IMMUNIZATION OF BULL CALVES WITH A GNRH ANALOG 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 (KSA -Cys-Gly-GnRH) conjugate to determine the effects of dose, adjuvant ty pe and interval between primary and booster injections on plasma testo sterone and LH concentrations, and testes and body growth. Friesian bu ll 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 factori al plan. Main effects were (i) conjugate dose (0.0, 0.1 or 1.0 mg HSA- Cys-GIy-GnRH), (ii) adjuvant (diethylaminoethyl-dextran or non-ulcerat ive Freund's adjuvant), and (iii) interval between primary (day 0) inj ection and booster injection (day 28 or 56). Plasma testosterone and L H concentrations and antibody titres were determined in blood samples collected at 14 day intervals during the experiment (140 days). Testic ular measurements were taken in situ every 28 days. Antibody titres (% binding at 1:160 dilution) were greater than or equal to 10% 28 days after booster injection and remained high for 140 days in 47 of 48 GnR H-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% bin ding, respectively; pooled SED 2.55%). Mean LH and testosterone concen trations were reduced (P < 0.05) in immunized animals compared with co ntrols. However, the 1.0 mg dose decreased mean testosterone concentra tions 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 decrea sed (P < 0.001) in immunized animals compared with controls; however, the 1.0 mg dose decreased (P < 0.001) testes parameters to the greates t 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 adju vant and interval between primary and booster injections affected anti body titres, (ii) the use of 0.1 or 1.0 mg of HSA-Cys-Gly-GnRH decreas ed LH and testosterone concentrations, and testicular development thro ughout the experiment, without adversely affecting body growth, and (i ii) an effective protocol is 1.0 mg GnRH-HSA conjugate, given in the a djuvant diethylaminoethyl dextran, with a primary-booster interval of 56 days.