Chapter 19: Digestion

1. Chapter 19: Digestion • Chapter overview: • Chapter 19 presents the physiology of digestion and nutrition, including: • anatomical views and digestive parts identification • the process of digestion and absorption • factors affecting digestibility of feeds

2. Digestion and Absorption: • The process of digestion includes: • The prehension of food or feed • The mechanical chewing and grinding • Mixing with digestive acids and enzymes to chemically break down the foodstuffs • The process of absorption includes: • Transport of the digested foods across the intestinal mucosa to the blood or lymph system

3. The General Mechanical Process: • Mastication - chewing • Deglutition - swallowing • Regurgitation - movement of digesta in reverse order from the stomach to mouth • Defecation - voiding the fecal, or waste, materials from the body

4. Animal Diet Types: • Species may be divided into dietary preference groups: • Herbivores consume plant materials, examples are horses and cattle • Carnivores consume flesh of other animals, examples are dogs and cats • Omnivores consume both plants and flesh, examples include the primates

5. Differing Digestive Tracts: • Farm animal species have a variety of digestive systems • Ruminants have four distinct stomach areas • examples include bovine, ovine and caprine • Nonruminants (also termed monogastrics) display considerable variety • hogs, dogs, and cats have a single, simple stomach • poultry have a two part stomach • horses have a large, functional cecum

6. Nonruminant Digestive System: • Mouth - prehension and chewing of food; some carbohydrate enzyme activity • Esophagus - route of food from mouth to stomach • Stomach - addition of hydrochloric acid and protein digesting enzymes, mixing and holding

7. Nonruminant Digestive System: • Small intestine - primary site of digestion and absorption • Large intestine - major site of water absorption and preparation of digesta for excretion

8. Nonruminant Variations: • Poultry • Mouth: no teeth for chewing • Esophagus: a “crop” is contained within the esophagus for food holding and moistening • Stomach: divided into proventriculus (glandular area) and ventriculus (crushing area) • Large intestine: short and exiting into the cloaca; two large ceca with limited function

9. Nonruminant Variations: • Horse • Large intestine differences: • cecum is very large (may contain 50% of digesta) • cecum provides some nutrients to the horse via microbial fermentation

10. Ruminant Digestive System: • Many parts of the tract are similar to nonruminant monogastric description • Significant differences include: • Mouth contains no upper incisors • Stomach is divided into four major parts • rumen • reticulum • omasum • abomasum

11. Ruminant Digestive System: • Rumen • Largest digestive area of the compound stomach • Majority of fermentation of feedstuffs occurs here • Majority of absorption of byproducts of fermentation - volatile fatty acids or “VFA’s”

12. Ruminant Digestive System: • Reticulum • Receives feed from esophagus • Initiates mixing, regurgitation, and eructation • Environment for fermentation of feedstuffs

13. Ruminant Digestive System: • Omasum • Third area of the stomach receives the digesta outflow of the rumen/reticulum • Some water absorption and further subdivision of feed particles may occur

14. Ruminant Digestive System: • Abomasum • The fourth and final stomach compartment, but very similar in function to the pig stomach • Acid and enzyme stomach • Final holding and mixing area before the small intestine

15. Ruminant Digestive System: • Selected terms: • Regurgitation - controlled reverse movement of coarse feedstuffs from reticulum/rumen via esophagus to mouth for rechewing • Eructation - expulsion of accumulated fermentation gases from rumen via esophagus • Rumination - refers to the processing of feedstuffs in the reticulum/rumen, to include fermentation, regurgitation, eructation

16. Chemistry of Digestion: • Digestion involves enzymes and acids produced by the host animal or microbes working in symbiosis with the host • Enzymes break specific chemical bonds in feeds • Domestic animals produce enzymes to digest nonfibrous carbohydrates, fats, and proteins • Only microbes have enzyme systems to digest fibrous carbohydrates, such as cellulose

17. Digestion of Carbohydrates: • General carbohydrate digesting enzyme (amylase) in the mouth begins digestion; little amylase is found in horses and none in ruminants • Carbohydrate digesting enzymes (amylase) from the pancreas, and intestinal mucosa (sucrase, maltase, lactase) complete carbohydrate digestion

18. Digestion of Proteins: • Protein digesting enzyme (pepsin) and hydrochloric acid in the stomach begin significant digestion • Protein digesting enzymes (e.g. trypsin) from the pancreas and intestinal mucosa complete digestion in the small intestine • Young nursing animals – rennin coagulates milk allowing more complete digestion

19. Digestion of Fat: • Fat digesting enzyme (lipase) in the stomach begins digestion • Fat digesting enzymes from the pancreas (lipase) and intestinal mucosa complete digestion in the small intestine • To assist in fat digestion, bile from the liver emulsifies fat into smaller droplets in the small intestine

20. Factors Affecting Digestibility: • Rate of passage - in general, increased rate of passage of digesta through the tract reducesdigestibility, factors increasing rate of passage include: • Increased level of feeding/intake (ruminants) • Finer processing (such as grinding) of feed Note: grinding grain usually increases digestibility but grinding hay decreases digestibility

21. Factors Affecting “Conversion”: • Feed “conversion” refers to the amount of productivity per unit of feed consumed • Factors impacting feed conversion include: • Age/weight - younger animals are more efficient • Level of feeding - limiting feed generally increases efficiency • Inheritance - feed conversion is moderately heritable; correlation between ADG and efficiency is high