It’s well-known that heat stress has a negative impact on cow fertility, but new research shows that it really hits repeat-breeders hard.

Research published in the May 2011 Journal of Dairy Science  compared the oocyte (egg) quality of heifers, peak lactation cows and repeat-breeders during summer and winter. Researchers also tracked environmental temperature and humidity, respiration rate, rectal temperatures, as well as skin temperatures for both seasons, to see what impact these factors have on reproduction.

Results indicate that the quality of oocytes can be significantly diminished for repeatbreeders during summer temperatures. Data also show that the blastocysts (the structures that form early in the embryo development process) were severely compromised by heat stress for repeat-breeders (22.5 percent) compared to those of animals in the other groups (7.9 percent). This helps explain why conception rate is lower for these animals.

The researchers conclude that these findings provide evidence of a greater sensitivity of repeatbreeders to summer heat stress. Heat stress abatement is extremely important for this subset of animals.