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
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 100Consumed
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 feces4. Errors in analyses5. Sorting of feedstuffs
– 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 x 100
% indicator in feces % nutrient in feed
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