THE INFLUENCE OF GILT AGE ON THE ENDOCRINE AND OVULATORY RESPONSE TO DIFFERENT DOSES OF ESTRADIOL BENZOATE

Fifty-five prepubertal Yorkshire × Landrace gilts were randomly assigned to one of six treatments involving the injection (i.m.) of estradiol benzoate (EB) at either 10 or 20 μg kg −1 body weight at gilt ages of 130, 150 or 170 d. Blood samples were taken by jugular vein puncture at 0, 24, and 36 h, and then at 6-h intervals until 84 h, after injection and assayed for plasma luteinizing hormone (LH) concentrations. The mean magnitude of the EB-induced LH peak was 6.1 ± 0.2 ng mL −1 , and was not affected by treatment. Fifty-six percent of gilts ovulated in response to EB injection, and 34% had a subsequent ovulation. The initial ovulation rate was unaffected by EB dose, but was higher (P < 0.05) in 170 d (9.9 ± 1.0) compared to younger gilts (4.2 ± 1.1 and 5.5 ± 1.1 for 130-dand 150-d gilts, respectively). Subsequent ovulation rates were unaffected by treatment. It is concluded that EB treatment will usually induce a phasic release of LH from prepubertal gilts, but this release is not necessarily accompanied by normal estrous cyclicity. Key words: Estradiol, gilts, ovulation