Diet Effects on the Equine Microbiome

Dr. Murray presented information about how diet affects the equine microbiome and offered suggestions for increasing microbiome stability.
Horses that eat a high-fiber diet of hay and pasture forage have greater microbiome diversity and stability. | Getty Images

Horse owners tend to be fairly casual about making changes to their horse’s diet. They often don’t realize the potential for substantial effects on the living microcosm within the intestinal tract. Jo-Anne Murray, PhD, MSC, PgDIP, PgCert, BSc (horse), BJSJ, RNutr, of the University of Glasgow is passionate about equine nutrition, including how diet affects the microbiome. She presented some basic tenets at the 2022 BEVA Equine Conference.

Fiber-Based Diets vs. Concentrate-Based Diets

Murray reviewed evolutionary tendencies of the equine diet. Horses evolved to graze intermittently for 16-20 hours per day. High fiber was a critical component of their diet. Domesticated horses now receive higher nutritional quality than their intestines were designed to accommodate. This leads to a reshaping of the intestinal microbiota. Horses digest fiber and structural carbohydrates in the hindgut to produce energy sources. However, non-structural carbohydrates (NSCs), composed of starch, fructan and simple sugars, are poorly digested in the small intestines. The small intestines can’t cope with a lot of starch at one time. When abundant sugars and starch spill over into the hindgut, they ferment and create significant problems. Not only does this modify the intestinal microbiome, but it also creates many metabolic issues as well as a risk for laminitis.

Horses eating a high-fiber diet have greater diversity and stability of the microbiome, says Murray. In addition, a fiber-based diet (hay and pasture forage) increases the number of fiber degraders as constituents of the microbial population. Beet pulp has levels of pectin, which is an easily digestible fiber. In contrast, a high-NSC diet (as with grains, for example) not only lessens diversity and stability of intestinal microbes but also increases the number of lactic acid-producing bacteria that decrease the intestinal pH. This further degrades the concentration of fiber-degrading microbes. She added that high-starch diets cause horses to be more reactive to stimuli and exhibit undesirable behaviors like “spookiness.”

The Affect of Hay and Pasture on the Equine Microbiome

Murray commented that even changing a batch of hay affects the equine microbiome. She noted that there are microbial changes associated with a combination of pasture and hay compared to just pasture, and that feeding concentrate is associated with a linear increase in the risk of developing colic.

Studies comparing hay to pasture to silage demonstrate a higher microbial diversity in pastured horses. The composition of the intestinal microbiome is affected by ambient temperature, feed quality, season and rainfall, all of which affect the amount of sugars in grass. Sugars are higher in May and decrease in August, March and October.

The Role of Probiotics for Microbiome Stability

Murray advised that adding yeast (20 g/500 kg body weight) as a probiotic can increase microbiome stability and diversity. This is especially helpful for a horse that transitions back and forth from grass to hay to grass to hay, such as might occur with a traveling competition horse. Yeast doesn’t colonize the gut so it must be fed daily. 

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