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Postpartum anestrous interval in beef cows is a major factor contributing to reproductive failure during a defined breeding season. Our objectives were to determine the ability of a controlled internal drug-releasing device (CIDR), or melengestrol acetate (MGA) to induce ovulation and to eliminate short estrous cycles. Multiparous beef cows (n = 75) were equally assigned by age, days postpartum, body condition, and body weight to one of three treatments: CIDR, MGA, or control. All cows were fed carrier (2 lbs•cow-1•day-1) with MGA (0.25 mg/lb) or without MGA for 7 days (day -6 to 0). On day -6, CIDR were inserted and were removed on d 0. Estrous behavior was monitored continuously from day -6 until 29 using HeatWatch electronic mount detectors. Blood was collected three times weekly from day -6 to 29. Treatment influenced (P = 0.03) the percentage of cows that were detected in standing estrus. Beginning on d 2, more CIDR-treated cows had exhibited standing estrus compared to control cows, but CIDR- and MGA-treated cows did not differ. The percentage of CIDR-treated cows that had ovulated was greater (P < 0.05) than the percentage of MGA-, or control-treated cows beginning on day 4. The percentage of cows that exhibited standing estrus before the first postpartum ovulation (CIDR = 65%, MGA = 57%, control = 30%) did not differ (P = 0.09) among treatments. Luteal lifespan following the first ovulation postpartum and the percentage of cows with a normal luteal lifespan (progesterone > 1 ng/mL for ≥ 10 d) was greater (P < 0.01) in CIDR-treated cows (14.0 ± 0.8 days; 20/20, 100%) compared with MGA- (6.2 ± 1.0 days; 3/13, 23%), or control-treated cows (6.1 ± 0.9 days; 4/17, 24%). In the present study, treatment of early postpartum suckled beef cows with CIDR-induced ovulation and initiated estrous cycles with a normal luteal lifespan in more cows than treatment with MGA, and treatment with MGA did not induce ovulation earlier than control cows.

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