Effects of elimination of short lactations and of adjusting for lactation length on the heritability of milk production in a Gir herd

Abstract

To assess the effects of reducing variation in lactation length, estimates of total first lactation (PLT) and 305 d milk yield (P305) were obtained based on 398 daughters by 50 sires in a Gir herd in Minas Gerais State. Discarding lactations shorter than 120 d caused a reduction in PLT and P305 h2 estimates, from 0.40±.18 and 0.38±.17 to 0.05±.14 and 0.01±.14, respectively. Estimated h2 of total and 305 d truncated lactation length were 0.59±.19 and 0.58±.19 when no data were discarded and 0.17±.16 and 0.28±.17 upon discarding short lactations. Adjusting for lactation length resulted in a negative sire variance component of milk yield. Although based on small numbers of sires and observations, these results confirm the presence of genetic variation for lactation length, thus neither adjusting nor discarding short lactations is recommended in genetic evaluation of Zebu cattle.