By Marianne Buza

As dairy farmers in Michigan know all too well, summers can be hot, humid and downright unbearable for both humans and cattle. As the temperature and humidity both rise, so does the risk for heat stress in dairy cattle.

Heat stress occurs when outside influences cause a cow’s body temperature to rise above the normal temperature of 100.9 to 101.5 degrees F. When this happens, cows are no longer comfortable and not able to reach their production or reproductive potential. Heat stress can occur at temperatures as low as 70 degrees with humidity over 65%. In Michigan, this can mean that your herd could be affected as early as April and as late as October.

Heat stress can have varying effects on cows, such as reduced dry matter intake, reduced milk production and delayed reproduction. Rumination, lactation and high feed intake result in dairy cattle having a higher body temperature from internally produced heat. As a result, the first instinct of a heat-stressed cow is to consume less feed to lower her body’s workload and temperature. A dairy cow will also increase her standing time in an attempt to cool her body down by increasing air surface area.

It does not take long after the decline in feed intake and decreased lying time for milk production to decrease. During the hottest times of the year, cows have been known to drop up to 10 pounds of milk per day. Milk production and fertility will start to decrease when a cow’s body temperature reaches 102.2 degrees. When the heat stress becomes so great that the cow’s body temperature increases to 104 degrees, an embryo in the first three days of conception will not develop.

While the most emphasis is put on lactating cows, dry cows and young stock are also susceptible to stress in the heat of summer. Research has shown for years that milk production of cows cooled during the dry period is significantly higher than their non-cooled counterparts. This is because cows who experience heat stress at any time during the dry period have lower cell metabolism, less cell growth and high cell death in the udder. Cows experiencing heat stress also have lower immunity. When these cows are vaccinated, they do not have the proper immune response to the vaccines. Colostrum quality from heat-stressed dry cows will in turn be reduced.

Heat stress can also have a lasting impact on the unborn calf. Cows that experience heat stress have lower birth weights and weaning weights for their calves when compared to cows that have cooling available in their environment.

Research has shown that calves from dams who experienced heat stress during the dry period have a lower IgG absorption than calves from dams who were cooled during the dry period. This occurred because in utero heat stress accelerated gut closure for those calves.

Many farms invest in cooling systems such as fans or sprinklers. These are preventive measures to ensure your cattle will stay cool. Cattle housed inside need plenty of water, adequate ventilation, fans and sprinkles to help keep them cool, while outside cattle should have shade and water available. It is important to make sure that the cooling system is working efficiently in your barn.

Michigan State University Extension suggests wind speed should range from 5 to 7 miles per hour, and water should soak animals but not pool on the ground. Fans should be no farther than 10 times their blade diameter apart. Fans should be angled at 10 to 15 degrees and point toward the bottom of the fan in front of it.

The Michigan State Extension dairy team is conducting a field study focused on heat stress in dairy cattle throughout Michigan. The team will be evaluating the extent to which cattle experience heat stress, along with the efficiency of the existing cooling systems. The research around heat stress in dairy cattle has been evolving rapidly over the last decade. It has become known that the only way to know the extent of heat stress is to take the temperature of the cow.

Through this field study, thermometers attached to vaginal implants will read every five minutes for five days. The implants, along with heat humidity monitors and wind monitors throughout the farm, will help collect data.

For more information on heat stress, contact MSU Extension dairy educator Marianne Buza, [email protected].

Buza is an MSU Extension dairy educator.