Luteal function and reproductive response in suckled beef cows after metestrus administration of a norgestomet implant and injection of estradiol valerate with various dosages of injectable norgestomet.

In an experiment replicated over 2 yr, 149 suckled beef cows were administered Syncro-Mate-B (SMB), a 6-mg Norgestomet (NOR) ear implant (in situ 9 d) in conjunction with an i.m. injection of 5 mg of estradiol valerate (EV), and either 3.0, 4.5, or 6.0 mg of NOR, 2 d after estrus. All cows were artificially inseminated at 48 h (timed insemination; TI) after implant removal (IR) and cows were reinseminated at any estrus subsequent to 24 h of TI through 30 d. Blood samples collected before treatment, every 3 d through IR, and at TI were assayed for progesterone (P4). At TI, 44, 39, and 12% of cows treated with 3.0, 4.5, or 6.0 mg of NOR, respectively, had serum concentrations of P4 greater than 1 ng/mL (3.0 and 4.5 mg vs 6.0 mg, P less than .01). Fifty-eight, 63, and 84% of cows treated with 3.0, 4.5, or 6.0 mg of NOR, respectively, exhibited a synchronized (within 5 d of IR) estrus (3.0 and 4.5 mg vs 6.0 mg, P less than .05). Pregnancy rates for the 5-d synchronized period were 38, 45, and 66% for cows treated with 3.0, 4.5, or 6.0 mg of NOR, respectively (3.0 and 4.5 mg vs 6.0 mg, P less than .05). First-service pregnancy rates were 66, 71, and 79% for cows treated with 3.0, 4.5, or 6.0 mg of NOR, respectively (P greater than .10).(ABSTRACT TRUNCATED AT 250 WORDS)