A 122-day trial was conducted with grazing beef heifers (N=120; 14 to 17 months of age) to determine the effects of a
silastic Norgestomet prototype implant on heifer
weight gain and on the suppression of ovarian luteal activity. The retention rate of the
Norgestomet implants and of
silastic placebo implants, which were either left untreated, or were topically treated with
oxytetracycline, was determined. Medication of the placebo implants with
oxytetracycline increased (P < 0.01) implant retention rate at both Day 56 and Day 122. Heifers with
Norgestomet implants had higher
body weight gains during the last 66 days (P < 0.05), and during the entire 122-day trial (P < 0.07) than heifers with placebo implants. Serum
progesterone concentrations on Day 111 or Day 112 indicated a decrease (P < 0.01) in percentage of heifers with ovarian luteal activity on the
Norgestomet treatment compared with heifers receiving
placebos (23.8% vs 87.7%, respectively). The
Norgestomet implant has the potential for suppressing ovarian luteal activity while improving
weight gain in beef heifers.