|Perry Jr, George|
|Smith, M - UNIV OF MISSOURI|
Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 24, 2004
Publication Date: March 1, 2004
Citation: Perry Jr, G.A., Smith, M.F., Geary, T.W. 2004. Ability of intravaginal progesterone and melengestrol acetate to induce estrous cycling status in postpartum beef cows. Journal of Animal Science 82(3):695-704. 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 have successfully induced ovulation and eliminated the occurrence of short estrous cycles. However, in the current study neither a high nor a normal dose of MGA was as effective at inducing ovulation in early postpartum anestrous beef cows as progesterone-releasing CIDRs. In addition, treatment with CIDRs resulted in a normal luteal lifespan following ovulation compared to a short luteal phase in cows treated with a normal dose of MGA and an intermediate luteal phase in cows treated with a high dose of MGA. Therefore, we conclude that not all progestins are equally effective at inducing ovulation and eliminating short estrous cycles in early postpartum anestrous cows.
Technical Abstract: The postpartum anestrous interval in beef cows is a major factor contributing to cows failing to become pregnant during a defined breeding season. Our objectives were to determine the ability of CIDRs (controlled internal drug-releasing device; 1.9 g progesterone), normal dose of melengestrol acetate (MGA; NM; 0.5 mg'hd-1'd-1), or high dose of MGA (HM; 4.0 mg'hd-1'd-1) to induce ovulation, and the effectiveness of each to eliminate short estrous cycles following the first postpartum ovulation. Multiparous beef cows (n = 100) were equally assigned to one of four treatments: CIDR, NM, HM, or control by age, days postpartum, body condition, and body weight. All cows were fed carrier (0.9072-kg'hd-1'd-1) with (NM 0.55 mg MGA'kg-1; HM 4.41 mg MGA'kg-1) or without MGA for 7 d (d -6 to 0). On d -6 CIDRs were inserted and were removed on d 0. Estrous behavior was monitored continuously from d -6 until 29 using HeatWatch (electronic mount detectors). Blood was collected on d -13, and three times weekly from d -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 HM or control treated cows, but CIDR and NM treated cows did not differ in percentage exhibiting estrus. In addition, the percentage of CIDR-treated cows that had ovulated was greater (P < 0.05) than the percentage of NM, HM, or control treated cows beginning on d 4. The percentage of cows that exhibited standing estrus before the first postpartum ovulation (CIDR 65%, NM 57%, HM 35%, and 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 d; 20/20 (100%)] compared to NM [6.2 ± 1.0 d; 3/13 (23%)], HM [9.6 ± 1.0 d; 8/14 (57%)], or control [6.1 ± 0.9 d; 4/17 (24%)] treated cows, and greater (P < 0.03) in HM compared to NM or control treated cows. In the present study, treatment of early postpartum suckled beef cows with CIDRs induced ovulation and initiated estrous cycles with a normal luteal lifespan in more cows than treatment with MGA. Treatment with MGA (normal or high dose) did not induce ovulation earlier than control cows, but a high dose of MGA did increase the percentage of cows with normal luteal lifespans following the first ovulation postpartum