External and internal markers for appraising site and extent of digestion in ruminants.

Digesta markers are used routinely to calculate fecal output and to estimate kinetics within the digestive tract. A marker suitable for estimating fecal output may not be a suitable kinetic marker because of problems with marker migration, phase separation, inhibition of digestion, osmotic effects within the gut, and quantitation. Marker validity should be checked when possible based on alternative methods (e.g., fecal output and ruminal evacuation). Certain parameters, such as pool size and passage rate, that can be estimated with markers cannot be measured by other noninvasive procedures. Nevertheless, only when marker results are verified by other methods can one evaluate the magnitude of error associated with assumptions inherent in marker mathematics (steady state, instantaneous mixing, and first-order kinetics). When marker results do not meet expectations, marker failure, rather than inadequate knowledge of gastrointestinal function or analytical difficulties, is blamed. No marker is ideal, but research to compare markers is useless if results are not related to direct flow or output measurements. Marker results often are adjusted for marker recovery based on the premise that analysis is precise and that losses are constant. Such adjustment is appropriate only if the error is analytical and proportional. Despite imprecision in marker procedures, inherent variation may be small relative to other sources of variation (e.g., gut physiology, diet, environment, and feed intake). Even though absolute values may be imprecise and inaccurate, marker-based estimates usually provide reliable information about the direction and extent of kinetic changes induced by treatments.