In a Virginia study, Holsteins in oestrus during the summer had about half the number of mounts of similar cows in oestrus during the winter, Researchers in
Louisiana, in the south central United States, reported that the average length of oestrus in a group of Holsteins and jerseys was 11.9 h, about 5h shorter than had been reported for cooler regions. In a later study, duration of oestrus was 20h for cows maintained in cooled conditions as compared to 11 and 14 h for heat-stressed cows in temperature control chambers or the natural summer environment, respectively.