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 as particulate markers for ruminant nutrition studies. RE elements have similar chemical properties. They are attractive for use as a multiple marker system because they bind tightly to plant materials. RE binding of plant cell walls is cyclic throughout the RE series, which can be explained by the filling of the 4 f electron shell. RE markers may migrate from labelled feedstuffs under gastrointestinal conditions and particularly under acidic conditions, and they may decrease the digestibility of labelled feedstuffs. The various binding techniques used for labelling particulate matter with RE elements are evaluated in order to increase the stability of bound RE in different gastrointestinal conditions. The soaking procedure of plant cell walls to be labelled in an RE chelate solution, followed by washing to remove loosely bound RE, is recommended.