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Heritability of the Length of the Gestation Period in Dairy Cattle¹

Department of Dairy Science, University of Illinois, Urbana

ABSTRACT

The data for this study consisted of 2,063 gestations representing the five major breeds of dairy cattle in the University of Illinois herd. These data were analyzed to determine the heritability and the variance of the length of the gestation period, so as to indicate the possibility of shortening the duration of pregnancy in dairy cattle. A comparison of the fiducial limits of the duration of pregnancy of each of the five breeds indicated breed differences in all possible comparisons, except between Jerseys and Holstein-Friesians. The effect of sex of fetus and the variance components for service sire and sire of the dam were estimated. The effects of the sex of the fetus of Ayrshires, Holstein-Friesians, and Jerseys were statistically significant and, in general, the male calves were carried about 1.5 days longer than the females.

Heritability was estimated by multiplying the paternal half-sib correlation by four and by doubling the regression of offspring on dam. The paternal half-sib estimate of heritability refers to gestation as a characteristic of the fetus, whereas the regression estimate refers to gestation both as a characteristic of the fetus and of the dam. The pooled estimates for the five breeds by the two different methods were 0.420 and 0.474, respectively. A comparison between the two heritability estimates, as well as a comparison of these estimates with repeatability estimates, demonstrated that the length of the gestation period was primarily a characteristic of the fetus.

Regressions of birth weight on the length of the gestation period indicated that calves carried one day less than average are approximately 1 lb. lighter than average at birth. It was predicted that the mean length of the gestation period would be decreased by almost ten days in three generations, if 5% of the male and 50% of the female calves resulting from the shortest gestations were saved as breeding stock. Experimentation is necessary to determine if any deleterious effects would be associated with this change.

¹ The data in this paper are from a thesis submitted by the senior author to the Graduate College of the University of Illinois in partial fulfillment of the requirements for the degree of Master of Science, June, 1958.