LAUSR

Abstract Background Racehorses commonly develop evidence of mild asthma in response to dust exposure. Diets deficient in omega‐3 polyunsaturated fatty acids (Ω‐3) might exacerbate this response. Hypothesis To compare dust exposure, bronchoalveolar lavage fluid (BALF) cytology, and plasma Ω‐3 and specialized pro‐resolving mediators (SPM) concentrations amongst racehorses fed dry hay, steamed hay, and haylage. Animals Forty‐three Thoroughbred racehorses. Methods Prospective clinical trial. Horses were randomly assigned to be fed dry hay, steamed hay, or haylage for 6 weeks. Measures of exposure to dust in the breathing zone were obtained twice. At baseline, week‐3, and week‐6, BALF cytology was examined. Plasma lipid profiles and plasma SPM concentrations were examined at baseline and week 6. Generalized linear mixed models examined the effect of forage upon dust exposure, BALF cytology, Ω‐3, and SPM concentrations. Results Respirable dust was significantly higher for horses fed hay (least‐square mean ± s.e.m. 0.081 ± 0.007 mg/m3) when compared with steamed hay (0.056 ± 0.005 mg/m3, P = .01) or haylage (0.053 ± 0.005 mg/m3, P < .01). At week 6, BALF neutrophil proportions in horses eating haylage (3.0% ± 0.6%) were significantly lower compared with baseline (5.1 ± 0.7, P = .04) and horses eating hay (6.3% ± 0.8%, P < .01). Plasma eicosapentaenoic acid to arachidonic acid ratios were higher in horses eating haylage for 6 weeks (0.51 ± 0.07) when compared with baseline (0.34 ± 0.05, P < .01) and horses eating steamed (0.24 ± 0.02, P < .01) or dry hay (0.25 ± 0.03, P < .01). Conclusions and Clinical Importance Steamed hay and haylage reduce dust exposure compared with dry hay, but only haylage increased the ratio of anti‐inflammatory to pro‐inflammatory lipids while reducing BAL neutrophil proportions within 6 weeks.