J. Anim Sci. 2007. 85:365-372. doi:10.2527/jas.2005-754
© 2007 American Society of Animal Science
Evaluation of reciprocal differences in Bos indicus x Bos taurus backcross calves produced through embryo transfer: I. Birth and weaning traits1
T. S. Amen,
A. D. Herring,
J. O. Sanders and
C. A. Gill2
Department of Animal Science, Texas A&M University, College Station 77843
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Abstract
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Angus (A) and Bos indicus (B; Brahman or Nellore) reciprocal backcross, embryo transfer calves, belonging to 28 full-sib families, were evaluated for differences in birth weight, gestation length, and weaning weight. Two methods were investigated; method I made no distinction between how the F1 parents were produced, whereas method II distinguished between the 2 types of F1 parents (AB vs. BA corresponding to A x B vs. B x A, respectively). Bos indicus backcross calves had a 4.3 d longer (P < 0.05) gestation length but did not differ in their average birth weight from A backcrosses. Among B backcrosses, B x F1 calves had a 5.2 d longer (P = 0.01) gestation length than F1 x B calves (290.5 vs. 285.3, respectively). Under method II analysis, there was a consistent trend for gestation length, in which BA F1 parents produced calves that ranked greater than calves from AB F1 parents, as sires and dams. Crosses with a greater proportion of B in the sire in relation to the amount in the dam had a heavier (P < 0.05) birth weight (F1 x A and B x F1; 38.1 and 38.4 kg, respectively) than their respective reciprocal crosses (A x F1 and F1 x B; 34.3 and 33.5 kg, respectively). The F1 x A and B x F1 crosses showed a large difference in birth weight between males and females (5.3 and 4.1 kg, respectively), whereas A x F1 and F1 x B crosses showed a small difference (P > 0.10) in birth weight between males and females (1.5 and 1.1 kg, respectively). Further examination within each sex showed a difference between male reciprocals that was generally much larger than that between female reciprocals. Calves with a greater percentage of B in the sire compared with the proportion in the dam ranked heavier for weaning weight as for birth weight, though these differences were not significant. In breeding systems involving B x Bos taurus crosses, even when using embryo transfer, not only does the breed composition of the calves affect their preweaning performance, but the particular cross that produces the calves also should be considered in making breeding decisions.
Key Words: beef cattle • birth weight • embryo transfer • gestation length • reciprocal cross • weaning weight
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INTRODUCTION
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Adaptation characteristics of Bos indicus (B) cattle make their inclusion in breeding programs appropriate, especially in the southern United States and other tropical and subtropical regions. Additionally, greater levels of heterosis exist in B x Bos taurus crosses than in crosses that only involve B or Bos taurus breeds. Research has shown that the manner in which B x Bos taurus crossbreeding programs are designed is extremely important (Roberson et al., 1986
; Thallman et al., 1993) due to a noticeable difference in the performance of reciprocal cross (B bulls bred to Bos taurus cows vs. Bos taurus bulls on B cows) calves for several traits of importance. In a study of Brahman and Hereford reciprocal cross calves, Ellis et al. (1965) found F1 Brahman x Hereford calves to be 8.9 kg heavier at birth than F1 Hereford x Brahman calves. In the same evaluation, the birth weight (BWT) differences between reciprocal backcrosses were found to be approximately half as large as the difference between F1 reciprocal crosses.
The objective of this research was to verify and expand on studies previously done concerning the performance of B x Bos taurus reciprocal cross calves. Using data collected from the Texas A&M University Angleton project, the aim was to better characterize differences in reciprocal cross performance for gestation length (GL), BWT, and weaning weight (WWT) in cattle that have been produced through embryo transfer (ET).
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MATERIALS AND METHODS
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All procedures involving animals were approved by the Texas A&M University Institutional Animal Care and Use Committee.
Birth weight (n = 511), GL (n = 509), and WWT (n = 499) were evaluated for Angus (A) x B reciprocal backcross ET calves. Sixteen purebred and F1 sires and 17 purebred and F1 dams representing A and B (Brahman or Nellore) breeds were crossed using 8 different mating strategies (Table 1) to produce 28 full-sib families, with the resulting progeny being 3/4 A 1/4 B or 3/4 B 1/4 A.
Calves were born in the spring and fall seasons of 1990 to 1996 at the Texas Agricultural Experiment Station at Angleton to recipient dams from 3 to 17 yr of age that were approximately 1/2 Brahman and 1/2 British breeds. Donor cows and recipients were synchronized, and donors superovulated with gonadotropins at a commercial facility, according to typical industry standards. Donor cows were bred 3 times by AI after the expression of estrus. Seven days after standing estrus, embryos were nonsurgically flushed and prepared for transfer to recipients. Bull calves were castrated at birth, and all calves were weaned at approximately 7 mo of age.
Statistical Methods
Birth weight, GL, and WWT data were studied through analysis of covariance using the MIXED procedure (SAS Inst. Inc., Cary, NC). Independent variables treated as fixed effects included sex, breed type (BT), sire type (ST) x dam type (DT) interaction nested within BT [ST x DT(BT)], the 3-way interaction of sex x ST x DT nested within BT [S x ST x DT (BT)], embryo quality (EQ), and embryo stage (ES). Weaning weight analysis included weaning age in days as a covariate. Preliminary analysis showed that age of recipient did not significantly affect calf traits, and it was therefore left out of the final analyses. Initial analysis for WWT included EQ and ES; however, neither was statistically important, so both were excluded from subsequent analyses.
For all analyses, sire, dam, and season nested with birth year were included as random effects. When an F-test was significant for a fixed effect, least squares means were separated by 2-tailed t-tests.
Sire type and DT were investigated using 2 methods of analysis. Method I pooled AB and BA parents into a single F1 category, whereas method II separated F1 parents into AB or BA categories (AB denotes F1 produced from A sires, BA denotes F1 produced from B sires). The number of calves represented for each family type is presented in Table 1
. Linear contrasts, as described by Steel and Torrie (1980), were utilized to evaluate the consistency of the calf sex effect on BWT within each reciprocal backcross through t-tests.
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RESULTS AND DISCUSSION
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Gestation Length
Results from the statistical analyses for GL as well as BWT and WWT are shown in Tables 2 (method I) and 3 (method II). Sex, BT, and ST x DT (BT) accounted for variation in GL for method I and method II. Gestation length was 286.8 ± 1.3 d for males, and 284.8 ± 1.3 d for females; A backcrosses averaged 283.6 ± 1.5 d, whereas B backcrosses averaged 287.9 ± 1.4 d.
Reciprocal cross differences for method I GL are presented in Figure 1. Gestation length among A backcross calves, was 282.7 ± 1.71 d for A x F1 calves and 284.6 ± 1.87 d in F1 x A calves (P = 0.365). Among B backcrosses, GL was 290.5 ± 1.65 d for B x F1 calves, which was 5.2 d longer (P = 0.01) than for F1 x B calves (285.3 ± 1.77 d). There was a consistent trend for longer gestations when the sire had a larger proportion of B, compared with the proportion of B in the dam.
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Figure 1. Least squares means (d) and SE for gestation length (method I), where A= Angus, and B = Bos indicus. The mean is given above each bar.
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Figure 2 shows GL differences for reciprocal cross parents using method II. Gestation length was 2.4 d longer (P = 0.16) for A backcross calves sired by A bulls (283.9 ± 1.84 d) when BA was the dam type than for calves with AB dams (281.5 ± 1.85 d). Calves sired by F1 bulls out of A dams had an average GL of 285.8 ± 2.02 d when the sire was BA as opposed to AB (283.5 ± 2.04 d; P = 0.21). For B-sired, B backcross calves, GL was 292.6 ± 1.89 d in calves from BA dams compared with 289.0 ± 1.76 d for those from AB dams (P = 0.04). For calves from F1 sires and B dams, GL was 286.6 ± 1.95 d in calves from BA sires, whereas calves from AB sires had an average GL of 284.2 ± 2.01 d (P = 0.23). In the A and B backcross calves, a trend for longer GL was exhibited when the F1 parent was BA as opposed to AB, although the difference was not statistically significant in all cases. It is important to note that BA F1 parents (all 3 sires and 2 out of 5 dams) had more Nellore influence than AB parents (4 sires and 4 dams), which had only Brahman influence. Nellore-influenced calves typically have a longer GL than calves influenced by other B breeds (Paschal et al., 1991; Thrift, 1997).
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Figure 2. Least squares means (d) and SE for gestation length (method II). For F1 sires and dams (AB and BA), the first letter denotes sire-type and the second letter denotes dam-type; A = Angus, and B = Bos indicus. The mean is given above each bar.
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The finding of shorter GL in calves whose dams had a greater percentage B in relation to the amount of B in the sire is consistent with results from a study of nonET calves by Reynolds et al. (1980). In reciprocal F1 A x Brahman crosses, Brahman x A calves were carried 2.6 d longer than A x Brahman. Lemos et al. (1984) reported similar results in Holstein x Guzerat cross nonET calves. Males and females were analyzed separately; however, similar results were observed for both sexes. The longest GL for both sexes was seen in calves sired by Guzerat bulls and out of F1 Holstein x Guzerat cows (290 and 288 d for male and female calves, respectively). Furthermore, reciprocal differences between sexes were analyzed by Thallman et al. (1993) in ET F1 crosses involving Brahman and Simmental. Among females, Brahman x Simmental calves were carried 2.3 d longer than Simmental x Brahman, whereas Brahman x Simmental males were carried 1.6 d longer than Simmental x Brahman males.
Birth Weight
Calf sex, the 2-way interaction of sire type x dam type nested within breedtype, the 3-way interaction of sire type x dam type x sex nested within breedtype, and ES accounted for significant differences in BWT for methods I (Table 2) and II (Table 3) analyses.
Male calves averaged 37.6 kg at birth, and female calves averaged 34.6 kg. This 3.0 kg difference between sexes is consistent with numerous other reports of sire breed x calf sex interactions on BWT, where large sex differences were found in natural service B x Bos taurus crossbred calves (Cartwright et al., 1964; Notter et al., 1978; Lemos et al., 1984). Sexual dimorphism tends to be greater in B sired calves out of Bos taurus dams with reported differences between bull and heifer calves ranging from 4.4 to 7 kg (Notter et al., 1978; Lemos et al., 1984; Paschal et al., 1991), compared with differences of only 2.5 to 2.7 kg between sexes for Bos taurus sired crossbred calves (Comerford et al., 1987; Paschal et al., 1991).
Calves from family types with a greater proportion B in the sire in relation to the amount in the dam had heavier BWT than their respective reciprocal crosses (Figure 3). For A backcrosses, A x F1 calves were 3.8 kg lighter (P = 0.03) than F1 x A calves (34.3 ± 1.54 kg vs. 38.1 ± 1.90 kg, respectively). Among B backcrosses, B x F1 calves averaged 38.4 ± 1.52 kg, which was 4.9 kg heavier (P = 0.003) than the F1 x B average of 33.5 ± 1.74 kg. Method II analysis also showed ST x DT (BT) to be responsible for variation in BWT. The only substantial difference occurred in A backcross calves from F1 sires. Calves with AB sires averaged 2.7 kg heavier than BA sired calves (P = 0.008). No other differences were found when comparing AB vs. BA F1 parental types (Figure 4).
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Figure 3. Least squares means (kg) and SE for birth weight (method I), where A = Angus, and B = Bos indicus. The mean is given above each bar.
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Figure 4. Least squares means (d) and SE for birth weight (method II). For F1 sires and dams (AB and BA), the first letter denotes sire type and the second letter denotes dam type; A = Angus, and B = Bos indicus. The mean is given above each bar.
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Variation in BWT for method I was also attributed to the 3-way interaction of sex x ST x DT nested within BT (Figure 5). Among A backcross calves from A sires and F1 dams, the BWT difference was 1.6 kg between male and female calves (P = 0.102), but when A backcross calves were produced from F1 sires and A dams the difference in BWT between males and females was 5.3 kg (P < 0.001). This difference between males and females (5.3 vs. 1.6 kg) was greater for F1-sired calves than for A-sired calves (P < 0.05). For B backcross calves produced from B sires and F1 dams, male calves averaged 4.1 kg heavier than females (P < 0.001), whereas when B backcross calves were produced from F1 sires and B dams the difference was only 1.1 kg (P = 0.340). Here, the difference between males and females was greater (P < 0.05) for B-sired calves than for F1-sired calves (4.1 vs. 1.1 kg). Calves from family types with a greater proportion B in the sire in relation to the amount in the dam (B x F1, F1 x A) showed a large BWT difference, whereas the 2 reciprocal crosses (F1 x B, A x F1) showed a small (1.1 and 1.6 kg) BWT difference (P > 0.10) between male and female calves.
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Figure 5. Least squares means (kg) and SE of birth weight (method I) for sex x sire type x dam type combinations, where A = Angus, and B = Bos indicus. The mean is given above each bar.
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Examination of reciprocal differences within each calf sex showed a difference between male reciprocals that was much larger than the complementary female comparison (Figure 6). Male F1 x A calves were 5.7 kg heavier than A x F1 male calves (P = 0.003), whereas the same comparison between female calves displayed only a 1.9 kg difference (P = 0.308). Among B backcross individuals, the B-sired male calves were 6.5 kg heavier (P < 0.001) than the F1-sired male calves, but the reciprocal female calves exhibited less difference with only 3.5 kg separating the 2 types (P = 0.06).
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Figure 6. Least squares means (kg) and SE within sex for birth weight (method I), where A = Angus, and B = Bos indicus. The mean is given above each bar.
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Sex differences involving B x Bos taurus crosses had been reported previously in nonET calves by Notter et al. (1978), Bailey and Moore (1980), and Lemos et al. (1984). In each of these studies, the average sex difference in BWT was greater for B-sired calves than for Bos taurus-sired calves. This is also consistent with results from Paschal et al. (1991), who reported a breed x sex interaction in a study of 5 different B sire breeds. In that study, A-sired male calves averaged 2.6 kg heavier than females, whereas B-sired males ranged from 4.4 to 7.0 kg heavier than females when these sire types were bred to Hereford cows. Similarly, Herring et al. (1996) reported that F1 Brahman-, Boran-, and Tuli-sired male calves were 5.9, 4.5, and 2.8 kg heavier, respectively, at birth than females when born to Hereford and A cows.
A 3-way interaction of sex x ST x DT nested within BT existed under method II also. Among A backcross calves with F1 sires, male calves with AB sires were 8.5 kg heavier than female calves of this cross (P < 0.001), whereas BA-sired males calves averaged only 1.5 kg heavier than females of that cross (P = 0.29). The sex difference (P < 0.05) in AB- vs. BA-sired calves (8.5 kg vs. 1.5 kg) was large. In relation to everything else, bull calves with AB sires and A dams were particularly large. Reasons for these extremely large differences are not clear but potentially could involve the interaction of A X and A Y chromosomes with some major gene from the B. Angus backcross calves from F1 dams exhibited a small difference in BWT between male and female calves (P > 0.10). Likewise, there was a small difference (P > 0.25) between male and female B backcross calves from F1 sires. However, B-sired B backcross males were 4.1 kg (P = 0.001) and 4.0 kg (P = 0.010) heavier than females when the dam was AB and BA, respectively (Figures 7 and 8).
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Figure 7. Least squares means (kg) and SE of birth weight by sex in A-backcross calves (method II). For F1 sires and dams (AB and BA), the first letter denotes sire type and the second letter denotes dam type; A = Angus, and B = Bos indicus. The mean is given above each bar.
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Figure 8. Least squares means (kg) and SE of birth weight by sex in Bos indicus-backcross calves (method II). For F1 sires and dams (AB and BA), the first letter denotes sire type and the second letter denotes dam type; A = Angus, and B = Bos indicus. The mean is given above each bar.
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Within sex analysis of reciprocal crosses (Figures 9 and 10) showed AB x A male calves outweighed A x AB male calves by 8.2 kg (P < 0.001), whereas AB x A female calves were only 1.5 kg heavier than A x AB females (P = 0.523). When BA was the F1 parental type, only 2.9 kg separated 3/4 A male calves and 2.6 kg separated female calves; again, calves from the cross with more B in the sire (BA x A) were heavier than those from the reciprocal cross (P > 0.10). These observed differences would be consistent with Y chromosome differences between A and B in the presence of an A X chromosome. However, this seems to be contradictory to what is seen in F1 calves with B sires and British dams where the bull calves are extremely heavy.
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Figure 9. Least squares means (kg) and SE for birth weight for method II in male calves. For F1 sires and dams (AB and BA), the first letter denotes sire type and the second letter denotes dam type; A = Angus, and B = Bos indicus. The mean is given above each bar.
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Figure 10. Least squares means (kg) and SE for birth weight for method II in female calves. For F1 sires and dams (AB and BA), the first letter denotes sire type and the second letter denotes dam type; A = Angus, and B = Bos indicus. The mean is given above each bar.
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Reciprocal differences also existed among 3/4 B male calves. Bos indicus x AB males averaged 6.7 kg heavier than AB x B males (P = 0.004), and B x BA males averaged 6.4 kg heavier than BA x B (P = 0.008). Among 3/4 B female calves, B x AB calves were 4.2 kg heavier than AB x B calves (P < 0.077) and B x BA females were 3.1 kg heavier than BA x B calves (P = 0.176).
No significant differences in BWT were detected when comparing reciprocal F1 parents (AB x A vs. BA x A, etc.), with the exception of A-backcross males with F1 sires. In this case, calves with AB sires averaged 6.2 kg heavier at birth than calves with BA sires (P < 0.001). However, as noted earlier, the BA sires had more Nellore influence than the AB sires; also, the A cows that were used to produce the Nellore-sired BA parents were smaller at maturity than the A that were used to produce the other crosses.
The B x Bos taurus reciprocal differences documented here in ET calves have been widely reported in nonET calves, and are also consistent with earlier reports in ET calves [with the exception of 3/4 A calves with BA F1 parents; although these calves with F1 sires were still heavier than those with F1 dams, the difference was small (P = 0.205)]. Reynolds et al. (1980) found A x Brahman natural service calves to average 5.0 kg lighter than Brahman x A calves. Roberson et al. (1986) found similar differences in Hereford and Brahman backcross calves produced by natural service. Backcross calves from Brahman sires and F1 dams were approximately 5.0 kg heavier than calves from F1 sires and Brahman dams. Hereford backcross calves from F1 sires and Hereford dams were about 5.1 kg heavier than Hereford-sired calves.
Similar trends were also reported by Thallman et al. (1993) in Brahman x Simmental reciprocal cross ET calves. Brahman x Simmental male calves averaged 13.7 kg heavier than Simmental x Brahman male calves, whereas Brahman x Simmental female calves were 7.7 kg heavier than Simmental x Brahman females. In the current study, the magnitude of the difference between males and females was similar to that of Thallman et al. (1993) in F1 calves. In each case, the difference between reciprocal male calves was nearly twice as large as the difference between reciprocal female calves. Likewise, Baker et al. (1989) found that Brahman x Hereford ET calves were 7.4 kg heavier than Hereford x Brahman calves when both were produced from Brahman recipients. When Hereford recipients were used, Brahman x Hereford calves were 11.9 kg heavier than their Hereford x Brahman contemporaries. Also Brahman x Hereford calves from Brahman recipients were 7.4 kg lighter than Brahman x Hereford calves from Hereford recipients. Similarly, Hereford x Brahman calves from Brahman recipients were 2.6 kg lighter than Hereford x Brahman calves from Hereford recipients. There is a trend, based on all these findings, for calves with a greater percentage of B in the sire (in relation to the amount of B in the dam) to have heavier BWT, even when produced through ET. As a result, this difference can not simply be due to a uterine maternal effect, although the exact genetic mechanisms remain to be determined.
Weaning Weight
Weaning age and calf sex were the only effects that accounted for significant variation in WWT (Tables 2 and 3). The differences observed between reciprocal crosses followed a trend similar to BWT because calves with a greater percentage of B in the sire compared with the proportion in the dam still ranked heavier, on average, at weaning (Figure 11). However, the difference between reciprocal crosses had not grown proportionately with the weight gain of the calves, so the differences detected were not statistically important. Among A backcross calves, F1-sired calves averaged 235.2 ± 5.6 kg and A-sired calves averaged 229.8 ± 4.8 kg. Among B backcrosses, B-sired calves averaged 236.1 ± 4.8 kg and F1-sired calves averaged 227.6 ± 5.5 kg (all P > 0.20).
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Figure 11. Least squares means (kg) and SE for weaning weight for method I, where A = Angus and B = Bos indicus. The mean is given above each bar.
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The differences between the sexes (male minus female) for A x F1, F1 x A, B x F1, and F1 x B were 9.8, 18.2, 21.1, and 16.8 kg, respectively. Similar to the BWT results, less difference existed between the sexes when the dam had more B influence (in relation to the amount in the sire). The F1 x A steers averaged 9.6 kg heavier than A x F1 steers, whereas no difference existed between the reciprocal cross females at weaning. The B x F1 steer calves outweighed the F1 x B steers by 10.6 kg, whereas the difference between these types of females was 6.3 kg. As with BWT, there is a larger difference in WWT between male reciprocal crosses than their female counterparts, though none of these differences were statistically important at this weight phase.
Differences associated with one F1 parent over the other were not apparent at this weight phase (Figure 12). In crosses in which the dam had more B influence than the sire (A x AB, A x BA, AB x B, and BA x B), there was no difference in the weight of calves at weaning. For crosses where the sire possessed more B influence than the dam (AB x A, BA x A, B x AB, and B x BA), the AB parent tended to produce heavier weaning calves (all P > 0.15). It may be important to note that the size of the difference between reciprocal crosses reported here for WWT is similar to differences reported as statistically significant in other studies.
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Figure 12. Least squares means (kg) and SE for weaning weight for method II. For F1 sires and dams (AB and BA), the first letter denotes sire type and the second letter denotes dam type; A = Angus, and B = Bos indicus. The mean is given above each bar.
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In a study of F1 reciprocal, nonET calves, Comerford et al. (1988) showed a significant sire breed x dam breed interaction. In crosses involving Simmental, Brahman, Polled Hereford, and Limousin, calves with Brahman dams outweighed their reciprocal for each breed combination (Simmental x Brahman 250.3 kg vs. Brahman x Simmental 249.5 kg, Limousin x Brahman 241.4 kg vs. Brahman x Limousin 232.0 kg, Hereford x Brahman 237.0 kg vs. Brahman x Hereford 215.5 kg). This also is consistent with nonET results from Peacock et al. (1978) involving A, Brahman, and Charolais, where F1 calves from Brahman dams outweighed their reciprocals at weaning (202.8, 198.5, 229.7, and 216.3 kg for A x Brahman, Brahman x A, Charolais x Brahman, and Brahman x Charolais, respectively). Similarly, Roberson et al. (1986) reported that F1 Brahman x Hereford dams weaned the heaviest calves, Brahman dams weaned heavier crossbred calves than Hereford dams, and purebred Brahman and Hereford calves were the lightest at weaning. It seems obvious that the postnatal maternal effect of the natural dam was the reason for the greater WWT of nonET calves with Brahman dams in these studies.
Klindt and Maurer (1986) reported an interaction of sex and reciprocal cross in F1 Red Poll x A ET calves that were raised on milk replacer. Angus-sired steers averaged 43 kg heavier than females at 150 d, whereas Red Poll-sired steers averaged only 1 kg heavier than females. Also, A x Red Poll steers were 33 kg lighter than Red Poll x A steers, but A x Red Poll females averaged 9 kg heavier than Red Poll x A females. Thallman et al. (1993) reported reciprocal differences at weaning for Brahman x Simmental ET calves born to Holstein recipients. Brahman-sired calves were heavier than Simmental-sired calves for both sexes; Brahman x Simmental bull calves averaged 46.5 kg heavier than Simmental x Brahman bulls; and Brahman x Simmental females averaged 23.7 kg heavier than Simmental x Brahman female calves. This is the same direction as the differences for BWT in their study and is consistent with the direction of the differences found in the current study.
Mitochondrial inheritance, genomic imprinting, Y-linked inheritance, or X-linked inheritance with non-random x inactivation were listed as possible causes of the reciprocal differences reported by Thallman et al. (1993). Herring and Sanders (1991) found no significant effect of the pedigree source of Y chromosome (B vs. Bos taurus) on BWT among natural service and ET Brangus bulls. Rohrer et al. (1994) analyzed BWT for 7,353 registered Brangus calves with breed of cytoplasmic origin fitted as a fixed effect. There was no evidence for an effect of mitochondrial inheritance on performance differences between animals. Gill et al. (2005) hypothesized that the reciprocal differences reported here may be due to a sex chromosome x autosome interaction and named XIST (X-inactive specific transcript) as a likely candidate for playing a role in the observed differences.
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IMPLICATIONS
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The breed advantages of British, Continental, and Bos indicus cattle have been studied at length, as has the benefit of crossing Bos indicus and Bos taurus breeds. This research suggests that in breeding systems in which Bos indicus and Bos taurus are used, not only does the breed composition of the calves affect their ultimate performance, but the particular cross that produces the calves does as well, even in embryo transfer calves, and particularly for gestation length and birth weight. Insight into the potential breed and sex effects and their interactions on calf performance would greatly assist producers in making improved breeding decisions.
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Footnotes
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1 Financial support for the resource population and phenotypic data collection were provided in part by the Texas Agricultural Experiment Station. 
2 Corresponding author: clare-gill{at}tamu.edu
Received for publication December 22, 2005.
Accepted for publication September 8, 2006.
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LITERATURE CITED
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Abstract
INTRODUCTION
