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The effects on pregnancy rate of three different treatments to remove intrauterine fluid were assessed in 1267 mares. The mares were mated and allocated, in strict rotation, to four treatment groups: 1) untreated controls, 2) intrauterine infusion of broad spectrum antibiotics, 3) intravenous injection of oxytocin, 4) intravenous injection of oxytocin followed by intrauterine antibiotics. The pregnancy status of the mares was determined 13 to 15 days and 27 to 30 days after ovulation by transrectal ultrasonography. The pregnancy rate of group 4 (72 per cent) was higher than that of group 2 (64 per cent, P < 0.01) or group 3 (63 per cent, P < 0.01). The pregnancy rates of groups 2 and 3 were higher than that of group 1 (56 per cent, P < 0.01). The treatment with antibiotics and oxytocin appeared to have an additive beneficial effect which suggested two different modes of action of the combination treatment, namely antibacterial activity and fluid drainage. In the untreated mares more fluid accumulated in the uterine lumen after mating, and this was the most likely reason for their lower pregnancy rate.

Although the ovulatory effects of prostaglandins are well documented in several domestic species included horses, there has been little attention paid to the use of this ovulatory effect for clinical purposes. Mares often grow large follicles during the luteal phase that may or may not ovulate before progesterone levels decline. Clinical observations of administering prostaglandins in diestrus mares with large follicles suggest that there may be a negative correlation between follicular diameter and interval from treatment to ovulation. The objectives of this study were two fold: to investigate the Cloprostenol dose rate effect on interval to ovulation and to confirm the negative correlation between follicular diameter and the interval to ovulation. The hypothesis tested was that high doses of Cloprostenol given in diestrus to mares with larger follicles would induce ovulation more rapidly than in mares given lower doses and or with smaller follicles. In order to test the hypothesis, a total of 1234 estrous cycles were induced with different doses of Cloprostenol (ranged from 8.75 to 625 µg). All mares had at least a follicle ≥ 28 mm. Dominant follicles were followed by transrectal ultrasound examinations every other day until ovulation was detected. There was a significant effect of dose (P = 0.000) and follicular diameter (P = 0.000) on the interval from treatment to ovulation. The shortest mean interval (2.4 days) was observed after administration of 625 µg in mares with follicles ≥ 36 mm whereas the longest (4.9 days) after 8.75 µg in follicles of 28-31 mm.

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