Non-linearity in the genetic relationship between milk yield and type traits in Holstein cattle

Non-linear genetic relationships between 305-day milk yield and 17 type traits in first lactating US Holstein cattle were estimated on the basis of a polynomial offspring-parent regression and paternal halfsib analysis. In total, 24 470 dam-daughter pairs and 177 188 halfsib records were analysed. Significant offspring-parent regression coefficients for the quadratic term were found for the relationship between milk yield and the type traits front teat placement, fore udder attachment, udder depth and final score. For these relationships, the genetic regression coefficients were calculated from the phenotypic regression coefficients according to formulae derived elsewhere. The transformation from the phenotypic offspring-parent regression to the genetic level resulted in rather strong non-linear relationships between milk yield and the respective type traits. Results for paternal halfsib analysis seemed unreliable which was most likely caused by milk yield data not being normally distributed due to selection. However, heritabilities for milli yield and type traits were consistent for both estimation methods. (C) 1998 Elsevier Science B.V. All rights reserved.