Prolonged interovulatory interval and hormonal changes in mares following the use of ovuplant™ to hasten ovulation
In its first year of commercial availability in the United States, reports from the field indicated that Ovuplant™ (a deslorelin-containing slow-release implant for hastening ovulation in mares) was associated with a delayed return to estrus in mares not becoming pregnant. Supposedly this effect was particularly prevalent in mares subsequently administered PGF2α to cause luteal regression after embryo collection. The present experiment was conducted 1) to determine if the field observations were repeatable under controlled experimental conditions, and 2) to gather endocrine data that might yield information on the underlying cause(s) of this observation. Twenty-five light horse mares were used. Ovaries of each mare were examined by transrectal ultrasonography daily during estrus until ovulation. Once a follicle >30 mm was detected, the mare received either Ovuplant (treated group; N = 13) at the recommended dosage or a sham injection (controls; N = 12); treatments were administered in a manner to ensure that they were unknown to personnel involved with data collection. On day 7 after ovulation, each mare received a luteolytic injection of PGF2∂. Mares were examined every other day until return to estrus or development of a 30 mm follicle, at which time daily examination was performed until ovulation. Jugular blood samples were collected daily. Two mares receiving Ovuplant did not return to estrus within 30 days and their data were not included in the statistical analyses; in contrast, no control mare exhibited such an extended interovulatory interval. For all other mares receiving Ovuplant, the interval between the first and second ovulations was longer (P = .0001) than that of control mares by an average of 6.2 days. In addition, plasma LH concentrations were lower (P < .05) in the treated mares on days 0 through 4, 9, 11, 18, and 19 after the first ovulation. Plasma FSH concentrations were also lower (P = .017) in treated mares from days 4 to 11 and on days 6 and 5 prior to the second ovulation (P = .005). Differences in progesterone and estradiol were observed but were less consistent than for LH and FSH. Mares receiving Ovuplant had fewer small (P =.026), medium (P = .003) and large (P = .045) follicles prior to the second ovulation. In conclusion, Ovuplant treatment at the recommended dosage decreased follicular activity after ovulation and increased the interovulatory interval in mares short-cycled with PGF2∂. These effects appear to be mediated by a hyposecretion of LH and(or) FSH.
Publication Source (Journal or Book title)
Journal of Equine Veterinary Science
Johnson, C., Thompson, D., Kulinski, K., & Guitreau, A. (2000). Prolonged interovulatory interval and hormonal changes in mares following the use of ovuplant™ to hasten ovulation. Journal of Equine Veterinary Science, 20 (5), 331-336. https://doi.org/10.1016/S0737-0806(00)70421-7