Fluid Markers

Digesta, particularly ruminal contents, is not a homogenous entity. Digesta phases have different composition and flow characteristics, which necessitates a compart-mental approach and the use of separate markers for as many digesta pools as can be reliably distinguished by physical or chemical means. With ruminal contents, the most common approach is fractionation into fluid and solid phases. The fluid phase includes not only solutes, but also small feed particles, which are densely populated with microbial cells and obey the kinetics of the fluid. Fluid markers should behave as ideal solutes and have a molecular weight high enough not to be absorbed throughout the digestive tract. A number of fluid markers have been proposed and employed with varying success.

Polyethylene glycol (PEG) with a molecular weight of 1000 Da or greater (usually 4000 Da) has been extensively, and relatively successfully, used as a solute marker. Studies with rabbits and sheep, however, showed incomplete (95%) recovery in digesta and feces. Reports also indicated that PEG was not completely associated with water in beet pulp tissues, can be precipitated by dietary tannins, and binds to particulate matter if digesta is frozen. PEG is assayed by turbidimetry.

Complexes of cobalt (Co) and chromium (Cr) with ethylenediamine tetraacetic acid (EDTA)[6,7] occupy larger fluid space in the rumen and have practically replaced PEG as fluid markers. Similar to PEG, both chelates are slightly absorbed (at approximately 5%) through the rumen wall. Adsorption of Cr EDTA to particulate matter has been reported at low concentrations and is affected by osmotic pressure in the rumen, which could lead to overestimation of water flow. Co EDTA is prepared as the sodium or lithium (Li) salt of the monovalent Co EDTA anion. Both compounds are readily soluble in water, relatively easy to prepare with a yield of approximately 90% (the Li salt in the case of Co EDTA), and are stable on drying. A common practice is to use Li/ Co EDTA as a fluid and Cr-mordanted fiber as a solid marker (see the following discussion). Cobalt and Cr are routinely analyzed by atomic absorption spectrophotom-etry, neutron activation, or plasma emission spectroscopy.

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