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College of Food, Agricultural, and Environmental Sciences


Heat Stress Impacts Not Just Plants

August 12, 2002

COLUMBUS, Ohio - Ohio farmers may be concerned over the effects warm, dry conditions are having on their crops, but they should also take note of the impacts heat stress can have on livestock.

Not every livestock animal species responds the same to heat stress, but keeping the animals cool and providing adequate water and nutrients is the common thread that helps keep livestock healthy.

Ohio State University Extension livestock specialists said during extended periods of high temperatures, lack of adequate cooling, water and supplemental nutrients can quickly lead to increased health problems, including death; poor reproductive performance; and decreased milk production, all of which result in overall reduced livestock production.

"Even though we've had some cooler days," said Roger High, an Ohio State Extension sheep specialist, "it's not just the management of heat, but also providing adequate nutrition that is important to keeping animals healthy." Livestock not only eat less during hot days, but high temperatures and dry weather reduce the amount of pasture grasses available for grazing, which can lead to inadequate feed intake or illness from consuming toxic plants and weeds that may be the only "greens" available for animals to consume.

For sheep, the poor nutrition combined with heat stress can result in an unsustainable pregnancy for ewes and infertility for rams.

"We are getting close to the breeding season and it's important to increase nutrition during that time to breed ewes and maintain that pregnancy," said High. "Ewes are not going to take on a pregnancy if they are not ready to sustain life themselves. For rams, the primary reason they will go infertile and won't breed ewes is because of heat stress." The same breeding problems hold true for swine suffering from heat stress.

"The primary characteristics of heat stress on the reproduction of female pigs are prolonged interval from weaning to estrus, reduced conception rates, reduced farrowing rates from sows mated during hot weather, delay in onset of puberty in gilts, reduced litter size, increase in abortions, and an increase in number of sows that fail to farrow," said Don Levis, an Ohio State Extension swine specialist. "Heat stress the last two weeks of gestation can increase the rate of stillborn piglets." Heat stress also has an impact on boars during breeding. With several days of 90-degree weather, fertility begins to drop in seven to 10 days and under severe heat-stressed conditions, boars can remain infertile or subfertile for as many as 50 days, said Levis.

"That can become a problem for producers who are using boars for artificial insemination or for natural services," he said.

In addition to reproductive problems, heat stress also has an impact on milk production, specifically in dairy cattle.

"As temperatures increase and the stress becomes more severe, milk production can drop 10 percent to 20 percent, even more," said Maurice Eastridge, an Ohio State Extension dairy specialist. "The main component leading into that is decreased feed intake. Cows may decrease intake by 10 percent to 15 percent and that corresponds into less milk being produced. The cows also may lose body condition to support milk production with the drop in dry matter intake." Reformulating rations with higher levels of protein, potassium and fat and providing high-energy feed with adequate fiber in the ration and providing plenty of water are some options farmers have to balance nutrient intake.

Keeping livestock cool through shading or more extensive cooling systems can also help minimize health problems and general discomfort associated with heat stress.

"The main thing if the animals are outside is to provide them with some sort of shade, whether it's shelter or trees," said Levis.

Housing, such as barns, should be well ventilated. "If a barn has curtains and windows, it would be a benefit to keeping them open to get as much breeze as possible," said Steve Boyles, an Ohio State Extension beef cattle specialist.

The Extension specialists also recommend using cooling mechanisms such as fans and sprinkler systems to help regulate body temperature.

"Swine don't have sweat glands, so they are more vulnerable to heat stress than any other livestock," said Levis. "So it's important to keep them as cool as possible." When using a sprinkler system, he added that producers should make sure the water is wetting the skin, not just the hair coat. "Water just sitting on the hair follicles is going to act as an insulation and won't cool the animal," he said.

High reminded sheep producers that wool also provides shelter for sheep. "Wool keeps them cool when it's warm and warm when it's cool. So it is not only a barrier from the heat, but also provides the sheep with more comfort," he said.

Livestock producers may find the following additional tips healthful when managing heat-stressed animals: * Don't overgraze in pastures. Typically, the taller the grass, the cooler the pasture will be.

* Consider feeding more at night rather than in the morning to shift heat fermentation to a cooler part of the day. The heat of digestion can place additional stress on the animal.

* Switch to frequent feedings since high moisture products, like silage, spoils more rapidly in hot weather. This will help control illnesses.

* Work cattle early in the morning to decrease the risk of heat stress. A danger sign in cattle is panting.

* Cattle in holding pens should be there no more than an hour. Holding pens and feed bunk areas should be well ventilated. Cool dairy cattle at feed bunks and as they exit milking parlors.

* Don't over-exercise or transport livestock during the hottest part of the day.

* Sheer rams 60 days prior to breeding to reduce additional stresses when they need to be fertile.

* When cooling down sows, be sure to keep piglets dry. Piglets require the heat.

* Keep finishing pigs as cool as possible. A comfortable finishing pig can increase in growth rate 10 percent to 12 percent, said Levis.

When dealing with potential heat stress on livestock, Normand St-Pierre, an Ohio State Extension dairy specialist, said the producers should keep in mind the weather factors contributing to heat stress and exactly how heat stress works.

"For one thing, I don't think most farmers measure humidity when it comes to determining the heat stress threshold of their livestock," he said. "Additionally, it's not the environment that is heating up the animals, it's just that the environment is no longer taking the heat produced by the animal and dissipating it at the same rate." Heat stress is brought about largely by a combination of temperature and humidity (what St-Pierre refers to as temperature humidity index) and it doesn't take much of an increase in the THI to place an animal in a potential health risk.

In Ohio, for example, the average maximum temperature in August is 83 degrees Fahrenheit and the average daytime relative humidity is 57 percent, resulting in an average THI of 77.

"Every livestock species in Ohio is experiencing some sort of heat stress during most of the day," said St-Pierre.

The heat stress threshold for milking cows is 70 THI units; 77 THI units for heifers; 72 THI units for beef yearlings; 75 THI units for beef cows; 72 THI units for finishing cattle; 74 THI units for sows and 72 THI units for feeder hogs. At 60 percent relative humidity, 70 THI units is reached at 73 degrees Farenheit, and 75 THI units at 80 degrees Farenheit.

Candace Pollock
Normand St-Pierre, Maurice Eastridge, Steve Boyles, Don Levis, Roger High