DIGESTIBILITY OF FEEDSTUFFS
PP 106 - 109
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
DIGESTION TRIALS
Standard Protocol
• Place animal in metabolism stall
• 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
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 %
LIMITATIONS OF STANDARD
DIGESTIBILITY TRIALS
1. 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
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
– Methods to determine digestibility in producing
animals
• Fecal pans
– Need to know individual feed intake
• Fecal bags
– Need to know individual feed intake
• Digestibility markers
– Markers
» Chromic oxide
» Titanium oxide
» Acid-insoluble ash
– Use
a. 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
b. Feed known concentration of marker in diet
• Collect feces for 3 to 7 days
• Analyze feces for marker
• DM dig. % = (1 – %markerfeed/%markerfeces) x 100%
3. 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
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)