Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2006
Publication Date: 4/1/2007
Citation: Bohmanova, J., Misztal, I., Cole, J.B. 2007. Temperature-humidity indices as indicators of milk production losses due to heat stress. Journal of Dairy Science. 90(4):1947-1956. Interpretive Summary: Temperature humidity index is a popular indicator of heat stress. Many indices with different weighting of temperature and humidity were proposed. However, none of them was specifically designed for Holstein cows. Identifying the most suitable temperature humidity index for Holstein cows held in either humid or semi-arid climate was objective of this study. Indices with higher weight on humidity were the most effective in identifying decline of milk production due to heat stress in humid climates. In semi-arid climates, humidity was not a causing factor of heat stress and indices with lower weight on humidity were more suitable.
Technical Abstract: Meteorological data (1993 to 2004) from two public weather stations in Phoenix, AZ and Athens, GA were analyzed with test day milk yield data from herds nearby the weather stations to identify the most appropriate temperature humidity index (THI) to measure losses in milk production due to heat stress in semi-arid climate of Arizona and humid climate of Georgia. Seven THIs with different weightings of dry bulb temperature and humidity were compared. Test-day data were analyzed using two fixed models to determine threshold of heat stress and rate of decline of milk production identified by a specific THI. Differences in thresholds of heat stress were found among indices and between regions. The index with higher weight on humidity was the best in the humid climate, while the index with larger weight of temperature was the best heat stress indicator in the semi-arid climate. Humidity is the limited factor of heat stress in humid climates while it is dry bulb temperature in dry climates.