| Abstract: | The first phase of this study was the production of contemporary straightbred (SB) and reciprocal crossbred (F1) bulls by mating Angus (A) bulls to A and Santa Gertrudis (SG) cows and SG bulls to SG and A cows. Of the bulls produced during the 4-yr period, those used for breeding included 15 A, 15 SG, 8 A X SG and 8 SG X A. For 205-d weight and weight/day of age (W/DA) postweaning, A X SG had higher (P less than .05) performance than SG X A bulls and SG had higher (P less than .05) performance than A bulls. There was no difference (P greater than .05) between SG and A X SG for 205-d weight, postweaning average daily gain (ADG) or postweaning W/DA. Heterosis estimates were 5.2 (P less than .10), 9.9 (P less than .01) and 5.8% (P less than .01) for 205-d weight, postweaning ADG and W/DA, respectively. The second phase of this study was the comparison of SB and F1 bulls for reproductive and progeny performance by exposing them as yearlings to 25 Polled Hereford cows each. There were no differences (P greater than .05) among the four sire groups for proportion of cows exposed that had a calf, had a live calf or weaned a calf. Reproductive performance of sires also was evaluated in terms of number of days (NOD) from the beginning of the breeding period until calves were born. The NOD for calves by SG was greater (P less than .05) than for calves by A or F1 sires. Compared with calves from SB sires, the NOD for calves from A X SG and SG X A sires were 5.0 and 10.6 d (P less than .05) lower. Differences among sires within year and breeding of sire were significant for all preweaning traits and for W/DA postweaning of their progeny. The SG-sired calves were heavier (P less than .05) for birth and 205-d weight and had higher (P less than .05) postweaning ADG than A-sired calves. Mean performance of calves by reciprocal F1 sires did not deviate (P greater than .05) from the mean performance of those by SB sires. Calves by A X SG sires had higher (P less than .05) 205-d weight, postweaning ADG and W/DA than calves by SG X A sires. Results indicated that the primary genetic effects responsible for differences in performance of calves sired by F1 vs SB bulls were mean transmitted and mean heterotic effects.(ABSTRACT TRUNCATED AT 400 WORDS) |
