Heat Stress

Heat Stress
By Ali Cullum, Anexa Vets, Morrinsville

Our cows suffer heat stress during the summer, especially on days with temperatures above 23°C, and with relative humidity above 80% (Waikato!). The heat stress or hyperthermia is caused by the fact that the cows can no longer cool their bodies down to their normal internal body temperature of 38.6 °C. Black cows absorb heat very quickly and become very hot. Jerseys in general are more heat resistant than Friesians. This may be due to the origin of the breeds: Jerseys come from a more tropical latitude than Friesians. Conversely, if you supply shade to hot cows, black cows will lose heat more quickly than white cows.

What does this do to the cow?

There are several immediate effects, a reduction in appetite, an increase in drinking, and most importantly an associated reduction in milk yield. Columbian research involving Holstein Friesians estimated that with each 0.5°C increase in body temperature above 38.6 °C, milk yield will decline by 1.8 kg/day (15 g MS/cow/day). Severe continued heat stress can also cause loss of pregnancies.

What are the cow signs of heat stress?

Like us they sweat! They breathe faster, and they extend their necks and pant. They drink more, and they seek shade (if it is available). They also change their behaviour. They will mob up and hang their heads to the ground. The mobbing behaviour creates some swirling air patterns which gives them minor relief from the heat. Higher producing cows produce more body heat of metabolism and therefore suffer most effects on hot days.

What can we do to help the cows?

We can supply shade! If you have a covered feed pad or stand off area, access to the shade during the day will help. Some farms have shady paddocks that they use as a sacrifice area for hot days. We can supply lots of water, both in the paddock and at the dairy/stand off pad. We can move the cows at cooler times of day, eg. if milking in the afternoon, delay until 5.00pm, rather than 3.00pm. If milking once a day, then early morning is usually a cooler time of day. We can use sprinklers on the collecting yard and that coat wetting will reduce body temperature for several hours after milking. Feeding lush food is not always possible when it is hot, but this type of food generates less digestive heat and therefore less heat stress to the cow.
Refer to: DairyNZ Farm Fact 3-15 https://bit.ly/2G9sjc6

Date Added: Tuesday, 5th February 2019