Sources of genetic variation for milk production in a crossbred herd in the tropics

Crossbreeding parameters and variance components were estimated for lactation and reproductive traits of crosses between the Sahiwal, Brown Swiss and Ayrshire breeds in a dairy herd in sub-humid coastal Kenya. An individual animal model was fitted to the data with a fixed effect for 20 distinct breed crosses. The estimates of breed cross means were then regressed on average breed content, expected average heterozygosity and recombination loss to determine the additive breed contributions, and the contributions of dominance and epistasis to heterosis. It was estimated that the Sahiwal contributed about 1345 kg (proportionally 0 . 33) less milk per lactation than the Brown Swiss. The Ayrshire was intermediate. The large amount of heterosis from the crosses of the Sahiwal (Bos indicus) and Bos taurus breeds more than compensated for the lower additive value of the Sahiwal when used in a three-breed rotational cross or synthetic. Heterosis among Bos taurus breeds was negligible. Estimates of the maternal heterosis and recombination loss (epistasis) were not significant, although the latter were consistently large and positive. It was concluded that the heterosis between the zebu and European breeds was mainly due to dominance effects. Estimates of heritability for milk yield traits were low (around 0 . 09 to 0 . 13) compared with other studies, although estimates of repeatability (around 0 . 29 to 0 . 33) were similar to results from the literature, indicating that the accuracy of estimated breeding values in this crossbred herd was reduced because of the non-additive genetic effects. The genetic improvement of crossbred herds is discussed.