Effects of short-term cooling on pregnancy rate of dairy heifers under summer heat stress

Thermal stress in cattle results in major decreases in dairy production and reproduction. A study was designed to evaluate the influence of temporary cooling on pregnancy rate in Holstein heifers during summer heat stress. Estrus was synchronized with two injections of prostaglandin (PG), administrated 11 d apart, and all heifers were housed in a shaded enclosed structure at the time of the second PG injection. After estrus detection, heifers were randomly divided into three groups: Control (C; n = 30), Sprinkler (S; n = 30) or Sprinkler and Fan (SF; n = 30). Rectal temperatures were measured 2.5 hours before AI, at the time of AI, and 1.5 and 3.5 hours after AI. Group C heifers receive no further treatment, but heifers in S and SF groups were exposed to short-term cooling from 2 hours before until 2 hours after AI with a sprinkler (S) or sprinkler and a fan (SF), respectively. Estrus detection and AI were performed by a single skilled technician; semen was from a bull of known fertility. Rectal temperature did not differ among groups 2.5 hours before AI, but at the time of AI was lower (P < 0.05) in SF group (39.3 ± 0.0°C) than in S (39.5 ± 0.0°C) and C (39.9 ± 0.1°C) groups which also differed (P < 0.05). At 1.5 and 3.5 hours after AI, rectal temperature remained lower in SF group (38.9 ± 0.0 and 38.7 ± 0.0°C, respectively) than in the S (39.4 ± 0.0 and 39.2 ± 0.0°C, respectively) and C (39.3 ± 0.0 and 39.3 ± 0.0°C, respectively) groups, which no longer differed. Pregnancy rate following AI was higher (P < 0.05) in SF group (56.7%) than in the C group (23.3%) with the S group (40%) intermediate and not different from either. The present study results indicate that cooling of dairy heifers for a short time before and after AI, especially with sprinkler and fan, can increase pregnancy rate during heat stress.