Changing your horse’s diet can affect him in many ways and diarrhea can sometimes be a concerning side effect. Only a few studies have been done in the area of dietary causes of diarrhea in adult horses, as most studies focus on infectious agents. However, many different dietary factors can induce diarrhea in horses.
Certain plants, such as corns, blue-green algae, castor beans and heather, are known to potentially cause diarrhea in horses if present in the hay. Other plants that may cause diarrhea are leafy spurge, wild iris, horsetail, bitter weeds and mustard plants. Usually, hay producers will be careful not to bale weeds or contaminants into the hay. The owner should look at the bales and examine them for any weeds, molds or discoloration. Other dietary changes, beyond changes in hay, such as increased grain feeding or change to pasture, have been associated with diarrhea, but no studies in the area are available.
When horses are on any type of diet, changes in dietary composition should be done gradually. Sudden changes in forage type, such as decreases in pasture to increases in hay, or decreases in hay to increases in pasture, should be done progressively over the period of a week. The reason is that the hindgut flora needs to adapt to the composition of the feed. In a study on risk factors for colic, the incidence of colic doubled when the hay type and/or supplier was changed over the year, demonstrating how changes in forage can lead to imbalances in the gastrointestinal flora. When grains were changed, risk for colic increased three-fold. When the diet composition is changed, there can be suppression or increase in microbial fermentation that can induce diarrhea. A change in the microbial flora and products of fermentation can facilitate the colonization with pathogenic bacteria, leading to diarrhea. Decreases in hindgut pH caused by changes in diet can reduce the resorption of water from the hindgut, causing diarrhea. This occurs when diets are increased in starch in grain or soluble carbohydrates (fructans or sugars in pasture). This is called osmotic diarrhea.
When changing forage diets, analysis of soluble carbohydrates could help in determining the risk of potential problems. Several nutritional labs can perform this analysis. Excessive amounts of rapidly fermentable or soluble carbohydrates are not properly digested in the small intestine and flow to the hindgut, where they are quickly fermented.
Diarrhea can be dangerous for the horse when caused by forage change if pathogenic bacteria multiply due to dysbiosis (death of normal bacteria and multiplication of lactic-acid-producing bacteria) or the horse gets laminitis. The best measure is to change diets slowly, over a week. When removing horses from pasture, plan ahead—introduce hay over a period of several days, increasing amounts progressively.
Tanja M. Hess, MV (DVM), MS, PhD, is an associate professor at The College of Agricultural Sciences, Equine Science, at Colorado State University. She received her degree in veterinary medicine at Federal Fluminense University, Brazil and her masters of science in equine clinics from Federal Rural University of Rio de Janeiro, Brazil, then her PhD in equine nutrition and exercise physiology from Virginia Polytechnic Institute and State University.
This article first appeared in the January 2018 issue of Dressage Today.