Animal Nutrition GI Track Digestive Systems — Key Points
Feed analysis and costs
Nutrition cost in production is high due to feed; start with feed analysis to balance nutrients.
Moisture vs dry matter: dry feed analysis removes water; moisture is the water removed; dry matter is the remainder.
Nutrient measurement and protein basics
Protein is calculated from nitrogen:
Some products may inflate protein numbers with nitrogen non-protein sources; beware in feed formulations.
Non-protein nitrogen (NPN) such as can feed rumen microbes, not directly the animal; microbes make microbial protein.
Nitrogen-free extract and ether extract are energy-related fractions; used for rough energy balance (not detailed in this course).
There is a large PDF listing feed stuffs and their analyses; use it to balance rations when you review feed types later.
Diet vs ration; dry matter basis vs as-fed basis
Diet: mix of feed ingredients; ration: amount fed to the animal.
Example: 2.2% corn in the ration; to feed a specific weight on a dry matter basis, multiply the DM percentage by the target DM weight. If feeding on a dry matter basis: .
Measurements come in two forms:
Dry matter basis: nutrients balanced on DM content.
As-fed basis: account for moisture to deliver actual feed to the animal.
Some forage reports use "as received" instead of "as fed"; they mean the same concept in paperwork.
Pasture phases and grazing management
Pasture growth phases:
Phase 1: short, very green and nutritious, small.
Phase 2: taller growth, roughly 8–10 inches before seed heads, best for grazing when maintained here.
Phase 3: seed heads form; higher structural carbohydrates (cellulose, hemicellulose, lignin) and lower digestibility.
Grazing strategy: target phase 2 for maximum volume and recoverability; phase 3 reduces digestibility; overgrazing harms regrowth and increases costs.
Overgrazing leads to weed invasion and higher future feed costs.
Animal types and digestive concepts
Digestive forms: physical (mechanical), chemical, and enzymatic digestion.
Peristalsis: smooth muscle contractions move digesta through the GI tract; essential for digestion.
Monogastric vs ruminant:
Monogastrics: single simple stomach (swine, poultry, horses to some extent).
Ruminants: multi-chamber stomach with rumen; extensive microbial fermentation.
Diet categories in nature: carnivores, herbivores, omnivores.
Swine and poultry are monogastric and omnivorous.
Dogs are vacillating carnivores (can digest plant matter but rely on meat).
Teeth and prehension:
Cows: long tongue, dental pad at the top; lower incisors shear forage; no upper incisors.
Goats/Sheep: similar setup; enough to pull and shear forage, tiny mouths.
Horses: strong incisors and powerful mouth; highly selective to plant parts.
Pigs: rooting behavior; omnivores with a different dentition.
Grazing behaviors by species
Cattle (ruminants): graze down to lower heights; strong when hungry; can cause low regrowth if overgrazed.
Goats: browsers; climb and reach higher vegetation; will browse leaves, branches, and vines; sift through complex vegetation.
Sheep: ground-oriented grazing; less exploratory than goats.
Horses: highly selective; can access high-quality growth; more capable of using tough plant parts via their teeth and jaw.
Pigs: rooting behavior; can cause soil disturbance; feces and rooting can impact fields/crops.
Feral hogs: reproducing rapidly (approx. ), causing significant crop damage and management challenges.
Digestive tract overview (mechanisms and sites)
Mastication (physical): mouth and teeth; chewing breaks down food; in monogastrics, saliva lubricates and buffers.
Salivation: lubricates, buffers rumen; helps maintain rumen pH to prevent acidosis.
Deglutition (swallowing): moving food from mouth to stomach.
Regurgitation: returning bolus in ruminants for cud chewing; part of normal rumen function.
Stomach (gastric): chemical digestion via hydrochloric acid and enzymes; begins protein digestion.
Splanchnic digestion differences:
Monogastrics: stomach -> small intestine -> large intestine; no cud chewing.
Ruminants: rumen fermentation supplements digestion; regurgitation enables cud chewing.
Enzymatic digestion and brush border:
Enzymes released in saliva and stomach; most enzymes act in the small intestine and along the brush border (villi).
Brush border enzymes are located on villi in the small intestine; absorption primarily in the jejunum/ileum.
Small intestine anatomy (three parts):
Duodenum: initial digestion with bile and pancreatic enzymes.
Jejunum: major nutrient absorption; pH rises as chyme moves forward.
Ileum: additional absorption near the junction with the large intestine.
Glandular features:
Monogastrics rely on gastric acid and enzyme action in the stomach; birds have a gizzard that physically grinds feed with grit after proventriculus.
Lumen structure: numerous villi and brush border enzymes increase surface area for absorption.
Poultry-specific anatomy
Gizzard (ventriculus): muscular grinding organ after the proventriculus; helps grind feed with grit.
Proventriculus (and ventriculus): chemical digestion precedes mechanical grinding in birds.
Practical notes on digestion efficiency
Energy cost of digestion: roughly consumed by digestion and absorption.
pH control is critical to maintain a healthy microbial population in the rumen; improper buffering can cause acidosis.
Enzymatic action depends on site and environment; shifts in digestion rate affect absorption efficiency.
Miscellaneous terms to remember
THI index: environmental stress indicator (heat/cold stress) affecting nutrient requirements.
Phase-based pasture management summaries and their impact on nutrient intake and forage availability.
Digestive efficiency depends on matching diet type to animal anatomy (e.g., ruminant vs monogastric) and feeding practices.
1. Feed Analysis and Nutrient Measurement
Feed Analysis Basics: Crucial for balancing nutrients and managing feeding costs.
Moisture vs. Dry Matter: Feed analysis involves removing water (moisture) to determine the dry matter (DM) content, which contains the actual nutrients.
Protein Calculation: Protein is calculated from nitrogen content: .
Non-Protein Nitrogen (NPN): Substances like provide nitrogen to rumen microbes, which then produce microbial protein, but don't directly feed the animal in the same way as true protein.
Energy Fractions: Nitrogen-free extract and ether extract are used for rough energy balance.
2. Diet, Ration, and Measurement Bases
Diet vs. Ration:
Diet: Refers to the overall mix of feed ingredients.
Ration: The specific amount of feed given to an animal.
Measurement Bases:
Dry Matter Basis: Nutrients are balanced based on the dry matter content of the feed. Example calculation: .
As-Fed Basis: Accounts for the moisture content to determine the actual weight of feed to be delivered to the animal (also known as "as received" for forages).
3. Pasture Management
Pasture Growth Phases:
Phase 1: Short, highly nutritious, but low in volume.
Phase 2: Taller growth (optimal at 8-10 inches before seed heads); best for grazing, offering maximum volume and good digestibility.
Phase 3: Seed heads form; characterized by higher structural carbohydrates (cellulose, hemicellulose, lignin) and lower digestibility.
Grazing Strategy: Target Phase 2 to maximize forage volume and promote pasture recoverability. Overgrazing, especially into Phase 1, harms regrowth, reduces digestibility, and can lead to weed invasion and increased future feed costs.
4. Animal Types and Digestive Concepts
Digestive Forms: Digestion involves physical (mechanical), chemical, and enzymatic processes, facilitated by peristalsis (smooth muscle contractions).
Monogastric vs. Ruminant:
Monogastrics: Animals with a single, simple stomach (e.g., swine, poultry, horses).
Ruminants: Animals with a multi-chamber stomach (e.g., cattle, goats, sheep) involving extensive microbial fermentation in the rumen.
Diet Categories: Carnivores, herbivores, omnivores.
Swine and poultry are monogastric and omnivorous.
Dogs are vacillating carnivores but can digest some plant matter.
Teeth and Prehension:
Cows: Long tongue, dental pad (no upper incisors), lower incisors for shearing forage.
Goats/Sheep: Similar setup, capable of pulling and shearing with smaller mouths.
Horses: Strong incisors, powerful mouth; highly selective grazers.
Pigs: Omnivores with dentition suitable for rooting.
5. Grazing Behaviors by Species
Cattle (Ruminants): Graze down to lower heights; can cause low regrowth if overgrazed.
Goats (Browsers): Climb and reach higher vegetation, browsing leaves, branches, and vines.
Sheep: Ground-oriented grazing; less exploratory than goats.
Horses: Highly selective grazers, capable of using tough plant parts.
Pigs: Exhibit rooting behavior, causing soil disturbance; feral hogs reproduce rapidly () and cause significant crop damage.
6. Digestive Tract Overview
Oral Digestion: Mastication (chewing) breaks down food; salivation lubricates and buffers (particularly important for rumen pH).
Swallowing: Deglutition moves food to the stomach.
Ruminant Specifics: Regurgitation and cud chewing are part of normal rumen function.
Stomach (Gastric): Chemical digestion begins with hydrochloric acid and enzymes, initiating protein breakdown.
Splanchnic Digestion Differences:
Monogastrics: Food passes stomach ( \rightarrow ) small intestine ( \rightarrow ) large intestine.
Ruminants: Rumen fermentation precedes and supplements other digestion.
Small Intestine: Primary site for enzymatic digestion and nutrient absorption.
Duodenum: Initial digestion with bile and pancreatic enzymes.
Jejunum: Major nutrient absorption; pH rises.
Ileum: Additional absorption near the large intestine junction.
Brush Border: Villi and brush border enzymes increase surface area for absorption.
Glandular Features: Monogastrics rely heavily on gastric acid and enzymes. Birds have a proventriculus (chemical digestion) followed by a gizzard (muscular grinding organ).
7. Digestion Efficiency and Miscellaneous Terms
Energy Cost of Digestion: Approximately of consumed energy is used for digestion and absorption.
pH Control: Critical for maintaining healthy microbial populations (e.g., in the rumen to prevent acidosis).
Digestion Efficiency: Depends on matching diet to