Retention of Rare Earths by Hay Particles Following Incubation in Fresh or Autoclaved Rumen Fluid

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Objectives were to assess the impact of labeling technique, microbial fermentation, and pH on retention of Yb and Ce by bromegrass hay incubated in vitro. Hay was chopped and labeled with Yb or Ce by spray or immersion techniques. One-half of each chopped and labeled hay then was ground. Within marking technique, chopped hay labeled with Yb and ground hay labeled with Ce or chopped hay labeled with Ce and ground hay labeled with Yb were combined and incubated in buffer and fresh or autoclaved rumen fluid. The buffer contained Co and Cr as EDTA complexes. One-half of the flasks were acidified with HCl after incubation. Flask contents were sieved to isolate residual chopped and ground particles. Effluent that passed a .07-mm screen was centrifuged to isolate fine feed and microbial residue and supernatant fractions. Distribution of Yb, Ce, Co, and Cr in all fractions was determined. Retention of Yb and Ce was not affected by marking procedure. Labeled hays incubated in autoclaved rumen fluid retained 65.0 and 51.2% of the initial amounts of Yb and Ce, respectively. Retention of Yb and Ce by hay incubated in fresh rumen fluid was lower than in autoclaved fluid (24.0 and 19.0% for Yb and Ce, respectively). Marker that migrated from the hay was found primarily in the fine feed and microbial residues isolated by centrifugation. Reducing media pH resulted in further elution of marker from labeled hay. The Co-EDTA and Cr-EDTA remained associated primarily with the fluid phase, regardless of pH and incubation media. Results suggest that Yb and Ce did not remain associated with originally labeled feed particles, thus raising serious questions about the suitability of these elements to estimate passage rate of feed from the rumen.