PERSISTENCE OF THE LUTEAL-PHASE FOLLOWING OVULATION DURING ALTRENOGEST TREATMENT IN MARES

Two experiments were conducted to test the efficacy of altrenogest tre atment in mares. The response to 15-d altrenogest treatment (Experimen t 1) was characterized in 20 mares that were given 22 mg daily of altr enogest in oil (n=10) or in gel (n=10) from Day 10 to 25 after ovulati on. In 17 mares, luteolysis occurred during altrenogest treatment (Day 17.7 +/- 0.5), while 2 mares retained their corpus luteum (CL), and 1 mare had a diestrous ovulation on Day 16, resulting in a prolonged lu teal phase. Ten of the 17 mares in which the CL had spontaneously regr essed returned to estrus after the end of treatment, and ovulated 5.7 +/- 0.8 d after the end of altrenogest treatment. Two of these 17 mare s ovulated 2 and 3 d after the end of altrenogest treatment but ovulat ion was not accompanied by estrous behavior, and 5 mares ovulated duri ng altrenogest treatment resulting in an interovulatory interval of 22 .4 +/- 1.1 d (range: 20 to 25d). Five mares which ovulated during altr enogest treatment and 2 mares which ovulated during silent estrus afte r the end of altrenogest treatment failed to regress the CL around 14 d post ovulation, and had a prolonged luteal phase. In Experiment 2, t he effect of altrenogest administered from luteolysis to ovulation on duration of the subsequent luteal period was analyzed. In 6 mares altr enogest was begun on Day 14 post ovulation and continued until the hCG -induced ovulation. The interval from ovulation during altrenogest tre atment to spontaneous luteolysis was 45.6 +/- 2.4 d (range: 40 to 54d) in altrenogest-treated mares and was significantly longer than in 10 untreated control mares (14.5 +/- 0.3 d, range: 13 to 16d). The result s suggest that the oil and gel altrenogest preparations are equally ef fective in modulating estrous behavior and time to estrus and ovulatio n. Altrenogest treatment started late in diestrus appears to result in a high incidence of ovulation during treatment and when luteolysis an d ovulation occur during treatment; the subsequent luteal phase is fre quently prolonged due to failure of regression of the CL.