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Consequences of removing the variation in lactation length on the evaluation of dairy cattle breeds and crosses

Fernando Enrique Madalena^I; Alvaro de Matos Lemos^II; Roberto Luiz Teodoro^II

^IDepartamento de Zootecnia, Escola de Veterinária, UFMG, Caixa Postal 567, 30161 Belo Horizonte, MG, Brasil. Send correspondence to F.E.M.
^IIEMBRAPA/CNPGL, 36155 Corone1 Pacheco, MG, Brasil

ABSTRACT

Three procedures commonly used for breed group comparisons were studied; (a) adjustment of milk yield for lactation length; (b) exclusion of short lactation considered abnormal; and (c) utilization of ali available observations, unadjusted for lactation length. These three procedures were applied in a comparison of six Holstein-Friesian: Guzera crossbred groups at farms in the southeast region of Brazil. Genetic group effects on milk and component yields were attenuated by procedures a and b, compared to procedure c, and this resulted in underestimation of heterosis for those traits, as weli as underestimation of the breed additive differences. These results were expected given the presence of genetic variation for lactation length and the high positive correlation between this trait and milk yield. This research suggests that procedures reducing variation in lactation length are inadequate for evaluation of breed groups. "Abnormal" lactation data also should be reported when "normal" lactation are analysed separately.

Keywords: Lactation length; Cattle breeds.

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