Heat stress (HS) causes hyperthermia, and at its most severe form, can lead to death. More commonly, HS reduces feed
intake, milk yield, growth rate and reproductive function in many mammals and birds, including the important cattle breeds
in Japan. Rectal temperatures greater than 39.0°C and respiration rates greater than 60/min indicate cows are undergoing
HS sufficient to affect milk yield and fertility. HS compromises oocyte quality and embryonic development, reduces
expression of estrus and changes secretion of several reproductive hormones. One of the most effective ways to reduce
the magnitude of HS is embryo transfer, which bypasses the inhibitory effects of HS on the oocyte and early embryo. It may
also be possible to select for genetic resistance to HS. Cooling can also improve reproductive performance in cows and
heifers, and probably, the most effective cooling systems currently in use are those that couple evaporative cooling with
tunnel ventilation or cross ventilation. Its effect on improving reproductive performance in Japan remains to be evaluated.