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The Crossbred Sire: Theory¹

Virginia Polytechnic Institute and State University, Blacksburg 24061

Abstract

Favorable effects of heterosis on performance of the crossbred market animal and the crossbred dam have been well documented and breeding plans have been developed to utilize these effects. However, a similar mandate for use of the crossbred sire does not exist. Given the common developmental factors involved in reproduction in both sexes, it appears likely that the same nonadditive effects that lead to improved reproductive fitness in the female would also have an effect in the male. Most studies that have evaluated heterotic effects in the male have observed heterosis for at least some components of male reproduction, but have been unable to document consistently favorable effects of the crossbred male on overall conception rate. This result may reflect the categorical nature of conception and possible limiting effects of the female on expression of male differences. Important effects of true paternal heterosis, parallel to maternal heterotic effects acting through the maternal environment provided by the dam, appear unlikely because of the less-intimate relationship between offspring and sire. However, if the relationship between underlying breeding value and realized phenotypic merit is not linear, substantial realized paternal heterosis may be observed due to optimization of gene frequencies through use of the crossbred male. This result is likely for categorical traits or traits with an intermediate optimum breeding value. At the production system level, the appropriate unit of evaluation is the cohort composed of a male and the set of females that are exposed to the male for breeding. At this level, potential advantages of the crossbred male may accrue due to both heterosis for male fertility and optimization of gene frequency.

¹ Presented at a symposium on "The Crossbred Sire" held during the 78th Annu. Meet, of ASAS, Kansas State Univ., Manhattan, July 30, 1986.

² Dept. of Anim. Sci.