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Comparative Performance of Straightbred and Crossbred (F₁) Sires^1,2,

University of Georgia College of Agriculture, Coastal Plain Station, Tifton 31793-0748

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<.05) performance than SG x A bulls and SG had higher (P<.05) performance than A bulls. There was no difference (P> .05) between SG and A x SG for 205-d wieght, postweaning average daily gain (ADG) or postweaning W/DA. Heterosis estimates were 5.2 (P<.10), 9.9 (P<.01) and 5.8% (P<.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>.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<.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<.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<.05) for birth and 205-d weight and had higher (P<.05) postweaning ADG than A-sired calves. Mean performance of calves by reciprocal F₁ sires did not deviate (P>.05) from the mean performance of those by SB sires. Calves by A x SG sires had higher (P<.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. There were, however, indications that additional effects were involved for some traits.

¹ The authors appreciate the cooperation of personnel at the Georgia State Prison, Reidsville, which permitted the conduct of the research. The authors acknowledge the help of J. B. Smith, deceased, who was an employee of the Anim. Sci. Dept. stationed at Reidsville and who carefully supervised the management of the cattle. Also, the authors gratefully acknowledge the assistance of Mr. B. G. Mullinix, Jr. for conducting the statistical analyses.

² Supported by State, Hatch and Regional S-10 funds.

³ Dept. of Anim. Sci.

⁴ Formerly Head, Dept. of Anim. Sci.; currently Assoc. Director Exp. Sta. and Resident Director Coastal Plain Exp. Sta., Tifton.