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Some Causes of Variation in Post-Partum Reproductive Activity in Hereford Cows¹

University of Wisconsin, Madison

Abstract

Analysis of variance of the component intervals of 61 service periods from four pairs of identical and eight pairs of fraternal Hereford twins showed interval to uterine involution to be significantly shorter in the first than in the second service period whereas the intervals to first ovulation and first heat were not significantly affected by parity.

A comparison of the variances within the two types of twin pairs showed that variation in involution rate was significantly less between identical twins than between fraternal twins. This same tendency was present but not statistically significant for the intervals to first ovulation and first estrus.

A non-significant correlation of 0.14 was found between intervals to involution and to first heat. The correlation between interval to involution and conception rate at first estrus and the corresponding standard partial regression were non-significant (r=0.08 and b=0.02). Conception rate at first service was significantly affected by interval to first estrus (r=0.40, P<0.05; b'=0.40, P<0.01). All animals were bred at first heat after calving.

¹ Paper from the Department of Genetics No. 750 and the Department of Animal Husbandry, published with the approval of the Director of the Agriculture Experiment Station.