To study heterogeneity of variances and to estimate the heritability coefficients at different levels of herd milk production and at different levels of standard deviation records on 13,053 lactations of 6,828 Holstein cows, sired by 811 bulls and distributed in 78 herds in the State of Rio Grande do Sul from 1988 a 1997 were analyzed. The herds were grouped by milk production and were stratified by the standard deviation of milk production (305 days) in three levels (low, medium and high). The variance components were estimated by Restrict Maximum Likelihood, using the program MTDFREML and a mixed animal model including the fixed effects: class of age, genetic group, herd / nucleus and year-season of calving and the random effects of animal, permanent environmental and residual. The genetic, residual and phenotypic variance components showed a positive relationship with production level and standard deviation class but the heritability coefficients did not present the same trend and the higher estimates occurred at the low production level and at the medium standard deviation class.
heterogeneity of variance; Holstein breed; production level
