A comparison of the voluntary intake and digestion of a range of forages at different times of the year by the sheep and the red deer (Cervus elaphus).

1. Comparisons were made between castrated male Scottish Blackface sheep and red deer (Cervus elaphus) of voluntary forage intake (VFI), digestibility and the mean retention time (MRT) of a particulate-phase marker (103Ru-phenanthroline) in the alimentary tract, when a range of forages: dried-grass pellets, chopped dried grass, fresh-frozen Agrostis-Festuca spp. and heather (Calluna vulgaris, L. Hull) were given at different times of the year. 2. On both the dried-grass-pellet and chopped dried-grass diets the red deer and sheep ate similar quantities. Both species had a higher VFI of dried-grass pellets in July than in November. The sheep digested the dried-grass-pellet diet better than the red deer and this was associated with a longer MRT of the particulate-phase marker in the alimentary tract. 3. The VFI of Agrostis-Festuca spp. and heather by the red deer was twice that of the sheep. The VFI of heather by the sheep increased by 32% between January and April, and the VFI of both the Agrostis--Festuca spp. and heather diets by the red deer increased by 65--70%. The sheep digested the Agrostis--Festuca spp. better than the red deer but the red deer digested the heather slightly better than the sheep. MRT of the particulate-phase marker was greater for the sheep than for the red deer on both the diets. The digestibility and MRT of both diets in the red deer did not decrease with the seasonal increase in VFI, suggesting a possible hypertrophy of the alimentary tract.