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Grading and Rotational Crossbreeding of Beef Cattle. III. Postweaning and Carcass Traits of Steers^1,2,

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

Abstract

Steers in this study were the product of three generations (Gl, G2, G3) of grading and rotational crossbreeding. Steers were sired by purebred Angus (A), Polled Hereford (PH) and Santa Gertrudis (SG) bulls and out of grade, two-breed and three-breed rotational cross cows (seven herds) of these breeds. The steers, which were not creep fed during the nursing period, were grown from weaning to slaughter entirely on forage, primarily oats and millet pasture. Herd differences, heterosis and various genetic components were evaluted for their effect on postweaning and carcass traits. During each generation, the average daily gains (ADG) of PH and SG exceeded A while those of PH and SG were similar. Also, during each generation, SG exceeded PH which exceeded A for final weight, carcass weight and carcass length. The A and SG additive and maternal breed effects were expressed as deviations from PH additive and maternal effects. For ADG, the additive effects for A were negative each generation, but significant only during G2, while those for SG were significant only during Gl. For final and carcass weight, additive effects for A increased in negative value and significance, while those for SG were positive but decreased in value as generations advanced. Heterozygotic effects due to breed crosses and breed maternal effects generally were not important for any of the traits studied. Average maternal heterosis effects were positive and significant for five important traits during G2 and positive, large but not significant for final and carcass weight in G3. Significant differences between rotations within herd occurred in the PH-SG and A-PH-SG rotational crosses for final and carcass weight. For the PH-SG rotational cross, SG-sired calves exceeded PH-sired calves during Gl and G2; and for the A-PH-SG rotational cross, PH-and SG-sired calves were similar and both exceeded A-sired calves during G2 and G3. Results indicated that the aggregate effect of specific maternal heterosis, epistasis, linkage and possibly other genetic effects had a significant affect on daily gain, final weight, carcass length, rib eye area and rib eye fat thickness.

¹ The authors appreciate the cooperation of personnel at the Georgia State Prison, Reidsville, which permitted the conduct of this research. We also acknowledge the help of T. M. Clyburn and J. B. Smith, employees of the Anim. Sci. Dept. Stationed at Reidsville, who carefully supervised the management of the cattle and collected most of the data. Further acknowledgement is made to B. L. Southwell and H. D. Chapman, former Anim. Sci. researchers, for their contributions.

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

³ Dept. of Anim. Sci.

⁴ Agr. Res. Statistician.

⁵ Formerly Head, Dept. of Anim. Sci.; currently Assoc. Director Exp. Sta. and Resident Director Coastal Plain Exp. Sta., Tifton.