Genetic component of heat stress in dairy cattle, parameter estimation

Our data included 119,205 first-parity, test-day records from 15,002 Holste ins in 134 Georgia farms with temperature and humidity data from 21 weather stations throughout Georgia. The test-day model included the effects of he rd test date, days-in-milk (DIM) classes, age, milking frequency, general a dditive effect, random regression on the heat-humidity index for heat-toler ance additive effect, general permanent environment, and the random regress ion on the heat-humidity index for a permanent environment. The general eff ects, which corresponded to effects in the current repeatability models, we re assumed to be correlated with the heat-tolerance effects. Variance compo nents were estimated by REML. For heat-humidity indices below 72, heritabil ity for milk was 0.17, and additive variance of heat tolerance was 0. For a heat-humidity index of 86 (which would correspond to temperatures of 36 de grees C at 50% humidity), the additive variance of heat tolerance was as hi gh as for general effect, and the genetic correlation between the two effec ts was -0.36. Results for fat and protein were similar. Current selection f or production reduces heat tolerance. Joint selection for heat tolerance an d production is possible.