Heritability of Reproductive Performance in Inbred and Linecross Beef Cows

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Abstract
Selection differentials on calf crop were found to be low for the cows at the Colorado San Juan Basin Station. The amount of selection pressure exerted at different ages ranged from 0 to 10.5%. The culling of open cows on fertility was not as great with the younger cows as with the older cows. Sires of dams, years of birth of dams, and ages of dams, when subjected to chi-square analysis, showed significant (P<.01) effects on percent of live calves born. There was an increase in percent calf crop with cow age up through 10 and 11 years. However, at these older ages numbers were too small to give definite conclusions. Inbreeding had a marked effect on calf crop when all ages of cows were studied. However, upon removal of the records of 2-year-old cows there was no significant difference between the inbred and linecross cows. This is consistent with the evidence that inbreeding effects are more pronounced in younger animals. Heritability of reproductive performance as measured in this beef herd was low. The respective heritability estimates found for contemporary cows were 0.15±0.09 and 0.14±0.12 for the inbreds and linecrosses. The ability of a cow to repeat her past performance was also low. The average repeatability was 0.08 on a contemporary basis. The results of this study indicate that reproductive performance will not respond greatly to selection.