Factors that affect fertility during oestrous cycles with short or normal luteal phases in postpartum cows.

We have used a model to study infertility in postpartum cows. In mated cows with short luteal phases, daily supplementation with progestagen, beginning on day 3, failed to maintain pregnancy, despite the fact that fertilization, early embryo development and transport of the embryo into the uterus appeared to be normal. Normal embryos were transferred, on day 7 after oestrus, into cows with short luteal phases that received daily supplementation with progestagen, and embryos from cows with short luteal phases were transferred, on day 6 after mating, into cows with normal cycles. Pregnancy rates in each case were about half those achieved using progestagen-pretreated cows with normal luteal phases. In mated cows with short luteal phases supplemented with progestagen, pregnancy rates were markedly improved by removal of the regressing corpus luteum on day 4 or 5 after mating. In cows with normal cycles, fertility was lower when preovulatory concentrations of oestradiol were high for more than 3 days. This may result from persistent follicles, which develop when progesterone is low and frequency of tonic pulses of LH is high. In cows with such follicles, oocytes may resume maturation before the surge of LH, and embryos died in the oviduct before the 16-cell stage. Finally, in both early postpartum cows with transferred embryos and cows with normal cycles, excessive follicular development and high oestradiol during the luteal phase were detrimental to embryo survival. Treatments designed to maximize fertility in early postpartum cows must provide high progesterone and low oestradiol before mating and not yield luteolytic influences such as PGF2 alpha and oestradiol, both early after mating and during maternal recognition of pregnancy.