The process of breaking down food into particles (molecules) small enough to permit absorbtion into the bloodstream. The process is both mechanical involving chewing, grinding, and mixing and chemical by means of digestive enzymes which cleave large molecules into smaller ones. In herbivores digestion is aided by the gut microflora which are able to break down fibre which is not susceptible to enzyme degradation.
The final part of digestion is absorbtion of available nutrients through the gut wall into the bloodstream and thence to the liver for further processing. The small veins leading from the gut empty into the large Hepatic Portal Vein which leads straight to the liver.
Digestion is continuous driven by rhythmic muscular contractions of the gut wall (peristalsis) pushing the contents along the tract which is lined by a protective layer of mucus. Once proteins, carbohydrates, fats, water, vitamins, and minerals have been digested and absorbed the remaining Indigestible material along with some microbes and gut lining cells is voided.
Rate of passage: following consumption of a meal most of it will have cleared the stomach within 2 hours, passed through the small intestine and reached the first part of the large intestine by 3 hours. Around 50% will have been voided after 24 hours and the remainder over the following 40 hours. The rate of passage is affected by the type of feed with pelleted feed passing more quickly than pasture, and hay moving more slowly and being retained for longer. One estimate for particles over 2cm in length is more than a week. Larger particles need more time to be fully digested. Processing feds by grinding, flaking, pelleting, and extrusion is designed to aid digestion by reducing particle size and facilitating enzyme attack.
The efficiency of digestion tends to decline in aged horses in conjunction with changes to the gut microbiome and these changes need to be considered when formulating suitable rations for the oldies. Provision of some high-quality protein along with easily digestible fibre sources and perhaps supplemental vitamins may be worthwhile.