Master of Science / Department of Animal Sciences and Industry / Teresa L. Douthit / Abruptly increasing concentrate in the ration of horses results in altered cecal dynamics which can culminate in digestive distress. Nine Quarter horses previously fitted with cecal cannulae were utilized for 3 consecutive 22-d experiments, each separated by 2 d of rest. During Exp. 1 and 2 horses were acclimated to the same ration for the initial 21 d of each period, followed by a concentrate challenge on d 22. The acclimation ration consisted of a morning meal of 0.5% BW concentrate (Omolene 200, Purina Animal Nutrition, LLC, Gray Summit, MO) fed with 1.5% BW prairie grass hay divided evenly between a morning and evening meal. On d 22 of Exp. 1, horses were fed a morning meal consisting solely of 1.0% BW concentrate while 1.25% BW concentrate was fed on d 22 of Exp. 2. Cecal samples were obtained through cecal cannulae from d 19 to 22 of each experiment every 4 h for h 24 following the morning meal each day. Cecal pH during Exp. 1 was recorded and decreased at h 12 following the concentrate meal on d 22 in comparison to cecal pH at h 12 on d 19 to 21 (P = 0.009). During Exp. 2 cecal pH increased at h 4 (P = 0.02) and decreased at h 12 and 20 (P < 0.0001) following this oncentrate challenge compared to cecal pH recorded at the same time points during the acclimation period. Experiment 3 differed from that of Exp. 2 only in the respect that during the acclimation period horses were fed, in addition to the acclimation ration, either a proprietary supplement (n = 5) or a placebo (n = 5). Cecal samples from d 19 to 22 were analyzed for pH, concentration of lactate, and concentration of VFA. Horses consuming the supplement had increased cecal pH at h 4 (P = 0.009), concurrently decreased cecal lactate (P = 0.02), increased ratio of (acetate+butyrate)/propionate at h 8 and 16 (P ≤ 0.006), and decreased VFA concentration at h 24 (P ≤ 0.05) compared to horses in the control group following the concentrate challenge.