Body temperature regulation in dairy cows is constantly challenged by a combination of environmental heat and that produced during rumen fermentation and nutrient metabolism. Heat stress occurs when cows cannot dissipate enough heat to maintain a core temperature below 101.3°F.
Internal heat production accelerates at higher dry matter intakes, therefore, your high producers are more sensitive to heat stress. Body temperatures elevated just 2.7°F have been shown to result in intake reductions of almost 13 pounds.
Midlactation drop severe
During lactation, cows start to drop intakes as temperatures exceed 75°F, with marked reductions above 86°F. Air velocity and relative humidity also need to be factored in. A thermoneutral zone can be assumed at 68°F and 50 percent relative humidity.
The thermoneutral zone is the temperature and moisture combination where the cow is comfortable and her organism does not need to resort to mechanisms to cool or warm up the body. When temperatures exceed 75°F, however, intake drops considerably even at 50 percent relative humidity. Intake is reduced more significantly in higher producing cows that consume more feed.
Intake reductions in groups of heat-stressed cows are most severe in midlactation while it is least severe early on in lactation. Early-lactation cows have the lowest intakes. However, they usually are the ones that produce more milk. Their advantage is that their metabolism is in “tissue mobilization mode” and can readily utilize body reserves to compensate the effects of environmental heat.
Second-lactation or greater cows that are pregnant have greater intake reductions than pregnant first-lactation cows. When cows are subjected to heat stress, a significant portion of their body heat is eliminated through the skin. As ambient temperatures rise, however, skin losses are reduced while panting hikes to augment heat emission through evaporation.
Continue reading this article published in Hoard’s Dairyman.
