The estimation of digestibility

Faecal collection on pasture

Advanced Feed Science2013

Rumen metabolism

Metabolic crate for sheep

Feed intake(I) = disappearance(X) + faecal output

1 = X + 0.2

X = 0.8

Digestibility = X / I = 0.8 / 1 = 0.8

Apparent digestibility of the total feed

Feed Faeces

10.2

X

Note: total feed and faeces are weighed for a given period!

Feed intake(I) = disappearance(X) + faecal output

0.4*1 = X + 0.3*0.2

X = 0.4 - 0.06 = 0.34

Digestibility = X / I = 0.34 / 0.4 = 0.85

Apparent digestibility of a feed component

Faeces

0.2

Feed

0.4 0.3

1X

Digestibility of a feed ingredient by difference

Diet 1: Basal feedDiet 2- : Basal feed + test feed at one or more levels

Calculations by difference or by regressionProblems: interactions between feeds

The effects of positive and negative feed interactions

50.0

52.054.0

56.058.0

60.0

62.064.0

66.068.0

70.0

15 25 35 45 55 65 75 85

Proportion of Feed A

Digesitib.%

No Neg. Pos.

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 fractionIs true fiber digestibility higher than apparent?

Factors affecting digestibility Feed qualityFeed interactions (examples)

starch: fibre digestibility in ruminantsprotein: fibre digestibility in ruminantslipids: Ca and Mg utilisation

Level of intake - effect on fibre utilisationPhysical form - effect on fibre utilisationFats: chain length, degree of saturationProcessing: proteins, starch, etc.Adaptation of gut microfloraAnimal age: e.g. starch, lipids, fibre

Use of markers - case studies1. Digestibility

- total digestive tract2. Flow

- ruminal3. Intake4. Methane emission5. 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 - 0.01/0.03)*100 = 67%

Note: The marker is given regularly and is not absorbed.

1. Digestibility: measured by a dosed marker or by a marker with known concentration in the feed

Feed Faeces Marker: 3%

100

Marker: 1%

100

Faecal flow is calculated as:

2/1 = 2.0 kg/d

Note: The marker is given regularly and is not absorbed.

2a. Intake during grazing: faecal output is first measured by a continuously dosed marker

Feed Faeces Marker:1 g/kg

Marker:2 g/d

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 = 0.01 mg/g methane

Methane produced = 3/0.01 = 300 g/d

4. Estimating microbial amino acid flow from the rumen

Rumen & reticulum

Omasum

Abomasum

Easophagus

Protein metabolism

Liver

Rumen

Feed and microbial

protein Small intestine

Ammonia Amino acids

Urine

Feed protein

Other organs

Amino acids

Urea

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

XanthineHypoxanthine

Uric acidAllantoin

Inosine

Urine

IMP

Guanine

Guanosine

NH3

Muscle turnover

MusclesCreatine-P + ADP ATP +

Creatine

LiverAmino acids + Creatinine +Creatine

Muscle turnover

UrineCreatinine, Creatine, (amino acids)

Other organs

Amino acids

Concentrations in urine and usefulness

Compound g/L Notes Usage

Urea ~20 Excretion depends on diet balance N efficiency

Creatine ~1

Creatinine ~1 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

1. You assume a constant creatinine excretion per day (g/d)

2. You calculate urine excretion (L/d) from #1 divided by creatinine concentration in urine (g/L)

3. You calculate allantoin excretion (g/d) from #2 times allantoin concentration in urine (g/L)

4. 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 cratesEquipment for monitoring methane

emissionMacro in vitro system (“Rörko”)Milk analysisMini-silosAerobic stability of silage