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Phenotypic and Genetic Relationships between Calving Ease, Gestation Length, Birth Weight and Preweaning Growth^{1 ,2,}

Montana State University, Bozeman 59717

Abstract

Records from the American Simmental Association were used to study calving ease score, percent assisted births, gestation length, birth weight, 205-day weight and average daily gain (ADG) from birth to weaning. The least-squares independent variables were management unit (herd weaning weigh data), sire within management unit, age of dam, sex of calf and percent Simmental of calf. They significantly affected all traits studied. Gestation length, when included in the model as a covariate, had a significant effect on all traits studied. Regression of birth weight on gestation length was .25 kg per day and percent assisted births increased .70% per day increase in gestation length. When both gestation length and birth weight were included as covariates, gestation length no longer had a significant effect on calving ease score or percent assisted births. Heritability estimates for calving ease score, percent assisted births, birth weight, gestation length, 205-day weight and ADG were .32 ± .04, .25 ± .04, .32 ± .04, .48 ± .04, .28 ± .04 and .28 ± .04, respectively. Genetic correlations and predicted correlated responses to selection indicated that selection for calving ease would increase ease of calving, decrease gestation length and have little affect on birth weight or 205-day weight. Direct selection against large birth weights would not be as effective in increasing the ease of calving, and would shorten gestation length and decrease birth weight and 205-day weight.

¹ Published with approval of the Director of the Montana Agricultural Experiment Station, Journal Series No. 821.

² Research supported in part by a grant from the National Association of Animal Breeders.

³ American Simmental Association, 1 Simmental Way, Bozeman, MT 59715.

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