Winter Diarrhea in Horses

end products called volatile fatty acids or VFA for short. VFA can be absorbed through the gut wall and transported to the liver via the blood stream where they are utilized as energy by the horse.
Starches and sugars in the hindgut are fermented very rapidly and the VFA can accumulate. This can cause a significant change in the pH environment of the cecum. Many of the beneficial bacteria living in the hindgut are very pH sensitive and may not survive a significant increase in acidity brought on by a sudden introduction of readily fermentable carbohydrates into the hindgut. Sensitive microbial populations die off and this can be detrimental to the health of the horse. Sudden diet changes for horses especially those including starches and sugars can lead to colic laminitis andor colitis. 
Complex carbohydrates such as fibre in the form of cellulose and hemicellulose as well as complex sugars such as fructan polysaccharides called fructan resist enzymatic digestion in the foregut and as a result are available for fermentation by the microbes living in the hindgut. These complex carbohydrates are usually more slowly fermented by the hindgut microbes thereby reducing the risk of an accumulation of acidic VFA. Hay is a rich source of complex carbohydrates contributing fructan cellulose and hemicellulose to equine diets. Horses are well adapted to utilize these complex carbohydrates as a safe energy source through healthy hindgut fermentation.
How Does This Relate to the Hay Analysis?
It is always a good idea to analyze the hay you plan to feed your horses for its nutrient content. A hay analysis will provide you with valuable information including many nutrients but for the horse with chronic diarrhea we want to pay particular attention to the carbohydrate fractions. These are expressed as NSC WSC starch ADF and NDF. 
Water soluble carbohydrate WSC is the sugar fraction of the hay and it includes both the simple easily digested sugars glucose and fructose as well as the complex storage sugars fructan polysaccharides. Nonstructural carbohydrate NSC is equal to the sum of the WSC and starch. Traditionally we have utilized NSC as a significant buying parameter for sugar sensitive horses or horses with chronic diarrhea.
NSC WSC starch where WSC is sugar plus fructan polysaccharides
Acid detergent fibre ADF and neutral detergent fibre NDF are a measurement of the fibrous portion of the forage. Typically the higher the ADF and NDF the lower the digestibility and