Estrous behavior and initiation of estrous cycles in postpartum Brahman- influenced cows after treatment with progesterone and prostaglandin F2α

Spring-calving, crossbred (1/4 to 3/8 Brahman) primiparous (n = 56) and multiparous (n = 102) beef cows were used to evaluate the effects of progesterone, delivered via a controlled internal drug-releasing (CIDR) device, and prostaglandin F2α (PGF2α) on estrous behavior, synchronization rate, initiation of estrous cycles, and pregnancy rate during a 2-yr period. To determine luteal activity, weekly blood samples were collected 3 wk before initiation of a 75-d breeding season. Treated cows received a CIDR for 7 d beginning on d −7 of the breeding season. On d 0, CIDR were removed, and cows receiving CIDR were administered PGF2α; control cows received no treatment. Cows were exposed to bulls, and estrous activity was monitored using a radiotelemetry system for the first 30 d of the breeding season. Treatment with CIDR-PGF2α increased (P < 0.05) the number of mounts received (22.5 ± 3.0 vs. 13.7 ± 3.9 for CIDR-PGF2α vs. untreated control cows, respectively) but did not influence duration of estrus or quiescence between mounts. Number of mounts received and duration of estrus were greater (P < 0.05) in multiparous compared with primiparous cows. Synchronization of estrus was greater (P < 0.05)