Submitted to: Research Update for Fort Keogh Livestock and Range Research Laboratory
Publication Type: Popular Publication
Publication Acceptance Date: 6/30/2005
Publication Date: 7/15/2005
Citation: Geary, T.W., Perry, G.W., Smith, M.F. 2005. Cidrs induce estrous cycles, melengestrol acetate (mga) does not. 2005 Research Update for Fort Keogh Livestock and Range Research Laboratory p.13. Interpretive Summary: The anestrous postpartum period and the occurrence of short estrous cycles are major factors in cows not conceiving during a defined breeding season, and treatment with some progestins before the breeding season can successfully induced ovulation and eliminated the occurrence of short estrous cycles. However, in the current study MGA was not as effective at inducing ovulation in early postpartum anestrous beef cows as CIDR treatment. In addition, treatment with a CIDR resulted in a normal luteal lifespan following ovulation compared with a short luteal phase in cows treated with MGA. Therefore, not all progestins are equally effective at inducing ovulation and eliminating short estrous cycles in early postpartum anestrous cows.
Technical Abstract: 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.