Endocrine control of ovulation rate in the cow
Price, Christopher Alan
Techniques to increase prolificacy in cattle have met with limited success; the aim of these studies was to investigate the potential of actively immunising cattle against certain gonadal hormones, and to examine the physiological basis of these treatments. Eight heifers were immunised against 8mg of a testosterone conjugate in Freund's Incomplete Adjuvant, & nine animals served as controls. These heifers were given one priming and two booster injections at four-month intervals. After the last booster injection, 7/8 animals had become anoestrous, and displayed significantly raised blood progesterone & mean LH concentrations,increased LH pulse frequency, & decreased mean FSH concentrations.Seven months after this treatment, 3/7 anoestrous heifers resumed ovarian cyclicity, with a mean ovulation rate of 2.7+0.7.To determine if the different ovarian responses observed above could be obtained by changes of ovarian steroid feedback seen during the oestrous cycle, groups of 6 heifers were implanted with large,medium or small sized oestradiol capsules during the luteal phase of the cycle. Five control heifers received empty implants. During the luteal phase of the cycle following implantation, all heifers were ovariectomised. The effect of the treatment on ovarian function and gonadotrophin secretion in the presence or absence of progesterone (PRID) was then determined. Increasing physiological concentrations of oestradiol reduced the number of large antral follicles and corpora lutea, but not the total number of antral follicles >lmm diameter. A combination of progesterone and oestradiol were fully effective in maintaining luteal-phase concentrations of LH and FSH, and follicular-phase concentrations of oestradiol alone were able to maintain LH and FSH concentrations within the physiological range. Thus changes of blood steroid levels similar to those seen during the oestrous cycle may interrupt ovarian function.Cattle were therefore immunised against a non-steroidal, partly purified fraction of ovine follicular fluid (PPFF) enriched ininhibin-like activity as measured in vivo and in vitro. Active immunisation against 0.4mg and 4mg ovine PPFF produced 1/5 & 3/5 heifers with multiple ovulations, respectively; this was not associated with changes of FSH secretion. To examine in more detail the endocrine responses to this treatment, and to investigate possible comparative aspects, cows were immunised against 4mg ovine, porcine or equine PPFF. No treatment increased ovulation rate, but the porcine-PPFF immunised heifers showed a 7-fold increase in mean LH secretion that could not be explained by alterations in pulsatile secretion or in steroid feedback.Collectively, these results suggest that the cow does not respond consistently to treatments so far designed to alter gonadotrophin secretion, that inhibin is not a major feedback hormone in this species, and that the heifer may possess an influential intra-ovarian control mechanism which ultimately determines ovulation rate.