Genetic (Co)variance between Age from First to Third Calving and Milk Yield of Holstein Cows in the Tropics

A total of 245187 records of the first three lactations of 131126 Holstein cows in tropical conditions were used to estimate the genetic (co)variance components between age at calving and its respective milk productions. A multivariate analysis of six traits was performed. Heritability for age at calving (h2AC) and milk yield (h2MY) decrease from the first (h2AC1=0.254±0.007; h2MY1=0.198±0.003) to the second (h2CA2=0.242±0.006; h2MY2=0.184±0.003) and third lactation (h2CA3=0.232±0.006; h2MY3=0.168±0.008). The genetic correlations (rg) between the AC1; AC2 and AC3 and between MY1; MY2 and MY3 were high and positive; however, the rg between age at calving and milk production were medium but negative. The first two eigenvalues accounted for 83% of the total genetic variance of the six traits. The corresponding eigenvectors were used as weighting factors,and two underlying expected breeding values (EBV) were estimated based on these components, allowing the identification of those sires that reduced ages at calving and increased milk yield in their daughters. A new approach was developed to estimate an equivalent of calving interval (CIeq) based on the differences between the EBV of the sire for two consecutive ages at calving.