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Animal Health

Summer brings heat stress potential

Written by Heather Smith Thomas

Heat stress and death loss from heat stress cost cattle producers millions of dollars each year.

Hot weather is hard on calves and fat cattle and also affects fertility and reproduction rates.

Calf impacts

Ram Kasimanickam of Washington State University says heat stress in calves can lead to dehydration. If a calf is sick from diarrhea, it will dehydrate quicker and more severely in hot weather, which creates more challenges for treating and saving the calf.

Producers can use a scoring system to identify the degree of dehydration.

“It is important to determine how severely the calf is dehydrated. If it’s from six to eight percent, the calf will have tight skin, dry mouth, weak suckling reflex and sunken eyes. If it is more severe, at eight to 10 percent, the eyes will be more dry and sunken, the mouth will be dry, and pulse rate will be increased,” he says.

The heart beats faster, trying to service the body’s needs with the inadequate fluid in the circulatory system.

“With extreme dehydration, there is 10 to 14 percent dehydration. The calf will not be responsive to stimuli and has cold extremities. It will slip into a coma and eventually die,” he says. “These are the signs we need to look for in a calf suffering from heat stress.”

Calves impacted by heat stress need additional fluids, supplemented with electrolytes. 

“Heat will interfere with growth rate of calves, as well. Energy needs will be diverted to tackle the stress rather than being utilized for growth. The long-term effects include stunted growth, delay in puberty and perhaps some impact on reproductive performance,” says Kasimanickam.

In the feedlot

Animals being grown or fattened for slaughter are very susceptible to heat stress because they have more body mass than a younger animal and more problems dissipating body heat.

“Every year there are some cattle lost during a severe heat wave. Dr. Terry Mader in Nebraska did heat stress studies looking at diet and indirect losses. He estimated between $4,000 and $5,000 lost in value of the surviving cattle for each animal that actually dies, and those values would be even higher now,” says Kasimanickam.

“Dr. Mader came up with recommendations on feeding systems and diets to mitigate heat affects,” he continues.

Mader used three groups of steers in one study, offering one group 60 percent roughage in a finishing diet at free choice. Another group was offered 85 to 95 percent roughage, and the third group 28 percent roughage diet, free choice.

“The cattle with the 28 percent roughage diet had lower respiration rates and significantly lower body temperature in hot conditions than the other two groups. This tells us that cattle cannot adequately dissipate heat produced during the fermentation digestion of roughages,” he explains.

Body temperatures

“Core body temperature usually peaks a couple of hours after the peak in ambient temperature, and it may take another six hours to dissipate the heat load. If we add a high body condition score, obese cattle take longer to dissipate the heat,” Kasimanickam explains.

If air temperature doesn’t drop much at night, these cattle may not get down to normal body temperature before morning. If hot weather lasts very many days, the rise in body temperature is cumulative, and they eventually suffer from severe heat stress.

“The accumulation has more impact on their body systems and may lead to acute death from heat stress. Producers need to be aware of weather patterns and try to prevent the losses. They can use the USDA Agricultural Research Service (ARS) forecasts for heat stress to help make management decisions,” he says.

Mitigation

“Water should be provided in several areas, so all cattle can have access,” Kasimanickam says.

Otherwise some of the dominant animals may stand near the water source and not allow the more timid animals to get a drink.

“It is also advisable to not handle, move or work cattle in extreme heat. It’s wise to avoid having cattle waiting in the processing area for more than 30 minutes. Standing in confined groups will add to the heat stress,” he says.

Hot air flow can also have a serious impact, and it may help to have some kind of mound or windbreak where cattle can stand on the other side of the flow.

“The impact of heat on cattle in a pasture will be different from the impact on cattle in the feedlot,” Kasimanickam explains. “The feedlot animals will be stressed the most.”

Reproduction problems

Hot weather can also reduce conception rates.

“The stress increases cortisol levels, and this, in turn, alters the hormones which are important for reproduction. During hot weather there is reduced ovulation,” says Kasimanickam.

“Heat stress can also affect early embryonic development if the cow does become pregnant. When we observe the conception and pregnancy rate on a farm, we notice the problem occurring not during the hot months, but a few months later, since it takes time to show these effects,” he explains.

Kasimanickam says that the gametes in both the cow and bull produced during hot months are effected. It takes 90 to 130 days to develop the gametes.

“This is why we see the impact later, such as in the fall, rather than during the hot months,” he says.

“Recent findings are also interesting, looking at the impact of maternal stress on the developing fetus,” he adds. “We now have a better understanding about how maternal heat stress might affect offspring.”

The fetus suffers stress during development that will have a lasting impact after birth on future reproductive performance of that calf when it grows up. The effects observed in later life are not so much due to how the animal was fed and raised or the environment it grew up in as due to the impact during fetal development.

“If the mother is suffering from heat stress, this can have a serious impact on the developing fetus. It may have a negative affect on the calf’s immune function and impair metabolic function and/or the future reproductive function,” Kasimanickam stresses. “There can be a multitude of adverse affects, so it is important to take good care of the pregnant animal to avoid these impacts on offspring. Their productive and reproductive life will be affected by maternal stress.”

Most people just see that the cow had a live calf and figure that everything will be okay.

“The production and reproduction abilities of both male and female fetuses will be affected when they become adults,” Kasimanickam comments.

Heather Smith Thomas is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to This email address is being protected from spambots. You need JavaScript enabled to view it..