Rare earth elements as markers for rate of passage measurements of individual feedstuffs through the digestive tract of ruminants.

Rumen and in vitro abomasal fluid incubations of four feeds, soybean meal (SBM), 0.3% formaldehyde-treated SBM, cottonseed meal and ground corn, each labeled with one of two markers (Sm and La) were conducted to estimate the extent of marker movement from the feeds. Feeds were marked by spraying acidic solutions of the marker movement from the feeds. Feeds were marked by spraying acidic solutions of the markers uniformly over the feeds. Concentration of marker on the feeds, on the feed residues remaining in polyester bags after 12 hours of incubation in the rumen, and on the feed residues after 12 hours of incubation in the rumen and 3 hours of in vitro incubation in abomasal fluid were determined by neutron activation. Initial concentration of the markers on the feeds was used as a covariable in analysis of the restricted randomized block design. Marker concentration of Sm and La, for all feeds, averaged 1115 +/- 44 and 1503 +/- 44 ppm after the rumen incubation and 959 +/- 54 and 1666 +/- 54 ppm after the combined incubations and differed (P less than 0.01) from the mean initial marker concentration of 1294 +/- 42 ppm. Abomasal incubations allowed estimation of marker movement (contamination) between bags. For both markers, increases (P less than 0.01) were observed in marker contamination when compared to initial levels. Marker contamination increased from 2.5 +/- 0.8 to 20.1 +/- 0.1 and 10.7 +/- 0.8 ppm for Sm and La, respectively. Data indicate significant marker movement and suggest that Sm and La, when applied to feeds by this method, are not suitable markers for estimating passage rates of individual feedstuffs.