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.
On the topic of cooling strategies, the University of Arizona presented results of a study using a passive cooling system rather than the typically used fans and sprinklers. They used heat exchangers buried 10 inches below the surface of the stall (with either sand or recycled manure solids) as components in a conductive system for cooling cows. For each kind of bedding material, they had control stalls (water OFF) and treatment stalls (water ON) that used water at 45°F passing through the heat exchangers. They tested the system in three different climates (hot dry, thermo neutral, and hot humid) using a climate controlled room. Sand bedding remained cooler than manure bedding in all environments and at all levels of cooling (water ON or OFF). Stall temperatures were lower and heat flux higher during the Sand ON bed treatment. They also detected a reduction in core body temperatures, respiration rates, and skin temperatures of those cows heat stressed during the Sand ON treatment. Dry matter intake and milk yield numerically increased during the Sand ON treatment for all climates. Authors concluded that this type of cooling system can be an alternative to traditional cooling systems and sand was a better bedding material to use in combination with heat exchangers.
Cornell University exposed cows to heat stress in a climate controlled room from 9:30 AM until 5:30 PM daily but moved to well-ventilated pens at night. During the time cows were under heat stress, experimental cows were conductively cooled by pumping chilled water through a waterbed in their stalls, but control cows had no heat abatement. They tested two ambient temperatures and two water temperatures in the stall. Results from the higher heat stress/lower water temperature treatment showed that conductive cooling removed approximately 60% of the total metabolic heat from the cow whenever the cow was lying down. This significantly reduced the effects of the heat stress; cooled cows produced about 20 pounds more milk per day than control cows, rectal temperatures for cooled cows were about 2°F lower and their respiration rates were 64 breaths per minute compared to 84 for control cows. Results indicated that conductive cooling shows promise for mitigating heat stress in lactating dairy cattle.
A study by the University of Missouri investigated the cost of heat stress to the dairy and beef industries in the U.S. The average annual economic loss across all states on a per cow basis for dairy producers was $89.01. Minnesota was below average at $33.71 per cow, but heat stress is still something that can influence dairy profitability and animal well-being in our state.