Editor’s note: This article was written by Peter Hansen, University of Florida distinguished professor of animal science and originally appeared in the Spring 2011 Dairy Update.

One of the major factors limiting optimal production and profitability in Florida is heat stress. During the summer, cows experience declines in milk yield of 15 to 20 percent and reduction in conception rate to values as low as 10 percent.

Recent research supported by the Southeast Milk Inc. Dairy Checkoff Program indicates that a cow’s ability to regulate its body temperature and prevent the negative effects of heat stress on cow function is determined in part by its genetics. What that means is that it should be possible to select cows genetically that are more resistance to the effects of heat stress.

Serdal Dikmen, a visiting scientist from Uludag University in Turkey, conducted the research on three dairies in north central Florida. During the months of June through September, Dikmen measured rectal temperatures of lactating Holstein cows housed in free-stall barns between the hours of 3 p.m. and 5 p.m., when body temperature is highest. Over the course of two years, he recorded values from 1,695 cows that were sired by 509 different bulls. Working with John Cole of the USDA Animal Improvement Laboratory and myself, he used the data to obtain estimates of the heritability of rectal temperature.

Heritability is an estimate of how much animals vary from each other because of difference in genetics.

The heritability of rectal temperature was found to be 0.21, which means 21 percent of the variation between cows in rectal temperature was due to differences in genes between the cows.

By comparison, the heritability of milk yield was 0.36 and the heritability of productive life was 0.16. The heritability of rectal temperature is high enough that you could expect to improve resistance to heat stress by selecting for rectal temperature in the summer. It is also likely that the bovine gene chip currently available for genetic testing of cattle could be used to speed up the rate of progress in selection for rectal temperature.

Our team also found that there was no genetic correlation between rectal temperature and milk yield but that cows that were genetically more likely to have low temperatures in the summer were also slightly more likely to have genes that improved somatic cell counts, productive life, daughter pregnancy rate and net merit.

What this means is you can select for rectal temperature without selecting for low milk yield and can expect some slight improvement in genetic merit for heath and reproduction traits.

Source: Dairy Update, University of Florida