Measuring the

rate of passage of feeds

Dr A T Adesogan

University of Florida

Fermentation

Degradation

Microbial synthesis

Intake

Undigested feed, digested

feed, unabsorbed VFA &

microbes

Absorptionka

(absorption rate)

kp

(passage rate)

kd (degradation

rate)

Note that kd was referred to

as ‘c’ in the notes on the in

situ technique

Definitions

Passage or flow rate (mass/time) = rate at which digesta leaves a compartment of the gut

Fractional outflow rate = proportion of a component of a feed that leaves the compartment per unit time

Turnover = Mean retention time= duration feed remains in compartment or whole gut

Time available for digy in each compartment = t½

Factors affecting passage rates

Feeding level

Animal species

Diet composition (e.g. concentrates vs forages)

Feed particle size / physical form

Moisture in feeds

How passage rates vary with performance

Animal type & performance

level

Feeding

level (L)

Passage

rate %/h

Sheep & cattle at maintenance

& low planes of nutrition

1 2

Growing sheep & cattle 2 5

High yielding dairy cows

(>15 kg milk/day)

3 8

(AFRC, 1993)

Effects of particle size of alfalfa hay on retention

time and fiber digestibility (Rodrigue & Allen,

1960)

Feed Mean size

(µ)

Retention

(h)

Fiber digy

%

Long hay … 54 44

Course grind 434 39 34

Medium

grind

393 34 31

Finely ground 280 27 22

Passage rate is the reciprocal of retention time

Passage rates of liquids & particles in different

animals (Uden, 1978; Van Soest et al., 1978)

Rumen retention time (h)

Spp. Body wt, kg Particles Liquid

Large heifers 555 47 15

Small heifers 243 38 16

Sheep 30 35 19

Goats 29 28 19

Rumen Evacuation Method

Manually empty rumen into barrel (in warm water) at

consecutive, >24 h intervals.

Each time, weigh contents, mix well and take sample

(2-3 kg) for analysis

Return rumen contents within 10 minutes, rumen must

not be empty for >2-3 minutes.

Rumen evacuation calculations

Pool size (kg) = mean weight of rumen contents

Intake rate (ki) = 1/24 x (intake, kg /d) / (pool size, kg)

Passage rate (kp) = 1/24 x (fecal output, kg /d) / (pool

size, kg)

Digestion rate (kd) = ki - kp

Marker Method

The theory:

– If an inert, unabsorbed marker is pumped into one section of the GIT long enough to reach equilibrium/steady state (i.e. the constant flow digesta rate any sampling point), then on average 100% of the daily dose of the marker will pass each subsequent section of the tract each day

Basic assumption

– Passage rate of potentially digested components equals that of the indigestible marker

– Particle outflow follows first order kinetics

(Hogan, 1980)

Procedure

Dose marker till steady state achieved, then withdraw it.

Measure in marker conc. with time at a distal site

Distal sites:

1. Rumen

• measure rate of dilution of marker in rumen

• Regress natural log of marker conc. in rumen on time

2. Feces

• measure rate of excretion of marker in feces

• Fit non-linear model to declining part of curve

Passage rate = slope of either of the curves above

Marker administration & sampling

Solid phase marker

– Add marker to dorsal rumen through a cannula 30-60 mins before feeding with a funnel

– For non-fistys, mix marker with concentrate, and feed ensuring complete consumption during feeding

– Collect feces after 12, 24, 27, 30, 33, 48, 54, 60,72, 96, 120 and 144

Liquid phase marker

– Pour into rumen via canulla, before feeding

– Sample rumen fluid after 0.5, 1,2,4, 6, 9, 12 &24 h

Ruminal marker profile

TIME

[MARKER]

Continuous infusion

Single pulse dose

Marker

withdrawal

Infusion

terminates

Slope =

passage rate

Ruminal marker dilution graph

TIME Post dosing

NATURAL LOG

OF [MARKER]

Slope = Dilution rate =

passage rate (kp)

Mean retention time (MRT) =

Turnover time = 1/slope

Half life (t½) = ln 2

dilution rate

turnover

½ life

2 8 14 20 26

Fluid volume (L) = Marker Dose

[marker at t=0]

Dilution method: Fecal marker

excretion curveCurve fitted to

declining

part of curve

Ellis et al., (1994)

Pulse dose

marker

Withdraw

marker

Marker issues

Dose marker till equilibrium / steady state achieved

implying:

– Digesta uniformly labelled

– Constant digesta flow rate through any sampling point

– Achieved in 6-8 days (depends on feed and feeding level)

Single pulse dose vs continuous infusion

– Single dose causes poor mixing of marker & digesta

Factors affecting results

Marker used

Single pulse dose vs continuous infusion

Achievement of steady state

Sampling site & frequency

Modelling

– Exponential model (equal outflow probability for all

particles regardless of size)

– Gamma lifetime (age dependent) model (Ellis ’94) –

assumes rumen particle outflow increases with time

Essential properties of markers(Kotb & Luckey, 1972)

Inert, non bulky and non-toxic

Quantitatively recovered (neither left in GIT nor

absorbed)

Physically similar or flow at same rate as substrate

Mix completely & distribute uniformly in feed

Not affect (or be affected by) GIT secretions, digestion,

absorption and microflora motility

Easy to analyze & inexpensive

No existing marker satisfies all of the criteria

Types of makers

External vs internal

Liquid vs Solid-phase

Dual phase (liquid + solid) markers often used since no

single marker moves in the same way as all the digesta

fractions

Internal markers

Component of the forage/feed

Pros

– Cheap & convenient

– Suited for free-ranging/feral animals

Cons

– Composition may be modified during digestion

Internal markers

Lignin –

– Poorly defined entity& low, inconsistent recovery

– Composition varies within plant parts / between plants

IADF

– Residual ADF after several days of in situ degradation

– Precipitated minerals may occlude bag pores

– Variable recovery rates

AIA

– Sample ash boiled in 2M HCl for 5 min; residue re-ashed

– Soil /dust contamination can affect results

13C or 14C

– Radioactive

Liquid-phase markers

Generally migrate less than solid-phase markers

Polyethylene glycol (PEG)

– excluded from some water space in feeds e.g. (S. beet)

– precipitated by tannins

– some absorption

CR-EDTA/ CO EDTA

– Lithium salt of the marker is used

– Some absorption through the rumen wall

– May bind to particulate matter

Solid-phase markers

Physical markers

– Powdered brazil nuts, charcoal, rubber, glass beads, cotton

knots, seeds, ball bearings, plastic pieces

– Not adsorbed to substrate, easily separated from digesta

– Similarity of flow of digesta & marker questionable

Chemical markers

– Most are adsorbed by particulate matter

– Barium sulphate, chromic oxide, rare earths, etc

Solid phase markers

Chromic oxide (Cr2O3) (chromic oxide)

– One of the most widely used for digy.

– Flow rate is b/w that of solid & liquid particles

– Administered either:

• Orally, gelatin capsules, mixed with ration

• Controlled release bolus

– Unsuitable for accurate estimations due to differential

flow rate.

Solid phase markersCr mordanted fiber

– Mordanting Process

• Remove soluble particles with ND reagent

• Soak fiber (mordant) in water overnight and wash to remove solubles

• Soak mordant in sodium or pottasium dichromate (12-14% of fiber wt)

• Heat suspension @ 100oC for 24 h; discard the poisonous liquid

• Suspend the mordant in tap water & ascorbic acid to give acidic pH

• Wash mordant several times in water, dry at 65oC for 24 h

• Grind dried mordant to pass 5 mm screen

– Higher specific gravity than digesta

– Can reduce cell wall digestion

Solid phase markers

Rare earths

– Metals from Lanthanum – Lutecium (atomic # 57-71) E.g.

Ytterbium, Samarium, Dysporium – commonly used

– Indigestible; but forms strong bonds with solid matter

– Ytterbium ($1500/kg) forms strongest complexes

– Analysis (atomic absorption spectrophotometry; plasma

emission spectroscopy or neutron activation analysis) can

be expensive/ difficult

References

– Marais, J. P. 2000. Use of Markers. In JPF D’ Mello (ed.) Farm animal metabolism and nutrition. CAB International. P255-277.

– Uden et al (1980) Journal of the Science of Food and Agriculture 31:625-632.

– Faichney (1980) Journal of Agricultural Science 94: 313-31

– Hogan, J P (1980) Estimating the sites and extent of digestion of ruminants. In: J L Wheeler and R D Mochrie (eds). Forage Evaluation. AFGC/CSIRO.

– Stern et al. (1997) J. Anim. Sci. 75:2256-276

– Hanson (1992) J. dairy Sci. 75:2605-2617

– Moore-Colyer et al., 2003. British Journal of Nutrition. 90: 109-118.