MANAGING HEAT STRESS

By Tom Wright

CANADA’S SUMMER FORECAST IS EXPECTED TO BE COOLER THAN RECENT YEARS; however, in May, many parts of Ontario were gripped by an unexpected heat wave.

It was a good reminder to farms to have a plan to manage heat stress for dairy cows, which respond to heat stress negatively. The effects are mostly lower milk yield and dry matter intake, but the impact of heat stress can have a lasting effect on a cow’s reproductive performance even after the summer is over.

The 2009 Code of Practice for the Care and Handling of Dairy Cattle included a chart showing a temperature-humidity index (THI) combining the effects of ambient temperature and relative humidity into an index describing their overall effect on cow performance. The THI chart showed cows do not experience heat stress when THI is below a value of 72.

More recently, many experts have recommended lowering the threshold to 68. A new study by University of Guelph researchers gathered Canadian weather data from NASA’s weather database, compared it with Canadian weather station data and then examined those data in combination with production data from Canadian cows from 2010 to 2019. The study used more than 20 million test day records from more than one million individual cows from 8,500 herds, grouping the results into five regions in Canada.