Three grasses and two legumes, each at two stages of maturity, were fed to three fistulated sheep and samples of the feed, and faeces and contents of the reticulorumen, abomasum and ileum were collected for the determination of particle size by wet sieving. Modulus of fineness (MF) was calculated for all particulate matter that failed to pass a 0-15 mm screen. Resistance to flow from the rumen of feed particles of different sizes was also calculated.The mean MF of the reticulo-rumen contents was 2-56 compared with 5-72 for the feed. Material leaving the rumen and found in the abomasum had a MF of l -67 and subsequent changes were small and not significant; ileum 1-63, faeces 1-80. Particles greater than 1-18 mm passed out of the reticulo-rumen although the quantity was small (1-3 %). The resistance to flow of particles of different sizes from the reticulorumen was closely related to particle size with no difference between grasses and legumes or between young and mature forages.It was concluded that in modelling the flow of particles from the reticulo-rumen a non-compartmentalized approach should be adopted, but if a simple two-compartment model is required then a critical sieve size of about 1-18 mm may be useful, since less than 5 % of the particulate material is retained on this sieve size.

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Pangola grass (Digitavia decumbens) and Rhodes grass (Chlovis gayana) cut as 6 and 12 week regrowths were separated into leaf and stem fractions and fed ad libitum to four cattle and eight sheep fitted with ruminal fistulae to determine the importance of particle size in controlling the retention time of feed in the rumeno-reticulum (rumen). Particle size was determined by using a wet sieving technique, and based on the cumulative dry matter distribution on the sieves of faeces from cattle and sheep; all particles >1.18 mm were described as large particles. The proportion of large particles was measured in the chopped diet offered, the masticated diet, the ruminal contents and the faeces. Chopped leaf and stem fractions contained 0.85 and 0.86 g/g large particles respectively. Mastication by cattle reduced the proportion of large particles in leaf and stem to 0.58 and 0.76 (P < 0.01), and mastication by sheep to 0.56 and 0.67 (P < 0.01). The proportion of large particles in the rumen of sheep was 0.236 and 0.249 for leaf and stem respectively (P > 0.05) and in cattle 0.272 and 0.345 (P < 0.05). The faeces contained relatively few large feed particles (< 0.045 for cattle and <0.018 for sheep). Cattle eating leaf and stem produced faeces containing 0.027 and 0.040 (g/g) large particles (P <0.05), compared with only 0,008 and 0.013 (P < 0.05) in sheep. Most of the large particles entering the rumen in the masticated feed disappeared in the rumen by breakdown to smaller particles or by digestion. In cattle the proportions that disappeared were 0.978 and 0.976 (P > 0.05) for leaf and stem, and in sheep 0.993 and 0.991 (P > 0.05). Large particles in leaf and stem were retained in the rumen of the sheep for 11.0 and 11.7 h (P > 0.05). Cattle retained large particles in all diets for a longer time (P < 0.01); 16.1 h for leaf and 20,2 h for stem (P > 0.05). Differences in retention time of large particles in the rumen did not appear to be the only factor controlling the retention of dry matter in the rumen and voluntary intake. A model was developed to describe the flow of large and small particles through the rumen. Intake simulation studies indicated that the most important factor influencing dry matter retention time in the rumen was the retention time of small particles (> 1.18 mm). Changes in the rate of breakdown of large particles had a small effect on dry matter retention time.

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The proportion of large particles (LP) broken down to small, insoluble particles by primary mastication (eating), rumination, digestion and detrition (rubbing) was determined for separated leaf and stem fractions of perennial ryegrass (Lolium perenne) and alfalfa (Medicago sativa) fed to cattle cannulated at the esophagus. Large particles were defined as those particles retained during wet sieving on a screen with an aperture of 1.18 mm. Reduction in weight of particles caused by solubilizing or digestion was not considered to be particle breakdown per se, and particles were corrected for this loss in weight. The proportion of LP in the forage broken down by primary mastication was 25 +/- 1.9% (means +/- SE). Breakdown of LP by rumination was calculated from the weight of total particles regurgitated and the proportion of LP in the regurgitated and swallowed remasticated material. The weight of LP regurgitated was corrected for the dry matter lost by digestion using lignin ratio in the LP entering the rumen and of the regurgitated digesta. Rumination accounted for 50 +/- 1.5% of LP breakdown. Fecal loss accounted for 8 +/- .8% of the LP in forage. Breakdown of LP by digestion and detrition was calculated as 17 +/- 1.3% from the difference between the LP eaten and those broken down by primary mastication, rumination and passing out in the feces. The significance of these results for predicting voluntary intake from laboratory analysis is considered.