DIGESTIBILITY OF FEEDSTUFFS PP 106 - 109

1. DIGESTIBILITY OF FEEDSTUFFSPP 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 TRIALSStandard 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% adlib 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% adlib 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 • Need to know individual feed intake • 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 • Collect feces for 3 to 7 days • Analyze feces for marker • 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)