The effect of size and density on mean retention time of particles in the gastrointestinal tract of sheep

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Abstract
The selective retention of particles in the reticulo-rumen and in the gastrointestinal tract distal to the reticulo-rumen was studied in fistulated sheep maintained on a roughage diet. Polyethylene glycol and plastic particles of different lengths (1 and 10 mm) and densities (092, 1.03, 1.22 and 1.44 g/ml) were either fed or were introduced into the omasum. The mean retention time in the reticulo-rumen (MRTRR) of 1 mm long particles with a density of approximately 1.0 g/ml was about 67 h, that is eight times longer than the MRTRR of fluid; the heavier particles were retained only three times longer than fluid. Particles with a length of 10 mm were retained in the reticulo-rumen 19–28 h longer than 1 mm long particles of the same density. Particles with a length of 10 mm were reduced to smaller particles (0.5–4 mm) due to rumination. Multiple regression analysis indicated that particle density and particle size accounted for 59 and 28% of the total variation of MRTRR respectively. The mean retention time distal to the reticulo- rumen (MRTGut) of 1 and 10 mm long particles with a density near 1.0 g/ml was 18–19 h, similar to that of fluid (16 h). The heavier particles were retained about 3–8 h longer.