Digestion In The Equine Gastrointestinal Tract

This second part of my blog focuses on nutrient digestion in the equine gastrointestinal tract. My goal is to ensure you have a good understanding of nutrient digestion in the various segments of the gastrointestinal tract.

WHAT IS IN HORSE FEED?
General Constituents Of Feed:

WATER
* Essential component of the diet
* Water required for:
  - Transport of nutrients around the body
  - Thermoregulation
  - Metabolism
  - Excretion of water products
  - Gut function
* 80% of foal's BM= water
* Water makes up 65-75% of a mature horse
* Water requirements depend on:
  - Age
  - Level of exercise
  - Type of feed (horses drink less on high moisture diets)
  - Environmental conditions (i.e. temperature & humidity)
* Water in food
  - Referred to as moisture content
  - Many horse feeds have high moisture content
  _ Dry matter calculated to compare nutrients

CARBOHYDRATE (CHO)
* Collective name for:
  - Starches
  - Sugars
  - Dietary fiber
* CHO Definitions
  - Structural (fiber)
  - Non-structural (sugar, starch and fructan)
* CHO primary ingredient required for energy

STRUCTURAL CHO
* Components constitute fibrous fraction of plant
  - Structural role in plants (hence name structural CHO)
  - Dietary fiber
  - Major energy source to horses
* Cell wall components
  - Cellulose
  - Hemicellulose
  - Pectin
  -  Lignin
* Degraded by microbes in LI
  - VFA
  - Gases
  - Not digested in SI
* Feeds high in fiber
  - Grass
  - Hay
  - Straw
  - Sugar Beet Pulp

LIGNIN
* Lignin is non-CHO polymer
* Closely associated with NSP fraction
* Negatively correlated to digestibility
  - Gives plant strength but highly resistant to chemical and microbial degradation
  - Also some plant CHO & proteins "bound" to lignin= lowering their digestibility
* Lignin content higher with maturity
* Mature plants, hay and straw rich in lignin

STRUCTURAL CHO- STAGE OF MATURITY
* Primary factor in nutritive value of forages
* High maturity= hugh structural material (i.e. stem)
* Result= lower nutritive value

FIBER ANALYSIS 
* Acid detergent fiber (ADF)
  - Measures cellulose and lignin
* Neutral detergent fiber (NDF)
  - Total cell wall (including ADF fraction)

NON-STRUCTURAL CHO
* Associated with cell contents
* Components constitute storage CHO of plant
* Storage CHO of forage crops:
  - Fructan
  - Starch

NON-STRUCTURAL CHO- FRUCTAN
* Major storage CHO of grasses
* Located primarily in the stem
* Polymers of fructose
* Soluble in H2O
* Digestion:
  - Fermented in LI
  - Horse has no enzymes in SI to break down fructose
  - Large intakes of fructan can cause digestive upset (laminitis)

NON-STRUCTURAL CHO- WSC FRACTION
* Grasses also store simple sugars
  - Glucose
  - Fructose
  - Sucrose
* Fructan + Simple Sugars= Water soluble CHO (WSC) fraction

WSC CONTENT OF GRASS= HIGHER VARIABLE
* Season
  - Early spring: Higher growth= lower WSC
  - Late spring/summer: Lower growth= higher WSC
  - Year effects

Notice in 2nd chart above (in blue) studies reported highest incidence of laminitis in May)

WSC CONTENT OF GRASS= HIGHER VARIABLE
* Time of day
  - Balance b/w photosynthetic activity & utilisation
  - Affected by light & temperature
  - Higher during the day
  - Peaks late afternoon/early evening
  - Declines during hours of darkness

WSC- FACTORS AFFECTING ACCUMULATION
* Environmental & management factors
  - Temperature: Lower temperature= higher WSC
  - Light intensity: Higher light= higher WSC
  - Water stress: Lower water= higher WSC
  - Fertilizer application: Higher fertilizer= lower WSC
  - Cutting/mowing: Higher cutting= lower WSC

STARCH
* Major source of NSC found in many plants
  - Cereal grains
  - Forage legumes
  - Roots & tubers
  - Leaves and stems of grass

NON-STRUCTURAL CHO- STARCH
* Starch content varies with plant species
  - Very low in leaves and stems of grasses
  - Forage legumes= 5% of DM
  - Cereal grains= 40-70% of DM
* Digestion
  - Broken down in SI
  - Amylases break starch down to simple sugars
  - Horse can digest limited amounts of starch in SI
  - Excess travel to LI= causing disturbance

FATS AND OIL
* Horse diets typically lower in fats and oils
* Fats and oils are digested in SI by lipases
* Horses don't require high levels of fat and oils in their diet
* Can add fats and oils to diet to increase energy content
* Double energy of CHO

PROTEIN
* Required for various functions
  - Structural (e.g. muscle, skin, hair)
  - Enzymes (catalysts that effect the rate of reactions in the body)
  - Hormones (chemical messengers that regulate metabolic processes)
  - Immune compounds (for fighting infection)
  - Transport compounds (e.g. transport of nutrients)
* Consists of chains of amino acids
* Essential and non-essential amino acids
* Non-essential: synthesised by the animal
* Essential (need to be supplied in the diet)
  - Lysine (first limiting: present in the least amount relative to requirement)
  - Methionine
  - Threonine
  - Isoleucine
  - Valine
  - Arginine
  - Trytophan
  - Histidine
  - Phenylalanine
* Protein breakdown begins in the stomach
  - Acidity denatures the proteins
  - Pepsin partially degrades proteins into smaller peptides
  - No protein absorption in the stomach
* Further broken down in SI
  - Proteases break down proteins to aa
  - Absorbed across SI gut wall
* Protein associated with the plant cell wall- fermented in LI
* Protein digested in SI- used by the horse
* No aa absorption in LI
* Microbes in LI degrade protein
  - Used for microbial growth
  - No biological value to horse

MINERALS
* Required for various functions:
  - Growth
  - Co-factors to enzymes
  - Involved in transport of energy
* Mineral digestion
  - Majority absorbed in SI
  - Majority of P absorbed in LI
* Two categories of minerals:
  - Macro-minerals
  - Micro-minerals
* Macro-minerals:
  - Required in relatively large quantities (g/day)
 * Micro-minerals:
  - Needed in very small quantities (mg/day)
* Amounts important- also ratios of minerals (e.g. ca:p)
* Two categories of minerals:
Macro Minerals
** Required in g/day
- Calcium (Ca)
- Phosphorus (P)
- Sodium (Na)
- Potassium (K)
- Magnesium (Mg)
- Sulpher (S)
- Chloride (Cl)
Micro Minerals
** Required in mg/day
- Cobalt (Co)
- Copper (Cu)
- Molybdenum (Mo)
- Zinc (Zn)
- Maganese (Mn)
- Iron (Fe)
- Floride (F)
- Iodine (I)
- Selenium (Se)
- Chromium (Cr)
* Amounts important- also ratios of minerals (e.g. ca:p)

VITAMINS
* Required in small amounts- but vital for many bodily functions
  - Vision, immunity, growth, bone development, blood clotting, etc
* Two categories
  - Fat soluble (A,E,D,K)
  - Fat soluble= associate with fat to be absorbed & transported in blood
 - Water soluble (C, B Complex)- dissolve easily in water
* Water soluble vitamins not stored
  - B vitamins need to be supplied regularly
  - B vitamins produced by microbial synthesis in LI
  - But- disruption to LI environment (e.g. high starch diets) can affect this
  - Vitamin C synthesised in the liver

Abbreviations used in this section:
ADF: Acid Detergent Fiber
BW= Body Weight
Ca= Calcium
CH4= Methane
CHO= Carbohydrate
CO2= Carbon Dioxide
DM= Dry Matter
G= Grams
GIT= Gastrointestinal Tract
H20= Water
LAB= Lactic Acid Bacteria
LI= Large Intestine
LW= Live Weight
MG= Milligrams
NSP= Non Starch Polysaccharide
P= Phosphorus
SI= Small Intestine
VFA= Volatile Fatty Acid
WSC= Water Soluble Carbohydrate

VIDEO "3D HORSE DIGESTION GUIDE"