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Title: NATIONAL GENETIC EVALUATION OF MILK YIELD FOR HEAT TOLERANCE OF UNITED STATES HOLSTEINS

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

Submitted to: Interbull Annual Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/2/2005
Publication Date: 8/1/2005
Citation: Bohmanova, J., Misztal, I., Tsuruta, S., Norman, H.D., Lawlor, T.J. 2005. National genetic evaluation of milk yield for heat tolerance of United States Holsteins. International Bull Evaluation Service Bulletin 33:160-162.

Interpretive Summary: Heat stress is an important factor that has a substantial impact on production of dairy cattle in the United States. Estimated total annual economic losses to the dairy industry due to heat stress range from $897 to $1500 million. Therefore selection for heat tolerance could be cost effective. However, a major obstacle is the availability of data for such selection on a national basis. A temperature-humidity index (THI) is a widely used indicator of external heat load caused by a combination of temperature and relative humidity. A method for genetic evaluation for heat tolerance based on average daily THI from 202 public weather stations across the United States was applied at the national level. Heat- tolerance evaluations of bulls ranged from –1 to 3 pounds of milk per THI unit above 72 per day; evaluations for daughter milk yield without considering heat tolerance were between -20 and 17 pounds of milk per day. Bulls with genetic extremes for tolerance to heat stress were identified. 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 type, worse dairy form, better udder and body composites, higher type-production indexes, longer productive life, and higher daughter pregnancy rate than did daughters of the least heat-tolerant sires. Much of the milk pricing in the southeastern United States is based on fluid milk and provides incentives to select for cows with high milk yield without advantage for high fat and protein content. Sires of such cows would be expected to transmit the least tolerance for heat stress. In a separate analysis, regional distribution of bulls was examined based on heat tolerance. Sires used in the southeastern United States had lower heat tolerance than the average U.S. bull. Continued selection for milk yield without consideration of heat tolerance may result in greater susceptibility to heat stress, and problems of heat stress in hot climates may be compounded by selection of less heat-tolerant sires.

Technical Abstract: Heat stress is an important factor that has a substantial impact on production of dairy cattle in the United States. Estimated total annual economic losses to the dairy industry due to heat stress range from $897 to $1500 million. Therefore selection for heat tolerance could be cost effective. However, a major obstacle is the availability of data for such selection on a national basis. A temperature-humidity index (THI) is a widely used indicator of external heat load caused by a combination of temperature and relative humidity. Holstein herds were assigned to hourly temperature and relative humidity records from 202 public weather stations across the United States. A method for genetic evaluation for heat tolerance based on mean daily THI was applied at the national level. The threshold for heat stress was assumed to be THI = 72. A random regression repeatability model was used for genetic evaluation of test-day yield and included effects for herd-test date, lactation length, calving age, milking frequency, general additive genetics, permanent environment, genetic and permanent environmental random regressions of heat tolerance. Heat-tolerance predicted transmitting abilities (PTA's) of sires ranged from -0.48 to 0.38 kg milk per THI unit above 72 per day; general milk-yield PTAs for sires were between -8.9 and 7.9 kg per day. Bulls with genetic extremes for tolerance to heat stress were identified. 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 type, worse dairy form, better udder and body composites, higher type-production indexes, longer productive life, and higher daughter pregnancy rate than did daughters of the least heat-tolerant sires. Much of the milk pricing in the southeastern United States is based on fluid milk and provides incentives to select for cows with high milk yield without advantage for high fat and protein content. Sires of such cows would be expected to transmit the least tolerance for heat stress. In a separate analysis, regional distribution of bulls was examined based on heat tolerance. Sires used in the southeastern United States had lower heat tolerance than the average U.S. bull. Continued selection for milk yield without consideration of heat tolerance may result in greater susceptibility to heat stress, and problems of heat stress in hot climates may be compounded by selection of less heat-tolerant sires.