Comparison of Forage Labeling Techniques for Conducting Passage Rate Studies

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Abstract
Studies were conducted to evaluate method of forage labeling for obtaining estimates of fecal output (FO), total mean retention time (TMRT) and the foUowing variables obtained from the fecal excretion curve (i.e., fecal marker concentration vs time postdosage) of cattle: k1 (turnover rate of large partiele rumination pool of the rumen), k2 (ruminal turnover rate) and time delay (time of first appearance of marker in feces). Comparisons between methods of forage labeling were (1) wheat forage (WF) labeled with ytterbium (Yb) by spray and immersion procedures and (2) sorghum-Sudan hay (SS) labeled with Yb by immersion and with chromium (Cr) by mordanting. In vitro dry matter digestibility (IVDMD) of unlabeled WF (77.4) was greater (P<.05) than IVDMD of WF labeled by spray (63.5) and immersion (65.7%) techniques. The rate constant, k1, was not different between treatments; however, k2 (4.9 vs 5.9%/h) and time delay (4.0 vs 5.6%/h) were greater (P<.05) when measured with WF prepared by the immersion technique. Estimated FO of 1.84 (spray) and 1.97 kg (immersion) were not different (P>.05). The lower k2 value obtained by spraying would indicate that back migration of marker to new feed entering the rumen may have occurred with the spray technique. The IVDMD of the unlabeled SS, Yb-labeled SS and Cr-labeled SS were 54.8, 35.1 and 9.5%, respectively. Rate constants k1 (13.4 vs 9.7%/h; P<.05) and k2 (2.6 vs 1.8%/h; P<.05) and estimated FO (93.4 vs 77.3% of actual, P<.05) were lower when estimated with Cr-mordanted SS. These data suggest that estimates of passage rate vary with choice of marker labeling technique. Of the techniques evaluated, immersion is preferred to spray or mordanting. Copyright © 1984. American Society of Animal Science . Copyright 1984 by American Society of Animal Science.

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