It is not a question of “if” we will have days in Northwest Iowa that cause heat stress in milking herds, but rather, how many days we will have. Heat stress decreases feed intake and milk production, decreases milk components including fat, lowers breeding efficiency and compromises the immune system plus the heat and humidity is a near perfect environment for mastitis pathogens to thrive.
Cows experience heat stress when the temperature/humidity index (THI) is above 72. The heat generated by a cow under those conditions cannot be dissipated without help.
Cows let us know there status if we pay attention. Mild to moderate heat stress signs include rapid shallow breathing, sweating and a decrease in milk production and feed intake. As the stress level increases, cows will begin panting and a 25 percent decrease in in milk yields and feed intake can happen.
In the ISUEO Fact Sheet “Reducing Heat Stress on Cows” author, Larry Tranel notes that cows generate heat to digest feed; and therefore eat less feed during periods of heat stress. Cows eat better at night and need a higher ratio of grain to forage dry matter. This change is helpful because the cow’s maintenance increases as she uses more energy to cool herself and eats less feed. Forages produce more body heat than grain when digested. Cows should eat at least 1-1.5 pounds of forage equivalent for each 100 pounds of body weight daily to minimize rumen acidosis or inverted fat protein tests.
Feed line soakers are an efficient way to help cow dissipate heat by the cooling effect of evaporation. With fans running continuously, wetting the hair coat for 2-3 minutes every half hour will supply the coat with 2.4 quarts of water for evaporation every hour. In addition, the sprinklers encourage cows to come to the bunk and eat. Fine misters should be avoided as they actually create a water barrier around the cow’s hide and hold in the heat. Concerning sprinkler heads, broken, worn or badly adjusted heads can waste a lot of water.
California research in 2016 showed no milk yield difference using a third of a gallon per minute verses soaking at 1.3 gallon per minute. With a soaking protocol of three minutes and then nine minutes off for cows at the feedline, water use was lowered by 73 percent.
Drinking water deters heat stress by cooling the animal internally and suppling water for evaporation. Plus, heat stress can double water requirements. Cows drink about half of their total daily intake immediately after milking, so providing adequate water as they exit the parlor is critical. We all like cool, clean water, so cleaning water tanks frequently and keeping them shaded will encourage drinking.
Tranel also noted that circulating fans are not needed in wind tunnel systems as they only circulate the moisture laden air within the barn. Ventilation fans, on the other hand, exchange the inside air for outside air thus removing the air generated as cows dissipate heat. Also, circulation fans or stir fans are much less efficient in moving air per watt than ventilating fans.
Cows standing in the holding pen are especially vulnerable so it should be a priority when considering ways to minimize heat stress. Using both properly angled fans and sprinklers are effective and will aide in cow comfort between milkings. However, adding water into a poorly ventilated holding pen will make it more humid and hot.
More detailed information is available at the Iowa State Extension Dairy Team website: http://www.extension.iastate.edu/dairyteam/