Effect of Monensin on the Estrogen-Induced LH Surge in Prepuberal Heifers3

Abstract

The effect of dietary monensin on the luteinizing hormone (LH) surge following estradiol-17β (E₂) injection was investigated in prepuberal Simmental ✕ Brahman-Hereford heifers. Ten heifers, weighing approximately 260 kg and approximately 10 mo of age, were equally divided by age and weight into two groups: control (C) heifers each received 1.8 kg/d of a concentrate diet plus Coastal bermuda grass hay ad libitum; monensin (M) heifers each received the same diet plus 200 mg monensin/d. All heifers were maintained in dry lots on their respective diets for 14 d before the E₂ challenge. On d 15, all heifers were injected im with 5 mg of E₂ in corn oil. Blood samples were collected via tail vessel puncture immediately before E₂ injection and at 2-h intervals until 48 h after the E₂ injection. The samples were processed for serum and stored at —20 C until LH concentrations were measured by radio-immunoassay. Mean concentrations of LH in serum differed (P<.005) between C and M heifers and with time after E₂ injection. A treatment ✕ sampling period interaction (P<.10) indicated that maximum serum concentrations of LH (LH surge) were detected earlier (P<.001) for M (17.2 ± 1.8 h) than C (27.0 ± 6.0 h) heifers after the E₂ injection. When the data were arrayed relative to the time of the LH surge, treatment (P<.05) and sampling period (P<.001) effects were significant, but a treatment ✕ sampling period interaction was not detected. Peak LH concentration was 23.1 ± 3.0 ng/ml for M heifers and 21.6 ± 4.2 ng/ml for controls (P>.10). Duration of the LH surge was 8.0 ± .9 h in M heifers and 4.8 ± 1.6 h in C heifers (P<.001). Area under the LH surge was greater (P<.001) in M heifers than in control heifers. We conclude that dietary monensin altered the estrogen induced LH surge in prepuberal heifers. Copyright © 1982. American Society of Animal Science . Copyright 1982 by American Society of Animal Science.