Estrous cyclicity in nonlactating and lactating Holsteins and Jerseys during a Pakistani 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.