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In any part of the digestive tract the time available for the breakdown and absorption of food is determined by the rate at which food residues pass through that piece of the gut. The reticule-rumen is a major site of digestion in the cow and the digestion of food, particularly the crude fibre fraction, is affected by changes in the rate of passage of the food through the reticulo-mmen, provided there are no compensating changes in the rate of breakdown of the food (Ralch, 1950; Balch & Johnson, 1950). Since undigested food can pass from the reticulo-rumen to other parts of the digestive tract only through the small reticulo-omasal orifice, the movements of this orifice are likely to influence the rate of passage of food.,The material passing through the reticulo-omasal orifice must come from the reticulum and possibly from the anterior part of the rumen. It will, therefore, tend to contain 90-95 yo water (Balch, 1950), particles of solid food and micro-organisms in suspension and substances in solution.Several observations suggest that there is a steady flow of digesta through the reticulo-omasal orifice throughout the day. Paloheimo (1939) slaughtered cows at intervals after giving them chromium sesquioxide, and found that the passage of this very heavy substance to the omasum had begun even in those cows killed immediately after administration. Phillipson (1946) suggested that there must be a constant flow of digesta through the reticulo-omasal orifice because the omasum, abomasum and intestines contain digesta at all times. In sheep the flow of digesta from the abomasum was found to be in gushes with usually less than 15 min. between each gush (Phillipson, 1948), which also suggests a continuous flow of digesta from the reticulorumen. The forces that activate the passage of digesta from the reticulo-rumen to the omasum are not fully understood. Wester (1926) and Schalk & Amadon (1928) considered that the movements of the omasum play an important part in drawing semiliquid digesta through the reticulo-omasal orifice into the omasum, but Phillipson (1946) did not support this contention. Both Wester and Schalk & Amadon described a sudden fall in pressure at the top of the pressure gradient in the omasum, illustrating the fall with kymograph records of varying clarity. Wester also gave a kymograph record of the contraction of the reticulo-omasal orifice.The present experiments were undertaken in order to study, in greater detail than was given by Wester or Schalk & Amadon, the relationship between pressure changes

The time taken for particles of roughage to be reduced to a size suitable for transfer from the reticulo-rumen to the omasum may be the major factor determining the retention time of roughage residues in the rumen and thereby may partly regulate the voluntary intake of roughage by the cow (Campling, Freer & Balch, 1961, 1962. The size of particles of hay can readily be reduced by grinding. An experiment was conducted, therefore, to examine the effect, on the voluntary intake of hay by cows, of grinding and pelleting the hay, and throughout this report the term 'ground hay' will be used to describe the ground, pelleted hay.The cost of grinding and pelleting roughages is high, but there has been some practical interest in the convenience with which the product can be handled. In evaluating this method of preparation, the voluntary intake and utilization of the product must be considered. Several studies have been made of the effects of grinding roughage on the digestibility of the roughage, the time of retention of food residues in the alimentary tract, energy metabolism and animal production (e.g. by Blaxter, appears to exert profound effects on the utilization of diets in which it is included and definition of the critical features in these changes will be an important stage in the understanding of ruminant digestion. E X P E R I M E N T A LThe voluntary intakes of long hay and ground hay were compared with four COWS (D, E, F and G) in a simple change-over design. Each of the two periods lasted 33 days and included a 3-week preliminary feeding period and 10 days during which voluntary intake, digestibility, and the retention time of stained food particles in the alimentary tract were measured. In a further 2 days the weights of the contents of the reticulorumen and the amounts of dry matter they contained were determined.I n a further two periods, comparisons were made between long hay and ground hay with two of the cows (D and E) given equal amounts of food dry matter (8.7 lb/day).The digestibility of each food was measured, together with the mean time of retention of food residues in the gut and the rate of breakdown of cotton thread in the rumen.

Cotton threads were used by Hoflund, Quin & Clark (1948) to study the rate of breakdown of cellulose in the rumen of sheep. These workers found that on a basic diet of poor quality grass hay, both cellulose breakdown and appetite were stimulated by small amounts of sugar in the diet, but markedly inhibited by excessive amounts of sugar. Excess casein suddenly introduced into a diet low in protein and carbohydrate caused a marked inhibition of cellulose digestion.When comparing between-cow differences in digestibility coefficients for crude fibre, Balch (1950) obtained data suggesting that a high ratio of water to dry matter in the total intake, and hence perhaps in the reticulo-ruminal digesta, favoured the breakdown of crude fibre. If so, it might be postulated that breakdown of the constituents of crude fibre would be lower among the dry digesta in the dorsal sac of the rumen than among the more fluid contents of the ventral sac. Confirmation of the importance of fluid conditions in determining the rate of breakdown of crude fibre in the reticulo-rumen would also offer a possible explanation for the depression of the digestibility of crude fibre known to result from the grinding of hay. Balch (1950) showed that this depression of digestibility of fibre was not the result of an accelerated rate of passage of ground hay through the reticulo-rumen.This communication is an account of studies done with two fistulated cows to check the hypothesis outlined above. The main technique used was a modification of the cotton-thread method of Hoflund et al. (1948), but the results were checked by one trial with hay suspended in the rumen in silk bags. METHODS

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