Intra-herd production level variances for milk yield of Zimbabwean Holstein cows

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, 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 (