Evaluation of reciprocal differences in Bos indicus × Bos taurus backcross calves produced through embryo transfer: I. Birth and weaning traits1

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 F^sub 1^ parents were produced, whereas method II distinguished between the 2 types of F^sub 1^ parents (AB vs. BA corresponding to A × B vs. B × 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 × F^sub 1^ calves had a 5.2 d longer (P = 0.01) gestation length than F^sub 1^ × B calves (290.5 vs. 285.3, respectively). Under method II analysis, there was a consistent trend for gestation length, in which BA F^sub 1^ parents produced calves that ranked greater than calves from AB F^sub 1^ 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 (F^sub 1^ × A and B × F^sub 1^; 38.1 and 38.4 kg, respectively) than their respective reciprocal crosses (A × F^sub 1^ and F^sub 1^ × B; 34.3 and 33.5 kg, respectively). The F^sub 1^ × A and B × F^sub 1^ crosses showed a large difference in birth weight between males and females (5.3 and 4.1 kg, respectively), whereas A × F^sub 1^ and F^sub 1^ × 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 × 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. [PUBLICATION ABSTRACT]