4 Apparent digestibility of the total feed Feed intake(I) = disappearance(X) + faecal output1 = X + 0.2X = 0.8Digestibility = X / I = 0.8 / 1 = 0.8X10.2FeedFaecesNote: total feed and faeces are weighed for a given period!
5 Apparent digestibility of a feed component Feed intake(I) = disappearance(X) + faecal output0.4*1 = X + 0.3*0.2X = = 0.34Digestibility = X / I = 0.34 / 0.4 = 0.85X10.20.3Feed0.4Faeces
6 Digestibility of a feed ingredient by difference Diet 1: Basal feed Diet 2- : Basal feed + test feed at one or more levelsCalculations by difference or by regressionProblems: 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 faecesMetabolic components consist of: crude protein, lipids, soluble carbohydrates and soluble ashMetabolic components are coming from the microbes and are soluble in neutral detergentsApp. dig. = true dig. - metabolic fractionIs true fiber digestibility higher than apparent?
9 Factors affecting digestibility Feed qualityFeed interactions (examples) starch: fibre digestibility in ruminants protein: fibre digestibility in ruminants lipids: Ca and Mg utilisationLevel 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
10 Use of markers - case studies Digestibility - total digestive tractFlow - ruminalIntakeMethane emissionMicrobial 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 markersInternal 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 feedDigestibility is calculated as:( /0.03)*100 = 67%Note: The marker is given regularly and is not absorbed.100100FeedMarker: 1%FaecesMarker: 3%
13 Faecal flow is calculated as: 2a. Intake during grazing: faecal output is first measured by a continuously dosed markerFaecal flow is calculated as:2/1 = 2.0 kg/dNote: The marker is given regularly and is not absorbed.FeedMarker: 2 g/dFaecesMarker: 1 g/kg
14 2b. Intake during grazing: intake is then estimated We now know flow of undigested feed (2a)We must determine digestibility separatelyFlow of indigestible feed = 2 kg/dDigestibility determined (hand clipped grass in vitro or in vivo) = 70%Intake of grass = 2/(1 – 0.70) = 6.7 kg/d
24 Concentrations in urine and usefulness Compoundg/LNotesUsageUrea~20Excretion depends on diet balanceN efficiencyCreatine~1CreatinineConstant muscle turnoverUrine volumeAllantoin~1.8Depends on microbial growthMicrobial synthesisHippuric acid~5Amino 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 cratesEquipment for monitoring methane emissionMacro in vitro system (“Rörko”)Milk analysisMini-silosAerobic stability of silage