Incidence of Postpartum Anestrus in Suckled Beef Cattle: Treatments to Induce Estrus, Ovulation, and Conception

Abstract

Conception during the first week of the breeding season is limited by the proportion of cows that have resumed normal estrous cycles (cycling) before the beginning of the breeding season. Only 54% of 3,269 beef cows studied were cycling before onset of the breeding season. Parity, days postpartum (DPP), and body condition score (BCS) influenced the proportion of cows cycling as assessed by concentrations of progesterone in blood serum. Compared with multiparous cows, fewer (P<0.01) primiparous cows were cycling, despite calving up to 22.9 ± 1.7 d earlier. Regardless of parity, cycling activity increased curvilinearly (P<0.001) from 9% at ≤30 d to a peak of 70% at 81 to 90 d postpartum. For every 10-d interval from ≤30 to >90 d, cycling activity increased (P<0.01) by 7.5 ± 0.7%. As BCS increased from ≤3.5 to ≥6.0 (1 = thin to 9 = fat), the percentage of cows cycling increased linearly (P<0.01) by 18 ± 2% for each unit increase in BCS. Ovulation was induced most successfully in noncycling cows after injection of gonadotropin-releasing hormone (GnRH) or GnRH plus a progestin [controlled internal drug release (CIDR®) (Pharmacia Animal Health, Kalamazoo, MI) insert containing progesterone for 7 d following GnRH or 14-d feeding of melengestrol acetate (MGA®) (Pharmacia Animal Health.) ending 12 d before GnRH]. Induction of ovulation after initial GnRH injections was limited in primiparous cows until BCS were ≥5.0. In older cows, induction of ovulation increased linearly (P<0.05) with increasing BCS. As BCS increased from ≤3.5 to ≥6.0, expressed estrus increased (P<0.05), as more cycling than noncycling cows expressed estrus during the first 5 d of the breeding season. Expression of estrus in cycling and noncycling cows was greatest after treatment with prostaglandin F2α(PGF2α), when preceded 7 d earlier with a GnRH injection plus a norgestomet (NORG) implant that was removed at the time of PGF2α administration. Compared with treatments consisting of MGA® before GnRH + PGF2α, GnRH + PGF2α, or two injections of PGF2α 14 d apart (2 × PGF), pregnancy rates after timed AI (TAI) were greater (P<0.05) for primiparous cows treated with the CIDR® or NORG for 7 d, if they were inserted or implanted just prior to GnRH injection and if those cows received PGF2α 7 d later at removal of the implant, with TAI occurring between 48 and 60 h later. In multiparous cows, treatments with GnRH + CIDR + PGF2α, GnRH + NORG + PGF2α, and MGA + GnRH + PGF2α, were superior (P<0.05) to the remaining treatments. Based on detected estrus, no treatments studied produced superior conception rates. Of the systems studied, anestrous suckled cows responded best to treatments that included GnRH plus a short-term progestin to maximize ovulation induction before PGF2α administration and expression of estrus and conception after PGF2α administration.