Evaluation of a continuous-release ytterbium bolus.

Four ruminally cannulated steers were used in a 4 X 4 Latin square design to determine the efficacy of a continuous-release bolus containing Yb203 for estimation of fecal output. One bolus was placed in the reticulum of each steer at the beginning of the experiment. Four diets were fed, including alfalfa, alfalfa treated with a commercial preservative, and each of the 2 alfalfa hays in a 50:50 mixture with concentrate. Each period of the Latin square consisted of 15 d for adaptation to diets, followed by 5 d of total fecal collection. Fecal output was estimated from marker concentration in grab (0800, 1700, and 0800 + 1700) and composite (from total collection) fecal samples using 3 methods of dose calculation (manufacturer-formulated release, 160 mg Yb/d; trial average, based on bolus weight change over entire trial; and period average, based on bolus weight change during the week preceding and week of sampling period). Percentage of actual fecal output estimated by each calculation method was not affected by diet (P>.10). Calculated Yb release from the week before and the week of collection provided estimates of fecal output that were not different (11% overestimation;P>.10) from total collection, while estimates using formulated or trial average dose differed (P&.10; 37 and 34% overestimation, respectively). Across dose calculation method, actual fecal output was overestimated by 31, 27, and 29% from 0800, 1700, and 0800 + 1700 grab samples, respectively. Estimates based on composite fecal samples overestimated actual fecal output by 22% averaged across dose method. Composite fecal samples and dose based on period average provided the best estimation of actual fecal output (9% greater than total collection values). Regardless of method of dose calculation or fecal sampling method used, estimates were variable and greater than total collection values.