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 studies cattle showed highly variable responses ranging from increased ovulation rate and fertility through to anovulation/anoestrous or superovulation. As a consequence, interest in manipulation of ovulation rate through this approach has declined and is now focused on immunological manipulation of endogenous inhibin following successful studies in sheep. Studies have concentrated on developing a prototype inhibin-based vaccine to be used for twinning in the Australian beef industry. The prototype vaccine (with recombinant ovine inhibin-alpha.3 fusion protein and Montanide:Marcol adjuvant) has proved to be very potent and control of the degree of ovarian stimulation has not been possible. The proportion of cattle with increased ovulation rate after inhibin immunization is affected by timing of booster vaccination within the ovarian cycle, time after vaccination, vaccine formulation and possibly genotype. Physiological studies show that cattle responding to the inhibin vaccine have increased plasma inhibin binding of native bovine inhibin, high plasma FSH concentrations, greater numbers of large (or = 8 mm) follicles and fewer small (5 mm) follicles during the preovulatory wave of follicular development compared with control or non-responding animals. Significant correlations among the response parameters (i.e. inhibin binding, plasma FSH concentrations, number of large follicles and ovulation rate) have been demonstrated. The results indicate that greater understanding of the various processes of folliculogenesis will be necessary to achieve a controlled increase in ovulation rate in cattle.