Effects of placement of intravaginal sponges on LH, FSH, estrus and ovarian activity in mares during the nonbreeding season.

Eight seasonally anestrous mares were administered intravaginal polyurethane sponges on December 15 and then weekly thereafter until February 1. Control mares received no sponges or genital contact. Sponge insertion caused an immediate surge in follicle-stimulating hormone (FSH) concentrations in jugular plasma in 50% of treated mares whereas no control mares had surges in FSH (P less than .05). The effect of treatment on luteinizing hormone (LH) concentrations was much less dramatic and only three treated mares appeared to have positive responses. Sponge-treated mares exhibited positive responses in FSH concentrations 11 times out of 32 mare-days and control mares zero out of 28 (P less than .05). The magnitude of the FSH response decreased rapidly with successive responses. Sponge insertion induced estrus in four of eight treated mares; no control mares exhibited estrus (P less than .05). Sponge insertion also increased ovarian size and the incidence of large follicles. When all mares were fed altrenogest for 14 d beginning February 1, there was no beneficial effect of sponge treatment on number of mares exhibiting estrus or on pregnancy rate. These data confirm earlier speculations that sponge treatment causes surges in gonadotropins and increased ovarian size in approximately 50% of anestrous mares. However, sponge treatment does not appear to provide a practical means of preparing mares for progestogen synchronization during the nonbreeding season.