Digestion and Absorption of Feed Why Do I Have To Know This Stuff? (Objectives) Because the Dean says so Learn basic parts of the digestive tract and their functions Compare digestive systems Learn how differences among animals influences their ability to digest and utilitze different feed ingredients First some definitions Digestion Any idea when digestion starts? Why is digestion important to the animal? Before animals can utilize nutrients they must be digested by the animal. Digestion includes all processes involved in breaking down food into units capable of being absorbed by the body Mechanical - Chewing, gut contractions Chemical - hydrochloric acid, bile Enzymes - from gut (SI or pancreas) or microorganisms   Absorption Absorption of nutrients includes all processes involved in entry of nutrients into the body from the intestinal tract The absorptive process may be active or passive depending on the nutrient active requires energy passive occurs by diffusion or osmosis   Types of Animals We can classify animal into three groups based on the type of feed they normally consume Carnivores Herbivores Omnivores Can animals consume and utilize feeds they don't usually eat?   Functions of the Mouth Grabbing Food Reduce that food to smaller particles by grinding and chewing Moisten food (saliva) Add digestive enzymes (saliva)   Functions of the Esophagus You tell me Transport food from mouth to stomach Food storage (Crop of chickens)   Functions of the Stomach Food Storage (animal and food dependent) Produce digestive fluids that break apart feeds Hydrochloric acid Protein digesting enzymes Site of first real digestive processes   Functions of the Small Intestine Produces many digestive enzymes Major site of nutrient digestion Major site of nutrient absorption   Functions of the Ceca Digestion by fermentation (animal dependent) Nutrient absorption (animal dependent)   Functions of the Large Intestine Digestion by fermentation (animal dependent) Nutrient absorption especially water (animal dependent)   Monogastric Animals Examples are carnivores and omnivores Stomach is simple in anatomy (one compartment) Some herbivores are considered monogastric (rabbit, horse) Monogastric is very general since there is a great deal of variation among animals to allow for differences in feed utilization   Digestive Tract of Monogastrics Pigs, Chickens, Horses Have similar parts but differ in size and relative function Prior to the small intestine the basic function is to reduce particle size and increase surface area for enzymatic activity Small intestine functions to split molecules (digestion) and absorb them into the body Large intestine absorbs water and forms undigested wastes into mass for excretion   Digestion in Monogastrics Enzymes are organic catalysts that speed up chemical reactions Always have a protein component and usually have a vitamin or mineral helping out Usually very specific for the type of reaction Usually named for the substrate they attack   Feed Passage in Monogastrics Mouth - varies in amount of chewing and saliva Esophagus - passage from mouth to stomach. In chickens an enlargement (crop) stores food Stomach - Secretes gastric juices and acid that activate digestion. Again chicken is different. They have two parts: proventriculus - glandular stomach gizzard - grinding   Role of the Mouth in Digestion Chewing stimulates saliva production which will act as a lubricant Saliva contains starch-digesting enzymes but due to a short residence time, there is little if any breakdown of nutrients   Digestion in the Stomach Secretes HCl and Mucin. HCl begins breakdown of protein while mucin helps protect the lining of the stomach from the acid Secretes enzymes Pepsin and Gastrin that act to breakdown proteins   Small Intestine Can you tell me what should be on this slide? Primary site of digestion and absorption (how does this occur?) Valved-off from upper and lower GI-tract Fairly similar in all animals except for length Presence of food triggers release of digestive aids (bile from liver, enzymes from pancreas and SI)   Digestion in the Small Intestine Breaks down fat,protein ans CHO's into individual molecules for absorption Specific absorption site for each nutrient Sites may be physically or chemically blocked by gums and other feed components. Damage by disease or parasites can impair absorption   Absorption and Transport of Nutrients Absorbed by both active and passive means (into blood or lymph) Generally transported to liver first for primary metabolism (fats) Liver functions as a major metabolic organ as well as a site for detoxification of waste products   Role of the Ceca in Digestion One of the major organ differences among animals All have one or a pair of ceca but they differ markedly in size and function In the chicken contributes little In the hog a moderate sized ceca aids in the digestion of fiber   In the horse the ceca is the major site of fiber digestion and allows utilization of forages Since the ceca is posterior to the small intestine, nutrients digested in the ceca may be poorly absorbed.   Cecal Fermentation In some animals post-gastric fermentation occurs in the Ceca (and is important) Name such an animal. Microbes in the Ceca act similarly as those in the rumen. That is they digest forages, produce volatile fatty acids (VFA's) which are absorbed for energy Extent of cecal fermentation varies   Polygastric Animals Polygastric or Ruminant Animals Most herbivores are polygastric Their stomach is a complex (like cinema 16) with several distinct compartments Again a great deal of variation depending on the animals adaptation to specific feeds   "Stomach" has foru parts/compartments Rumen (largest) Reticulum Omasum Abomasum Other than the "stomach" the GI-tract is very similar to that of a monogastric Differ in size and functionality of Ceca   Rumination Polygastrics typically eat forage rapidly then latter "regurgitate" a bolus (chewing their cud) This allows more thorough chewing and mixing with saliva (decrease particle size) This process known as rumination   Functions of the Rumen Largest portion of "stomach" Large fermentation vat filled with bacteria and protozoa These beasties are how ruminant digest cellulose and othe complex CHO's Microbes are also capable of producing protein from simple N compounds   Microbes produce B-complex vitamins Microbes eventually die and are digested and absorbed for nutrients in the SI Very useful for the digestion of forages but inefficient in the use of starches and proteins   Digestion in Ruminants In mature polygastric animals predigestive fermentation takes place in the rumen The bugs (bacteria and protozoa) thrive due to moisture, warmth and food supply Microbes are an essential source of nutrients Excess bugs are killed in the abomasum and digested in the _______ to provide amino acids and energy Microbes ferment CHO's to provide volatile fatty acids that are absorbed through the rumen wall and used for energy This whole process is highly desirable for forage digestion but is not efficient for easily digested nutrients   By-products of Fermentation Methane and CO2 are produced as byproducts of fermentation Normally released by eructation Occasionally eructation mechanism doesn't work and can lead to bloat (dietary source) Gas pressure builds up on heart and lungs and can result in death   Function in the Reticulum Honeycomb appearence Interacts with rumen to mix and stir feed Provides additional area for fermentation   Function of the Omasum Has many folds and fissures Not considered to have major role in digestion Possibly some grinding of feed and nutrient absorption Filters large particles   Function of the Abomasum "True" stomach Same function as in monogastrics Kills microbes and begins digestion of protein   Young Polygastric Animals Based on what you know about the two types of digestive tracts we've studied (and their strengths and weaknesses) what does a young ruminant eat? Young nursing animals have digestive tracts which act like those of a monogastric Food (milk) bypasses the rumen and enters directly the omasum/abomasum (esophageal groove) This allows milk to bypass fermentation which we have already mentioned was an inefficient way to use high quality feeds   Digestion in Ruminants Fermentation of fats and CHO's produces volatile fatty acids (VFA's) such as acetic, butyric and proprionic acid VFA's are absorbed through the rumen wall into the blood stream and are used as energy sources Great for poorly digested CHO's but not efficient use of easily digested carbohydrates or fats (time)   Energy Pathways in the Ruminant Due to microbes little if any glucose or fat is absorbed directly into the body (blood glucose reduced) Most are metabolized to VFA's and can be converted back to glucose in the liver (as needed) VFA's are the major source of energy in the ruminant (Fig. 16.8)   Protein Pathways Microbes can incorporate very simple nitrogen sources into microbe protein Easily digested proteins (soybean meal, milk) are extensively digested Protein quality of microbes may be less than that supplied in the diet Endproducts of protein metabolism (urea, ammonia) are excreted in saliva and urine On to bigger and better things -- Poultry