DIGESTIBILITY OF FEEDSTUFFS
PP 96 – 101
PURPOSES FOR DETERMINING
FEEDSTUFF DIGESTIBILITY
• Evaluate and quantify available nutrients from individual
feed ingredients
• Evaluate and quantify available nutrients from diets
(complete food matrices)
• Quantify the available energy concentration of feedstuffs
and or feeds
• Partition digestion of nutrients in different compartments
of the digestive tract
DIGESTION TRIALS
Standard Protocol
• Place animal in metabolism stall
Without Metabolism Stalls/Crates
Adjustment Period
• First 10 to 14 days (can be shorter or longer
depending on species, experimental design,
discipline, etc…)
• Free digestive tract of prior undigested feed
• Accustom animals to environment
• Feed to a constant rate
• Isocalorically (maintain body weight)
• Feed animal to attain 10% waste to determine ad libitum feed
intake (typical for production animal trials)
• Do not collect feces or urine
Collection Period
• Next 4 to 7 days
• Non ruminants: 3 – 5 days
• Ruminants: 5 – 7 days
• Carefully record feed intake
• Feed at 90% ad lib intake (if feeding ad lib)
• Collect all orts
• Must have accurate intake data
• Collect a diet sample – subsample
• Collect all feces excreted, weigh
• Large animals - collect a 5 to 10% subsample
• Small animals (dogs, cats) – 100% fecal sample
• (Optional) Collect urine, measure volume, collect a 5 to 10%
subsample, and add to a composite urine sample
Important Measures
• Accurate feed intake
– Include weight of all orts
• Accurate fecal output
– (must know 100% weight even if you are subsampling)
• Appropriate diet samples
• Accurate urine volume (if included)
Post Collection
• Determine chemical composition and/or energy
concentration of feed samples, feces, orts
• (Optional) Determine chemical composition and/or energy
concentration of urine
• Calculate digestibility
Digestibility (%) = Consumed – Excreted x 100
Consumed
DIGESTIBILITY CALCULATIONS
• Dry matter intake (DMI)
– Average amount of feed x %DM
• Fecal output (FO)
– Average amount of feces excreted x %DM (feces)
• DM digestibility (DMD)
– (DMI – FO)/DMI x 100%
• Individual nutrient digestibility
– Example: Protein
– (DMI x %CPdiet – FO x %CPfeces)/(DMI x %CPdiet ) x 100%
Application
• Concentration of the target digestible nutrient in diet
(Example: Protein)
• Crude Protein (%DM) x Protein Digestibility % = Digestible protein, %DM
• Can be done for each nutrient of interest
LIMITATIONS OF STANDARD
DIGESTIBILITY TRIALS
1.
Apparent Digestibility (Apparent Digestion Coefficient)
– Feces composed of undigested nutrients and endogenous materials
• Endogenous materials
– Sloughed mucosa cells
– Bacteria
– Enzymes and bile salts
– How do you determine True Digestibility?
• Endogenous materials must be accounted for and removed from FO
• Accomplished by: Ileal cannula (swine, dogs) or ceca removal poultry
• Used for amino acid digestibility in non-ruminants
2. Limiting feed intake to 90% ad lib will slow rate
of passage of digesta in the digestive tract
3. Spillage and wastage of feed or feces
4. Errors in analyses
5. Sorting of feedstuffs
Production or Group Housing
When animals cannot be placed in metabolism stalls/crates
– Methods to determine digestibility in producing animals
• Fecal pans, bags
– Need to know individual feed intake
• Digestibility markers and indicators
– Characteristics
– Physiologically inert
– Contain no element under investigation
– Will not diffuse
– Totally indigestible
– Pass at uniform rate
– Readily determined chemically
– Examples
– Carmine
– Titanium oxide
– Acid Insoluble Ash (AIA)
– Chromic oxide
Apparent Digestibility (%) =
100 – % indicator in feed x % nutrient in feces
% indicator in feces
% nutrient in feed
x 100
Factors that Affect Digestibility
•Physiological stage of the animal
•Particle size/processing
•Disease state (parasites, antibiotic treatment, etc…)
•Feed source and composition
•Level of intake
•Rate of passage
– Too slow = high fermentation and putrefactive compounds
– Too fast = incomplete digestion
•Nutrient imbalance (excess or deficiency)
•Matrices might be different than individual feedstuffs used
Balance Trials
• Account for losses associated with urine
• More precise measure of nutrient retention
• Commonly done for nitrogen to measure protein
gain or loss
• Measurement:
Nitrogen balance =
= (Nitrogen intake) – (fecal nitrogen + urine nitrogen)