Three yearling and three 2-year-old Saddlebred horses were used to determine seasonal differences in apparent digestibility of Ky 31 tall fescue (Festuca arundinacea Schreb.) pasture by horses. Three trials were conducted with collection periods beginning August 27, 1984; January 8, 1985; and June 3, 1985. Each 72 h collection period followed a 2 wk preliminary grazing period. The horses were given a pulse-dose of Yb impregnated fescue forage at 0 h, and fecal grab samples were taken at 6, 12, 18, 24, 30, 36, 48, 60, and 72 h after dosing. Fecal output was estimated from Yb concentration in the feces. Apparent digestibilities of dry matter (DM), acid detergent fiber (ADF), neutral detergent fiber (NDF), crude protein (CP), and cellulose were estimated using indigestible NDF as an internal marker. Forage DM intake was calculated using the formula (fecal output/(100- % forage DM digestibility) x 100). Estimated DM intake and fecal DM were 4.5 and 1.7 kg/d for August; 7.5 and 2.7 kg/d for January; and 6.4 and 2.8 kg/d for June, respectively. Percent ADF and NDF were highest (P<.05) and apparent digestibilities of these components were lowest (P<.05) for the spring forage. / Master of Science