1 540 986 first parity milk yield test-day records on 153 805 Holstein cows from 670 farms and weather information from 51 public weather stations from various parts of the United States were used to investigate differences in level of heat stress among states from Southeast and Southwest. A fixed model was used to identify threshold and intensity of heat stress and an animal model was used to estimate variance components and genetic parameters of heat tolerance. Threshold of heat stress was 70 in most states of Southeast and 72 in Southwest. Declines of milk yield due to heat stress varied significantly among states. Additive variance for heat tolerance is almost constant across states. However, significant differences in Spearman correlations between both additive effects were observed. The correlations were the highest in states with highest additive genetic variance for heat tolerance.

M. Freitas, Ignacy Misztal, J. Bohmanova, R. Torres

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume , , 01.11, 2006
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