Use of rare earth elements as external markers for mean retention time measurements in ruminants

The present review deals with the utilisation of rare earth (RE) elements a s particulate markers for ruminant nutrition studies. RE elements have simi lar chemical properties. They are attractive for use as a multiple marker s ystem because they bind tightly to plant materials. RE binding of plant cel l walls is cyclic throughout the RE series, which can be explained by the f illing of the 4 f electron shell. RE markers may migrate from labelled feed stuffs under gastrointestinal conditions and particularly under acidic cond itions, and they may decrease the digestibility of labelled feedstuffs. The various binding techniques used for labelling particulate matter with RE e lements are evaluated in order to increase the stability of bound RE in dif ferent gastrointestinal conditions. The soaking procedure of plant cell wal ls to be labelled in an RE chelate solution, followed by washing to remove loosely bound RE, is recommended.