In Vitro Digestibilities of Summer Forages Utilized by the Rivière George Caribou Herd
The Riviere George caribou herd (northern Quebec-Labrador, Canada) is thought to be regulated by forage limitations in its summer range. In such a situation, digestibilities of plants may strongly affect the diet choice and physical condition of animals. In vitro dry matter disappearance (IVDMD) of the most important summer forages of the Riviere George caribou herd was determined during fermentation periods of 12, 24, and 48 h using rumen fluid collected from a Holstein cow. IVDMD values for Cyperaceae (Eriophorum angustifolium and Carex rariflora), and shrubs (Betula glandulosa and Vaccinium uliginosum) collected in July and August were higher for long fermentation periods (48 h) than for shorter ones (12 and 24 h). Plants collected in early summer were also more digestible than those collected in late summer. Contrary to my prediction, both Cyperaceae were more digestible than the two shrubs in mid-July. However, no difference occurred in early August. The fermentation period did not affect the IVDMD of lichens; maximum digestibility was attained after 12 h. Alectoria ochroleuca and Cetraria spp. were more digestible than Cladina rangiferina, Cladina stellaris, and Stereocaulon paschale, likely because of their lower fibre content. These results suggest that the preference of caribou for Cladina spp. is not based on digestibility, but probably on the fact that these species are abundant in the Riviere George area. Variations in IVDMD seem to be explained by plant phenology, because longer fermentation periods were necessary to attain a high level of digestibility as summer progressed. Plant digestibility alone cannot explain caribou summer diet. Other variables such as plant constituents (e.g. protein) and relative abundance must also be considered.