Increased Postweaning Gain of Beef Heifers Enhances Fertility and Milk Production1

Increased Postweaning Gain of Beef Heifers Enhances Fertility and Milk Production1 D. D. Buskirk, D. B. Faulkner, and F. A Ireland Department of Animal Sciences, University of Illinois, Urbana 61801 ABSTRACT Four hundred fifty-two (452 weanling heifer calves (192f23 kg) were used to determine the effect of postweaning weight gain on subsequent reproductive performance and lactation. Heifers grazed stockpiled tall fescue pastures and were fed a high ( H) or low ( L) amount of ground corn supplement (3.68 and 2.99 kg/[animal.dl, respectively) during a postweaning treatment period of 136 d. Postweaning gain of heifers receiving L and H was.43 and.62 kg/d, respectively. These treatments resulted in a range of postweaning gain from.07 to 1.17 kg/d. Heifers receiving H were 26 kg heavier ( P <.01), had.l5 cm more fat thickness ( P <. Ol), 5% greater pelvic area ( P <.Ol), and tended (P =.08) to have greater hip height than heifers receiving L at yearling. More heifers in the H group were pubertal before the start of the breeding season (70.9 vs 61.3%) ( P <.05). No significant differences were found in pregnancy rate or first-service calving rate between treatment groups. Mean milk production was 10% greater for H- than for L-fed heifers ( P <.01) and resulted in heavier calves at 54, 104, and 153 d of age (P <.05). Regression analysis revealed that as weaning weight and postweaning gain increased, so did the probability for reaching puberty before the breeding season. Increasing weaning weight also increased the probability of calving to the first AI service. As heifer weaning weight and postweaning gain increased, subsequent milk production also increased. Increasing postweaning weight gain of lightweight heifers from.07 to 1.17 kgld increased both reproductive and lactation performance. Key Words: Beef Cattle, Heifers, Supplementary Feeding, Gain, Milk Production, Fertility J. him. Sci. 1995. 73:937-946 Introduction The economic success of a replacement heifer development program is dependent on subsequent heifer fertility and lactation ability. Heifers that conceive early in their first breeding season have a greater lifetime calf production (Lesmeister et al., 1973) and those with increased milk production utilize less feed energy per unit of calf weaning weight (Freking and Marshall, 1992). These production traits can be influenced by manipulation of nutrition before breeding. A high plane of nutrition postweaning results in earlier puberty (Wiltbank et al., 1969, 1985) and increased conception rates (Short and Bellows, 1971; Patterson et al., 1989) compared with a low plane. However, overfeeding heifers before breeding has been demonstrated to have detrimental 'The authors wish to thank the staff athe Dixon Springs Agricultural Center, Simpson, IL for care of the experimental animals. We would also like to acknowledge D. J. Kesler for assistance with the progesterone analysis. Received March 10, 1994. Accepted December 6, 1994. effects on pregnancy rates (Ferrell, 1982). In addition, supplemental feeding of beef heifers before puberty has been shown to reduce lifetime calf weaning weights (Martin et al., 1981) due to impaired milk production (Hixon et al., 1982). Limited research exists that examines reproduction or milk production in beef heifers as influenced by a continuum of postweaning gain. The objectives of this study were to determine the effects of two levels of supplementation and a range of postweaning gain of lightweight beef heifers on subsequent fertility and milk production. Experimental Procedures Four hundred fifty-two (452) Angus and Angus- Hereford lightweight weanling heifer calves (192f23 kg) were used to determine the effect of postweaning weight gain on subsequent productivity. The heifers were assigned randomly to two levels of nutrition from weaning to breeding in a completely random design. These spring-born calves were purchased from various sources in the southeastern United States during October 1991 (Yr 1) and were placed on endophyte- 937

938 BUSKIRK Table 1. Supplement composition and intake on DM basisa Low High Item Ground corn, B 96 98 Urea, %l 4 2 Vitamin A IUkg of supplement 11,023 11,023 Rumensinb, mg/(animal,d)cd 150 150 Supplement intake, kgi(anima1.d) 2.99 3.68 aanimals had ad libitum access to trace mineralized salt, composition (%): NaCl, 20-24; Ca, 14.5-16.5; P, > 8; Mg, > 1.1; S, >.71; K, > 2.24; Fe, >.25; Zn, >.25; Mn, >.25; Cu, >.03; CO, >,003; I, >,004; Se, >,0026 and Vitamin A, > 529,100; Vitamin D3, > 88,183; Vitamin E, > 441 IUkg. bcomposition of high intake supplement does not include addition of an average 5% white salt. CElanco Animal Health, Indianapolis, IN. dconcentration in supplements were adjusted every 28 d to maintain 150 mgi(anima1.d). infected tall fescue (Festuca arundinacea Schreb.) pastures at the Dixon Springs Agricultural Center, Simpson, IL until they were assigned to treatment in November 1991. During the postweaning treatment period of 136 d (November 5, 1991, to March 20, 19923, heifers grazed stockpiled tall fescue pastures and were fed a high ( H) or low ( L) amount of ground corn supplement (3.68 and 2.99 kg/[animal.dl, respectively) (Table 1). Heifers receiving H were given ad libitum access to the supplement with intake limited by addition of an average of 5% white salt (DM basis). Heifers receiving L were limit-fed in bunks daily. Urea was added to both supplements to assure adequate N intake. All heifers had ad libitum access to trace mineralized salt. These treatments were designed to elicit a range in weight gain. The beginning and end of the treatment period are referred to as weaning and yearling, respectively. The period from weaning to yearling is referred to as the postweaning period. Beginning and ending weights for the postweaning period were calculated as the mean of full weights taken on two consecutive days. Full weights were also obtained in the fall of 1992 (Yr 2) and spring and fall of 1993 (Yr 3). Fat thickness between the 12th and 13th ribs was measured using a real-time linear array ultrasound instrument (Model 210 DX, Johnson and Johnson Ultrasound, Englewood, CO) at weaning, yearling, and in the fall of Yr 2. Hip height was measured to a point directly over the hook bones at weaning, yearling, and in the fall of Yr 2 and 3. Horizontal and vertical internal pelvic measurements were obtained by the procedure of Rice and Wiltbank (1972) with a Rice Pelvimeter (Lane Manufacturing, Denver, CO). The two measurements were multiplied to yield an estimate of pelvic area. Pelvic area was obtained in the spring and fall of Yr 2 and spring of Yr 3. Body condition score ( BCS) on a 1-to-9 scale (Wagner et al., 1988) was assigned in the spring and fall of Yr 3 by an experienced evaluator. ET AL. Concentrations of progesterone were determined in samples of serum collected 10 d before and on the 1st d of the breeding season in Yr 2. Blood was collected via jugular venipuncture from all heifers; samples were immediately placed in crushed ice to prevent progesterone metabolism (Wiseman et al., 1982) and serum was separated by centrifugation at 1,000 x g for 30 min within 6 h of collection. Serum was stored at -20 C until it was assayed for concentrations of progesterone as determined by a validated enzyme immunoassay (Kesler et al., 1990). Heifers were considered pubertal if one or both of the serum samples contained progesterone concentrations 2 1.5 ng1ml. After collection of blood samples for progesterone data, 33 of the lightest-weight heifers were removed from each of the treatment groups. Heifers were then visually sorted into Angus and Angus-Polled Herefordcross groups and artificially inseminated and exposed to Polled Hereford and Angus bulls, respectively. Heifers were further divided into four groups (morning and afternoon of two consecutive days) for estrus synchronization and AI. Estrus was synchronized with Syncro-Mate-Be' ( SMB; Sanofi Animal Health, Overland Park, KS) as described by Favero et al. (19931, and heifers were artificially inseminated approximately 48 h after removal of the implants. Artificial insemination was performed by one of three inseminators experienced in timed AI. Inseminator and service sire were randomly assigned at the AI. Twelve days following AI, all heifers were exposed to multiple bulls in their respective breeding groups for the remainder of the 60-d breeding season. All heifers were managed alike after this time and were maintained on tall fescue pastures through the fall of Yr 2. Pregnancy was determined by palpation per rectum 160 d after AI and all nonpregnant heifers were removed from the study. In addition, 18 pregnant heifers that weighed less than 300 kg and had a hip height less than 120 cm were removed because of practical management considerations. A total of 70 heifers from the L treatment and 63 from the H treatment were removed at this time. Heifers grazed stockpiled tall fescue pastures with pre- and postcalving winter supplementation consisting of tall fescue hay and ground corn. Heifers were group-fed and supplemented at the same rate (2.5 kgld ground corn). At parturition, heifers were scored according to degree of calving difficulty ( 1 = no difficulty, 2 = minor difficulty, 3 = major difficulty, 4 = Caesarean section, 5 = abnormal presentation) (BIF, 1990). Only calving ease scores of 1 to 4 were included in statistical analysis. Calves were identified and weighed within 24 h after birth and male calves were castrated. Calving date was used to estimate calving rate to the timed AI (283+11 d from AI). Heifers that did not raise a calf or did not raise their own calf were removed from the study. Calves nursed their dams

POSTWEANING GAIN OF BEEF HEIFERS 939 throughout the preweaning period without access to SNF, fat, and protein were calculated as the mean supplemental feed. Heifers grazed tall fescue, red component percentage for each treatment x mean clover ( Trifolium pratense L. ) pastures postpartum. daily milk production. Ash concentration was assumed Calves were weaned at 214k18 d of age and weight to be 6.3% on a DM basis (NRC, 1982). Production of and hip height were recorded. organic matter was calculated as (SNF production + Milk production estimates were obtained at 54, 104, fat production) x,937. Lactose production was calcuand 153+18 d postpartum for heifers bycalf weigh- lated as organic matter production - protein producsuckle-weigh procedures. Due to the large number of heifers, milk production estimates were performed on tion - fat production. Experimental procedures were conducted according to those approved by the Univerthree consecutive days in a facility with 24 sorting sity of Illinois Laboratory Animal Care Advisory pens. At approximately 1400, dam-calf pairs were Committee. sorted into groups of 10 or fewer and calves were Statistical Analysis. Effects of dietary treatment separated from their dams. Beginning at 1900, groups were analyzed using the GLM procedures of SAS of calves were paired with their dams, allowed to ( 1985) for a completely random design. Individual nurse for 15 min, and separated until the following animal was considered the experimental unit. Due to morning. This preliminary separation period and removal of animals during the study, data were subsequent nursing was designed to leave only analyzed by least squares procedures for unequal residual milk in the mammary gland at the beginning subclass numbers. Model sums of squares were of the measurement period. Calves were denied access partitioned into treatment effects and treatment to feed and water after the initial separation for the means were separated using a significant F-test. duration of the measurement period. Dams were Heifer fertility data including pubertal before breeding allowed access to feed and water between nursings. season, pregnancy and first-service calving rates were Beginning at 0700, groups of calves were weighed to reanalyzed using the CATMOD procedures (SAS, the nearest.09 kg with an electronic scale (Mettler- 1985). In all instances, GLM and CATMOD proce- Toledo Model 2158, System Scale, Indianapolis, IN) dures yielded the same probability values. Statistics and paired with their dam. Pairing was done in a presented here are results of GLM procedures. large open lot so that calves paired quickly and cross- Multiple logistic regression techniques were used to nursing was prevented. Immediately after nursing, evaluate the effects of weaning weight and postweaneach calf was reweighed and returned to its dam. The ing gain on subsequent heifer productivity. Heifer difference in calf pre- and post-nursing weight was fertility was analyzed using logistic regression by the multiplied by 2 to provide estimate an of CATMOD procedures of SAS ( 1985). Measures of 24-h milk production. The three milk production heifer fertility were assumed to follow a binomial estimates were averaged to yield an estimate of distribution with the probability of the ith heifer being average daily milk production. fertile equalling Pi. To accommodate binomial Milk composition was determined at 140f19 d responses, logistic regression models were used to postpartum for a subsample of heifers by milking produce maximum likelihood estimates of regression machine. Heifers were placed in 20 blocks determined coefficients. The following expression was used: by their postweaning gain. One heifer from each block was randomly selected for measurement of milk composition. At approximately 1400, dam-calf pairs were sorted into groups of two and separated. Beginning at 1900, groups of calves were paired with their dams, allowed to nurse for 15 min, and separated until the following morning. Dams were allowed access to feed and water. Beginning at 0700 heifers were restrained in a chute and injected i.m. with 100 U.S.P. units of oxytocin induce milk letdown, immediately before machine milking. During milking, each gland was massaged until milk flow ceased to ensure that complete milkout had been obtained. The milk was thoroughly mixed and a 120-mL sample was obtained. A preservative (2-bromo-2-nitropropane-1,3 diol, MicrotabsTM, & D F Control Systems, San Ramon, CA) was immediately added to the samples and they were refrigerated at 4 C before analyses. Solids-notfat ( SNF) concentration was determined according to the procedure of Golding (1959). Milk fat and protein concentrations were determined by infrared analysis (Dairy Lab Services, Dubuque, IA). Production of Pi = 1 ( i = 1, 2,..., n), [ OO+iOA] 1 + exp H where n is the number of heifers and 60 is the intercept with k regression coefficients ( associated with individual or combinations of weaning weight and postweaning gain regressor variables ( XLJ) (Bergmann and Hohenboken, 1992). Models including linear, quadratic, and two-way interaction terms for regressor variables were fit for each fertility variable. Regressor variables that did not account for significant variation ( P >.05) in the fertility variables were removed from the model. Maximum R2 improvement, stepwise multiple regression procedures (SAS, 1985) were used to develop models to describe the effects of postweaning gain and heifer weaning weight on subsequent body measurements. The same stepwise procedures were used to develop models to describe the effects of

940 BUSKIRK ET AL. Table 2. Arithmetic means, standard deviations, minimum and maximum values of observed variables Variable n Mean Maximum SD Minimum Postweaning gain, kgjd 448.52.l8.07 1.17 Weight, kg Weaning 452 192 22.5 142 274 Yearling 448 263 386 34.7 176 Fall Yr 2 376 293 33.8 210 373 Spring Yr 3 211 347 34.9 269 447 Fall Yr 3 205 330 35.3 239 429 Hip height, cm Weaning 452 97 104 3.4 116 Yearling 448 108 122 4.5 86 Fall Yr 2 374 117 130 3.9 104 Fall Yr 3 205 121 4.6 107 133 Fat thickness, cm Weaning 452.51.06.30.70 Yearling.71 447.l8.oo 1.4 Fall Yr 2 376.38. l5.oo.80 Body condition scorea Spring Yr 3.72 12 3.6 2 5 Fall Yr 3.6 205 3.4 2 5 Pelvic area, cm2 Spring Yr 2 373 160.7 18.2 114.0 217.5 Fall Yr 2 376 185.6 26.9 104.5 252.0 Spring Yr 3 206 260.5 32.2 135.0 332.5 Calving ease scoreb 243 1.5.9 1 5 Calf Birth wt, kg 244 29.4 4.6 13.6 41.7 54-d wt, kg 52.3 12.5 22.9 87.8 104-d wt, kg 204 78.8 16.4 36.3 128.7 153-d wt, kg 202 107.5 20.1 56.5 160.0 214-d wt, kg 139.9 201 23.3 81.6 197.8 214-d hip height, cm 96.4 201 81.3 5.5 110.5 Age at weaning, d 201 214 18.2 174 250 al to 9 scale. 1 to 5 scale. postweaning gain on subsequent milk production and calf growth. These models included mean calf age, at the mean milk production estimate, as a covariate. Models included linear, quadratic, cubic, and two-way interaction terms for regressor variables. Regressor variables that did not account for significant variation ( P >.05) were removed from the model. Although use of prediction equations was limited to values within the range observed in the study, some extrapolation of the response surfaces (Figures 1 and 3) exists when a combination of variables is used for simultaneous prediction (i.e., a combination of minimum and maximum values for both traits may not have existed) (Freking and Marshall, 1992). Results and Discussion Average daily supplement consumption for L- and H-fed groups was 2.99 and 3.68 kg per animal, respectively (Table 1). These treatments resulted in a range of postweanin gain from.07 to 1.17 kg/d (Table 2). Postweaning gain for L- and H-fed groups of heifers was.43 and.62 kg/d, respectively (Table 3). Postweaning gain for both groups was lower than expected and may have been due to negative associative effects of the concentrate and forage diets. Granger et al. (1990) observed decreases in DM, fiber, and CP digestibilities with supplementation of ground corn to hay diets of weanling heifers. In addition, grazing of endophyte-infected tall fescue and muddy conditions may have contributed to decreased gains. Heifers that received H weighed 26 kg more ( P <.01) than those that received L at yearling. In the fall of Yr 2, heifers in the H group still weighed 19 kg more ( P < -0 1) than those in the L group. There was no difference ( P >.lo) in heifer weight, due to treatment, after the fall of Yr 2. Heifers receiving H tended (P =.08) to have increased hip height at yearling compared with those receiving L; however, there was no difference after this time. As expected, fat thickness was 23 and 26% greater ( P <.O 1) for H fed heifers at yearling and fall Yr 2 measurements. By Yr 3, there was no difference ( P >.lo) in BCS between treatment groups. Body condition of these heifers was poor and may have been due to the consumption of endophyteinfected tall fescue (Peters et al., 1992). Heifers in the H group also had 5.3 and 4.9% greater ( P <.01) pelvic

POSTWEANING GAIN OF BEEF HEIFERS 941 Table 3. Heifer growth as influenced by supplement intake during the postweaning period Item P-valuea intake High Low intake Treatment gain, Postweaning kg/d 448.62.43 rt:.01,001 f.01 Weight, kg Weaning 192.7 452 191.8 f 1.5 f 1.5.69 448 Yearling f 2.2.001 275.9 f 2.1 Fall Yr 2 376 283.9 f 2.4,001302.9 f 2.4 347.8 211 Spring Yr 3 f 3.5 347.0 f 3.3.87 Fall Yr 3 205 327.7 331.3 f 3.6 f 3.4.46 Hip height, cm Weaning 104.1 452 f.2.40 103.9 f.2 448 Yearling f.3 108.5 f.3.08 Fall Yr 2 117.2 374 k.3 117.8 f.3.l4 Fall Yr 3 205 121.3 f.5.54 120.9 f.4 Fat thickness, cm Weaning.51 452 f.004 51 f.004.37 Yearling.64 447.79 f.01 f.01.001 Fall Yr 2.34 376.43 f.01 f.01.001 Body condition scoreb Spring Yr 3 3.6 212.27 f.07 3.5 f.06 Fall Yr 3 205 3.4 f.07 3.3 f.06.46 Pelvic area, cm2 Spring Yr 2 156.6 373 164.9 f 1.3.001 f 1.3 Fall Yr 2 181.2 376 190.0 f 1.9,001 rt: 1.9 Spring Yr 3 206 256.8 263.8 f 3.3.l2 f 3.1 af robability of observing a greater F-value. 1 to 9 scale. area as measured in the spring and fall of Yr 2. Other researchers have shown a positive relationship between postweaning gain and pelvic area (Short and Bellows, 1971; Fleck et al., 1980). By 73k18 d postpartum there was no difference in pelvic area. Calving ease scores did not differ ( P >.lo) for L and H groups (Table 4). Fleck et al. (1980) found heifers with high postweaning gain to have fewer calving problems. However, calf birth weight is the single most important factor in causing dystocia (Rice and Wiltbank, 1972; Johnson et al., 19881, which was similar (P >.lo) between treatments. Nearly 10% more heifers in the H group were pubertal before the start of the breeding season (70.9 vs 61.3%). It has often been noted that increasing rate of postweaning gain will decrease age at puberty (Wiltbank et al., 1969; Short and Bellows, 1971; Ferrell, 1982). Short and Bellows (1971) and Patterson et al. (1989) observed increased pregnancy rates for heifers on a high plane of postweaning nutrition. Early puberty did not result in an increase in pregnancy or first-service calving rates for heifers that received H. First-service calving rates were low for both groups of heifers. Although a large proportion of these lightweight heifers were cycling prior to the SMB procedure, this may have been one of their first estrous cycles. The fertility of the pubertal estrus in beef heifers is lower than that of third estrus (Byerley et al., 1987). Additionally, fertility of th estrus induced by the SMB procedure is variable (Odde, 1990) and may be low due to luteal dysfunction (Favero et al., 1988). Milk solids-not-fat, fat, and protein percentages were not different between treatments. The H-fed heifers produced 13% more milk ( P <.O 1) at 54 d of lactation and tended (P =.08) to produce more milk at 153 d. Mean milk production was 10% greater ( P <.Ol) for H-fed than for L-fed heifers. Milk production for both groups was lower than previous reports for Angus and Angus-Hereford heifers (Hixon et al., 1982; Sacco et al., 1987). This was likely caused by the detrimental effects of endophyte-infected tall fescue, which can suppress milk production of beef heifers by as much as 50% (Schmidt et al., 1986). The observation that increased supplemental intake during the postweaning period increases milk production is in contrast to that observed for younger creepfed heifers. Angus and Hereford females that were creep-fed for 90 d before weaning produced 28% less milk at 120 d of lactation (Hixon et al., 1982). In a similar comparison, Martin et al. ( 198 1) found that creep feeding Angus heifers for 90 to 120 d before weaning decreased their lifetime calf production by 89 kg. Reported effects of postweaning nutrition on milk production of beef heifers has been variable. Fleck et al. ( 1980) found no differences in milk production of Polled Hereford heifers fed to gain from.09 to.9 kg/d during their first winter. However, Ferrell ( 19821 fed heifers of different breeds to gain.4,.6, and.8 kg/d postweaning. Mean milk production during their first

~ 942 BUSKIRK ET AL. Table 4. Heifer fertility, calving ease, milk production, milk component production, and as influenced by supplement intake during the postweaning period calf performance Treatment Item n Low intake High intake P-valuea Pubertal before breeding, % Pregnancy Yr 2, % First-service calving rate, % Calving dateb Calving ease scorec Pregnancy Yr 3, % Milk component, % Solids-not-fat Fat Protein Milk production, kgjd 54-d 104-d 153-d Mean Milk component production, kgid Organic matter Solids-not-fat Fat Protein Lactose Calf weight, kg Birth 54-d 104-d 153-d 214-d Calf hip height at weaning, cm 380 377 359 244 238 205 19 20 20 204 202 244 212 207 205 204 204 aprobability of observing a greater F-value. bcalculated as the number of days from the birth of the first calf. 1 to 5 scale; 1 = no difficulty, 2 = minor difficulty. 61.3 f 3.4 67.8 f 3.5 17.8 i 2.7 35.1 f 1.6 1.4 f.07 79.2 f 3.8 7.95 f.07 3.98 f.l3 3.08 rt.l0 3.8 f.l 3.1 f.l 1.8 rt.l 2.9 f.l.32 f.01.23 f.007.l2 i.004.09 5.003.l2 i.004 29.0 f.42 50.9 f.9 76.7 f 1.4 104.8 k 1.8 137.7 f 2.3 95.4 f.5 70.9 2 3.4 68.1 t 3.4 12.4 F 2.6 36.4 f 1.6 1.5 2.07 86.2? 3.6 8.07 2.07 4.28?.l2 3.05 k.l0 4.3 k.l 3.3 4.l 2.1 f.l 3.2 F.l.37 i.01.26 rt.007.l4 i,004.l0 f,003.l4 f,004 29.8 i.41 53.5 f.9 80.7 k 1.3 109.9 f 1.7 141.9 k 2.1 97.2 f.5.05.95.15.59.33.18.22.12.84,008.15.08,006,001,002,001.01.001.17.04.03.04.19.o 1 lactation was highest for heifers gaining.6 kg/d. Additionally, Lemenager et al. ( 1980) and Ferrell (1982) found that increased winter gains up to.52 and.60 kg/d, respectively, resulted in increased progeny performance. Therefore, it is likely that heifers in the H group were near optimal postweaning gain (.62 kg/d) for expression of maximal milk production. Estimates of milk component production for H-fed dams were all greater (P <.O 1) than those for L-fed dams. Increased milk production from dams that received H resulted in calves that weighed more ( P <.05) at 54, 104, and 153 d of age (Table 4). Although hip height at weaning was greater (P <.05) for calves of H-fed heifers, there was no difference in calf weaning weight. Calves of L-fed dams may have Table 5. Pearson product-moment correlations among calf weight and dam and milk component production milk production Milk production, kgid Milk component production, 54-d 104-d 153-d Mean OM SNF~ Fat Protein Lactose kgid anull hypothesis is 11. I = 0 bsnf = solids-not-fat. *P <.05. **P <.01. ***P c,001.

E 'OSTWEANING GAIN OF BEEF HEIFERS 943 G<- i a * m01 00 * * 00 d r- * 9 99 c 9 * m W 3 I l l / l + ' I 1 I I I I I R * * X * * * m hlrn! 91 3 * W rl I I f l I l l I I l + 1 + 1 1 I 0 or-? y& Q, W m* x * * * x * x * * * * * x * * *Q)* Q) * * W t-mr( m o m 0 * W m 9:: 999 9 99 9

+ exp[6.40-.0294(weaning + exp[5.91-.021(weaning 944 BUSKIRK ET AL. Figure 1. Logistic regression of probability of reaching puberty before the start of breeding (Pi) on heifer weaning weight and postweaning gain; Pi = 1 wt)-2.84(postweaning daily gain11 1 (SEpl =.0057, SE62 =,731. increased their forage consumption to compensate for their lower milk intake in late lactation, resulting in similar weaning weights. Calves in drylot have been shown to increase forage intake when milk production of their dam is reduced (Buskirk et al., 1992). Mean milk production of the dam accounted for 24% of the variation (r =.49) in weaning weight of the calf I I 1 I I 1 50 175 200 225 250 275 Weaning weight, kg Figure 2. Logistic regression of probability of calving to the first AI service (Pi) on heifer weaning weight; Pi = 1 (SE61 =.0071). wt.)] 1 Figure 3. Response surface of mean milk production on heifer weaning weight and postweaning gain. (Table 5). This is within the range of 9% (Marston et al., 1992) and 77% (Totusek et al., 1973) previously reported and emphasizes the importance of milk production in beef cows. Similar to othereports (Totusek et al., 1973; Beal et al., 19901, milk component production did not aid explaining variation in calf weaning weight beyond that of mean milk production. Probability of reaching puberty before breeding as influenced by weaning weight and postweaning gain is shown in Figure 1. As weaning weight and postweaning gain increased, so did the probability for reaching puberty before the breeding season. Figure 2 demonstrates the importance of weaning weight on the probability of calving to the first-service AI. Increasing weaning weight from 150 to 275 kg increased the probability of calving to the first-service from 5.8 to 45.5%. Bergmann and Hohenboken ( 1992 reported that weaning weight was related curvilinearly to pregnancy of Angus heifers. The predicted probability of fertility reached a plateau at 210 kg and was maximized for heifers weighing 245 kg at weaning. Regressions of pregnancy rate on weaning weight and postweanin gain were not significant. Regression equations are given in Table 6 that describe heifer growth characteristics as influenced by weaning weight and postweaning weight gain. Weaning weight and postweaningain were positively correlated (R =.20 to.66) with subsequent weight, hip height, fat thickness, and pelvic area. Regression equations are given in Table 7 for calf growth characteristics and milk production of their dams. Mean milk production as influenced by weaning weight and postweaning weight gain i shown in

POSTWEANING GAIN OF BEEF HEIFERS 945 Table 7. Regressioncoefficient estimates ofcalf growth and milk production on heifer weaning weight and postweaning gain Regression coefficient estimatesa Postweaning Postweaning Weaning wt, kg x Variable n Intercept Calf d age, gain, kgid gain, kg/d3 postweaning gain, kg/d R2 Calf shrunk weight, kg 54-d -2.14.46 f.03*** -21.75 f 10.79* -.l8 f.05**.51 104-d 204 12.33.53 5.05*** - -.l1 +_.03***.39 153-d 202 37.93.53 f.07*** - -.l3 f.04***.28 214-d 201 86.39.36?.09*** - -.l4 f.05*.l3 Calf hip height, cm 214-d 201 84.23.073 f.02*** - -.041 f.04***.l3 Milk production, kg/d 54-d 4.92 -.021 f.005*** - -.01 +..003***.l6 104-d 204 3.31 -.011?.005* - -,0091 f.002***.09 153-d 202 3.80 -.021 k.005*** - 1.11 f.45* -.l2 Mean 3.85 -.017 f.003*** - -,0090 f.002***.l9 aregressor variables weaning weight and postweaning gain are correlated; r =.12, P <.01. *Accounts for variation explained by the model (P <.05). **Accounts for variation exdained bv the model P <.O 1 ). ***Accounts for variation explained by the model (P <,001). Figure 3. This graph reveals that as weaning weight and postweanin gain increased, subsequent milk production increased. Increasing weaning weight from 150 to 275 kg and postweaning gain from.20 to 1.00 kg/d doubled milk production from 2.3 to 4.6 kg/d. Several trials with dairy heifers have shown an inverse relationship between heifer average daily gain and subsequent milk production (Swanson, 1960; Sejrsen, 1978; Little and Kay, 1979). High rates of gain in dairy heifers causes a decrease in parenchyma (functional tissue of the mammary gland) growth (Sejrsen et al., 1982; Harrison et al., 1983), which has been implicated as the cause of reduced subsequent milk production. Heifers in the present study, being of a different biological type, may have been capable of greater daily gain without impairing milk production. Implications Increasing rate of postweaning gain of lightweight heifers up to 1 kg/d by supplementation results in earlier puberty and larger pelvic areas. 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