One of the analytical divisions of plant carbohydrate it includes sugars (mono and disaccharides) longer chain oligosaccharides and polysaccharides including the fructans. When the WSC content is added to the Starch content this approximates the NSC Non Structural Carbohydrate.
WSC is important as it is a potent driver of the insulin response. Some so called Low Starch feeds may indeed have a low starch value (<18% for example) but if the WSC content is high then this will raise the NSC level and much of the low starch advantage will be lost. High WSC feeds should be minimized and used with caution in horses prone to Laminitis, P.P.I.D. horses and those with Equine Metabolic Syndrome