Mild heat stress starts to occur at a THI value of about 72, which corresponds to 77°F and a typical Tulare morning humidity of 50%, or at 82°F at a typical Tulare afternoon humidity of 20%. Even at THI values in this “mild” category, reproductive losses have been documented and milk production can be decreased by approximately 6 lbs/cow/day. Recent studies in high-producing cows suggest that smaller detrimental effects can be at THI values as low as 68. Inexpensive thermometer/hygrometer can be purchased to determine THI values at different locations on the dairy.
How can I tell when my cows are heat stressed?
Certainly cows that are panting with open mouths are in severe heat stress, but at low to moderate levels producers may not observe obvious signs. As described in a review from the University of California, cows do consistently exhibit certain behaviors when trying to mitigate heat. With rising air temperature and solar radiation cows will increasingly seek shade.
Even cows that have deprived of lying for 12 hours will prefer to stand in the shade rather than lie in the sun, sometimes even engaging in aggressive behavior to gain access to it. Either in or out of shade cows will spend as much as 10% more time standing, possibly to increase air flow around their bodies.
Cows will reduce their feed intake, in one study up 5% per day in at 90°F. In a Florida study cows without access to shade ate 19% more at night, but were not able to make up of the decreased feed intake, still eating 13% less then cows with shade. Unshaded cows may also spend 2 to 3 times more time standing around the water trough. The most sensitive and practical measurement of heat stress is respiration rate, which unlike rectal temperature, increases in response to heat load with little or no lag in time.
Normal respiration rates for cattle are about less than 40 breaths per minute. If five cows out of 10 have respiratory rates exceeding 100 breaths per minute immediate action is indicated. The University of Missouri has released a smart phone app that allows beef and dairy producers to merge their cows’ breathing rates with local heat and humidity data to instantly determine heat stress rating and recommendations.