1. Sixteen sheep, each fitted with cannulas in the rumen and proximal duodenum, were given four diets in the chopped or ground and pelleted form, at fixed intakes at intervals of 2 h. The sheep were closely shorn and exposed to temperatures of 22–25° or 1–4° for four periods of 45 d. Flow of duodenal digesta by reference to the markers CoEDTA and 103Ru-phenanthroline, chewing behaviour and particle size of rumen and duodenal digesta were measured.
2. Apparent digestibility of organic matter (OM) in the gastrointestinal tract was depressed (P < 0.05) by grinding and pelleting the diet, and by exposure of sheep to cold ambient temperatures. This was attributable to depression (P < 0.01) by 0.1 of OM digestion in the reticulo-rumen. No effects on intestinal digestion of OM were observed.
3. Cold ambient temperatures did not affect the content, but increased the rate of digestion for pelleted diets but not for chopped diets, of potentially-degradable cell-wall constituents of ground dietary material incubated in nylon-bags in the rumen.
4. Retention times of markers of the particulate and liquid phases of rumen digesta were not significantly (P < 0.05) affected by ambient temperature, despite significant (P < 0.001) increases in the rate of contraction of the reticulum. Retention time of 103Ru-phenanthroline in the intestines was not affected by cold exposure.
5. Cold exposure was associated with depression (P < 0.05) of volatile fatty acids concentration in the rumen and elevated (P < 0.05) pH. Molar proportions of acetic and isovaleric acid were reduced (P < 0.01), accompanied by increased (P < 0.001) proportions of propionic acid during cold exposure.
6. Cold exposure and pelleting of the diets were both associated with reduction in digesta particle size in the rumen. Duodenal particle size was not affected by cold exposure. Pelleting of the diet markedly reduced (P < 0.001) duration of chewing and number of chews/d during eating and rumination. Cold exposure of sheep resulted in a faster (P < 0.01) rate of eating of the diets.
7. When allowed to express their voluntary feed consumption during a 10 d period, intakes of chopped diets were increased by 0.13 (P < 0.01) by cold exposure, in contrast to lack of significant change in sheep given pellets.
8. The results did not support the hypothesis that the effect of cold exposure on digestion was dependent on the physical form of the diet given at fixed intake, but did indicate that increased voluntary feed consumption was a result of increased clearance of digesta from the rumen throdgh decreased rumen particle size and increased reticulum motility.