Most forage quality models were developed for ruminant nutrition, and may not apply to the horse. This two-part study evaluated the relationship between forage chemical composition and dry matter digestibility (DMD) using an in vitro method with equine feces as the inoculums. The first experiment determined that compared to 48 h of incubation, 72 h of incubation resulted in higher DMD for some forages. As a result of experiment 1, incubations in experiment 2 were conducted using 48 and 72 h incubation periods at 38 degrees C. The second experiment evaluated the effect of chemical composition on DMD. Thirty-one hay samples were used that ranged from 33% to 71% for NDF, 21% to 44% for ADF and 6.7 to 25.6% for CP (all on DMB). There were inverse relationships between ADF and DMD ( r = -0.826 at 48 h; -0.841 at 72 h) and NDF and DMD (r = -0.779 at 48 h; 0.812 at 72 h). There was a positive relationship between CP and DMD (r =0.572 at 48 h; 0.615 at 72 h). Forage chemical composition, particularly ADF and NDF, has potential to predict digestibility of forages by horses.