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By Heather Smith Thomas Hot weather can adversely affect cattle. Heat stressed cattle don't eat well and have poor gains; in some situations they may die. This series will look at various ways to prevent or minimize heat stress, and factors involved - types of feed and feeding times, water, shade, use of fans, sprinklers, and other methods to reduce stress caused by high temperatures and humidity. Dr. Tom Welsh, Department of Animal Science, Texas A&M says cattle regulate body temperature to stay within a thermo-neutral zone, until the ambient (surrounding) temperature becomes too high. "Things that affect body temperature include metabolism (the body processes produce heat), heat of digestion, shade, water, etc. Research has looked at whether cattle should be fed a high quality or low quality forage when experiencing heat stress, and has found that high quality forage generates less heat during digestion than does the fermentation breakdown of low quality forages containing more fiber. So that's a way to put less heat load on the animal. Heat stress can cause a temporary delay in growth, and we have to rebuild that after the animal recovers and becomes rehydrated." Since cattle don't sweat as much as horses or humans, they cool themselves by increasing their respiration rate (for more evaporation in the air passages) and they often sling saliva over their backs to produce more evaporation from the body. Dr. Robert Collier of the University of Arizona says this creates more saliva production and loss, "which means the cattle are also losing bicarbonate. Bicarbonate in saliva is used in the rumen to buffer the digestive acids. If they are using a lot of saliva to try to cool themselves, this puts them at greater risk for acidosis." Several things happen, behaviorally, when cattle try to cool themselves. "They reduce their feed intake and performance, spending more time just trying to stay cool. When an animal is breathing up to 120 breaths per minute, it doesn't want to move, and it's difficult to eat," says Collier. Time of Feeding One of the most important things in a feedlot is to keep cattle comfortable. "Anything that can be done to improve environmental protection for these animals will improve their performance," says Collier. Cattle always eat more when they are cooler. At pasture, they graze in early morning and late evening, avoiding activity during the heat of the day. They try to conserve energy and not generate much body heat. "We need to take advantage of that behavioral shift in hot weather, and change our feeding time to accommodate their behavior," says Collier. There's a delay in rise of body temperature when cattle are exposed to summer heat. People tend to think cattle are hottest in mid-day when air temperature is highest, but cattle accumulate heat and there is a delay of several hours. If they have been eating, there's also an increase in metabolic heat produced. They don't reach peak body temperature until four to five hours after they finish the feed. So the longer you wait in the afternoon to feed (until sundown, for instance) the better, so the metabolic heat load they generate - which is a significant part of the problem - comes at night when it's cooler. Cattle are often fed in the morning, but that's the worst time, since heat production from eating will peak in mid-day. Cattle will eat more if fed very early or very late; they don't like to eat when it's hot. Dr. Welsh says, "To keep cattle eating at optimum levels, feed them at least two hours after the peak ambient temperature, shifting more of the feeding to evening. Then when they generate the heat of digestion, it's not during the hottest time of the day." Ration The ration fed can also make a difference. A total mix ration (TMR) is preferable in hot weather, says Collier. Then the cattle won't separate out and leave the bulkier roughage. "They tend to eat the high energy items and avoid bulky feed; the latter creates more heat during digestion since it must be fermented. They endanger themselves for acidosis and other rumen problems," he says. If you know a heat wave is coming, it's also wise to cut down the amount fed. He recommends feeding about 80 percent of the usual ration. In a life or death situation, you'd rather have them gain less for a few days, than die. Salt and minerals are also important and should always be provided in ample amount during hot weather. If the animals can't replace the electrolytes lost through sweat and other body fluids such as saliva, they are more stressed in the heat. Adjusting to the heat If cattle are unaccustomed to hot weather, or if it comes on suddenly, it takes awhile for them to adjust to it, making functional and behavioral changes. Dr. Don Spiers of the University of Missouri, Columbia says that in Missouri, they often go through a period of about a week with heat stress, then it will cool down, then have another period of heat. "We go through several cycles like that each summer. We've found that when it cools down, cattle go into a compensatory phase, where they overeat to try to make up for lack of intake. Then if you get a second or third hot spell, the animals have more problems," says Spiers. "They may have greater heat stress because they've been eating more; their metabolism is racing. We're trying to figure out how much cool time they need in order to prevent this. And how many days it takes to recover from the first heat wave, before they get hit with the next one, so we can give recommendations as to how much cool-down time they need, and how to feed them. It all depends on how cool it gets. We're looking at feed intakes, how much they change, and how that is affected by the ambient conditions," says Spiers. If hot weather comes on suddenly, it takes cattle three to four days for their metabolism to drop to lower levels, (producing less heat). "The problem, in feedlot cattle, is that you are still trying to get full feed into them, and that keeps the metabolism revved up. It's a tough call, whether to tell people to go easy on the feed. But if there's risk of losing the animals, this may be what they have to do," says Spiers. © The rest of this series will include articles on water, air movement, sprinklers, shade, risks for heat stress and heat stroke and working cattle. |
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