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Fourteen lactating and cycling Holsteins in each of two summers were assigned randomly to pens in a free-stall barn either with or without overhead fans to study the effect of fan cooling on certain endocrine and behavioral responses during the estrous cycle. After an adjustment period of 8 d in the first summer and 21 d in the second summer, jugular cannulas were inserted, and 25 mg of PGF2 alpha were injected. After injection, blood samples were collected frequently for 84 h in the first summer and 88 h in the second summer, followed by collection three times weekly for 3 wk thereafter each summer. Rectal temperatures were lower in the group cooled by fans than in the control group each summer. Luteal progesterone secretion tended to be greater in the fan group each summer; area under the luteal phase curve was significantly higher than for controls during the second summer. There was tendency for more preovulatory surges of LH and higher estrous responses rates in the fan group during the second summer. Thus, fan cooling of lactating dairy cows for several weeks before anticipated breeding provides potential for more efficient reproductive performance during the summer.

Data from 36 cows were used in a 2 x 2 factorial arrangement to determine the effects of breed and lactation status on estrous cyclicity during a Pakistani summer (June through October). Cows were selected from a herd of Holsteins and Jerseys imported from the United States 5 yr earlier. Ambient temperatures were highest in June and declined in the ensuing months with the onset of the rainy season. Relative humidity increased in July and August and them remained stable until the end of the study. Although early morning rectal temperatures gradually declined from June through October, late afternoon rectal temperatures were highest in August. Average early morning rectal temperatures were higher in Holsteins than in Jerseys (38.5 vs. 38.3 degrees C). Even though all cows were cyclic throughout the study, as indicated by patterns of progesterone secretion, observed expression of estrus was low (36.8%) and unaffected by breed or lactation status. Average serum progesterone concentrations were lower (2.4 vs. 3.1 ng/ml), and cortisol concentrations were higher (4.1 vs. 3.9 ng/ml), in Holsteins than in Jerseys. Breed by lactation status interactions were significant for lengths of the luteal phase and estrous cycle. Lengths for lactating Holsteins were longer than those of other groups.