Heterogeneity of variance and its implications on dairy cattle breeding
Milk yield data (n = 12307) from 116 Holstein-Friesian herds were grouped into three production environments based on mean and standard deviation of herd 305-day milk yield and evaluated for within herd variation using univariate animal model procedures. Variance components were estimated by derivative free REML algorithm, and significance tests done using the Fmax procedure. Phenotypic, additive genetic and residual variances were heterogeneous across production environments. The estimates of variances in kg2 were 708349.9, 966853.7 and 2229338.9; 112266.9, 209949.9 and 689942.1; and 512660.0, 734854.5 and 1317808.3 for phenotypic, additive genetic and residual variances, respectively for production environments 1, 2 and 3. The heritability estimates were 0.15 ± 0.04, 0.22 ± 0.05 and 0.31 ± 0.03, respectively. The magnitude of estimated breeding values for sires and their ranking was influenced by production environment. Each production environment had a unique set of superior sires. The more variable production environments contributed more bull-dams at various selection intensities. Effects of heterogeneous variances need to be accounted for in genetic evaluation for Holstein-Friesian in Kenya.
Keywords: Variance components; heterogeneity; milk yield; Holstein-Friesian
South African Journal of Animal Science Vol. 37 (3) 2007: pp.170-175