Using publicly available weather station data to investigate the effects of heat stress on milk production traits in Canadian Holstein cattle

Heat stress imposes a challenge to the dairy industry, even in northern latitudes. In this study, publicly available weather station data was combined with test-day records for milk, fat, and protein yields to identify the temperature-humidity index (THI) thresholds at which heat load starts affecting milk production traits in Canadian Holstein cows. Production loss per THI unit above the threshold for each trait was estimated. Test-day records from 2010-2019 from 166,749 cows raised in Ontario and from 221,214 cows raised in Quebec were analyzed. Annual economic losses due to heat stress were estimated from the average losses of fat and protein yields based on the annual average of 156 days with THI exceeding the calculated thresholds. Average thresholds for the daily maximum (THI_max) and daily average (THI_avg) THI estimated across lactations in both provinces were THI_max (THI_avg) 68 (64), 57 (50), and 60 (58) for milk, fat, and protein yield, respectively, indicating that milk components are more sensitive to heat stress. An economic loss of about $34.5 million per year was estimated. Our findings contribute to an initial investigation into the impact of heat stress on the Canadian dairy industry and provides a basis for genetic studies on heat tolerance.