Estimated Genetic Improvement in Weaning Weight of Beef Cattle

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Abstract
Weaning weight records collected at the San Juan Basin Research Center, Hesperus, from 1946 through 1971 were used to evaluate phenotypic, environmental and genetic trends in inbred and linecross Hereford populations. Records on 1,534 inbred and 1,874 linecross calves representing 14 inbred and 14 corresponding linecross groups were used. Inbreeding coefficients averaged 33.1 and 21.8% for inbreeding of calf and dam over the 26-year period in the inbred population. Phenotypic regressions of adjusted weaning weight on years were −.346, .355 and .575 kg/year for inbred, inbred adjusted for inbreeding of calf and dam, and linecross groups, respectively. The environmental trend was estimated from repeat mating information and a regression value of −1.520 kg/year was obtained. Estimated genetic regressions obtained by subtracting the environmental regression from the phenotypic regressions were 1.174, 1.874 and 2.094 kg/year for the same three groups. Estimates of genetic trends include the effects of selection within and between lines since six of the 14 lines were terminated during the study period. Plotting genetic trends by years indicated that most of the improvement occurred during the last two-thirds of the time period studied. Genetic response was also obtained by line and linecross groups in a similar manner and indicated variable response by line. Averages of the 14 genetic regressions within lines were .569, 1.575 and 1.873 kg/year for the three groups. These values are smaller than estimates from the overall analyses and should more nearly reflect expectation without culling of poorer lines. Estimated genetic changes in the remaining eight line and linecross groups should reflect the past performance of the present herd. Average regression values over these eight line and linecross groups were 1.245, 2.035 and 2.606 kg/year, respectively, for the three groups. It appears that a breeding system combining selection within and between lines with inbreeding produces fairly rapid improvement. However, more traits in this herd and more studies need to be evaluated. Copyright © 1976. American Society of Animal Science . Copyright 1976 by American Society of Animal Science.

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