Effect of reduction of lactation length variation on genetic evaluation of crossbred dairy cattle

Abstract

From data of milk control obtained from Brazilian Association of Girolando Breeders, the effects of removing variation in lactation length on the comparison of productive performance of various Holstein x Gir crossbred genetic groups were studied through comparison of results obtained from the use of three methods: (a) adjusting milk yield by lactation length; (b) excluding short lactations ( < 120 days of length); (c) use of all records not adjusting by lactation length. Estimates of (co)variance components were obtained through the method of the Restricted Maximum Likelihood (REML) under animal model. The heritability estimates for milk yield by the use of the methodologies "a", "b" and "c" were of 0.24 ± 0.029, 0.31 ± 0.034 and 0.27 ± 0.029, respectively. It was concluded that the adjustment of milk yield by lactation length could take to mistaken practices in the comparison of Holstein x Gir genetic groups for milk yield and in the ranking of the animals by genetic merit. However, the exclusion of the short lactations was shown appropriate decision once it did not reduce the genetic variability and it reduces the residual variance, contributing to the improvement of the quality of the genetic evaluations.