CORPUS-LUTEUM GROWTH AND FUNCTION IN LACTATING HOLSTEIN COWS DURING SPRING AND SUMMER

Corpus luteum growth and function were monitored daily for a complete estrous cycle by ultrasonography and peripheral serum progesterone in lactating, cyclic Holstein cows during spring (n = 8) and summer (n = 8). For spring and summer, respectively, daily ambient temperatures (X BAR +/- SEM) were 21.2 +/- .9 and 31.1 +/- .3-degrees-C. In summer com pared with spring, early morning rectal temperatures were higher on d 1, 2, and 3, when the corpus luteum was forming, and on d 15 and 19. T he length of the luteal phase and the corpus luteum cross-sectional ar ea were similar for the two seasonal groups. Central luteal cavities w ere observed during spring and summer. Serum progesterone secreted bet ween d 6 and 18 was lower during summer. In addition, progesterone con centrations during the entire luteal phase and the peak magnitude of p rogesterone tended to be lower during summer. Suppressed luteal functi on may contribute to low fertility when cows are inseminated during su mmer.