When cows experience heat stress, milk production goes down — everyone knows that. But much of the drop in milk production has to do with physiological factors that are independent of feed intake.

Researchers at the University of Arizona, in an experiment, found that reduced feed intake accounted for only 50 percent of heat-stress-induced decreases in milk yield, while physiological factors accounted for the rest. In the study, one group of cows was subjected to temperatures ranging from 85 degrees to 102 degrees F for 14 days (with a bovine somatotropin injection on day 8). Another group of cows was kept in thermoneutral conditions at a constant 68 degrees F for 14 days (again, with a bovine somatotropin injection on day 8). The cows kept in thermoneutral conditions had their feed intake restricted to mirror that of the heat-stressed cows. Milk yield in the heat-stressed and thermoneutral cows dropped 27.6 percent and 13.9 percent, respectively, indicating that reduced feed intake accounted for only 50 percent of the decreased milk production. Shifts in post-absorptive metabolism may be responsible for a large portion of the remainder, the researchers wrote in the February 2010 edition of Journal of Dairy Science.

"Heat-stressed cows have increased basal and stimulated insulin levels, resulting in decreased adipose tissue lipid mobilization and apparently increased glucose utilization by peripheral tissues," they wrote.

Under normal conditions, glucose from the liver makes its way to the mammary gland where it influences lactose production and, hence, milk production. But under heat-stress conditions, some of that glucose gets diverted to muscles and organs instead as the cow goes into survival mode. During heat-stress, "anything we can do to maximize glucose production in a safe manner" is a good management strategy, says Lance Baumguard, one of the University of Arizona researchers in the study who has since moved to Iowa State University. For instance, feeding Rumensin improves the production of propionic acid in the rumen, which results in higher blood glucose levels as the liver converts propionate to glucose.

Read abstract.