Genetic Parameters for Gestation Length, Birth Date and First Breeding Date in Beef Cattle

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Abstract
Genetic and environmental variability in gestation length (GL), birth date (BD) and time from first day of breeding season to first observed estrus (FE), were analyzed using 12 yr of breeding and calving records on cows sired by Simmental, Maine-Anjou, Limousin and Gelbvieh bulls. All cows were artificially inseminated. Statistical analyses were performed separately on first, second and last parity records. Number of records in the three sets were: 4,474, 2,803 and 2,504 for GL; 4,228, 2,688 and 2,429 for BD and 4,242, 2,688 and 2,429 for FE. Year and the interaction of year and season affected all three traits. Sex of calf or twin pregnancy affected both GL and BD. Paternal half-sib estimates of heritability for GL as a trait of the calf were .41 ± .07, .45 ± .09 and .36 ± .07 in the first, second and last parity, while the estimates for BD were .09 ± .03, .17 ± .03 and .24 ± .06, respectively. Time to first estrus was treated as a trait of the cow. The heritabilities were .05 ± .02, .10 ± .04 and negative in the three parities, respectively. Heritabilities for maternal additive effects were small for GL and BD. The correlation between direct and maternal additive effects were negative for GL and BD in parity 1 and for GL in parity 2. If selection were to be practiced on GL or BD, an optimal selection scheme that would maximize the sum of the direct and maternal breeding values should be considered. Copyright © 1987. American Society of Animal Science . Copyright 1987 by American Society of Animal Science