Feeding strategies for high-producing dairy cows during periods of elevated heat and humidity.

Summary A re-evaluation of the temperature humidity index (THI) confirmed that current THI values underestimate the severity of heat stress levels. Previously, a THI equal to or greater than 72 has been used to define onset of heat stress, but our research demonstrated that a THI greater than or equal to 68 is low enough to cause adverse affects when cows suffer from heat stress. Furthermore, we were able to demonstrate that when average THI is 68 or higher, the milk yield loss becomes highly significant after 17 hours of exposure. Supplementation of lactating dairy cows with encapsulated niacin proved to alleviate some affects of heat stress during acute thermal stress. This was observed through increased evaporative heat loss, increased water intake to support the increased sweating rate, and decreased rectal and core temperatures. Milk production was not affected by the supplementation of encapsulated niacin.