The stomach is adapted to a continuous feed intake, and has a comparatively small volume (15-20 litre for a medium sized, 500 kg, horse). The stomach can be subdivided into three main parts:
1. The oesophageal part which has no glands
2. The fundic part which has glands producing hydrochloric acid, enzymes and mucus
3. The pyloric part which has glands producing mucus
The oesophageal part of the stomach usually has a high concentration of microbes (up to 1000 millions per gram). These ferment (break down) ingested feeds. The high temperature in the stomach, and the liquids which are mixed into the feed during chewing, together with the neutral or moderately acidic pH provides an excellent environment for bacteria. This part of the stomach is also inhabited by the bots fly larvae (Gastrophilus) which lives and overwinters in the stomach of the horse. When fully grown, the 1-2 cm long larvae are passed with faeces, pupate in the soil and emerge as flies which lay their yellow eggs on the legs of the horse.
This microbial activity leads to a partial digestion of carbohydrates and sugars, and begins the hydrolysis of soluble proteins. The fermentation process produces lactic acid, short-chain fatty acids, and some carbon dioxide gas. Gas is potentially harmful because the horse has a poor ability to expel it from the stomach.
Fermentation stops as the gastric contents gradually become acidified by the fundic secretions which contain pepsin, a protein-cleaving enzyme, and hydrochloric acid which rapidly reduces the pH of the ingested feed to between 2 and 3. It is in the pyloric part of the stomach (the lower half) that the muscles in the wall first produce sufficiently strong contractions to mix the contents fully. In horses fed infrequent, large meals, gas production and over distension of the stomach can become a problem. Horses should therefore be fed many small meals, particularly when they are exercised heavily and need large quantities of hard feed.
In the horse's stomach, the only breakdown of carbohydrates, such as starch, occurs as a result of microbial fermentation since the stomach does not produce any carbohydrate- or fat-digesting enzymes.
The secretion of hydrochloric acid is regulated by the hormone gastrin in a negative feedback manner. Increased secretion of gastrin leads to increased concentrations of HCl in the stomach, which in turn reduces the secretion of gastrin. The secretion of gastrin is primarily stimulated by the presence of food in the stomach through specific receptors in the stomach wall which react to being stretched and to chemical substances such as amino acids in the feed. The lower section of the stomach is the pyloric part where glands produce mucus which lubricate the digesta.
The stomach starts to empty its contents into the small intestine whilst the horse is still eating. Stomach contents with a dry matter percentage below 18% will pass rapidly, and may overtake fibrous contents by following the small curvature of the stomach towards the pylorus. Generally, fibre-rich diets will remain longer in the stomach. Proper mixing of the feed with digestive juices is necessary for normal digestion. Failure in mixing may be caused by:
- Too few gastric secretions, often resulting from too much physical or psychological stress soon after feeding.
- The feed being consumed too rapidly (the horse is too hungry)
- Too much feed being given (in particular concentrates and grains)
- Concentrates containing cooked (micronized) wheat or rye-starch which may prevent the hydrochloric acid from sufficiently reducing the pH of the gastric contents to stop microbial fermentation, resulting in excessive gas formation, stomach dilatation and colic.