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1. Six closely shorn sheep were given brome grass (Bromus inermis) pellets at the rate of 59 or 98 g dry matter (dm)/h and maintained at ambient temperatures of 2–5° and 22–25° for 35 d. Measurements of digestion, rate of passage of digesta, and nitrogen transformations were made during the last 13 d of temperature exposure.2. Cold exposure at the lower level of intake reduced the apparent digestibility of dm and organic matter (om) approximately 0.055 units. Apparent digestibility of dm and om was further decreased approximately 0.03 units with the higher level of food intake in the cold. Apparent N digestibility was significantly depressed from 0.62 to 0.59–0.60 for sheep exposed to cold at both levels of intake.3. Exposure of sheep to cold resulted in a decrease in the turnover time of the particulate marker, 103Ru, from 19 h to 10.12 h in the rumen, a decrease in rumen volume, and a significant increase in dm and om which escaped digestion in the stomach. Volatile fatty acid and methane production in the rumen were highly correlated with the amount of om digested in the stomach. Methane production in the rumen comprised 0.81 of total production in warm sheep, and 0.68–0.74 of total production in cold-exposed sheep.4. More om and non-ammonia-N were apparently digested in the intestines of sheep exposed to cold than in warm sheep at the same food intake, but the apparent digestibilities in the intestines of dm, om and non-ammonia-N leaving the abomasum did not change significantly between treatments. The retention time of 103Ru in the intestines was 17.18 h in sheep given 59 g dm food/h at both exposure temperatures, but was reduced to 12 h for cold-exposed sheep given 98 g dm/h. Methane production in the postruminal tract was increased at the higher food intake, but there was no difference between warm and cold-exposed sheep at the same food intake.5. The rate of irreversible loss of plasma urea and rumen ammonia was measured by infusion of [15N]urea and [15N]ammonium chloride. Exposure to cold reduced the irreversible loss of plasma urea from 0.85 to 0.75–0.77 g N/g N intake, and the irreversible loss of rumen ammonia from 0.66 to 0.57–0.61 g N/g N intake. The transfer of plasma urea-N to the rumen ammonia pool was significantly greater (9.5 g N/d) in the cold-exposed sheep than the value (7.3 g N/d) in warm sheep.6. The efficiency of microbial synthesis in the rumen was increased in cold-exposed sheep, and was related to the amount of N recycled through the rumen ammonia pool from intraruminal sources. The effect of dilution rate and fermentation patterns on efficiency of microbial synthesis is discussed.

Results from recent in vitro studies indicate that in excess of 20% of the energy expenditure of skeletal muscle, duodenal epithelium and liver of domestic ruminants is to achieve Na+ and K+ transport across the plasma membrane. The energy cost of active Ca2+ transport is less clear but is likely less than 10% of the total expenditure of skeletal muscle at rest. Energy expenditure on Na+ and K+ transport was quite sensitive to the physiological state of the animal. During lactation, Na+ and K+ transport accounted for nearly half of the in vitro O2 uptake of skeletal muscle, duodenal epithelium and liver. The energetic cost of supporting Na+ and K+ transport was also elevated in young, as compared with older animals, by feed intake and by exposure to cold. Na+ and K+ transport appears to be a substantial component of the maintenance energy expenditure of ruminant tissues. Its variation, therefore, implies that change of maintenance energy expenditures with physiological state of the animal warrants serious attention.

I. 15NH4C1 was continuously infused for periods of 120-216 h into the rumens of sheep which were allowed to feed 2 out of every 10 min. These treatments achieved steady metabolic states and allowed the assessment of nitrogen conversions by means of tracer methodology. The sheep were given either a barley diet or one of three hay diets. I n two trials, the flow of abomasal material was determined using lignin and polyethylene glycol as markers. The amounts of dry matter (DM), gross energy, total N, soluble N, microbial N and microbial DM in abomasal digesta, and the concentration of ammonia in the rumen liquor were measured. The concentrations of lSN in the N of urine, faeces, rumen and abomasal bacteria and protozoa, rumen and abomasal bacterial and protozoal protein, abomasal particulate matter and in rumen ammonia were determined.2. Comparisons of the steady-state concentrations of I6N in the microbes with that in rumen ammonia indicated that from 50 to 65 % of the bacterial N and from 31 to 55 % of the protozoal N were derived from rumen ammonia in vivo.3. An amount of N equivalent to 60-92 % of the daily intake was transformed into ammonia N in the nunen. 4. Some 17-54% of the ammonia was absorbed from the rumen, but this was not readily converted into urea.5. Microbial growth in the rumen resulted in the assimilation of 1.7-2.6 g N/IW g DM fermented.6. The generation-time of bacterial protein in the rumen was calculated from the rate of increase of 16N concentration in this fraction, and values of 38 and 42 h were obtained for sheep given barley and hay diets respectively. 7.The combined results allowed quantitative estimates to be made of the complete metabolism of rumen N, and from these the possibility of fixation of N gas in the rumen was suggested.Ammonia may be produced in the rumen from either protein (Blackburn & Hobson, 1960), or urea (Huhtanen & Gall, 1955) which enters in the saliva (Somers, 1961) or by diffusion across the rumen wall (Houpt, 1959), or from non-protein nitrogen sources other than urea (Blackburn, 1965).The results of numerous experiments have shown that rumen micro-organisms utilize ammonia for cell synthesis (Hungate, 1966). Bryant & Robinson (1962) found that ammonia is a preferred source of N for a large number of rumen organisms, and that it is essential for growth in some species.The experiments described in this paper were designed to study in vivo the quantitative importance of rumen ammonia for the synthesis of nitrogenous material in rumen microbial cells, a topic which Hungate (1966) described as 'one of the most intriguing problems in rumen ecology'. In addition, two experiments were undertaken to determine the quantity of plant and microbial material passing through the abomasum. 24

A mathematical representation of the energy-requiring processes of protein turnover and Na+,K+-transport in the tissues of growing lambs is described. This model was then used to examine the relative contributions of these processes to ATP expenditure at two different growth rates (90-230 g/d). Protein turnover accounted for 19% of whole-body ATP expenditure at both growth rates examined, with the gastrointestinal tract (GIT), accounting for 25-27%, muscle for 21-26%, skin for 23-26% and liver for 13% of total protein turnover energy costs. The contribution of Na+,K+-transport increased from 18 to 23% of whole-body heat production as growth rate increased, with the GIT accounting for 39 and 50%, muscle for 17 and 10% and liver for 18 and 23% of total Na+,K+-transport costs at low and high nutrient inputs, respectively. Thus, protein turnover accounted for 19% of the increment in ATP expenditure due to the increased nutrient input at the higher rate of growth, while Na+,K+-transport accounted for 39%, and fat turnover and accretion accounted for 25%, leaving 17% of the ATP increment unaccounted for.