EFFICACY OF SHORT-TERM ADMINISTRATION OF ALTRENOGEST TO POSTPONE OVULATION IN MARES

Estrogen from a growing follicle stimulates the preovulatory surge of luteinizing hormone (LH) while progesterone (P) is known to suppress L H. The possibility exists that administration of P, in the presence of an ovulatory follicle, would sufficiently suppress LH and, therefore, delay ovulation. The objective of this research was to elucidate the potential for oral administration of altrenogest (17-Allyl-17 beta-hyd roxyestra-4,9,11-trien-3-one) to postpone ovulation of a preovulatory follicle (35 mm) for approximately two days. Fourteen light-horse mare s, ranging in age from two to 19 years, were randomly assigned to one of three treatments (A - .044 mg/kg BW altrenogest for two days; B - . 088 mg/kg BW altrenogest for two days; and C - no altrenogest). Mares began treatment when a 35-mm or greater follicle was observed via real -time transrectal ultrasonography. Both number of days until ovulation and follicular maintenance differed between treated and control mares . Number of days until ovulation was increased (P < .05) for mares in treatment A when compared with the control mares. Follicular diameter maintenance, a measurement of follicular diameter throughout treatment , also increased (P < .05) for mares in treatment A when compared with the control mares. Mean LH concentration was not different between ma res treated with altrenogest at either treatment dose when compared wi th the control mares. Pregnancy rates and embryonic vesicle size chang e were also measured to determine potential effects of altrenogest adm inistration. No differences (P > .05) were found in either characteris tic. Short-term administration of altrenogest increased the number of days to ovulation. Further study is warranted to prove conclusively th at altrenogest increases follicular maintenance, alters the preovulato ry LH surge, and has no detrimental effects upon reproductive efficien cy.