You may feel sorry for him, but keeping him off grass may be best.

There are few prettier sights than a horse grazing happily, up to his knees in a luxuriant late-spring pasture. However, a rear view of this horse may reveal something not quite so nice—diarrhea.

There’s a commonly held myth that spring-grass diarrhea is caused by the high water content of the pastures, but horses with a normally functioning intestinal tract efficiently absorb water without developing any diarrhea. The excess is excreted into the urine, not the horse’s digestive tract.

Rapid introduction to unrestricted grass access is part of the problem. A move from all hay to growing grass is a feed change. The pasture not only contains a wider variety of plant types than the horse is probably used to, but the composition of the grass is also different.

Spring grasses are often high in protein, 20% or more. Protein that isn’t completely digested or absorbed in the small intestine reaches the hindgut and is worked on by the organisms there. Excessive gas and frothing may result, along with the diarrhea. Young grasses are also often highest in simple sugars and starches (clover), which can throw the balance of bacterial populations out of whack, even change the acidicity of the large intestine and cause irritation.

Sudden overgrowths of some populations of bacteria may also lead to greater-than-normal levels of bacterial toxins. Another problem is the low level of fiber in young grasses. Many of the most beneficial organisms in terms of health of the colon are those that slowly ferment fiber. Without an adequate supply of their normal “food,” numbers will drop.

Although pasture-related diarrhea is usually more of a cosmetic problem than a medical one, diarrhea is never normal, and more serious problems with obvious colic or even laminitis may develop. Even horses that are pastured 24/7 and accustomed to grazing may be susceptible to the problem when spring weather conditions favor a very rapid and heavy growth of grass.

Limiting access by keeping them off the pasture part of the day until it matures, or the use of grazing muzzles, will help. Increasing fiber intake, in the form of a mature-cutting of hay, will also help prevent loss of the beneficial fiber-fermenting bacterial strains. Make sure the horse having diarrhea problems is confined off pasture long enough to consume a minimum of 5 to 10 lbs. of a mature-cutting hay every day to create a better environment in the bowel.

Ration Plus (www.cytozyme.com, 801-533-9208) may be an aid in combating the diarrhea. Probiotics will help if the organisms are going into an environment where the level of acidity and food supply is favorable for them. Otherwise, controlling grass intake and providing more fiber is the best way to go.