Crossbreeding for dairy production in the lowland tropics of Kenya - I. Estimation of individual crossbreeding effects on milk production and reproductive traits and on cow live weight

Crossbreeding parameters for milk production and reproductive traits and for cow live weight (LW) of crosses of Ayrshire (A), Brown Swiss (B), Friesian (F) and Sahiwal (S) cattle were estimated using data from a dairy herd in the lowland coastal tropics of Kenya. An individual animal model was fitted to the data to estimate the breed cross means of 25 genotypes. The means were then regressed on gene proportion of breeds and on the coefficients of heterosis (or dominance) and recombination (or epistasis) using the dominance, Dickerson and Kinghorn models and a model that included dominance and additive x additive interaction effects (Model D). When comparing models, estimates of crossbreeding parameters showed similar patterns, although the magnitude of the estimates differed between models. Relative to A, F showed significantly positive breed effects for lactation milk yield (MY) (ranging from 1033 to 1139 kg for different models), annual milk yield (AMY) (884 to 1210 kg), calving interval (CI) (-17 to -45 days) and MY expressed per unit of metabolic weight (12.45 to 13.36 kg). B showed favourable, but smaller than F, additive breed effects for these traits. S showed the smallest additive breed effects for MY(-768 to -795), AMY(-551 to -562 kg) and MY expressed per unit of metabolic weight (-5.89 to -6.08). Significant heterosis in the cross B x S was only found for MY (296 to 426 kg), AMY (382 to 409 kg), CI (-15 to -22 days) and MY expressed per unit of metabolic weight (3.07 to 3.30 kg) in the dominance and Dickerson models. Negative recombination and additive x additive interaction effects were obtained for MY and MY expressed per unit of metabolic weight in the crosses A x B and B x S in the Dickerson model and Model D. Epistasis effects were also negative for MY and MY expressed per unit of metabolic weight in the Kinghorn model. The Dickerson model or Model D seem most suitable for analysing these crossbred generations. (C) 2000 Elsevier Science B.V. All rights reserved.