Heat stress takes its toll on your herd in several ways.
With heat stress season right around the corner, I was fortunate to be included in discussions last week with Dr. Robert Collier of the University of Arizona.
Dr. Collier and his colleagues (including Dr. John Smith, who passed away in March) have been at the forefront of heat stress research in dairy cattle.
Here are some bullet points from my notes:
• Heat stress begins to affect dairy cows producing more than about 75 lb. of milk at a temperature humidity index (THI) over 68. (THI table below). Death rate increases when the minimum THI during the day is 70 and the maximum THI is 80 or higher.
• Respiratory rates over 60 breaths/minute indicate heat stress in cows.
• Hot cows don’t lie down, especially when the ground or bedding is hot. A percentage of your cows will be standing when the THI is over 70. Sand is much cooler than manure bedding. When a cow’s core body temperature reaches 39 C (102.2 F), she will get up and go to a soaker, if available, until her temperature reaches 38 C (100.4 F).
• It takes two days after the onset of heat stress to observe the maximum effects on production, due to reduced feed intake.
• Heat stress can reduce the birth weight of calves by 10 lb. due to the direct effect of heat on the dam and the growing fetus and indirect effect of a shortened gestation.
• You can use an infrared gun to help determine whether a cow is hot. Point it high on the flank. Skin temperatures over 35C (95 F) indicate heat stress.
• Cooling dry cows will result in at least 1,000 pounds more milk in the next lactation. Minimum protection for dry cows includes shade.
• Make sure hospital barns have excellent heat abatement. The reasons are obvious.
• When the air temperature is close to the cow’s body temperature, evaporative cooling is the only effective method of cooling. Blowing air that is hotter than the cow’s core body temp (101 F) on a cow only makes her hotter.
• Measure the summer-winter difference in average milk production of your herd to get a gauge on your heat abatement. In Arizona herds, it’s currently about a 9 lb. difference. In Israel, the difference is down to less than 2 lb., indicating what can be done to alleviate heat stress.
• Cows with the "slick hair gene" (from the Senepol breed) are more heat tolerant. Although cows don’t sweat well, slick hair cows have more sweat glands. With less hair, air flow over the skin results in more effective cooling.
• Cows follow shade by instinct. Because of this, stationary cooling systems under east-west facing shades are less effective as the shade moves. Cooling systems that follow the shade help.
For those who closely observe cow behavior, many of her responses to heat stress are obvious. Why do cows eat less when they’re hot? Try eating a bowl of Cheerios when you’re breathing 120 times/minute, says Dr. Collier.