In experiment 1, 57 cyclic beef heifers
were randomly assigned to one of two groups according to the type (new or previously
used for 9 days) of norgestomet ear (NORG) implant. At the time of NORG implant
insertion, the heifers were treated with 2 mg of intramuscular estradiol benzoate. Eight
days later, the NORG implants were removed, and the heifers received an intramuscular
administration of 150 mg of D-cloprostenol, 300 IU of equine chorionic gonadotropin, and
0.5 mg of estradiol cypionate. The heifers had their ovaries scanned every 12 hours from
the time of NORG implant removal to 96 hours after verifying the occurrence and timing of
ovulation. No difference (P ¼ 0.89) was observed in the ovulation rates between the two
treatments (new ¼ 80.0%; 24/30 vs. used ¼ 81.5%; 22/27). However, the heifers treated
with a used NORG implant had (P ¼ 0.04) higher proportion (36.4%; 8/22) of early
ovulation (between 36 and 48 hours after NORG implant removal) compared with the
heifers treated with a new NORG implant (8.3%; 2/24).