Flow paths of plant tissue residues and digesta through gastrointestinal segments in Spanish goats and methodological considerations 1 L. S. Walz*, W. C. Ellis† 2 , T. W. White*, J. H. Matis†, H. G. Bateman*, C. C. Williams*, J. M. Fernandez* and L. R. Gentry* *Louisiana State University Agricultural Center, Baton Rouge 70803-4210 and †Texas A&M University, College Station 77843-2471 ABSTRACT: A sequence of eight twice-daily meals, each marked with different rare earth elements, was fed to 24 Spanish goats (BW = 20.6 ± 1.94 kg) to produce meal-based profiles of rare earth markers within seg- ments of the gastrointestinal digesta on subsequent slaughter. Accumulative mean residence time and time delay of rare earths and segmental and accumulative mean residence times of indigestible NDF (IDF) were estimated for each sampled segment. Diets consisted of ad libitum access to bermudagrass hay with a limit feeding of one of four supplements: 1) minerals (basal, B); 2) B + energy (E); 3) B + CP (CP); or 4) B + E + CP for 84 d. Mean daily intake (g/kg of BW) during the 5 d before slaughter differed (P < 0.05) via diet for DM but not for IDF (8.0 ± 0.35 g/kg of BW). Larger estimates of cumulative mean residence time for IDF vs. rare earths were suggested to be the consequence of a meal- induced bias in the single measurement of IDF pool size by anatomical site. The rare earth compartment method was considered more reliable than the IDF pool dilution method because it yielded flow estimates based on the flux of eight meal-dosed rare earth markers over 4 d and was independent of anatomical definitions of Key Words: Digesta, Goats, Methodology, Rumen, Turnover 2004 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2004. 82:508–520 Introduction The nutritive potential of meals is determined by the rate of escape (or conversely, the segmental mean retention time) and rates of digestion of nutritive enti- ties of masticated fragments within segments of di- gesta. Various forces propel and constrain the flow of plant tissue fragments undergoing digestion within 1 Approved by the Director of the Louisiana Agric. Exp. Stn. as publication No. 02-11-0115. 2 Correspondence: 2471 TAMU (phone: 979-845-5063; fax: 979-845- 5292; e-mail: w-ellis@tamu.edu). Received October 3, 2002. Accepted October 14, 2003. 508 pool size. Statistically indistinguishable estimates for gastrointestinal mean residence times for IDF and rare earths conform to assumed indelibility for the specifi- cally applied rare earths and indigestibility of IDF. The potentially digestible NDF (PDF):IDF ratio of dietary fragments (0.8) progressively decreased in the following order: caudodorsal reticulorumen (0.390) > crainodorsal reticulorumen (0.357) ≈ reticulum (0.354) > mid-dorsal reticulorumen (0.291) ≈ ventral reticulorumen (0.286), to that within the omasal folds and in the abomasum (0.259). Such a gradient of progressively aging mixture of plant tissue fragments is consistent with age-depen- dent flow paths established in the reticulorumen and flowing to the omasum and abomasum. Such heteroge- neity of fragment ages within the reticulorumen is also indicated by the superior fit of marker dose site ≠ marker sampling site model assumptions. Additionally, cyclic meal- and rumination-induced variations in es- cape rate occur. Estimates of mean escape rates over days, needed for the practice of ruminant nutrition, must consider the complex interactions among plant tissues and the dynamics of their ruminal digestion of PDF. successive segments of the gastrointestinal tract of her- bivores. Driven by nutrient deficits at the tissue level (Ellis et al., 1999, 2000), continuing intake of sequential meals causes a flux of undigested residues being pro- pelled by gastrointestinal motility through successive segments of the gastrointestinal tract. Various forces constrain this flux in different segments of digesta and cause differences in segmental loads and mean resi- dence times of undigested fragments. Because of its diverticular arrangements of entry and exit flow (Moir, 1965) and well-organized and regulated motility cycles of the reticulorumen (Erhlein, 1980), primary emphasis has been directed to defining mechanisms that con- strained flux of undigested fragments within the reticu- loruminal digesta (Ellis et al., 1994). Quantitative esti-