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Understanding bow rate of passage estimates are influenced by procedural variations may facilitate standardization of methodology and enhance comparisons among studies. Therefore, 12 ruminal-fistulated beef steers (P wt. = 294 kg) were used in two 6-day grazing trials to evaluate influences of sampling site, httraruminal mixing, and mathematical model on particle passage rate estimates. Steers grazed a 13&a pasture of immature crested wheatgrass. We estimated particle passage rate from the rumen by intraruminal administration of a pulse dose of Yb-iabeled forage followed by serial collection of ruminal digesta or fecal samples. Treatments were (1) rectal sampling; (2) rumburl samphngruminal contents mixed before subsampling digestai (3) nuninal sampling-ruminal contents not mixed before s&sampling digesta. All steers were fitted with vibracorders to monitor grazing time before and during sampling periods. Fecal Yb curves were fitted with a one compartment, time-dependent (ICMPT-TD), a two=__t8nadepadcd_(2CMPTTD), and a two-compartment, time-independent (ZCMPT-TI) model. All ruminal Yb curves were fitted with a single exponential decay model. Comparisons among models were limited to rate constants associated with the slower escape process. Intranmunal mixing did not alter (E70.10) passage rates. The ZCMPT-TD model failed to fit some fecal profiles. Particle passage rates from the 2CMPT-TI model were greater (p<O.os) than those from the ICMPT-TD model. Similarity among passage rate constants derived from fecal Yb curves and those derived by semilogarithmic regression of ruminal Yb concentration on time depended on the model used to fit fecal Yb curves. Grazing time decreased wO.01) during intensive samplhtg periods. We conclude that for steers grazing immature grass pastures, intraruminal mixing before subsamplhtg does not significantly alter rate of pasaap estimates; however, site of sampling and mathematical model may be hnportant factors to consider in chooshtg appropriate methodology for estimating rate of passage.

Examination of Methods for Estimating Rate of Passage in Grazing Steers