Drought '99 Ohio State University Extension Hot Days Can Melt Away Milk Production, July 15, 1999 By Kyle Sharp COLUMBUS, Ohio -- This summer's hot temperatures are no day at the beach for dairy cows. Sharp increases in temperature can cause a cow's milk production to drop by as much as 10 percent, said Ohio State University Extension dairy nutrition specialist Maurice Eastridge. From May 3 through July 11, temperatures in Ohio were above normal eight of 10 weeks. The thermometer often topped 90 degrees Fahrenheit during that span. For dairy cows, which begin to adjust their metabolism to deal with excess heat in their environment when temperatures rise above 70 degrees, such high temperatures can lead to problems, Eastridge said. Cows can begin to experience mild heat stress at 75 degrees and 75 percent humidity. Moderate stress can occur at 85 degrees and 75 percent humidity. At 95 degrees and 75 percent humidity, most cows will experience severe heat stress if not managed properly. Temperatures above 100 degrees with high humidity levels can be fatal. When cows begin experiencing heat stress, they eat less feed, Eastridge said. Cows that eat less also produce less milk. Dairy nutritionists generally estimate that 1 pound of dry feed intake will supply enough nutrients for 2 pounds of milk, Eastridge said. A 15-degree-Fahrenheit temperature increase, from 70 to 85 degrees, could cause an average cow to eat about 5 percent, or 3 pounds, less feed per day. That equates into a loss of 6 pounds of milk production per day -- a 10 percent loss for the average cow. Cows are more likely to experience health problems when heat stress occurs. Bacteria in livestock houses thrive during high temperatures and could cause udder infections, Eastridge said. Too much heat also could impair a cow's fertility, causing reproductive problems in the fall. Heat stress can harm the development of follicles on a cow's ovary where eggs form. Eggs that later develop on these follicles may not be fertile. "Dairy farmers should be cautioned that if they don't take measures to reduce heat stress in their cows, it can cause a drop in milk production, lead to problems with animal health and hinder reproductive performance," Eastridge said. "Producers should use any economically feasible means to keep their cows as comfortable as possible and observe them regularly to see how well they are coping with the heat." The highest milk-producing cows are the most vulnerable to heat stress because they have the highest metabolic rate, resulting in more heat to transfer from their bodies, he said. Cows already with health problems also are at greater risk. In addition to decreased feed consumption, signs of heat stress in cattle include labored breathing or panting, excessive salivating, and standing with tongues hanging out. Cows eat less when they are hot because the more they eat the more heat is produced internally during digestion which must eventually be transferred from their bodies, Eastridge said. The blood flow to their extremities also increases, allowing more heat to transfer into the environment. "Normally, dairy farmers should begin thinking about how they will cope with summer heat stress in May," he said. "But if they haven't yet, it's just early July, and we could have more hot days in July and August, so there still are some things they can do to minimize heat stress." Some things producers can do to relieve heat stress in dairy cattle and minimize the loss of milk production during the summer months are: -- Provide adequate shade, either under trees for grazing cows or in some form of housing. -- If housing is used, make sure it is well ventilated; side walls should be open so wind can blow across the barn, or the ridge at the top of the barn should be open so rising heat can escape. -- If natural wind is not moving enough to keep facilities cool, use fans. Guidelines are available from county offices of Ohio State University Extension on how many and what sizes of fans to use. -- Provide plenty of clean water out of direct exposure to the sunlight. -- Minimize the amount of time cows spend in the holding pen prior to milking because cows are often bunched together and standing, which can cause temperatures to rise faster than normal. The holding pen should be ventilated. -- Provide mechanisms to encourage cows to eat, such as placing fans or water misters over feed bunks. Cattle only sweat 10 percent as much as humans, so the extra water sprayed on their hair will evaporate and remove more heat. Be sure the water is sprayed at appropriate levels. Too little water will only increase humidity around a cow, and too much could roll off the hair, minimize heat transfer and get the udder wet, increasing the potential for disease or infection. -- Spread out the feed ration to provide two or more feeding times per day. This keeps feed fresher, stimulating consumption. In fenceline feeding systems, push feed up to the manger more often. -- Adjust the diet to feed less forage and more nutrient-dense feeds, such as concentrates. More heat is generated in the digestion of forage than grain. Also, feeding supplemental fat can increase energy density of diets while minimizing heat produced during fermentation. -- Reduce walking distances to the milk parlor, feed bunks, water and shade. Less walking means more energy to transfer heat from the body and produce milk. -- Provide most of the feed ration during cooler periods of the day to minimize heat production when temperatures are higher. -30- All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status. Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension. TDD No. 800-589-8292 (Ohio only) or 614-292-1868   | C.O.R.N. | Newsletter | Ohioline | Publications |