Genetic component of heat stress in dairy cattle, parameter estimation.
Our data included 119,205 first-parity, test-day records from 15,002 Holsteins in 134 Georgia farms with temperature and humidity data from 21 weather stations throughout Georgia. The test-day model included the effects of herd test date, days-in-milk (DIM) classes, age, milking frequency, general additive effect, random regression on the heat-humidity index for heat-tolerance additive effect, general permanent environment, and the random regression on the heat-humidity index for a permanent environment. The general effects, which corresponded to effects in the current repeatability models, were assumed to be correlated with the heat-tolerance effects. Variance components were estimated by REML. For heat-humidity indices below 72, heritability 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 degrees C at 50% humidity), the additive variance of heat tolerance was as high as for general effect, and the genetic correlation between the two effects was -0.36. Results for fat and protein were similar. Current selection for production reduces heat tolerance. Joint selection for heat tolerance and production is possible.
Ravagnolo
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Animal and Dairy Science Department, University of Georgia, Athens 30605, USA. olga@nce.ads.uga.edu
Misztal
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D000818:Animals; D001947:Breeding; D002417:Cattle; D002418:Cattle Diseases / Q000235:genetics*; D005260:Female; D005845:Georgia; D018882:Heat Stress Disorders / Q000235:genetics / Q000662:veterinary*; D006358:Hot Temperature / Q000009:adverse effects*; D006813:Humidity / Q000009:adverse effects*; D007774:Lactation / Q000235:genetics; D008297:Male; D008892:Milk / Q000737:chemistry; D008954:Models, Biological; D012641:Selection, Genetic; D014887:Weather