1. The estimation of digestibility
Faecal collection on pasture

2. Rumen metabolism
Advanced Feed Science
2013

3. Metabolic crate for sheep

4. 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!

5. 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

6. 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

7. The effects of positive and negative feed interactions

8. 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?

9. 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

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

11. 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.

12. 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%

13. 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

14. 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

15. 3 a. Methane emission
Gas sampling device

16. 3 b. Methane emission
SF6 delivery device
SF6 permeable membrane

17. 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

18. 4. Estimating microbial amino acid flow from the rumen

19. Rumen & reticulum
Easophagus
Omasum
Abomasum

20. 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

21. 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

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

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

24. 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

25. 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

26. 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