I . Three cows were given 3, 5 or 7 kg hay once daily in a Latin-square design, and samples of digesta from four sites in the rumen and onc site in the reticulum were taken at six times within the 24 h period after the feed. Dry-matter content and distribution of particle size and of particle density were measured for each sample. The changcs in thcse mcasurements with time were studied, The incidence of rumination was also recorded.2. Dry-matter contents of samples ranged from 15 to 3 yo. Valucs for samples from the dorsal sacs of the rumen were considerahly higher than those for samples from the ventral sites; they also changed more with time after feeding and with level of feeding.3. Particlcs were fractionated by sieving into six size groups with mcan dimensions (mm) of 9.3 x 0.8, 4.4 x 0.6, 2.6 x 0 . 3 , 1.6 x o'zj, 0.5 x 0.1 and smaller than 0.5 x 0.1. Coarse particles occurred at highest concentrations in the dorsal sacs of the rumen and responded to effects of time and level of feeding; smaller particles showed less response. 4.. Particle densities ranged from 800 g/l to 1500 gil. The proportions of low-density particles were higher in samples from thc dorsal sacs of the rumen than in samples from ventral sites ; the latter samples had higher proportions of high-density particles than of low-density particles. The changes which occurred are discussed. .The density of coarse particles tended to he low and that of fine particles tcnded to be high.6. Rumination started at the time of maximum Concentration of particles of low density and minimum concentration of particles of high density. Conversely, rumination ended when the concentration of the low-density particles was a minimum and that of thc dense particles a maximum.7. The rcsults are discussed in relation to the possible movement of particles within the reticulo-rumen and the kinetics of particle breakdown.Schalk 8i Amadon (1928) described in some detail the structure of the reticulorumen, made observations on its functioning and referred to some of the physical characteristics of its contents for animals given different types of ration. Since then a number of other workers have added to this basic information (e.g. Magee, 1932;Balch, 1949Balch, , 1958Balch & Kelly, 1950;Nichols, 1959;Campling & Freer, 1962;Welch, 1965;Pearce, 1967;Troelsen & Campbell, 1968).The rate of passage of digesta through the reticulo-rumen and the rate of breakdown of digesta in that organ are important determinants of the efficiency of digestion and of the rate of rumen emptying (Balch, 1950). Both rate of passage and rate of breakdown appear to be markedly influenced by the physical characteristics of the digesta. Knowledge of the factors influencing the physical characteristics will be useful in understanding the effects of practices such as grinding, pelleting and wafering of roughages, the ' protection' of nutrients from digestion in the rumen, the granulation of trace minerals and the capsulation of drugs, medicinal compounds and inert fillers.

The nutritional significance of rumination was investigated in relation to certain aspects of rumen function, by using:(a) a chaffed roughage ration on which rumination was either allowed to occur normally or was restricted by means of a muzzle, and (b) a finely ground ration with either the normal rumination or with the addition of polyethylene flakes to stimulate rumination. An apparatus for recording jaw movements was used to measure the extent of rumination. Compared with the chaffed ration, grinding resulted in a shorter retention time of stained particles in the digestive tract, and lower apparent dry matter, organic matter, and crude fibre digestibilities. The addition of polyethylene flakes to the ground ration tended to further decrease retention time and caused a lowering of apparent dry matter, organic matter, and crude fibre digestibilities. The effect of muzzling, to restrict rumination, was to markedly increase the retention time, and this was accompanied by higher apparent dry matter, organic matter, and crude fibre digestibilities than when rumination was not restricted. Supplementary rumen metabolic data are presented. The importance of the mechanical activity in the digestive process is stressed.

The physical disintegration of five mature, chaffed roughages (barley straw, oat straw, pea straw, ryegrass hay and lucerne hay) in chewing during eating was examined using four oesophageal-fistulated steers. Measurements were made of intake, chemical composition, modulus of fineness and grinding energy of the feeds, and of particle size distribution, modulus of fineness and the proportion of particles greater than 1 mm in the oesophageal extrusa. Significant differences between steers and between feeds occurred in modulus of fineness and the proportion of particles greater than 1 mm. Between steers, there were decreases, with chewing, of 30-40% in the mean proportion of particles greater than 1 mm. The order of proportionate particle size reduction for the different roughages was as follows: pea straw <: oat straw < ryegrass hay < lucerne hay < barley straw. Expressed in terms of the modulus of fineness, the mean decreases in particle size ranged from 46 to 52% for the different roughages. Some interactions occurred between the different roughages and individual steers. The critical characteristics responsible for the variation that occurred have not been defined either in the roughages or in the steers.

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