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Title: TEST-DAY MODEL THAT ACCOUNTS FOR HEAT STRESS OF HOLSTEINS IN THE UNITED STATES

Author
item BOHMANOVA, J - UNIV OF GEORGIA
item MISZTAL, I - UNIV OF GEORGIA
item TSURUTA, S - UNIV OF GEORGIA
item Norman, H

Submitted to: Journal of Dairy Science
Publication Type: Abstract Only
Publication Acceptance Date: 2/28/2005
Publication Date: 7/24/2005
Citation: Bohmanova, J., Misztal, I., Tsuruta, S., Norman, H.D. 2005. Test-day model that accounts for heat stress of Holsteins in the United States [abstract]. Journal of Dairy Science. 88(Suppl. 1):353-354.

Interpretive Summary:

Technical Abstract: Genetic evaluations for heat tolerance of US Holsteins were developed with national data. Hourly temperature and relative humidity records were available from 202 public weather stations across the United States. Production data included 57,315,661 first-parity test-day records from 1993 through 2004 for 6,906,815 Holsteins. Herds were assigned by distance from the nearest weather station. Hourly temperature-humidity indexes (THI) were calculated as [1.8(°C) + 32] - [0.55 - 0.0055(relative humidity percentage)][1.8(°C) - 26]. A daily temperature-humidity index (THI24) was calculated as mean of hourly THI on recording day. The threshold for heat stress was assumed to be a THI of 72. The test-day model contained effects for interaction of herd with test day, days-in-milk classes, calving age, milking frequency, additive genetics, permanent environment, and random regressions on THI24 for additive genetic and permanent environmental effects. Breeding values were calculated by BLUP90IOD in 144 rounds and 8 hr. Breeding values for heat tolerance of sires ranged from -0.48 to 0.38 kg milk per THI unit above a THI24 of 72. Breeding values at higher THI24 would be greater by (THI24 - 72) kg. On average, the 50 most heat-tolerant bulls were 3 yr older than the 50 least tolerant. National genetic evaluation for heat tolerance is feasible. Recent bull selection practices may have resulted in selection against heat tolerance.