Respiratory

For Horses With Asthma, Steaming Hay May Not Be a Lung Savior

By Adam Marcus

New findings throw cold water on the notion that steamed hay can reduce respiratory problems in horses with severe asthma.

The study, conducted by researchers in Canada, showed asthmatic horses experienced similar declines in lung function and increases in airway inflammation on a steamed-hay diet when compared with animals fed dry hay.

The results came as something of a surprise to the researchers, who said they went into the experiment thinking that the steamed fibers would be easier on the airway. “We were expecting to have poor lung function with the dry hay, and the steamed hay was supposed to be better—but it wasn’t,” said Mathilde Leclère, DVM, PhD, DACVIM-LAIM, professor of equine internal medicine at the University of Montreal, in Quebec, and the co-director of the institution’s equine asthma research laboratory.

For the crossover study, Dr. Leclère and her colleagues compared the effects of dry and steamed hay on the lungs of 9 horses (median age, 18 years) with confirmed severe equine asthma. There was a 2-week period of acclimatization in a stable in which all horses ate alfalfa pellets. After that period, some animals received a diet of steamed hay for 4 weeks, and others were given dry hay for 4 weeks. That period was followed by a 2-week washout interval, during which the horses returned to the pellet diet until their airway symptoms resolved. Once in remission, the horses were switched to the form of hay they had not been eating during the initial intervention period and remained on the new feed for 4 weeks.

To assess the effects of hay on the lungs, the researchers used a combination of several measures, including cytologic examination of a bronchoalveolar lavage specimen, tracheal mucus score and lung function, as well as a 23-point weighted clinical score (WCS) with parameters such as abnormal auscultation, cough, respiratory rate and nasal discharge. These assessments were performed before and after the 4-week feeding periods.

Despite previous studies showing steamed hay reduces exposure to dust and might improve lung function in some horses, the researchers found no differences between the wet and dry feed. Both hays were linked to declines in pulmonary function and increases in markers of airway inflammation during the feeding periods, although the associated clinical signs were mild.

Although the reasons for the lack of benefit are unclear, a variety of factors could play a role, including how horses feed, according to Dr. Leclère.

“It’s not only what’s in the hay but how the horses behave,” she said. “Some will bury their head in the hay and spend a lot of time chewing in that position, thereby, increasing their exposure to allergens.”
But Dr.Leclère said even considering the new data, she is not discouraging owners from steaming hay for their horses with asthma.

“If an owner tells me, ‘It’s really helping when I steam the hay,’ then I’m not going to tell them not to do it,” she said. “But there are a lot of things that can affect the exposure to dust,” including mold in stalls, overall dust in the barn, and behavior.
On the other hand, she does advise giving the practice a trial run before fully committing. “I tell my clients to rent a steamer for a couple of weeks before buying one. I think that would be a very reasonable approach,” she said. “But it’s a big purchase if you’re not sure it’s going to work.” MeV

For more information:
Raïsky C, Vives BM, Leduc L, et al. Steamed hay for the prevention of severe equine asthma exacerbations. Equine Vet J. 2025 Sep 9. doi: 10.1111/evj.70089. Online ahead of print. https://beva.onlinelibrary.wiley.com/doi/epdf/10.1111/evj.70089