Effects of cold exposure on digestion, microbial synthesis and nitrogen transformations in sheep.

I. 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, lOSRu, from rg h to IC-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 o81 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 nonammonia-N leaving the abomasum did not change significantly between treatments. The retention time of losRu in the intestines was 17-18 h in sheep given 59 g DM foodlh 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 [16N]urea and [W]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 (0.5 g N/d) in the coldexposed 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.