The present study evaluated the effect of the type of norgestomet ear implant (new vs.
used) on the ovarian follicular response (experiment 1) and pregnancy per artificial
insemination (AI) (P/AI; experiment 2) of beef heifers subjected to an estradiol plus progestin
timed artificial insemination (TAI) program. 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). In experiment 2, at the beginning of
the synchronization protocol, 416 beef heifers were randomly assigned into two groups, as
described in the experiment 1. Two days after the NORG implant removal, the heifers were
reassigned to be inseminated at 48 or 54 hours after NORG implant removal. There was an
interaction (P ¼ 0.03) between the type of NORG implant and the timing of TAI on P/AI. The
timing of insemination only had an effect (P ¼ 0.02) on the P/AI when the heifers were
treated with a used NORG implant [(TAI 54 hours ¼ 41.9% (44/105) vs. TAI 48 hours ¼ 58.6%
(58/99)]. In conclusion, beef heifers synchronized with a used NORG implant plus estradiol
exhibited a higher proportion of earlier ovulations, and TAI in these heifers should be
performed 48 hours after removal of used NORG implants.