This has been a hot summer in many parts of the U.S.

In addition to monitoring impact of heat stress on dry matter intake and milk production, it’s important to monitor milk composition, points out Maurice Eastridge, professor and extension dairy specialist at Ohio State University.

He suggests looking at these possible effects:

•    Milk fat percentage often decreases during heat stress. Cows may be sorting through the feed, reducing fiber intake. Or, cows may be losing saliva via panting, which could mean less buffer entering the rumen.

•    Milk protein percentage also may decrease during heat stress. With the reduction in dry matter intake, there is reduced amino acid intake and consumption of fermentable organic matter, which can reduce microbial protein synthesis.

•    The somatic cell count often increases during the summer, which results in reduced milk yield and can lead to a reduced price for the milk. In addition to the pathogen load increasing, the cow’s ability to fight the infection may be compromised during heat stress.

•    Milk urea nitrogen often changes during the summer. Typical concentrations should be 10 to 15 mg/dl; however, changes even within this range can signal problems.

Changes in milk composition should be monitored as frequently as the data are available, Eastridge says. Changes in milk fat may be occurring within groups but not the bulk tank (or the overall herd average from DHI), so changes should be monitored within groups and for the herd.