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Evaluation of Phenotypic and Genetic Trends in Weaning Weight in Angus and Hereford Populations in Virginia¹

Virginia Polytechnic Institute and State University³, Blacksburg 24061

Abstract

Weaning weight records of 27,774 Angus calves in 13 herds and 14,738 Hereford calves in 11 herds born during 1953 through 1983 in Virginia were analyzed using regression techniques and maximum likelihood (ML) procedures to estimate phenotypic and genetic trends for adjusted weaning weight (AWWT), weaning weight ratio (WWR) and deviations of AWWT from the mean AWWT of the contemporary group (DEV). Phenotypic trends for AWWT in the Angus and Herefords were .96 ± .02 and .82 ± .03 kg/yr, respectively. In the Angus breed, estimates of one-half of the sire genetic trend obtained from the ML procedure for WWR and DEV were .40 ± .04 ratio units/ yr and .72 ± .07 kg/yr, respectively; corresponding values for Herefords were .25 ± .06 ratio units/yr and .45 ± .12 kg/yr. Estimates of one-half of the dam trends for the respective traits were .32 ± .02 ratio units/yr and .55 ± .04 kg/yr for Angus and .21 ± .03 ratio units/yr and .30 ± .07 kg/yr for Herefords. Estimates of sire and dam genetic trends from the regression analyses were slightly higher than estimates from the ML procedure, but adjustments to eliminate bias due to non-random mating and culling from the regression analyses increased the similarlity of the results from the two procedures. Average annual genetic trends over the entire study period from the ML procedure for AWWT were 1.27 kg/yr for Angus and .75 kg/yr for Herefords. Genetic trends were not linear over the entire period. Total genetic trends in AWWT for Angus and Hereford, respectively, were .30 and –.61 kg/yr before 1971 and 2.18 and 1.98 kg/yr after 1970.

¹ The authors express their appreciation to the Virginia Beef Cattle Improvement Assoc, the Amer. Angus Assoc, and the Amer. Polled Hereford Assoc, for providing the data used in this study.

² Present address: Dept. of Anim. Sci., Univ. of Guelph, Guelph, Ontario, Canada N1G 2W1.

³ Dept. of Anim. Sci.