., 2014). In addi tion, increased ADG resulted in increased body fat and increased total percentage of carcass weight lipids (Waldman et al., 1971; Kempster et al., 1976), which may be involved i n the process of puberty onset by the critical body fat hypothesis (McShane et al., 1989 ) H owever th is study revealed no difference ( P = 0.09 4 ) in ADG among breeds for Angus ( 1.5 0.26 kg ) Brangus ( 1.43 0.26 kg ) and Braford ( 1.39 0.25 kg ) heifers A ttainment of puberty can be impacted by nutritional management, particularly in Bos indicus cross breeds that are known to reach puberty at later ages (Day et al., 1986; Berg and Walters, 1983). Feed efficiency may also influence attainment of puberty, sin ce less feed efficient heifers have been reported to have a decreased age at puberty (Shaffer et al., 2010). However, Pearson correlations indicated a negative relationship between ADG and FTG (r = 0.810; P < 0.001 ; Table 3 4 ); therefore, there was no effect ( P = 0.41 4 ) of breed on FTG for Angus ( 9.29 1.84 kg of DMI/kg of BW gain ) Brangus ( 9.62 1.23 kg of DMI/kg of BW gain ) and Braford ( 9.30 1.83 kg of DMI/kg of BW gain ) heifers A moderate genetic control of RFI and a genetic variation for RF I exists in Bos indicus and Bos taurus breeds of beef cattle (Pitchford, 2004) ; however, in the current study no differences (P < 0.05) were detected for RFI of Angus, Brangus and Braford was detected ( P = 0. 80 5 ; Figure 3 2 ).
38 On d 0 and d 70 Angus and Brangus heifers had greater ( P < 0.001 ) BW than Braford heifers. In addition, o n d 0 Angus and Bra ngus heifers had greater ( P = 0.008) BCS than B raford heifers but on d 70 BCS was similar ( P = 0.995) among breeds P revious reports indicate that there was a degree of fatness, or a body composition that must be achieved before puberty may be attained in replacement heifers (Grass et al., 1982; Nelson et al., 1982) ; however conflicting results have also been reported (Brooks et al., 1985). Thus, BCS may be correlated with puberty but may not be a primary factor causing the onset of puberty. Pregnancy rate s w ere derived from pregnancy diagnosis 28 d following the conclusion of the artificial insemination period as well as 37 d following bull removal. Pregnan cy rates to AI were similar ( P = 0.732) among breeds 21.0% for Angus, 26.7% for Brangus, and 23.8% for Braford. Overall pregnancy rates were also similar ( P = 0.3166) among breeds for Angus (80.3%), Brangus (72.6%), and Braford (66.9%) heifers In addition estrus expression was similar ( P = 0.653) among breeds Thus, there was no effect of breed on establishment of pregnancy in heifers ; however, since pregnancy rates are a binary observation, this study was not designed to conclusively determine differences among breeds and there is a likelihood that a type II statistical error may have occurred. Heifer fertility was also evaluated based on heifer RFI classification (Low, Medium, or High ; Table 3 3) P henotypic independence between RFI and body weight have been reported in weaned heifers, as well as a similar range in RFI values between the most and the least efficient heifers ( 1.25 to 1.87 kg/d, respectively; Kelly et al., 2010a). Since the mo del for RFI is phenotypically independent of body weight and growth, it is not surprising that ranking heifers into Low, Medium and High RFI groups was not associated with initial BW ( P = 0.952) or final BW ( P = 0.447 ). The RFI classification also was not influenced by initial BCS ( P = 0.5654) or
39 final BCS ( P = 0.5877). R esults indicate d an effect of RFI group ( P = 0.048) on ADG ; however there was no difference between Low RFI and High RFI ( P = 0.905), or Low RFI and Medium RFI ( P = 0.1 60 ), but th ere wa s a tendency ( P = 0.056) for the High RFI group A DG ( 1.49 0.25) to be greater than the Medium RFI group ADG (1.37 0.25) Reports have indicate d a trend for low RFI cows to calve 5 d later compared to high RFI cows (Arthur et al., 2005), and dams producing Low RFI progeny were reported to calve 5 to 6 d later in the year ( P < 0.001) than cows that produced Medium and High RFI progeny (Basarab et al., 2007); however there was no reported difference s in pregnancy rate among RFI groups. Neverthe less, late calving cows which tend to become pregnant later in the subsequent breeding season may fail to become pregnant in subsequent years. In contrast our results revealed that heifers in the Low RFI group (387.15 43.6 d) attained ( P = 0.016) pubert y at a younger age than heifers in the high RFI group (414.9 42.89 d), whereas the M edium RFI group (401.72 43.13 d) was intermediate. This agreed with survival analysis which indicated differences in age of puberty for Low RFI group compared to Medium and High RFI group (P < 0.01; Figure 3 4); Similarly, Pearson correlations indicated a positive relationship between a ge of puberty and RFI (r = 0.215; P = 0.00 4 ) A greater (P < 0.001 ) percentage of Low RFI heifers attained puberty during weeks 1 and 12 of the study compared to M edium RFI or High RFI heifers (Table 3 1) In addition, heifer age at the beginning of the study did not differ among RFI group, providing further support that attainment of puberty was not related to age, but was associated t o heifer RFI classification. T herefore, more efficient heifers attained puberty earlier compared to less efficient heifers which indicates the potential for increased lifetime productivity since lifetime productivity is heavily dependent o n a
40 lity to reach reproductive competence, to conceive earlier during the breeding season, and calve earlier in the first calving season (Lesmeister et al., 1973). Pregnancy rate s w ere also evaluated based on heifer RFI classification (Low, Medium, or High). Pregnancy rates to AI were similar ( P = 0.633) among RFI groups with 18.4% for Low RFI, 27.0% for Medium RFI, and 26.0% for High RFI. Overall pregnancy rates were also similar ( P = 0.350) among RFI groups for Low RFI (80.0%) Medium RFI (66.2%) and High RFI (73.6%) heifers. In addition, estrus expression was not influenced by RFI group ( P = 0.65 7 ). Thus, this data indicates that there was no effect of RFI group on pregnancy rates of heifers in agreement with other reports (Arthur et al., 2005 Basarab et al., 2007) Feed I ntake Heifer feed intake was evaluated based on heifer breed ( Angus, Brangus and Braford; Table 3 2 ). Body weight and age at puberty are influenced by nutrient intake (Short and Bellows, 1971). A phenotypic correlation exists between RFI and DMI using diverse breeds of cattle, indicating the selection for a more favorable RFI phenotype should result in a reduction of feed intake (Arthur et al, 2001 a, b ; Hoque et al., 2005; Nkrumah et al., 2007). In the current study, Pearson correlations indicated a positive relationship between a ge of puberty and DMI (r = 0.182; P < 0.014). A n effect was detected between breed and DMI ( P = 0.0 0 4 ), where Braford heifers (12.8 1.4 kg ) had a lower (P = 0.003 ) DMI compared to Angus heifers (13. 6 1. 5 kg ) and a tendency (P = 0.05 4 ) exist ed for Braford heifers to have reduced DMI compared to Brangus heifers (13. 4 1. 5 kg ) ; however, there was no effect of breed on heifer RFI as reported previously. Pearson correlations indicated a positive relationship between FTG and DMI (r = 0.317; P < 0.001) but no effect of breed on FTG ( P = 0.413). I n young Bos taurus crossbred heifers, ADG and BW accounted for 77% of the variation in DMI (Kelly et al., 2010a); however, reports also indicate that 30% of the variation in daily
41 DMI was justified by ADG and BW in Bos indicus and Bos taurus tropically adapted cattle (Elzo et al., 2009). A lth ough Pearson correlations indicated a positive relationship between ADG and DMI (r = 0.256; P = 0.0005) i n agreement to our previous reports on BW and ADG t h e reduced DMI was likely associated with heifer BW, where Braford heifers had lower BW compared to Angus and Brangus and no difference i n ADG between breed was detected Heifer feed intake was also evaluated based on heifer RFI classification (Low, Medium, or High ; Table 3 3 ). S ystematic reductions in DMI for decreasing RFI rank groups have been repor ted for growing cattle (Elzo et al., 2009; Lancaster et al., 2009a; Shaffer et al., 2010) Pearson correlations indicated a positive relationship between RFI and DMI (r = 0.833; P < 0.001); therefore, o ur results indicated an effect of RFI group on DMI ( P < 0.001) The Low RFI group (11.9 1. 5 kg) of heifers had the lowest ( P < 0.001) DMI compared to Medium RFI group ( 13. 2 1.5 kg ) and to the High RFI group ( 14. 7 1. 5 kg) Pearson correlations indicated a positive relationship between RFI and FTG (r = 0.461; P < 0.001); thus, as expected, there was an effect of RFI group on FTG ( P < 0.001). Heifers in the Low RFI group had decreased FTG conversion (8.3 1. 9 kg of DMI/kg of BW gain ) compared to the High RFI group (10.1 1.8 kg of DMI/kg of BW gain ; P < 0.001) and Medium RFI group (9. 8 1.8 kg of DMI/kg of BW gain ; P = 0.001). These results further support that more efficient heifers a re consuming less feed and maintain similar gain s compared to less efficient heifers.
42 Figure 3 1. Yearly schematic for data collection of heifers BCS = body condition score; BW = body weight; RFI = Residual feed intake ; FE Facility = Feed efficiency facility
43 Figure 3 2. Residual feed intake of replacement heifers associated with breed. AN = Angus, n = 98; BN = Brangus, n = 60; BR = Braford, n = 21 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 RFI, kg Angus Brangus Braford
44 Figure 3 3. Percentage of pubertal heifers during the 120 day s of development and breeding phase for Angus, Brangus and Braford heifers. 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 110 120 Percentage pubertal Days from initiation of blood sampling until completion of the breeding season Angus Brangus Braford Breeding season
45 Figure 3 4. Percentage of pubertal heifers during the 120 days of development and breeding phase for Angus, Brangus and Braford heifers. 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 110 Percentage pubertal Days from initiation of blood sampling until complition of the breading season High Medium Low Breeding season
46 Figure 3 5 Mean age at puberty onset of replacement heifers selected by High RFI, Medium RFI and Low RFI 350 360 370 380 390 400 410 420 430 High RFI Medium RFI Low RFI Mean age at puberty onset, d
47 Table 3 1 Fertility and performance data based on Angus, Brangus and Braford heifers Heifer breed Item Angus Brangus Braford SEM P value No. of heifers 98 60 21 Fertility Mean age at puberty onset d 393.4 42 407.9 42 402.4 43 9.2 8 0. 103 Estrus expression % 71.4 63.3 76. 2 0.246 0.65 4 Pregnancy to AI % 21 .0 26. 7 23. 8 11.4 0.732 Overall pregnancy rate % 80.3 72. 6 66. 9 9.336 0.31 7 Animal performance Initial BW, kg 354.41 32.69 357.47 32.97 327.20 32.93 6.994 <0.001 Final BW, kg 438.95 37.39 437.69 37.77 405.67 37.61 8.463 <0.001 Initial BCS 6. 3 6 .0 0.406 0.008 Final BCS 6.9 6.9 6.9 0.552 0.995 ADG, kg 1.5 0.26 1.4 3 0.26 1. 39 0.25 0.0 8 7 0.0 9 4 RFI, kg /d 0.06 1.23 0.1 3 1.24 0.1 0 1.19 0. 133 0. 80 5 F : G 9.2 9 1.84 9. 62 1.23 9.3 0 1.83 0.77 3 0. 41 4 DMI, k g /d 13.57 1.47 13.37 1.48 12.80 1.42 0.340 0.004 Mean age at puberty = the age of the week in w h ich the first rise of P4 was detected Estrus expression = percentage of animals that showed estrus during heat detection Pregnancy to AI = Percentage of heifers pregnant after 30d following AI BW = Body weight ; ADG = Average daily gain ; RFI = Residual feed intake ; FTG = Feed to gain ; DMI = Dry matter intake Significant differences of Least Squared Means within a row (P 0.05 )
48 T able 3 2 Fertility and performance data based on heifer rankings considered as Low, Medium and High feed efficiency categories RFI Classification Item Low Medium High SEM P value No. of heifers 51 66 62 Fertility Mean age at puberty onset d 414.9 42. 9 7.23 0.016 Estrus expression % 66. 5 74.5 69.3 24.04 0.657 Pregnancy to AI % 18.4 27 .0 26. 1 9.641 0.633 Overall pregnancy rate % 80 66.2 73. 6 7.111 0.350 Initial BW, kg 346.05 32.69 345.44 32.8 347.60 32.84 5.326 0.952 Final BW, kg 428.63 37.41 421.88 37.37 431.81 37.39 6.695 0.448 Initial BCS 6.2 6.2 6. 2 0.406 0.565 Final BCS 7 .0 6.9 6.9 0.552 0.588 ADG, kg 1.47 0.25 1.37 0.25 1.49 0.25 0.079 0.049 RFI, kg /d 0.101 <0.001 F : G 0.742 <0.001 DMI, kg /d 0.309 <0.001 Cows were sorted and placed into Low (< 0.5 SD), Medium (< 0.5 SD >), and High (> 0.5 SD) efficiency groups based on their RFI values, with more negative values (Low) being efficient and positive values (High) inefficient. Mean age at puberty = the age of the week in which the first rise of P4 was detected Estrus expression = percentage of animals that showed estrus during heat detection fers pregnant after 30d following AI FTG = Feed to gain; DMI = Dry matter intake Significant differences of Least Squared Means within a row (P 0.05 )
49 Table 3 3 Pearson correlation coefficients (top number) and P value (bottom number) among feed efficiency and fertility traits for replacement beef heifer N = 179, Prob > |r| under H0: Rho = 0 Item Age of puberty, d RFI, kg ADG FTG DMI, kg/d Age of puberty, d 1.00 0.215 0.0037 0.037 0.619 0.052 0.481 0.182 0.014 RFI, kg 1.00 0.005 0.938 0.461 <0.001 0.833 <0.001 ADG 1.00 0.810 <0.001 0.256 0.0005 FTG 1.00 0.317 <0.001 DMI, kg/d 1.00 RFI = Residual feed intake; DMI = Dry matter intake
50 Table 3 4 Nutritional values of feeds offed during the two years of project Ingredient, DM Basis Yr1 Yr2 DM, % 94.9 92.1 TDN, % 65 69 CP, % 13.7 14.3 ADF, % 40.9 36.6 NDF, % 63.4 54.4 NEm (Mcal/kg) 0.62 0.71 NEg (Mcal/kg) 0.36 0.44 Sample composition analyzed via Near Infrared Reflectance Analysis (NIR) Yr1 year 1 from January to May 2017 Yr2 year 2 from January to May 2018
51 CHAPTER 4 CONCLUSION Timing at puberty onset has important implications for livestock production. Lifetime productivity is heavily dependent upon the ability of a beef heifer to reach reproductive maturity. Feed costs are responsible for two thirds of the annual cost of produc tion within the beef industry (Arthur et al., 2001a) and the relationship of fertility and feed efficiency has barely been researched. This study revealed that there were no differences in BW, BCS, estrus expression and overall pregnancy dependent of RFI c lassification. In addition, although age at puberty was not affected by breed, our results indicated an improvement in age at puberty for Low RFI (more efficient) heifers. This experiment reveals that by selecting heifers for feed efficiency may also impr ove age at puberty onset and thus improve reproductive performance of the herd. Understanding the relationship between fertility and feed efficiency remains an important factor in the selection of replacement females. While this study provides positive ins ight to the phenotypic relationships between RFI and puberty onset, more research is needed to fully understand the impact of feed efficiency selection on the entire beef cattle production system.
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60 BIOGRAPHICAL SKETCH Luara Bertelli Canal was born in S o Paulo, Brazil, in 1990 to Ivo Canal and Sandra Bertelli. S he grew up in a small town helping her family in their veterinary clinic and always showed a great affection for animals. This led her to the So Paulo State University, where she obtained her B.S. degree in Veterinary Medicine. She began to develop herself as a scientist by helping with resea rch projects and conferences related to beef and dairy cattle production and as a cattle expert by working at the dairy facility on campus for four years. In the s pring of 2017, she began working with Dr. Lamb and Dr. DiLorenzo as a n M.S. Student, assessin g the relationship between feed efficiency and attainment of puberty in beef heifers. This has increased her desire to support the agricultural community, and for that, she intends to pursue her career as an extension specialist