# The estimation of digestibility

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The estimation of digestibility
Faecal collection on pasture

Rumen metabolism Advanced Feed Science 2013

Metabolic crate for sheep

Apparent digestibility of the total feed
Feed intake(I) = disappearance(X) + faecal output 1 = X + 0.2 X = 0.8 Digestibility = X / I = 0.8 / 1 = 0.8 X 1 0.2 Feed Faeces Note: total feed and faeces are weighed for a given period!

Apparent digestibility of a feed component
Feed intake(I) = disappearance(X) + faecal output 0.4*1 = X + 0.3*0.2 X = = 0.34 Digestibility = X / I = 0.34 / 0.4 = 0.85 X 1 0.2 0.3 Feed 0.4 Faeces

Digestibility of a feed ingredient by difference
Diet 1: Basal feed Diet 2- : Basal feed + test feed at one or more levels Calculations by difference or by regression Problems: interactions between feeds

The effects of positive and negative feed interactions

True and apparent digestibility
True digestibility is always higher than apparent if any metabolic or endogenous components are excreted in the faeces Metabolic components consist of: crude protein, lipids, soluble carbohydrates and soluble ash Metabolic components are coming from the microbes and are soluble in neutral detergents App. dig. = true dig. - metabolic fraction Is true fiber digestibility higher than apparent?

Factors affecting digestibility
Feed quality Feed interactions (examples) starch: fibre digestibility in ruminants protein: fibre digestibility in ruminants lipids: Ca and Mg utilisation Level of intake - effect on fibre utilisation Physical form - effect on fibre utilisation Fats: chain length, degree of saturation Processing: proteins, starch, etc. Adaptation of gut microflora Animal age: e.g. starch, lipids, fibre

Use of markers - case studies
Digestibility - total digestive tract Flow - ruminal Intake Methane emission Microbial amino acid absorption

Use of markers to estimate digestibility, intake and flows
Markers must not be absorbed or interfere with the digesta or adhere to tissues, etc. External markers: known amounts of dosed markers Internal markers: components of the feed or produced in metabolism.

Digestibility is calculated as:
1. Digestibility: measured by a dosed marker or by a marker with known concentration in the feed Digestibility is calculated as: ( /0.03)*100 = 67% Note: The marker is given regularly and is not absorbed. 100 100 Feed Marker: 1% Faeces Marker: 3%

Faecal flow is calculated as:
2a. Intake during grazing: faecal output is first measured by a continuously dosed marker Faecal flow is calculated as: 2/1 = 2.0 kg/d Note: The marker is given regularly and is not absorbed. Feed Marker: 2 g/d Faeces Marker: 1 g/kg

2b. Intake during grazing: intake is then estimated
We now know flow of undigested feed (2a) We must determine digestibility separately Flow of indigestible feed = 2 kg/d Digestibility determined (hand clipped grass in vitro or in vivo) = 70% Intake of grass = 2/(1 – 0.70) = 6.7 kg/d

3 a. Methane emission Gas sampling device

3 b. Methane emission SF6 delivery device SF6 permeable membrane

3 c. Methane emission SF6 delivery = 3 mg/d
SF6 concentration in collected air = mg/g methane Methane produced = 3/0.01 = 300 g/d

4. Estimating microbial amino acid flow from the rumen

Rumen & reticulum Easophagus Omasum Abomasum

Feed and microbial protein
Protein metabolism Feed and microbial protein Rumen Small intestine Feed protein Ammonia Amino acids Amino acids Liver Other organs Urea Urine

Digestive tract metabolism of nucleic acids
Rumen Feed nucleic acids Nucleic acids Metabolized Nucleic acids (RNA) Bacterial synthesis Hydrolysis Acid stomach Small intestine Nucleosides & bases To liver

Liver metabolism of the purine bases
NH3 AMP & GMP Xanthine Hypoxanthine Uric acid Allantoin Inosine Urine IMP Guanine Guanosine

Muscle turnover Muscles Creatine-P + ADP ATP + Creatine Liver
Amino acids Liver Amino acids + Creatinine +Creatine Other organs Urine Creatinine, Creatine, (amino acids)

Concentrations in urine and usefulness
Compound g/L Notes Usage Urea ~20 Excretion depends on diet balance N efficiency Creatine ~1 Creatinine Constant muscle turnover Urine volume Allantoin ~1.8 Depends on microbial growth Microbial synthesis Hippuric acid ~5 Amino acids ~0.8

Estimation of microbial amino acid uptake
You assume a constant creatinine excretion per day (g/d) You calculate urine excretion (L/d) from #1 divided by creatinine concentration in urine (g/L) You calculate allantoin excretion (g/d) from #2 times allantoin concentration in urine (g/L) You finally calculate microbial amino acid absorption (g/d) assuming a relationship between microbial amino acid absorption and allantoin excretion in urine. The basis for that is an assumed RNA/amino acid ratio in rumen bacteria

Demonstrations and visit to Kungsängen research facilities
Rumen fistulation – film (optional) Digestibility - sheep and cattle crates Equipment for monitoring methane emission Macro in vitro system (“Rörko”) Milk analysis Mini-silos Aerobic stability of silage

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