Gestation Length in an Inbred Jersey Herd

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Abstract
Records of 1353 gestation periods of 427 cows of the U. California inbred Jersey herd were used. Inbreeding ranged from 0 to 62% and averaged 11% for cows and 19% for calves. The mean length of uncorrected gestation periods was 282.7 days; standard deviation was 5.4 days. Sex and calving sequence were important sources of variation. Male calves were carried in utero 2 days longer than female calves. First calves were carried 1.3 days less than 2d calves; 2d calves 1.2 days less than 3rd calves. From the 3rd calving sequence on, no significant differences or trend was found. Neither year nor season had significant effects on gestation length. The significant year X season interaction was interpreted to indicate short-term time trends not associated with season of year. A highly significant regression of birth weight on gestation length of 0.36 lb. per day was found. There was no evidence that calf inbreeding affected length of time it was carried in utero. Inbreeding of the dam was positively correlated with gestation length, but there was evidence that this relationship might be confounded with other variable factors, including, possibly, milk production. A partitioning of the phenotypic variance yielded the following estimates of relative importance of the sources of variability: additive genetic differences between calves, 30% additive genetic differences between dams, 31%; permanent differences between dams other than those included above, 0%; unanalayzed residual effects, 39%. Positive correlations were found between milk production and time to settle a cow, and between milk production and gestation length. The latter is statistically significant as the 5% level of probability; the former is not. An hypothesis is discussed in which relationships between the above variables are interpreted in terms of genetic homeostasis.

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