The use of progestins in regimens for fixed-time artificial insemination in beef cattle

Abstract

Four experiments were conducted to investigate modifications to gonadotropin releasing hormone (GnRH)-based fixed-time AI protocols in beef cattle. In Experiment 1, the effect of reducing the interval from GnRH treatment to prostaglandin (PGF) was examined. Lactating beef cows ( n=111) were given 100 mg gonadorelin (GnRH) on Day 0 (start of treatment) and either 500 μg cloprostenol (PGF) on Day 6 with AI and 100 μg GnRH 60 h later, or PGF on Day 7 with AI and GnRH 48 h later (6- or 7-day Co-Synch regimens). Pregnancy rates were 32/61 (53.3%) versus 26/50 (52.0%), respectively ( P=0.96). In Experiment 2, cattle ( n=196) were synchronized with a 7-day Co-Synch regimen and received either no further treatment or a CIDR-B device (Days 0–7). Pregnancy rates were 32/71 (45.1%) versus 33/77 (42.9%) in cows ( P<0.8), and 9/23 (39.1%) versus 17/25 (68.0%) in heifers ( P<0.05). In Experiment 3, 49 beef heifers were randomly assigned to receive 12.5 mg pLH on Day 0, PGF on Day 7 and 12.5 mg of pLH on Day 9 with AI 12 h later (pLH Ovsynch), or similar treatment plus a CIDR-B device from Days 0 to 7 (pLH Ovsynch+CIDR-B), or 1 mg estradiol benzoate (EB) and 100 mg progesterone on Day 0, a CIDR-B device from Days 0 to 7 (EB/P4+CIDR-B), PGF on Day 7 (at the time of CIDR-B removal) and 1 mg i.m. EB on Day 8 with AI on Day 9 (52 h after PGF). Pregnancy rate in the EB/P4+CIDR-B group (75.0%) was higher ( P<0.04) than in the pLH Ovsynch group (37.5%); the pLH Ovsynch+CIDR-B group was intermediate (64.7%). In Experiment 4, 266 non-lactating cows were allocated to a 7-day Co-Synch protocol (Co-Synch), a 7-day Co-Synch plus 0.6 mg per head per day melengestrol acetate (MGA) from Days 0 to 6 inclusive (Co-Synch+MGA) or MGA (Days 0–6) plus 2 mg EB and 50 mg progesterone on Day 0, 500 μg PGF on Day 7, 1 mg EB on Day 8 and fixed-time AI 28 h later (EB/P4+MGA). Pregnancy rates ( P<0.25) were 44.8% (39/87; Co-Synch), 47.8% (43/90; Co-Synch+MGA), and 60.7% (54/89; EB/P4+MGA). In conclusion, a 6- or 7-day interval from GnRH to PGF in a Co-Synch regimen resulted in similar pregnancy rates in cows. The addition of a progestin to a Co-Synch or Ovsynch regimen significantly improved pregnancy rates in heifers but not in cows. Progestin-based regimens that included EB consistently resulted in high pregnancy rates to fixed-time AI.