stalled horse eating hay

The equine gut microbiome is made up of bacteria, protozoa, and fungi. Most of the research concerning the microbiome has focused on bacteria; however, protozoa and fungi also play significant roles. All three can break down material in the GI tract to provide volatile fatty acids (VFAs), an energy source for the horse.

During the British Equine Veterinary Association Congress, held Sept. 7-10 in Liverpool, U.K., Jo-Anne Murray, BSc(Hons), PhD, MSc, PgDip, PgCert, BHSII, RNutr, FAfN, FRSB, PFHEA, professor of equine nutrition at the University of Glasgow, in Scotland, spoke about evaluating your horse’s diet with the microbiome in mind to promote beneficial gastric health.

Focus on Fiber

Horses consuming diets high in fiber from forage have greater microbial diversity in their GI tract than horses eating less forage. When you add concentrates to the diet, the number of fiber-degrading bacteria in the gut decreases, allowing harmful lactic acid bacteria to proliferate. This can lead to colic, laminitis, and hindgut acidosis (high acidity in the large intestine and colon). Therefore, Murray suggests basing a horse’s diet around forage and only adding starch-containing concentrates when necessary helps keep the microbiome healthier.

Make Changes Slowly

Changes in the diet can disrupt a previously stable microbiome. When this happens, Murray said the horse’s risk of colic and other gastrointestinal concerns rises. Dietary changes that impact the microbiome are not limited to the concentrate element of the diet;­ forage plays a role, too. Providing your horse with a consistent forage source helps minimize disruptions. However, even a horse that grazes on pasture will inevitably experience microbiome changes due to the varying nutrient content of grass. Within the same pasture, grass undergoes changes over months, days, and even hours due to temperature, season, and rainfall.

Murray said the sugar content in pasture is usually highest in March in the U.K. because “at that time of the year, in early spring, grass growth is slowing down, but … the plant is still photosynthesizing—it’s still producing sugars, but because it’s not growing, it’s not using the sugars.” This causes a buildup of sugar content in the grass.

Murray added that domestication has altered what horses would typically eat, inevitably reshaping the equine intestinal microbiota. “Horses are grazing on what we call improved pastures—higher nutritional quality than they are designed to eat,” she said. For optimal microbiome health, horses should be grazing on a lower-quality but higher-fiber forage than what owners typically provide.

Even switching horses between pasture grass and hay creates changes in the microbiome. Typically, moving from grass to hay is riskier than moving from hay to grass. Murray said this is crucial to consider going into winter. “Sometimes when we manage horses we think, ‘We are just going to bring them in now and get them in at night because the weather is changing.’ But if we do that, bring them in, say, early evening, and they are in the stable for a significant period of time and they are getting hay, that is a sudden dietary change,” she explained.

Murray recommended transitioning horses from turnout to stalls slowly, if possible, to avoid sudden diet changes and maintain a healthy microbiome.

Add Probiotics

Supplementing a horse’s diet with probiotic (live microorganism) products might aid digestion and support a stable microbiome. Probiotics for animals contain bacteria and strains of Saccharomyces cerevisiae yeast. Murray said that adding yeast to a horse’s diet increases the digestibility of fiber. Yeast might also use up oxygen in the gastric system, making oxygen-sensitive lactic acid more difficult to produce and resulting in a beneficial higher pH level.

Additionally, yeast helps the microbiome remain healthier when the horse goes through stressful situations, such as traveling. “Yeast appears to support stability through times of change,” said Murray. Adding yeast to the horse’s diet when competing and on the road might help minimize negative physical effects on the gastrointestinal tract.

Take-Home Message

To create a healthy and stable equine microbial gastric population, focus on feeding a high-fiber diet. Only add high-starch concentrates to the diet as needed, make dietary changes slowly, and consider adding a yeast supplement to help maintain gastric health.

Of course, some change is inevitable, said Murray, “but there are some things we can do to mitigate the risk.” She said she focuses on “trying to educate horse owners about what changes they might not naturally think of and what we can do to help reduce that risk of disturbance occurring in the hindgut and then impacting the development of other issues.”