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Abstract
Although the ovulatory effects of prostaglandins are well documented in several domestic species including 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 diestrous 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 twofold: to investigate the cloprostenol dose rate effect on interval to ovulation and to confirm the negative correlation between follicular diameter and 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 or with smaller follicles. To test the hypothesis, a total of 1,234 estrous cycles were induced with different doses of cloprostenol (ranging from 8.75 to 625 μg). All mares had at least one follicle of 28 mm or larger. Dominant follicles were followed by transrectal ultrasound examinations every other day until ovulation was detected. There was a significant effect of dose ( P < .000) and follicular diameter ( P < .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 or larger, whereas the longest (4.9 days) occurred after 8.75 μg in follicles of 28 to 31 mm.
Highlights
Prostaglandin (PGF) and its analogues are used largely as a luteolytic drugs in animal reproduction, especially in horses and cattle
There was a significant effect of Cloprostenol dose on interval to ovulation (P = 0.000)
The linear regression showed a negative effect of Cloprostenol dose on interval to ovulation (P = 0.000) with larger doses inducing shorter intervals to ovulation
Summary
Prostaglandin (PGF) and its analogues are used largely as a luteolytic drugs in animal reproduction, especially in horses and cattle. During the first three to four days post ovulation, the corpus luteum (CL) is thought to be refractory to the recommended dose rate of PGF (9 mg / kg) and its analogues (1 ml of Estrumate, 250 μg Cloprostenol)[1,2] so that conventionally PGF is not administered until the CL is at least 5 days old. At this stage any developing follicles are less than 20 mm in diameter and may take up to 12 days to develop and ovulate (unpublished data). The result is that the follicle may ovulate soon after PGF administration, depending on its diameter and degree of maturity
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