Digestion

Definition: The primary method of digestion involving physical processes.
Example: Chewing food into smaller pieces; applies to animals that require complex breakdown (e.g., herbivores cannot absorb hay directly).

Involves the use of enzymes to break food down into smaller components.
Definition of Enzymes: Biological catalysts that facilitate chemical reactions by breaking the bonds between molecules.
Chemical Digestion in the Small Intestine:
- Breakdown process: Large molecules are converted to smaller, absorbable units.
- Mechanism: Big molecules (long chains) convert to smaller molecules (short chains).

Function: Activates pepsinogen to pepsin, facilitating protein digestion in the stomach.

Definition: Bile is not an enzyme but aids in digestion, specifically of fats.
Mechanism: Bile emulsifies fats, facilitating their mixing with digestive fluids.
Bile Production:
- Made by the liver and stored in the gallbladder.
- When fats are detected, the gallbladder releases bile into the duodenum to begin emulsification.
Unique Case: Horses do not have a gallbladder but continuously produce and release bile into the small intestine.

Emulsification:
- Process involving bile salts breaking down large fat droplets into smaller droplets.
Hydrolysis:
- Lipase breaks down triglycerides into free fatty acids (FFA) and monoglycerides using water ( $(H_2O)$ ).
- Formation of micelles: Micelles are formed to help transport FFA and monoglycerides across the intestinal barrier (similar to micellar water).
Transport:
- Once inside intestinal cells, FFA and monoglycerides are reassembled into triglycerides.
- Formation of Chylomicrons:
  - Structure: Bilayer of phospholipids; transports fats through lymphatic system instead of bloodstream due to their size.
Absorption into Bloodstream:
- Chylomicrons enter the lymphatic system, eventually reaching the bloodstream and moving toward the liver for processing.

Structure: Composed of four chambers:
1. Rumen: Resembles a pear, where fermentation occurs.
2. Reticulum: Honeycomb structure, acts as a fermentation vat.
3. Omasum: Filter-like structure.
4. Abomasum: Acts as the true stomach, secretes acids and digest proteins.

Definition: Regurgitating, remasticating, and swallowing food.
Relationship with Saliva: Ruminants produce significant amounts of saliva (up to 50 gallons for cattle) containing bicarbonate that neutralizes acids produced during fermentation.
Example Incident: Ruminating goats may appear to be in distress when they are actually performing normal behavior.

Microbial Role: Microbes digest cellulose and produce volatile fatty acids (VFAs).
- **Types of Microbes: **
  - Cellulolytic Bacteria: Break down fiber (cellulose); produce acetate.
  - Amylolytic Bacteria: Digest starches and sugars; prefer lower pH levels.

Balance of feed affects bacterial populations and energy sources available to the ruminant.
- High-fiber diets support cellulolytic bacteria, producing more acetate for energy.
- High-starch diets favor amylolytic bacteria, creating propionate as an energy source.

Acetate: Direct energy source, can convert to ATP.
Propionate: Indirect energy source; requires conversion to glucose via gluconeogenesis before ATP production.

Concept of Grain Overload and Ruminal Acidosis: Eating excessive grain negatively affects pH in the rumen leading to acidosis and potential damage to tissues.
Symptoms: Reduced rumination and digestive issues.
Management Solutions: Use of bicarbonate, providing hay, and reducing grain.

Microbial Utilization: Microbes utilize dietary protein for their own function and reproduction, allowing for microbial protein to become a key protein source for ruminants.
Non-Protein Nitrogen: Microbes can use non-protein nitrogen sources (like urea) as they primarily need nitrogen for growth rather than complete protein.
Microbial protein: Similar amino acid composition to ruminant requirements and provides nutrition post-digestion in the abomasum and small intestine.

Some proteins cannot be digested by microbes (rumen bypass proteins) and must undergo normal digestive processes in the abomasum.

Maximum % of fat in the diet: 5% due to negative effects on microbial populations and digestibility.
Differences from Horses: Ruminants can more effectively utilize poor-quality forage feed due to their front-loaded digestive process, providing them better access to microbial protein.

Mouth: Mechanical and chemical digestion (starch only); Salivary amylase (omnivores).
Stomach/Abomasum: Mechanical and chemical digestion (protein); Pepsin is released.
Small Intestine: Chemical digestion (starches, sugars, lipids, proteins); enzymes primarily from pancreas and small intestine.
Large Intestine/Rumen Reticulum: Microbial digestion, focus on fiber digestion, no animal enzymes directly involved for digestion.
Importance of fiber digestion leads to the creation of volatile fatty acids.

Discussion on the efficiency of ruminants in utilizing poor-quality feed compared to horses due to differences in digestive strategies and microbial functions.
There will be further discussion and review in upcoming sessions.