Tall Fescue Adds to Heat Stress in Cattle

By Linda Geist

Cattle rid their bodies of heat in three ways: radiation, convection and evaporation. “If your cattle can’t use all three methods, they’ll overheat fast,” says University of Missouri Extension state beef nutrition specialist Eric Bailey.

Summer heat knocks weight off calves and pounds off milk, Bailey says. Losses come in the form of less gain, weaker fertility and other health issues. “Heat stress costs real money,” he says.

Producers can reduce heat stress by providing adequate shade, proper fly control, access to water and the right choice of pasture grass.

Tall fescue pastures turn up the heat

Cattle grazing on tall fescue grass when temperatures rise is a recipe for disaster.

Toxic endophytes in fescue can turn up the cattle’s body heat. Their efforts to keep cool can melt profits. But there are practical fixes that keep weight and profits steady, Bailey says.

Most fescue pastures in the Fescue Belt carry endophytes that make toxic ergot alkaloids, which mess with your cattle’s ability to stay cool, he says.

These toxins tighten blood vessels, cut blood flow to the skin and prevent heat from escaping. They make it hard for cattle to sweat and pant, essential tasks to rid their bodies of heat. Third, ergot alkaloids delay shedding by lowering prolactin. This leaves cattle with shaggy coats that trap summer heat.

Summer temperatures, especially heat waves, make cattle on fescue vulnerable to problems. Normally, cattle can tolerate temperatures of 31 C or 88 F. Cows likely consume enough ergovaline by Memorial Day to make heat stress worse during the summer.