Characteristics of the estrous cycle and antioxidant status of lactating Holstein cows exposed to heat stress.

Follicular growth, lifespan of the corpus luteum, and antioxidant status of lactating Holsteins that experienced heat stress were monitored. Eleven multiparous cows, 60 to 110 d in milk, were maintained from 0800 to 1800 h daily in environmental chambers from d 11 to 21 of the estrous cycle. Cows were randomly assigned to a heat stress (mean dry bulb temperature peaked at 38.3 degrees C) or control treatment (mean dry bulb temperatures varied from 20.8 to 25.6 degrees C). Rectal temperature and respiration rates of heat-stressed cows were higher at 1600 h than were those of control cows. The length of the estrous cycle and the interval from estrus until luteolysis were not different between treatments. Two of 6 control cows and 1 of 5 heat-stressed cows had extended cycles (> 24 d). Heat-stressed cows had more class 1 (2 to 5 mm) follicles from d 11 to 15 of the estrous cycle. Numbers of class 2 (6 to 9 mm) and class 3 (> or = 9 mm) follicles were similar between treatments. Plasma progesterone concentrations were higher for heat-stressed cows until d 19 of the estrous cycle. Treatment did not affect concentrations of alpha-tocopherol, beta-carotene, retinol, retinyl palmitate, or total protein in plasma or concentrations of malondialdehyde in muscle. In conclusion, heat stress did not extend luteal function or the length of the estrous cycle of lactating Holstein cows but did affect follicular growth and progesterone concentrations in plasma. Heat stress did not appear to increase lipid peroxidation or decrease lipid-soluble antioxidant concentrations in blood.