Measurement of mineral availability

The availability of mineral elements is commonly high in young animals fed on milk and milk products but declines as the diet changes to solid foods. An additional complication is that the absorption, and hence apparent availability, of some mineral elements is under homeostatic control (determined by the animal s need for them). Iron absorption, discussed in Chapter 8, is the clearest example of this effect, but in ruminants the efficiency of calcium absorption also appears to be dependent on the animal s requirements. 

Whilst no attempt is made here or in Chapter 8 to provide a complete list of factors affecting the availability of mineral elements, those mentioned serve to illustrate why availability coefficients are not included in tables of food composition. The availability of a mineral in a particular food depends so much on other constituents of the diet and the type of animal that average availability coefficients would be of little significance. 

In a typical digestibility trial, the food under investigation is fed to animals for 21-30 days, with faeces being collected for the last 7-10 days. 

If food intake and/or faeces output cannot be measured quantitatively, digestibility may be estimated indirectly from the relative concentrations of an indigestible substance in the food and faeces known as an indicator. 

The digestibility of foods may be estimated in the laboratory (in vitro) by incubating them in rumen liquor or various chemical and enzymatic techniques. Near-infrared reflectance spectroscopy (NIRS) is now used routinely to estimate the digestibility of foods for farm advisory work. 

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