Consumer discrimination against meat products from animals treated with hormones has provided the impetus to investigate alternative ways of boosting feed conversion efficiency and growth rates of animals and to manipulate the reproductive capacity of animals. In view of the longevity of the immune response to specific antigens and the realization that antibodies are capable of neutralizing or mimicking the actions of hormones, vaccination techniques have been explored as alternatives to hormone administration to boost animal productivity. To date, only two commercial vaccines are available to improve the efficiency of production, although neither is related to the stimulation of growth efficiency: Fecundin (registered product, Glaxo Pty Ltd) is designed to improve lambing percentages, and Vaxstrate (registered product, Arthur Webster & Co.) to prevent reproductive activity. In this review, recent advances in vaccine development are explored together with some of the problems encountered in research in this field.

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Methoxinine (O-methyl-DL-homoserine) was given to 18 Merino sheep by intravenous infusion or injection in amounts of 52-100 mg/kg body weight. Effects on strength, morphology and growth rate of wool fibres were studied.On average, methoxinine reduced the strength of bundles of wool fibres to about one-third of pretreatment values, but the magnitude of the effect varied considerably between sheep. Methoxinine also reduced the staple crimp frequency over a distance of c. 5-30 mm. A loss of cuticle scale pattern on wool fibres was observed following dosing; other abnormalities included grooved cuticle scales, degraded sections of fibre and distorted fibres . ..wool growth was temporarily reduced after methoxinine administration to c. 60% of the pretreatment rate. Effects were greater and more rapid on length growth rate than on fibre diameter.None of the above effects of methoxinine was prevented by the concurrent administration of cysteine and the efficacy of the concurrent administration of methionine was equivocal. A continuation of the infusion of methionine for 4 weeks following methoxinine administration prevented the depression of wool growth and either prevented or reduced the effects on wool fibres.Following a dose of methoxinine (60 mg/kg) the concentration of methoxinine in blood plasma was 500-800 {lmolll, at the end of a 2-day infusion or 5 h after an injection. The concentration in plasma declined slowly thereafter and was 50-80 {lmolll after 5 weeks. The effects of methoxinine were not mediated via copper deficiency as judged by plasma copper concentrations and the failure to cause depigmentation of black wool.

This experiment examined the effects of immunizing against ACTH (adrenocorticotrophic hormone) and GnRH (hypothalamic peptide gonadotrophic releasing hormone) on growth and carcase composition of entire ram lambs from 6 to 35 weeks of age. There were four experimental groups (each of 17 animals) in a 2×2 factorial design: (1) a non-immunized control group, (2) immunized against ACTH, (3) immunized against GnRH and (4) immunized against both antigens. After booster injections, experimental lambs exhibited antibody titres to the two antigens presented, resulting in significantly lower cortisol and testosterone levels (P<0·001) in peripheral blood. The fall in blood cortisol levels in response to ACTH immunization was later matched by a similar decline in control animals to a point where they were not different.In GnRH-immunized animals, antibodies disrupted the pituitary-gonadal axis, thereby impacting severely on testes development in growing lambs to the degree that by day 119 their testes were smaller than at the commencement of the experiment. Towards the end of the growth phase of the animals, GnRH-immunized lambs had significantly lower body weights at day 189 (P<0·05) and day 204 (P<0·01) while ACTH-immune lambs were not different in weight from control animals. Also, GnRH-immunized lambs were found to have greater GR measurement (P<0·05). Although ACTH immunization was capable of lowering cortisol for a brief period during the current experiment, the results were variable and detrimental to the efficacy of the immuno-castration vaccine (GnRH).