Intra-herd production level variances for milk yield of Zimbabwean Holstein cows
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As evidence has shown that dairy cattle from more variable herds may be over-evaluated, the effect of heterogeneous variances across herd production levels on genetic evaluation was investigated using lactation milk production records of Zimbabwean Holstein cows. A total of 12420 first lactation milk production records of Zimbabwean Holstein cows were split into three groups (Low, Medium and High) according to herd production level. Preliminary analysis to determine the fixed effects of herd average group, herd, class of cow, year of calving, month of calving and age at calving on milk yield was done. An animal model containing additive genetic effects pertaining to cows, dams and sires was implemented in AI-REML package to estimate additive genetic, residual and phenotypic variances and heritability estimates. The F-max test was used to test for homogeneity of variances. All fixed effects significantly affected milk yield (P < 0.05). Estimates of phenotypic, genetic and environmental variances increased with herd production level. Additive genetic variances were 200 275kg2, 320 400kg2, 376 900kg2 for Low, Medium and High herd production levels, respectively. Heritability (h2) also increased with herd production level. Estimates of heritability were 0.25,0.29,0.30 for Low, Medium and High herd production levels, respectively. Breeding values for the top ten sires varied with herd production levels suggesting a re-ranking of sires due to herd production level. This study showed that intra-herd variances are heterogeneous. Methods for data transformation to account for heterogeneous variances should be developed.