Regulation of follicle waves to maximize fertility in cattle.

Cattle have recurrent follicular waves every 7-10 days in most physiological situations; an FSH increase is associated with emergence of the wave and LH pulse frequency determines the fate of the dominant follicle. To control oestrus with hormones it is necessary to ensure that either induced corpus luteum regression or the termination of a progestogen treatment coincides with the selection of the dominant follicle during the wave, to give a precise onset of oestrus and high fertility. The exogenous administration of progesterone or progestagen blocks the normal turnover of the dominant follicle once the corpus luteum regresses. Thus, the effects of duration of dominance of the preovulatory follicle on onset of oestrus and fertility were examined. The variation in onset of oestrus was reduced but occurred 5-9 h later after 4 versus 8 days of dominance; pregnancy rate was also affected with dominance periods of 2-4, 4-8 and > 10 days resulting in 0, 10-15% or 20-50% reduction in pregnancy rates, respectively. The necessity for short duration of dominance of the preovulatory follicle means that to ensure high fertility the follicular wave needs to be regulated when using hormones to control oestrus. Two approaches were examined, namely the use of GnRH or oestradiol at time of progesterone intravaginal releasing device insertion. The effect of 250 micrograms of synthetic GnRH on the fate of an existing follicle wave was to ovulate the dominant follicle (20/20 cows) and a new wave emerged 1.6 +/- 0.3 days later; however, there was no effect of GnRH on the wave if administered before dominant follicle selection. The effect of oestradiol concentrations on suppression of FSH in ovariectomized heifers showed that increasing oestradiol to 10-15 pg ml-1 caused a 37 +/- 6.9% decrease in FSH for 24 h, with a subsequent increase to pretreatment values by 57 +/- 13 h. In cyclic heifers, increasing oestradiol to > 10 pg ml-1 in conjunction with progesterone treatment at emergence of the first wave of the cycle affected the current follicle wave by either preventing dominant follicle selection or decreasing diameter of the dominant follicle, without consistently affecting the interval to new wave emergence. Increase of oestradiol after dominance, however, delayed new wave emergence by 2-5 days. A better understanding of the hormonal control of follicle waves will lead to development of improved hormonal regimens to control oestrus sufficiently to give high pregnancy rates to a single AI without recourse to detection of oestrus.