Animal Science Department

 

Date of this Version

January 1990

Comments

Published in J. Anim. Sci. 1990. 68:1877-1888.

Abstract

The potential of ovulation rate before 18 to 21 mo of age in puberal heifers as an indirect selection criterion for twinning rate was considered. Heritability (h2) was .07 ± .03 for single observations and .34 ± .18 for the mean of 7.9 estrous cycles per heifer. Estimated repeatability (r) of ovulation rate was ≤h2, indicating negligible permanent environmental effects. Expected h2 for mean ovulation rate (assuming h2 = r = .07 for single observations) for increasing numbers of estrous cycles would be as follows: 4, .23; 6, .31; 8, .38; and 10, .43. About 50% of the heifers produced no multiple ovulations, but 27% produced multiple ovulations in more than 15% of their estrous cycles. Ovulation rate varied seasonally and increased about .01 per month of age (P < .05). Genetic correlation of mean ovulation rate with adjusted 368-d weight was low (.08 ± .32). Ovulation rate in postpartum cows was higher (P < .05) in fall than in spring (1.15 vs 1.08). In postpartum cows, estimated h2 = .24 ± .13 and r = .17. Mean ovulation rate for postpartum cows was 1.12 vs 1.09 in puberal heifers, accounting in part for the higher h2. Pregnancy rate was higher (P < .O5) in multiple- than in single-ovulating cows. Effects of ovulation rate on embryonic survival were small (P > .05). Unilateral and bilateral multiple ovulations were not diffemt in embryonic survival. Accuracy of ovulation rate determination by palpation per rectum was lower in multiple- than in single-ovulating postpartum cows, because some unilateral multiple corpora lutea, especially, were recorded as singles. Results suggest that use of ovulation rate in puberal heifers should permit effective indirect selection for twinning rate among yearling heifers based on individual performance and among young sires based on ovulation rate of sibs and daughters.

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