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Background: Steaming hay significantly reduces respirable particles and provides a palatable alternative to dry hay for horses with asthma, but there are few prospective studies demonstrating the clinical efficacy of steamed hay., Objectives: To compare the efficacy of alfalfa pellets and steamed hay in improving lung function and inflammation of horses with severe asthma (SEA)., Study Design: Controlled crossover study., Methods: Ten horses with SEA were enrolled and nine completed the study. Horses were housed indoors and fed hay. Once in exacerbation, they were fed pellets and steamed hay for 4 weeks, in a crossover design. Weighted clinical scores and lung function were recorded weekly. Bronchoalveolar lavage fluid (BALF) cytology and mucus scores were recorded before and after each diet., Results: Based on linear mixed model (LMM) analysis, weighted clinical scores significantly improved over time (p < 0.001, no diet effect or time-diet interactions). With pellets, weighted clinical scores decreased from (median (interquartile range)) 13 (5.5) to 2 (1.5), while with steamed hay, they decreased from 10 (9.5) to 6 (8.5). With pellets, lung resistance decreased significantly from a baseline of (mean (SD)) 2.62 (0.77) cmH 2 O/L/s to 1.17 (0.66), 0.79 (0.54), 0.70 (0.20), 0.62 (0.18) on Weeks 1-4, respectively (LMM with post hoc tests, p < 0.001). With steamed hay, the resistance decreased significantly from a baseline of 2.34 (0.93) cmH 2 O/L/s to 1.38 (0.49) and 1.51 (0.66) on Weeks 1 and 2, respectively (p < 0.03). Neutrophils BALF decreased significantly with both diets (pellets: 40.2 (24.4) to 20.1 (11.0) %; steamed hay 30.9 (20.2) to 25.7 (17.6) %; LMM, p = 0.006)., Main Limitations: A small number of horses in a research setting. Dust was not measured in the stalls., Conclusions: Clinical scores, lung function and BALF inflammation of horses with SEA improved with steamed hay and pellets, but the effect on lung function was more pronounced with pellets., (© 2024 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.)