1. DIGESTIBILITY OF FEEDSTUFFS PP 106 - 109

2. PURPOSES FOR DETERMINING FEEDSTUFF DIGESTIBILITY
Quantify the availability of nutrients
Quantify the available energy concentration of feedstuffs
Partition metabolism of nutrients in different compartments of the digestive tract

3. DIGESTION TRIALS Standard Protocol
Place animal in metabolism stall

4. First 10 to 14 days (Adjustment period)
Feed animal to attain 10% waste to determine ad libitum feed intake
Do not collect feces or urine
Next 5 to 7 days (Collection period)
Feed at 90% ad lib intake (Collect weighback if necessary)
Collect an equivalent amount of diet daily and add to a composite diet sample daily
Collect feces, weigh, collect a 5 to 10% subsample, and add to a composite fecal sample daily
(Optional) Collect urine, measure volume, collect a 5 to 10% subsample, and add to a composite urine sample
Post-collection
Determine DM, chemical composition and/or energy concentration of feed and feces
(Optional) Determine chemical composition and/or energy concentration of urine

5. DIGESTIBILITY CALCULATIONS
Dry matter intake (DMI)
DMI = Average amount of feed offered x %DM
Fecal output (FO)
FO = Average amount of feces excreted x %DM
DM digestibility (DMD)
DMD, % = (DMI – FO)/DMI x 100%
Individual nutrient digestibility (Example: CP)
Protein dig % =
(DMI x %CPdiet – FO x %CPfeces)/(DMI x %CPdiet ) x 100%
Concentration of digestible nutrient in diet (Example: Protein)
Digestible protein, %DM = CP, % of DM x protein dig %

6. LIMITATIONS OF STANDARD DIGESTIBILITY TRIALS
Feces composed of undigested nutrients and endogenous materials
Endogenous materials
Sloughed mucosa cells
Bacteria
Enzymes and bile salts
Therefore, the digestibility percentage determined by a standard digestibility trial should be referred to as the ‘apparent digestibility’ or ‘apparent digestion coefficient’
The ‘true digestibility’ can only be determined if the endogenous materials are quantified and subtracted from the fecal output
Inserting and collecting digesta at a cannula in the ileum can be used to avoid errors associated with bacteria
Used for amino acid digestibility in nonruminants

7. 2. Limiting feed intake to 90% ad lib will slow rate of passage of digesta in the digestive tract
Apparent digestibility coefficients determined in standard digestion trial may be higher than apparent digestion in producing animals

8. Methods to determine digestibility in producing animals Fecal pans
Need to know individual feed intake
Fecal bags
Digestibility markers
Markers
Chromic oxide
Titanium oxide
Acid-insoluble ash
Use
Feed known amount of marker
Collect feces for 3 to 7 days
Analyze feces for marker
Fecal output = Amount of marker fed per day/
Fecal concentration of marker
Feed known concentration of marker in diet
DM dig. % = (1 – %markerfeed/%markerfeces) x 100%

9. Some feedstuffs can not be fed as sole diet
Digestibility may be determined as a ‘partial digestion coefficient’
Procedure
Feed test ingredient at varying percentages in a diet
Determine apparent DM digestibility of each diet
Graph (or run regression analysis) of each digestibility on a line vs the test feed ingredient’s concentration in the diet
Apparent digestibility of test feed ingredient will be at the intercept
Assumption
Concentrations of other ingredients in the diet do not affect digestibility of test ingredient
No associative feed effects

10. NITROGEN BALANCE A measure of protein gain or loss Measurement:
Nitrogen balance = DMI x %N, DM basis –
(FO x %N + Urine volume, L x N, gm/L)