Heat stress is one of the primary obstacles to efficient animal production. It can cause major negative effects on milk production and reproduction, influencing profitability of the dairy operation. Some estimates indicate the cost of heat stress across animal species is over $3 billion a year in the U.S. In addition, heat stress is a major animal welfare concern, as pointed out by Temple Grandin on her list of animal welfare priorities during the recent International Beef Welfare Symposium in Iowa.
There were about 100 abstracts related to heat stress in animals, mostly dairy cattle, during the annual Dairy and Animal Science Meetings in Kansas City, July 2014. There were presentations about investigating the effect of heat stress during the dry period, the mechanisms of heat stress effects on cow physiology, the use of alternative cooling options, cost of heat stress in various states, etc.
University of Florida and University of Georgia summarized the overall benefits of providing evaporative cooling to dry cows to be approximately 8.5 pounds per cow per day during the following lactation. In the 11 studies summarized, the benefit ranged from 3 to 16.5 pounds per day. In addition, heat stress during the dry period appeared to increase the incidence of uterine diseases, but cooling did not reduce the impact, despite improving measures of immune function. Heat stressed cows consume less feed and have compromised mammary growth. Maternal heat stress of cows during the dry period also has been shown to have negative effects on their calves. Calves from heat stressed cows had lower birth weight, impaired passive immunity and high mortality prior to weaning. Their daily gain and overall growth was compromised. They also can become less productive heifers in their first lactation; in one study, producing about 9 pounds less milk per day than heifers born from cooled cows.
Iowa State University research indicated that there are still numerous knowledge gaps on how heat stress affects animals. We used to think that the negative effect on milk production was a result of reduced feed intake. However, there is a need to better understand the tissue- and organ-specific consequences of heat stress, as it is not as simple as just a reduction of intake. For example, how do the liver, muscle, adipose, mammary, and ovarian systems respond to heat stress and resultant endotoxemia and inflammation? What about gut health? Baumgard summarized that the reduction in feed intake only accounts for about 50% of the reduction in milk yield. We don't know yet what else is going on in the cow and we have to learn more in order to develop effective cooling strategies.