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Title: EFFECTS OF ACCOUNTING FOR HEAT STRESS ON GENETIC EVALUATION OF US HOLSTEINS FOR MILK BY A TEST DAY MODEL

Author
item BOHMANOVA, J - UNIV OF GA, ATHENS
item MISZTAL, I - UNIV OF GA, ATHENS
item TSURUTA, S - UNIV OF GA, ATHENS
item Norman, H
item LAWLOR, T - HOLSTEIN ASSOCIATION

Submitted to: Journal of Dairy Science
Publication Type: Abstract Only
Publication Acceptance Date: 2/17/2006
Publication Date: 7/9/2006
Citation: Bohmanova, J., Misztal, I., Tsuruta, S., Norman, H.D., Lawlor, T.J. 2006. Effects of accounting for heat stress on genetic evaluation of US Holsteins for milk by a test day model [abstract]. Journal of Dairy Science. 89(Suppl. 1):137-138(abstr. 163).

Interpretive Summary:

Technical Abstract: The purpose of this study was to quantify the effect of heat stress on genotype by environment interaction among different regions of the United States. The national data consisted of 55.5 million first parity test-day (TD) milk yield records on 5.8 million Holstein cows. Subsets of the national data were from the Northeast (NE) with 12.5 million TD records on 1.3 million cows, and from the Southeast (SE) with 3.5 million TD records on 0.36 million cows. Meteorological data from 202 public weather stations were matched with herds based on distance. Daily mean temperature-humidity index (THI) was calculated from temperature and relative humidity records three days prior to TD. The first model that lacked the effect of heat stress included fixed effects of herd-test date, age at calving class, frequency of milking, and DIM x season class, and random genetic additive (regular breeding value) and permanent environmental effects. The second model that accounted for heat stress included two additional random regressions on degrees of heat stress (t=max[0,THI-72]), one for additive genetic (heat breeding value) and one for permanent environmental effect. Breeding values (BV) were computed by BLUP90IOD. Correlations involved sires with at least 300 daughters in regions being compared. When heat stress was ignored, the correlations of regular BV between NE and SE were 0.86. When the heat stress was considered, the correlation increased by 0.01. The correlation between heat BV for NE x SE was 0.72. Heat stress effect as applied explains only a fraction of differences in sire ranking between SE and the NE. The real impact of heat stress may be higher because THI as used accounts only for a fraction of variability due to heat and low correlations are in part due to limited accuracies.