Non-pharmaceutical methods are desirable to treat or prevent gastric ulceration in horses. This two-period, randomised, double-blinded placebo-controlled study was designed to evaluate the efficacy of dietary supplementation (25 g once daily in feed for 30 days) with Fermaid®Ease 187 (FE, Lallemand Australia Pty Ltd, Maroochydore, Australia) on gastric squamous and glandular mucosal ulcer scores in horses. Gastroscopy of 120 horses presented to the Veterinary Clinical Centre at Charles Sturt University (CSU; Bathurst, Australia) or at local training establishments identified 60 horses with spontaneous gastric ulcer disease (50%). Of horses eligible for inclusion, 29 horses were recruited and randomised to receive either the FE or placebo for 30 days. Effects were assessed by repeat gastroscopy (Day 31), at which time horses started the reciprocal treatment. Eleven horses successfully completed both treatment periods. Treatment with FE showed a significant decrease in squamous ulcer scores in period 1 (P=0.008), with a similar effect observed in period 2. No change was observed in squamous ulcer scores for horses receiving placebo treatment in period 1, but increased squamous scores were observed in horses receiving placebo treatment in period 2 (P=0.062). Squamous ulcer scores on Day 31 were significantly lower (P=0.005) following FE treatment than for horses receiving the placebo treatment. No effects were observed on glandular ulcer scores. This study supported the use of FE in horses predisposed to ulceration of the squamous gastric mucosa.