1. Digesta passage and retention were measured in heifers, sheep, goats, equines and rabbits of varying body-weights when given timothy (Phleum pratense) hay.2. Two passage markers were compared, cobalt (111) ethylene diamine tetraacetate (CoEDTA) and chromiummordanted timothy fibre for liquid and solid phase respectively. Both markers were injected into the rumen of the ruminants and into the caecum of the equines and rabbits.3. In ruminants, two different sets of rate constants (k, and k,) were derived from a two-pool model tor marker passage, using a graphical approach and a computer-based non-linear least-squares curve-fitting technique. 4. Retention times, due to unidirectional Row through the gastrointestinal tract (transit time) and due to pool effects (mean retention time, MRT), were calculated.5. Curve titting was only successful for the excretion of liquids in ruminants. The two-pool model was not applicable to the passage of solids.6. Apparent retention of liquid was always shorter than for solids in all species, except in rabbits. However, absorption of CoEDTA was too large in the rabbits to determine liquid retentian accurately. Times for first appearance of the two markers were similar within animal groups. 8. Liquid retention seemed to decrease somewhat with increasing body-weight in the ruminants. Solids retention decreased with decreasing body-weight in the ruminants, but sheep had longer retention times than goats of similar size. Equines exhibited large individual variation in retention of the liquid or solid markers, seemingly unrelated to size. No effect of size was seen in the retention of solids in the rabbits.Digesta rate of passage in the herbivore is of great importance to the nutrition and feeding strategy of the animal. There is reason to believe that the small herbivore wilI have a more rapid rate of passage than the large herbivore, due to a higher food intake relative to body-weight and gut volume (Parra, 1978). It appears that the ruminant gut architecture imposes more restrictions to the passage of digesta than is found in the non-ruminant herbivore, affecting the relative ability of the ruminant to consume large quantities of fibrous feeds.A previous paper (UdCn & Van Soest, 1982) reported the digestive efficiencies of ruminants, equines and rabbits to utilize the fibre components of timothy (Phleumpratense) hay.The objectives of these experiments were to: (a) study the effects of gut architecture and body size on the retention of digesta (liquid and solid), (b) compare the retention and passage of liquids and solids and (c) compare different techniques for calculating the rate of passage and retention times. E X P E R I M E N T A L

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