Direct measurement has been made of the daily production of microbial protein in the rumen. When sheep were fed on a virtually protein-free purified diet, increases in the intake of nitrogen from 2 to 9 g/day increased linearly the production of protein in the rumen from 32.5 to 50.0 g/day. There was no further increase in protein production when the nitrogen intake was raised to 16 g/day. The amount of total nitrogen flowing out of the rumen showed a net increase over that ingested at daily nitrogen intakes of 2 and 4 g, no change at 9 g, and a substantial net loss at 16 g. At the lowest nitrogen intake at least 4 g recycled nitrogen was utilized by the rumen microorganisms daily. The yield of protein from the ruminal fermentation increased from 9.1 g/100 g organic matter digested in the rumen when nitrogen was most limiting, to 13.3 g/100 g when nitrogen was in excess of requirements. It was calculated that protein production in the rumen at the highest nitrogen intake was less than the potential production from the energy supplied to the microorganisms. Possible limiting factors are discussed.

Compensatory growth can be due to increased protein deposition, reduced maintenance and greater feed intake. However, the contribution to and interaction between these mechanisms during compensatory growth is not clear. It was hypothesized that initial compensatory growth was due to reduced maintenance requirement and greater deposition of protein, after which compensatory growth was due to greater feed intake. Changes in the composition of sheep and cattle were measured during nutritional restriction and subsequent compensatory growth, and compared with the changes in control animals fed ad libitum throughout. At the end of the experiment the restricted cattle had compensated completely, and there was no difference in the body composition of the restricted and the control cattle. The restricted sheep did not compensate completely and were leaner than the control sheep. During nutritional restriction there was differential weight loss of carcase tissues in both the sheep and the cattle. The greatest losses were in the liver and the digestive tract in both species and in the skin of the sheep. It was concluded that the loss of these tissues reduced the maintenance requirement of the restricted animals and that the lowered maintenance requirement persisted during re-alimentation until these tissues had been fully repleted. Further, the repletion of these tissues required an increase in protein deposition, and it was a combination of these two mechanisms that was responsible for compensatory growth during the first 12 weeks of re-alimentation.

Sheep and cattle often exhibit compensatory growth following nutritional restriction. Complete compensation, that is the same weight at the same age as non-restricted contemporaries, has often been observed in sheep but not in cattle. In this experiment the compensatory growth of sheep and cattle was measured after their nutrition had been restricted sufficiently to induce losses in body weight. The growth, feed intake and feed conversion efficiency of the compensating sheep and cattle, measured during re-alimentation, was compared to control animals fed ad libitium throughout the experiment. A high-quality diet was used to maximize the opportunity for compensatory growth. The cattle exhibited compensatory growth for the 11 months between re-alimentation and the end of the experiment, and were able to compensate completely. Compensatory growth did not persist as long in the sheep as in the cattle, and they remained lighter than the controls at the end of the experiment. During the first 12 weeks of re-alimentation there was no difference in the feed intake of the compensating and control animals in both species. Compensatory growth during this time was due to the greater efficiency of the compensating animals. After this initial 12 weeks the feed intake of the compensating animals increased, and the subsequent compensatory growth could be fully accounted for by greater feed intake. The greater persistence of compensatory growth in the cattle was due to their intake remaining elevated longer.

Daily infusion of 7, 35, and 70 g of glucose m solution (1, 5, and 10 per cent. of the ration) into the rumen of Merino sheep maintained on an adequate diet resulted in the ruminal pH falling 2–4 hr after feeding, from a mean of 5.66 without added glucose to 5.40 at the higher glucose levels. The concentration of ammonia also decreased, whereas that of volatile fatty acids increased. At the 10 per cent. glucose level, food intake was depressed and the ruminal pH in this treatment did not fall below that at the 5 per cent. glucose level. A significant daily fall in the ruminal pH minima was found during the 3-day sampling period. No consistently significant differences in the concentration of ciliate Protozoa were found at four different sites in the rumen, either before or 1 hr after feeding, although there were significant differences between sheep. Samples taken from the bottom of the rumen immediately inferior to the ruminal cannula, before feeding and 1, 2, 4, 6, 8, 12, and 16 hr after feeding, showed: (a) Only oligotrichs in the ciliate population; (b) Marked diurnal fluctuation in the ciliate population, the concentration after feeding falling to as low as one-third of the prefeeding levels; (c) A diurnal cycle for dividing Protozoa which indicated that their capacity to multiply was strongly inhibited by the low pH experienced 2–4 hr after feeding. A depression of the average ciliate concentration daily during the 3-day sampling period was also demonstrated. This was related to a consistent but inexplicable fall in the daily ruminal pH minima over this period. It was concluded that the minimum pH within the rumen is an important factor controlling rumen protozoal concentrations in the sheep, and the need for cognizance of this phenomenon in studies of rumen Protozoa is stressed.

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.