Summary: Equine gastric mucosal ulcer syndrome (EGUS) is a well-known disease entity in equine medicine. Many studies have evaluated this disorder, by investigating prevalence, etiology and treatment. The effect of feeding different roughage-based diets to weanling foals has not been examined to date. In this study, 89 weanlings aged between 172 -174 days were randomly allocated to two feeding trials. Trial 1 consisted of three groups: Group 1 alfalfa chaff (group 1), hay (Group 2) and Group 3: a total mixed ration (TMR). There were 20 foals in each group. Trial 2 consisted of two groups: Group 4: molassed alfalfa chaff and Group 5: hay. There were 19 foals in each group. Gastroscopic examination was performed immediately before weaning and after a feeding period of 15 -16 days. In the first part of the study (or trial 1) the prevalence of gastric mucosal lesion was 44 % with a low severity score (median 0; 25/75 % percentile: 0/0) before weaning. There was a significant increase in the number of lesions in the Pars nonglandularis of the Curvatura minor in all groups of trial 1 post weaning. The group 1 (alfalfa chaff) exhibited significantly more mucosal lesions at the pylorus compared to group 2 (hay) and 3 (TMR). Significant healing of gastric ulcers was seen in the hay and TMR group. Results of the second part of the study (trial 2) indicated a prevalence of 86 % before weaning increasing to 97 % after the feeding period. There was an increase in the number of lesions at the Curvatura major and minor of both Pars nonglandularis and Pars glandularis in both groups (hay and alfalfa chaff). A significantly higher number of lesions at the pylorus were seen in the molassed alfalfa chaff group only. Despite the high number and severity score of the lesions none of the weanlings showed clinical signs associated with a gastric disorder.Keywords: EGUS / feeding / alfalfa chaff / weaning / foal / nutrition Citation: Fedtke A., Pfaff M., Volquardsen J., Venner M., Vervuert I. (2015) Effects of feeding different roughage-based diets on gastric mucosa after weaning in warmblood foals. Pferdeheilkunde 31, 596-602