Cows can tolerate greater temperatures during the day when ambient temperature during the night drops below 70°F. This suggests that whenever necessary (e.g., successive hot days), strategies are needed to reduce temperature during the night. In addition, it has also been reported that intake and milk production drops on a given day are more related to the climate of the two previous days (West, 2003).
Intake reduction is less accentuated in cows with lower feed intake and/or lower productivity. Cows around mid-lactation (100-180 days in milk, DIM) are those affected the most, followed by those at the end of the lactation (180-260 DIM), and finally cows in early lactation (less than 100 DIM). Cows in early lactation had the lowest DMI; however, they were the ones that produced the most milk. This suggests that cows at the beginning of the lactation use body reserves to compensate for the effects of environmental heat. In addition, the effects of heat stress also depend on the number of lactations. Pregnant multiparous cows in mid-lactation subjected to heat stress had greater intake reductions (22%) than first-lactation cows (9%) of similar lactation and gestation stage.
As discussed before, heat stress reduces intake and in turn milk production. However, experiments at the University of Arizona have demonstrated that the intake reduction caused by heat alone is responsible for only 40% to 50% of the drop in milk production. Other biological changes, such as those in the endocrine system and the increase in maintenance requirements, also contribute also to the reduction in energy available for milk production. In addition to the intake reduction, cows under heat stress look for shade and circulating air, increase water intake, reduce physical activity, increase respiratory cycles (panting), and increase sweating.
Improving the Facilities
Providing clean, fresh water, enough shade, and adequate air circulation is critical to maintain production. When cows are subjected to heat stress, a significant portion of their body heat is eliminated through the skin. However, as ambient temperatures increase, heat losses through the lungs increase, and those through the skin are reduced. Cows increase their respiration cycles (panting) to augment heat elimination through evaporation. Heat losses through the skin increase when both skin and coat are soaked. The body temperature leads to water evaporation from the surface, dissipating heat. One of the most effective strategies to improve intake and milk production is to install water sprinklers along the feed bunk. These sprinklers eject big drops that are distributed along the cow's back while she is eating in the bunk. Water drops must be large enough to soak the coat and skin of the animal and have to be applied intermittently to allow time for them to evaporate before the next wetting cycle begins.