Ytterbium Chloride as a Marker for Particulate Matter in the Rumen

Abstract

Binding strength and capacity of various feedstuffs for ytterbium (Yb), potential for Yb release from feed binding sites and migration to particulate and soluble fractions, and the effect of Yb labelling on in vitro and in situ digestion of feedstuffs were evaluated in four in vitro and two in vivo experiments. All feedstuffs tested formed bonds with Yb. Feeds higher in crude fiber or crude protein bound more Yb (76 to 205 µmol Yb/g feed). Each feedstuff exhibited at least two types of Yb binding sites. Weaker sites within feedstuffs had molar association constants of 4.5 to 9.2 x 10⁵ compared with 1.2 to 3.8 x 10⁷ for stronger sites. Water dialysis of Yb-tagged and washed feedstuffs detached Yb from binding sites at rates ranging from .09 to .57%/h. Ytterbium migration in the rumen from labelled corn grain to unlabelled prairie hay increased Yb content of hay to a level equal to .18% of the corn after 96 h. Release of Yb from feeds was not well correlaed (R² = —.55) with percentage of weak binding sites in feedstuffs. Certain soluble organic compounds formed bonds with Yb. Formation of complexes between rare earths and soluble organic compounds makes it necessary to minimize marker release from labelled feeds. Competition for Yb against five feedstuffs, ranked various solutes from greatest to least, as follows: lactate>acetate>lysine>glucose>glycine>sucrose. Saturating feedstuffs with Yb reduced (P<.05) in vitro and in situ digestibility of dry matter. Ytterbium disappeared from nylon bags at a rate about one-half that of dry matter and totaled 9.3 to 25.3% after 96 h of incubation. Reduced feedstuff digestibility and potential for Yb migration makes the method of Yb application critical for collection of interpretable data. Copyright © 1984. American Society of Animal Science . Copyright 1984 by American Society of Animal Science.