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 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 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%

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)