Fertility and hormonal responses to temporary relief of heat stress in lactating dairy cows.

The influence of temporary cooling on pregnancy rate and ovarian secretion of cortisol, estradiol and progesterone in Holstein cows was evaluated during the months of June to September. Cows were randomly assigned to one of five treatment groups: 1) evaporative cooling, 8 d; 2) evaporative cooling, 16 d; 3) refrigerated air conditioning, 8 d; 4) refrigerated air conditioning, 16 d and 5) controls maintained outdoors with access to shade. Estrus was synchronized by giving two injections of prostaglandin (25 mg). Cows in the cooled groups were placed in box stalls at the time of the second prostaglandin injection, allowing them to be cooled for approximately 3 d prior to breeding. Blood samples were taken on days 0, 5, 10, 15, 20 and 25 postbreeding and serum was analyzed for estradiol, cortisol, progesterone and a pregnancy-specific protein. Pregnancy rates as determined by palpation per rectum at 40 to 60 d post breeding were not different between cows in cooled treatments, regardless of duration or type of cooling. Pregnancy rate was higher (P<0.05) in cooled as compared to control cows. Serum progesterone concentrations were higher on Day 15 in cooled cows as compared to control cows. No differences in serum estradiol or cortisol concentrations were observed between cooled and control cows. Pregnancy-specific protein determinations indicated that 50% of both cooled and control cows conceived. Results indicate that temporary cooling increased pregnancy rates by increasing embryonic survival rates.