Like the equine gut, forage has a microbiome that’s teeming with microscopic life representing a wide variety of species, said Simon Daniels, BSc (Hons) Equine Science, PhD, senior lecturer of Equine Management and Science at the Royal Agricultural University in Cirencester, the U.K. And like with the gut microbiome, it’s important to ensure this “haybiome” hosts a healthy diversity of species and includes an abundance of beneficial bacteria compared to disease-causing bacteria.

Owners can help favor a positive haybiome balance by steaming their hay in water temperatures close to boiling, said Daniels. “Our findings show that steaming is the most effective way to reduce both aeroallergens and bacteria from hay,” he said.

Compared to soaking or leaving hay dry, steaming conserves the beneficial blend of microorganisms found in fresh, good-quality hay, while targeting bacteria that can cause dental or respiratory disease, Daniels said. “High-temperature steaming dry hay does not alter the bacterial diversity, making it more like the dry forage horses traditionally ingest, and it does not alter the nutrient content. However, it does reduce potential pathogens (disease-causing organisms), making it the most beneficial pretreatment of hay for horses.”

In their study, Daniels and his fellow researchers ran genetic sequencing on samples of four kinds of meadow and ryegrass hay after it had gone through either 12 hours of soaking in water at 16°C (61 F) or one hour in a commercial steamer that heated the hay to at least 95°C (203 F) for 10 minutes. They sequenced samples of the same hay left dry for the same period and compared results.

They found the dry hay had an abundance of “good” bacteria, such as Verrucomicrobia, Fibrobacters, Actinobacteria, and Firmicutes, which help with digestion of plant materials, said Daniels. These good bacteria were still present in the hay after both steaming and soaking, although diversity was lower in soaked hay.

But dry hay also contained populations of respiratory-disease-causing bacteria, such as Pseudomonas spp. and Stenotrophomonas spp., and dental-disease-causing bacteria, such as Prevotella nigrescens, Prevotella melaninogenica spp., and Porphyromonas spp, he said. These bacteria survived soaking and stayed present on the soaked hay. However, they were killed in the steaming process, making the steamed hay free of these disease-causing pathogens.

They also detected a toxin, cyanobacteria (blue-green algae), in the dry hay, he said. Both soaking and steaming significantly reduced the presence of cyanobacteria. But when soaked, the mixture of different microorganisms simply transferred into the water, making the wastewater a toxic hazard. Steaming, however, effectively killed the pathogen.

Meanwhile, soaking reduced the hay’s nutritional content, he added. While that might be desirable for owners seeking to reduce the amount of water-soluble carbohydrates their overweight or insulin-dysregulated horses consume, it’s important to remember that soaking for other purposes, such as reducing allergen content, will also compromise nutritional value.

“When looking to reduce the respirable particles in hay for horses with asthma, both soaking and steaming provide a way to reduce those aeroallergens,” Daniels said. “The problem with soaking hay is you can lose nutrients and the water you are left with is a biohazard to dispose of.”

Ideally, horse owners—especially those with horses prone to respiratory or dental disease—should know what’s in their animals’ hay. “We would always recommend getting the nutritional profile of hay tested, and the hygiene profile can be tested too,” Daniels said.

“It is logical that the bacterial profile of a hay will be influenced by lots of things, such as the environment—the soil and the weather conditions and how that grass crop has been managed,” he said. “Another thing to think about are the number of grass species, as previous studies have shown that monoculture grasses have fewer bacteria whereas mixed meadow grasses harbor more diverse bacteria because different bacteria will reside on different grass species.

“It is important to remember that many of these bacteria pose no harm to our horses, and some are beneficial and therefore normal to be ingested,” he added. “For those horses with respiratory conditions, while having good low-dust hay is important, finding a suitable way to reduce aeroallergens on hay (by steaming, for example) and in the environment in general is the most important strategy, as many of the bacteria on the hay will pose no harm to the horse and form part of those normal gut bacteria.”