6.5: Monogastric (Nonruminant) Simple Stomach

Pigs are the only common farm animals that secrete salivary amylase in their saliva. This enzyme begins starch breakdown; however, it provides little nutritional benefit because feed passes through the mouth quickly, and the low stomach pH inactivates the enzyme.

The pig's gastric juices contain enzymes typical of monogastrics. Protein digestion begins in the stomach with pepsin but is completed in the small intestine, where most fat digestion also occurs.

Pancreatic juice plays a crucial role by providing:

• Digestive enzymes for carbohydrates, proteins, and fats
• Sodium carbonate and sodium bicarbonate are used to neutralize stomach acid
• Insulin (produced by the pancreas) for carbohydrate metabolism

Bile, formed in the liver and stored in the gallbladder, assists in fat digestion and absorption while aiding in the absorption of fat-soluble vitamins (A, D, E, and K).

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Birds lack teeth and possess several unique digestive structures:

Crop – A dilation of the esophagus that stores and moistens food; some fermentation occurs in certain species

Proventriculus – The glandular stomach equivalent to the mammalian stomach or ruminant abomasum; produces gastric juice (HCl and pepsin) with a pH of 4. Ingesta passes through in seconds, so minimal digestion occurs here.

Gizzard (Ventriculus) – A highly specialized, muscular grinding organ that mechanically breaks down feed. Free-ranging birds may consume grit to aid grinding, but modern preground rations make this unnecessary. No enzymes are secreted in the gizzard, although enzymes from the proventriculus continue to work here.

Ceca – Two large pouches located at the junction of the small and large intestine. Size varies with diet—larger ceca develop when animals consume high-fiber diets. The ceca digest fiber, though modern chickens fed low-fiber diets make minimal use of them.

Cloaca – The common orifice for waste elimination (feces and urine), copulation, and egg laying in females

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