Efficacy of short-term administration of altrenogest to postpone ovulation in mares

Abstract

Estrogen from a growing follicle stimulates the preovulatory surge of luteinizing hormone (LH) unlike the suppressive effects of progesterone. The possibility exists that administration of progesterone, in the presence of an ovulatory follicle, would sufficiently suppress LH thereby delaying ovulation. The objective of this experiment was to elucidate the potential for oral administration of altrenogest (17-Allyl-17phydroxyestra-4,9,1 1-trien-3-one) to postpone ovulation of a preovulatory follicle (35mm) for greater than 48h. Fourteen light horse mares, ranging in age from 2-19y were randomly assigned to three treatments (A-.044 mg/kg BW; B-.088 mg/kg BW; and no altrenogest) over three consecutive estrous cycles. Mares received the treatment dose when a >35mm follicle was observed via transrectal ultrasonography. Control mares were bred on the first day a 35mm follicle was first detected and every other day until ovulation. Treated mares were given altrenogest for the first two days. Beginning on the fifth day after initiation of treatment, the mares were bred every other day until ovulation. LH was determined during estrus on daily blood samples collected via jugular venipuncture using a double-antibody RIA. The intra-and inter-assay coefficients of variation were 1.39% and 5.39%, respectively. When number of days to ovulation, follicular activity, and mean LH concentration were analyzed, no difference was found due to a period by treatment interaction. Data were analyzed by period to determine effect of altrenogest administration on the three previously mentioned factors. Period I was most representative of a true response to administration of altrenogest. In period 1, both number of days until ovulation and follicular activity exhibited differences. Number of days until ovulation demonstrated an increased (P<.05) number of days for mares in treatment A when compared with the control mares. Follicular activity also exhibited an increase in follicular diameter maintenance (P<.05) for mares in treatment A when compared with the control mares. Mean LH concentration did not show significant difference in values between mares treated with altrenogest when compared with the control mares. Pregnancy rates and embryonic vesicle size change were also measured to determine possible effects of altrenogest administration. No differences (P>.05) were found in either parameter.