IMMUNOLOGICAL MANIPULATION OF OVULATION RATE FOR TWINNING IN CATTLE

Unlike in sheep, in which immunization against androstenedione causes mild and reasonably controlled increased ovulation rate, in similar st udies cattle showed highly variable responses ranging from increased o vulation rate and fertility through to anovulation/anoestrus or supero vulation. As a consequence, interest in manipulation of ovulation rate through this approach has declined and is now focused on immunologica l manipulation of endogenous inhibin following successful studies in s heep. Studies have concentrated on developing a prototype inhibin-base d vaccine to be used for twinning in the Australian beef industry. The prototype vaccine (with recombinant ovine inhibin-alpha.3 fusion prot ein and Montanide:Marcol adjuvant) has proved to be very potent and co ntrol of the degree of ovarian stimulation has not been possible. The proportion of cattle with increased ovulation rate after inhibin immun ization is affected by timing of booster vaccination within the ovaria n cycle, time after vaccination, vaccine formulation and possibly geno type. Physiological studies show that cattle responding to the inhibin vaccine have increased plasma inhibin binding of native bovine inhibi n, high plasma FSH concentrations, greater numbers of large (greater t han or equal to 8 mm) follicles and fewer small (<5 mm) follicles duri ng the preovulatory wave of follicular development compared with contr ol or non-responding animals. Significant correlations among the respo nse parameters (i.e. inhibin binding, plasma FSH concentrations, numbe r of large follicles and ovulation rate) have been demonstrated. The r esults indicate that greater understanding of the various processes of folliculogenesis will be necessary to achieve a controlled increase i n ovulation rate in cattle.