Studes on genetics of heat stress in us holsteins
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The objective of this study was to explore the genetic component of heat stress in U.S. Holsteins using national milk yield data consisting of 57 million first-parity test-day records of 6 million Holstein cows that calved from 1993 through 2004 and weather records from 202 public weather stations. Seven temperature humidity indices were compared in a humid and semi-arid climate for their ability to detect a decline of milk yield due to heat stress. The index with a higher weight on humidity was the best in the humid climate. The index with a larger weight on temperature was the best heat stress indicator in the semi-arid climate. National genetic evaluation for heat tolerance was conducted using a repeatability test-day model. Based on estimated heat tolerance PTAs, the 100 most and 100 least heat-tolerant sires were selected. For each of the 200 sires, official U.S. PTAs from February 2006 were obtained. Sires that were the most heat tolerant transmitted lower milk yields with higher fat and protein contents than did sires that were the least heat tolerant. Daughters of the most heat tolerant sires had better udder and body composition, better type, lower dairy form, slightly higher TPI, longer productive life, higher daughter pregnancy rate, were easier calving and had better persistency than did daughters of the least heat tolerant sires. Heat stress was evaluated as a factor in the genotype x environment interaction on milk production in the United States. Data for the Southeast and Northeast were extracted from the national data set and analyzed separately. Two repeatability models with and without the effect of heat stress were implemented. Both models were fitted with the national and regional data sets. Correlations between breeding values of sires with • 100 and • 300 daughters in two regions were calculated. When heat stress was ignored (first model), the correlation of regular breeding values between regions for sires with • 100 (• 300) daughters was 0.85 (0.87). Heat stress as modeled here explains only a small amount of genotype by environment interaction, partly because test day records provide only snapshots of heat stress over a hot season.