similar in terms of apparent The fecal material from all cows appeared to be While the approximate variation in moisture content and gross consistency. t, excretion daily fecal output between experimental animals was 25 percen ty of material quanti the to d relate rates for labeled sand particles were not tract. that passed through the gastrointestinal Retention Time conThe mean retention time can be calculated very simply if the cow gut is and sidered as.a one-compartment model where mixing of ingesta is very rapid n retentio mean the on, Under this assumpti emptying is by first-order kinetics. times 1.44 or , time would be the reciprocal of the first-order rate constant For these data, however, the biological half-time (Johnson and Lovaas, 1971). able the area under the curve before excretion begins (Figure 1) is a consider a such and s), portion of the total area (especially for the 20-um particle particle on of calculated retention time would be useful mainly in comparis Other The biological half-time is equally useful in this regard. kinetics. and (Balch proposed methods for calculating a mean retention time have been on variati however, 1971); Campling, 1965; Phillipson, 1970; Johnson and Lovaas, of ation ve manipul among animals in this experiment indicates that exhausti A summary of retention time values from this these data is not warranted. The definitions of the various terms are experiment is given in Table 7. given in the footnotes to Table 7. The average retention time, or average time required for experiment cows to excrete 67% of the particles, is shown as curve 2 of Figure 2. This information is of value for initial estimates of dairy cow retention times for ingested particles. The information on variability among animals, Tables 4 through 7, should be noted in making such estimates. The average retention time is more useful than the biological half-time in estimating the period during which particles are subjected to digestive solubilization and absorption processes because it includes the initial period of retention in the rumen. Discussion Ruminant stomachs are of a compound nature and consist of four compartments, designated as rumen, reticulum, omasum, and abomasum. The anatomy and physiology of these compartments and the remainder of the gastrointestinal tract are thoroughly described in the literature (Annison and Lewis, 1959; Balch and Campling, 1965; Dukes, 1955; and Phillipson, 1970). Food materials remain in the cow rumen longer than in other segments. Once food material is passed to the fourth compartment, the abomasum, about 90% is excreted within 30 hours; from the rumen, about 100 hours are needed for 90% excretion (Phillipson, 1970). Digestive processes tend to allow accumulation of foreign debris in the reticulum and abomasum, while very little accumulates in the rumen and omasum. The material in the reticulum is liquid, so heavy foreign matter tends to settle out. Because the inlet and outlet of the reticulum are well above the fundus, heavy foreign matter is expelled with difficulty (Dukes, 1955). A fairly wide variety of markers (e.g., dyed hay, plastic beads, and soluble unabsorbed radionuclides) have been used frequently to provide information for nutritional studies. Typical results for cattle fed marked food materials 161