Ovulation records have been collected by ovarian palpation per rectum at Waikite station from experimental cows intensely screened for twTnhThg and from two private herds with high twinning rate. The selected cows had a double ovulation rate of .30, .13 greater than among controls. Large differences among cows were found. The highest rate in mature cows (.40) was found among Friesians. Mean ovulation rates of cow groups recorded in two successive years were similar. Heifers born in the experiment had a higher double ovulation rate at 2 yr than at 15 to 18 mo of age. The possibility of a single gene effect cannot be discounted. A bimodal distribution of double ovulations was found in one paternal half sib group. A Milking Shorthorn herd has been found to contain especially high rates of double ovulation (.32) and twinning (.06) from eight sire groups. Cows with a history of twinning had higher double ovulation rates than those which had never twinned. A similar finding on ovulation was obtained in a Friesian herd with high twinning rate.

C. A Morris, A. M Day

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XI. Genetics of reproduction, lactation, growth, adaptation, disease, and parasite resistance., , 96–100, 1986
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