Immunization of bull calves with a GnRH analogue-human serum albumin conjugate: effect of conjugate dose, type of adjuvant and booster interval on immune, endocrine, testicular and growth responses.

Bull calves were immunized with GnRH analogue-human serum albumin (HSA-Cys-Gly-GnRH) conjugate to determine the effects of dose, adjuvant type and interval between primary and booster injections on plasma testosterone and LH concentrations, and testes and body growth. Friesian bull calves aged between 8 and 10 weeks (n = 72) were blocked according to age and weight and, within block, randomly assigned to 12 treatment combinations (n = 6 per treatment combination) in a 3 x 2 x 2 factorial plan. Main effects were (i) conjugate dose (0.0, 0.1 or 1.0 mg HSA-Cys-Gly-GnRH), (ii) adjuvant (diethylaminoethyl-dextran or non-ulcerative Freund's adjuvant), and (iii) interval between primary (day 0) injection and booster injection (day 28 or 56). Plasma testosterone and LH concentrations and antibody titres were determined in blood samples collected at 14 day intervals during the experiment (140 days). Testicular measurements were taken in situ every 28 days. Antibody titres (% binding at 1:160 dilution) were > or = 10% 28 days after booster injection and remained high for 140 days in 47 of 48 GnRH-immunized bulls. The mean titre was higher (P < 0.05) in response to the 1.0 mg dose compared with the 0.1 mg dose (37.7% versus 29.6% binding, respectively; pooled SED 2.55%). Mean LH and testosterone concentrations were reduced (P < 0.05) in immunized animals compared with controls. However, the 1.0 mg dose decreased mean testosterone concentrations by a greater extent (P < 0.001) than either the 0.1 mg or 0.0 mg doses. Testes length and depth, and scrotal circumference were decreased (P < 0.001) in immunized animals compared with controls; however, the 1.0 mg dose decreased (P < 0.001) testes parameters to the greatest extent. There was no effect of conjugate dose on average daily gain in body mass. It is concluded that (i) dose of conjugate, type of adjuvant and interval between primary and booster injections affected antibody titres, (ii) the use of 0.1 or 1.0 mg of HSA-Cys-Gly-GnRH decreased LH and testosterone concentrations, and testicular development throughout the experiment, without adversely affecting body growth, and (iii) an effective protocol is 1.0 mg GnRH-HSA conjugate, given in the adjuvant diethylaminoethyl dextran, with a primary-booster interval of 56 days.