Studies have shown that rearing regimes that result in heifers of heavier liveweights give rise to greater subsequent lactation yields than those regimes resulting in heifers of average liveweight. Rearing heifers to heavier weights is more costly because of additional management inputs and reduced pasture utilisation. The objective of this research was to quantify some relationships between immature weights and subsequent milk yields and compositions in each of the first three lactations. Genetic and phenotypic correlations between liveweights at 9, 15, and 21 months of age (LW9, LW15, LW21) and lactation yields were obtained from records on 2365 mixed breed dairy cows. Genetic correlations between first lactation yields of milk, fat, and protein (MY1, FY1, PY1) and liveweights were all positive and ranged from 0.14 (LW9 and FY1) to 0.45 (LW15 and PY1). Genetic correlations between weights and milk and fat yields tended to be slightly lower at second and third lactations as compared to first lactation, but were associated with high standard errors. LW15 had the highest genetic correlations with yield traits for the three lactations. Phenotypic correlations between liveweights and yields were of similar size to the genetic correlations. The effects of rearing management were estimated from regressing average lactation yields of groups of contemporary cows in subsequent years on average liveweights of the same groups as immature heifers. Average weights and yields were adjusted for differences in genetic merit between groups before regression analysis. Management that resulted in a 1 kg increase in LW15 caused a 6.7 litre increase in MY1, 0.27 kg increase in FY1, and 0.18 kg increase in PY1. Profit per lactation was estimated to increase by $0.48 per kg increase in immature weight.
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