Luteal Function and Reproductive ResDonse in Suckled Beef

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 a t 48 h (timed insemination; TI) after implant removal (IR) and cows were reinseminated a t 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 (P3. 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 > 1 ng/mL (3.0 and 4.5 mg vs 6.0 mg, P c .011. Fifty-eight, 63, and 8 4 % 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 e .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 c .051. 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 z .lo). Syncro-Mate-B with 6.0 mg of injectable NOR resulted in fewer cows with functional CL (i.e., P4 > 1 ng/mL at TI), more cows exhibiting estrus within 5 d of IR, and a higher 5-d pregnancy rate compared with SMB with 3.0 or 4.5 mg of injectable NOR.