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Factors Associated with the Duration of Gestation in Dairy Cattle

Institute of Genetics, Ohio State University, Columbus

ABSTRACT

The relationship between certain factors and the duration of pregnancy was investigated in a purebred dairy herd composed of the five major breeds of dairy cattle. Data used were restricted to gestations terminated by the birth of a living, single normal-appearing calf. Although not included in the data used in this study, the gestations preceding the birth of twins, in which at least one of each pair was alive at birth, averaged 3.4 days less than gestations terminated by single births.

The means of the Brown Swiss and Guernsey gestations were 288.4 and 282.7 days, respectively. The mean duration of pregnancy for each of these two breeds was different from that of each other and from that of each of the other three breeds at the 1 per cent level. The means for Ayrshires, Holstein-Friesians and Jerseys, which were 278.2, 278.6 and 277.9 days, respectively, were not significantly different from each other. The variances of the mean gestation length were significantly less in the two breeds where more line breeding had been practiced.

Upon comparing the mean gestation lengths resulting from the use of different sires within four of these breeds, inter-sire difference was significant in three breeds but not in the Ayrshire breed in which the sires were more closely related.

Correlations of + 0.32 and + 0.20, significant at the 1 and 5 per cent levels, respectively, were found between the time the dam spent in utero and the time she carried her progeny. Apparently this is the first time that correlation coefficients have been reported between the duration of pregnancy of a dam and of her progeny. The difference of 1.5 days by which the mean gestations of cows 5 yr. old and over exceeded that of 2-yr.-olds was highly significant.

Gestations preceding the birth of 631 male calves averaged 0.77 days longer than 625 gestations preceding the birth of female calves. Although this difference was small, it was found to be significant. The correlation between gestation length and birth weight was significant in the Ayrshire, Brown Swiss and Jersey breeds but not in the other two breeds. The gestations of spring-calving cows averaged 2.07 days longer than fall-calving cows. This difference was significant at the 1 per cent level.

The following evidence indicates genetic influence on gestation length: (a) differences between breed means, (b) smaller variances within the breeds in which line breeding was practiced, (c) inter-sire differences within breeds, (d) decrease in inter-sire differences when sires are closely related, and (e) positive correlation between time the dam and her progeny spent in utero.

¹ Department of Dairy Science.

² Department of Zoology and Entomology.