Morphology of Starch Digestion in The Horse

Abstract

Summary Structure of starch in feed and chyme of horses (7 with a cannula at the caudal end of the jejunum and 2 with a cecal fistula) after feeding high starch diets (maize: whole, broken, ground, expanded and as silage, oats: whole, rolled or ground, rolled barley, raw potatoes, and tapioca) was investigated by light and electron microscopy. Structure of feed starch and morphology or starch degradation in the chyme corresponded to data on preileal starch digestibility which was investigated in a parallel study. Barriers for starch digestion in the gastrointestinal tract of the horse were structure of the plant storage organ, as, for example, a tight connection between starch granules in maize grains as well as the structure of the starch granules itself. The highly digestible oat starch was degraded by exocorrosion around the grains, whereas in other, less digestible, starch types degradation occurred by endocorrosion via pin holes. The number and size of the pin holes increased with increasing preileal starch digestibility. The effect of various ways of decomposition on preileal digestibility increased with advanced destruction of the original starch structure. Expanding was most effective. The granules were destroyed completely and the starch became soluble. Simple examination by light microscopy is a fast method to evaluate the degree of starch decomposition in the feed.

Starch digestion in horses: The impact of feed processing

“…Scanning electron microscopic examinations showed that the corrosion characteristics of the starch granules by amylase in the equid foregut were typical for grains. Endocorrosion through pin holes was observed in granules of corn, whereas on oats exocorrosion all around the surface was noted, while barley starch granules presented sporadic pin holes and small round structures on the surface (Kienzle et al, 1997).…”

“…Kleffken in 1993 (cited by Kienzle et al, 1997) postulated that the soluble starch, liberated by extruded corn, could flow with the liquid phase and thus have a shorter MRT than the insoluble form. These observations emphasized the major role that processing can have on the transit of digesta.…”

“…Recently de Fombelle et al (2003) measured gastric starch disappearance from 95.9% to 97.9%, 65.6% to 70.7% and 23.9% to 27.2% for oats, barley and corn respectively. Because of the weak activity of the saliva amylase, it was hypothesized that the abundant gastric microflora (Kern et al, 1974;Kienzle et al, 1997;de Fombelle et al, 2003) could contribute to the gastric disappearance of starch in horses. The gastric pH of equines which is highly variable (from 1.5 to 7.0) (Merrit, 1999) and can provide favourable conditions for microorganism development.…”

“…Mechanical processing, and especially grinding, alters the physical form of grain primarily by breaking down the macrostructure of connected starch granules and secondarily by destroying the structure of the starch granules themselves (Kienzle et al, 1997). The surface area is thus increased which allows a greater exposure of the granules to digestive and microbial enzymes and enhances the rate of starch digestion.…”

Influence of management on equine digestion

“…Starches and sugars in processed (flaked, rolled, steamed or popped) grains are more rapidly and efficiently digested than in whole, unprocessed grains (Hoekstra et al, 1999;Julliand et al, 2006, NRC, 2007, Keinzle et al 1997 though the overall benefits may be negligible to the actual biology of the horse, conferring only a 10% increase or less in most cases. Oat starch is more digestible than corn starch (Kienzle et al, 1997, NRC, 2007 which may account for the general perception that oats are safer than corn to feed in large amounts. If fed in large amounts to horses unaccustomed to the feed, cornstarch will be more likely to bypass digestion and absorption than oat starch, and be delivered in large amounts to the cecum and large colon where the resultant disruption of normal fermentation can result in altered digestion and even colic and laminitis (DeFombelle et al, 2004, Julliand et al, 2001Medina at al, 2002;Meyer et al, 1993;Moore and Dehority, 1993).…”