Effect of herd environment on the genetic and phenotypic relationships among milk yield, conception rate, and somatic cell score in Holstein cattle

A total of 248,230 primiparous records of Holstein cows calving from 1987 t o 1994 (daughters of 588 sires in 3042 herds) was used to evaluate potentia l genotype by environment interactions among mature equivalent milk yield, lactation mean somatic cell score, and conception rate at first service. He rds were classified into low and high environmental groups using three diff erent criteria: standard deviation of herd mature equivalent milk yield, a combination of herd mature equivalent milk yield mean and standard deviatio n, and the herd mean of body weight at first calving divided by age at firs t calving. Genetic parameters were modeled by using multiple-trait linear m ixed models and were fitted using the multiple-trait derivative-free softwa re. Heritabilities for mature equivalent milk yield, lactation mean somatic cell score, and conception rate at first service were 0.221, 0.106, and 0. 015 in low environment herds and 0.300, 0.093, and 0.009 in high environmen t herds, respectively. Genetic (and phenotypic) correlations between mature equivalent milk yield and lactation mean somatic cell score, mature equiva lent milk yield and conception rate at first service, and lactation mean so matic cell score and conception rate at first service were 0.277, -0.417, a nd -0.209, (-0.049, -0.180, and -0.040) and 0.173, -0.318, and -0.144, (-0. 087, -0.166, and -0.035) in low and high environment herds, respectively. T he genetic correlations between pairs of traits were consistently smaller i n high environment herds, suggesting that differences in management between the two environment levels lessened the antagonistic genetic association b etween the traits studied. A long-range plan for low environment herds shou ld focus on improving the level of management, which would greatly reduce t he unfavorable correlated changes in lactation mean somatic cell score and conception rate at first service associated with the genetic improvement of mature equivalent milk yield.