Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2007
Publication Date: 2/1/2008
Citation: Bohmanova, J., Misztal, I., Tsuruta, S., Norman, H.D., Lawlor, T.J. 2008. Short Communication: Genotype by Environment Interaction Due to Heat Stress. Journal of Dairy Science. 91(2):840-846.
Interpretive Summary: Test-day milk yield records of US Holsteins were analyzed by 2 test-day models with or without including the effect of heat stress. The analyses involved US national data and data only from the Southeast and the Northeast. The proportion of records under heat stress was 10% nationally, 7% in the Northeast and 27% in the Southeast. The effect of heat stress accounted for a small fraction of differences between regional evaluations. Estimated breeding values for heat tolerance were similar across regions for sires with a large number of daughters.
Technical Abstract: Heat stress was evaluated as a factor in differences between regional evaluations for milk yield in the United States. The national data set (NA) consisted of 56 million first-parity test-day milk yields on 6 million Holsteins. The Northeastern subset (NE) included 12.5 million records on 1.3 million first calved heifers from 8 states, and the Southeastern subset (SE) included 3.5 million records on 0.4 million heifers from eleven states. Climatic data were available from 202 public weather stations. Each herd was assigned to the nearest weather station. Average daily temperature-humidity index (meanTHI) three days prior to test date was used as an indicator of heat stress. Two test-day repeatability models were implemented. Effects included in both models were herd-test date, age at calving class, frequency of milking, DIM x season class, additive genetic (regular breeding value) and permanent environmental effect. Additionally, the second model included random regressions on degrees of heat stress (t = max[0, meanTHI-72]) for additive genetic (breeding value for heat tolerance) and permanent environmental effects. Both models were fitted with the national and regional data sets. Correlations involved estimated breeding values (EBV) from SE and NE for sires with >= 100 and >= 300 daughters in each region. When heat stress was ignored (first model) the correlations of regular EBV between SE and NE for sires with >= 100 (>= 300) daughters were 0.85 (0.87). When heat stress was considered (second model), the correlation increased by up to 0.01. The correlations of heat stress EBV between NE and SE for sires with >= 100 (>= 300, >= 700) daughters were 0.58 (0.72, 0.81). Evaluations for heat tolerance were similar in cooler and hotter regions for high reliability sires. Heat stress as modeled explains only a small amount of regional differences, partly because test-day records depict only snapshots of heat stress.