In the present study, we investigated the effects of reproductive status, size of follicles and plasma
progesterone concentrations of mares at PRIDTM insertion on the efficacy of the treatment, estrous cycle
patterns, plasma concentrations of progesterone and LH. The progesterone-releasing device (PRIDTM) was
administered intravaginally to 28 Haflinger mares for 11 days at different reproductive stages: anestrus
(n = 6), estrus (n = 11) and diestrus (n = 11). Plasma concentrations of progesterone at insertion (Day 1)
of PRIDTM differed among treatment groups (anestrus: 0.2–0.6 ngmL−1, estrus: 0.2–0.5 and diestrus:
1.6–10.8 ngmL−1; P < 0.001). Total secretion of progesterone (area under curve (AUC)) during treatment
period revealed highest values in diestrus (38.2±3.1 ngmL−1 h−1) followed by estrus (25.1±2.7) and
anestrus (21.0±0.4 ngmL−1 h−1; P < 0.05). Progesterone area under curve (AUC) was positively correlated
with initial progesterone concentrations (R = 0.5; P < 0.05), but it did not correlate with the interval
from PRIDTM removal to ovulation. Plasma concentrations of LH during treatment period, were significantly
lower in anestrous mares (184.6±28.6 ngmL−1 h−1) when compared to estrous and diestrous mares
(349.7±53.3 and 370.5±40.3 ngmL−1 h−1; P < 0.05). Follicular size at PRIDTM insertion had no effects
on the intervals from PRIDTM removal to subsequent estrus and ovulation. Follicle diameters at removal
of PRIDTM were significantly correlated with the interval from coil removal to estrus (R =−0.55, P < 0.05)
and ovulation (R =−0.72, P < 0.0004) in cyclic mares. In anestrus 0 of 6 (0%) mares, in estrus 5 of 11
(45.5%) and in diestrus 6 of 11 (54.5%) mares ovulated within a defined interval of 1 day before to 1 day
after mean interval from PRIDTM removal to ovulation. In cyclic mares, response to treatment was signif-