| Summary: | There is an increasing interest in the manipulation of ovarian follicular populations in large domestic animals because this could prove beneficial for assisted reproductive techniques such as ovum pick-up (OPU). The aim of the present study was to evaluate the effects of deslorelin slow-release implants (SRI) on the interovulatory interval, antral follicle count (AFC), number of follicles of different size ranges and plasma anti-Muellerian hormone (AMH) concentration in mares. To synchronize their estrous cycles, Haflinger mares (<i>n</i> = 12) were treated twice with a PGF<sub>2</sub><sub>α</sub> analogue. One day after the second injection (day 0), mares received a 9.4 mg deslorelin SRI (group DES, <i>n</i> = 6) or 1.25 mg deslorelin in a short-acting formulation (CON; <i>n</i> = 6), respectively. Regular transrectal ultrasonography of the genital tract was performed and blood samples were collected for the analysis of progesterone, AMH and gonadotrophins. The interval from implant insertion to the first spontaneous ovulation was 23.8 ± 10.5 days in group DES compared to 17.0 ± 3.9 days in group CON (<i>p <</i> 0.05). For the concentrations of LH, FSH and AMH, interactions between time and treatment were detected (<i>p</i> < 0.05). The AFC and the mean number of follicles with 5 to 10, 10 to 15 and 15 to 20 mm in diameter changed over time (<i>p</i> < 0.05). A time x treatment interaction was demonstrated for follicles of 10 to 15 mm in diameter (<i>p</i> < 0.05). The changes in this follicular subpopulation were reflected by increased plasma AMH concentration in group DES. In conclusion, 9.4 mg deslorelin implants show minor effects with regard to estrus suppression in mares, whereas the changes in the subpopulation of small ovarian follicles could be a promising tool for preparation of mares for OPU.
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