PASSAGE OF CHROMIUM-MORDANTED AND RARE EARTH- LABELED FIBER: TIME OF DOSING KINETICS I K. R. Pond 2, W. C. Ellis 3, J. H. Matis 4 and A. G. Deswysen 5 Texas A&M University, College Station 77843 ABSTRACT Coastal bermudagrass hay was labeled with Cr by the Cr-mordant procedure and with 177Lu applied to the same fiber. Neutral detergent fiber prepared from the same Coastal bermudagrass hay was labeled with Yb, 169yb, Tb and 16t~b by soaking overnight following by thorough washing and drying. Wood chips were similarly labeled with Sm or 147 141 La, and Solka Floc was labeled with Nd and Ce. The carriers, labels and times of administration to cattle were: bermudagrass fiber with both Cr and 177Lu, bermudagrass fiber with 169yb and Solka Floc labeled with t47Nd at 0 h; bermudagrass fiber with Yb, Solka Floc with 141Ce and wood chips with Sm at 24 h; wood chips with La at 48 h; and bermudagrass fiber labeled with 16~ at the beginning and labeled with Tb at the end of a meal. Fecal collection followed and passage characteristics were determined with a two- compartment, age-dependent model. Markers labeling the different fiber sources had different (P < .01) passage rates (Solka Floc > Coastal bermudagrass > wood chips), but there was no difference within fiber source for rare earth passage. There also was no difference between the passage characteristics of Cr-mordant and 177Lu. However, passage rate of particles administered at the beginning of the meal (~6~ was 42% higher than for particles at the end of the meal feb). These results indicate that the flow characteristics of rare earths applied by the soak and rinse procedure were the same as those of the more strongly bound Cr-mordanted fiber and that the kinetics of passage can be affected by the time of dosing relative to the consumption of the meal. (Key Words: Markers, Rare Earth Elements, Kinetics, Transit Time.) J. Anim. Sci. 1989. 67:1020--1028 IntroclucUon Voluntary intake (VI) of grazing animals generally is computed from estimates of digestibility (DIG) and fecal output (FO), (VI=FO/(1-DIG)). Fecal output can be deter- 1Approved by the director of the Texas A&M Agric. Exp. Sta. TA-1792. Special appreciation is expressed to L. A. Nelson, Statistics Dept. and to S. A. WesL Computing Sci. Center, North CarolinaState Univ., for advice in the statistical analysis. 2Current address: Dept. of Anim. Sci., North Carolina State Univ., Box 7621, Raleigh 27695-7621. 3Dept. of Anim. Sci. 4Inst. of Statistics. 5PostdoctoralNATOFellow,University CatholicLou- vain, Belgium. Received February 12, 1988. Accepted August 31, 1988. mined directly using collection bags or with external markers (including Cr and several of the rare earth elements). Markers usually are administered by continuous infusion and daily administration (to obtain a steady state) or by a single-dose procedure (Galyean et al., 1987). The single-dose procedure also provides estimates of passage rate, mean retention time and gastrointestinal tract fill (Pond et al., 1988). However, construction of the marker appearance curve requires frequent sampling, which may affect the animal's normal behavior (Fisher et al., 1986). Wollscheid et al. (1979) proposed estimat- ing passage rate from the ratio of two markers dosed at different times. The solute markers utilized were not biologically identical and they unsuccessfully estimated passage from the marker ratio. Passage of rare earth-labeled particles may be less variable than passage of 1020