Effects of summer sheep grazing on browse nutritive quality in autumn and winter

Prescribed livestock grazing in summer may improve the nutritive quality of autumn and winter browse for wild ungulates. We examined the effects of early-summer versus late-summer sheep grazing on autumn ana winter browse quality in northern Idaho. Nutritive quality of 6 shrub species collected in September (autumn) and November (winter) was measured following early-summer (May-June) sheep grazing, late-summer (August) sheep grazing, and no grazing in 1993 and 1994. Shrub samples were analyzed for crude protein (CP), available protein, neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin. Relative to the ungrazed control, early-summer sheep grazing improved both autumn and winter browse quality in redstem ceanothus (Ceanothus sanguineus), ninebark (Physocarpus malvaceus), rose (Rosa spp.), and snowberry (Symphoricarpus spp.). Early-summer sheep grazing improved browse quality of thimbleberry (Rubus parviflorus) in autumn but had no effect on its nutritive quality in winter. In contrast, late-summer sheep grazing reduced both autumn and winter browse quality in redstem ceanothus and ninebark. Late-summer grazing reduced autumn browse quality in snowberry and rose but improved browse quality of rose in winter. Scouler willow (Salix scouleriana) was not readily selected by sheep in either grazing season, which explains why the nutritive quality of Scouler willow in autumn and winter was unaffected by the grazing treatments. For most browse species, fiber content in autumn and winter was higher in 1993 than 1994, due to weather conditions. We conclude that prescribed sheep grazing can either improve or reduce autumn and winter browse nutritive quality for wild ungulates, depending upon weather conditions and the intensity of sheep browsing. For transitory ranges of the Inland Northwest, we suggest browse quality will likely improve if moderate sheep grazing (40-55% relative utilization) in summer ceases by mid- to late June. Autumn and winter browse quality should be largely unaffected if moderate sheep grazing in summer ceases by mid-August. Browse quality in autumn sill probably be lowered if sheep grazing occurs in late August, but moderate sheep grazing in late August will have relatively minor effects on browse quality in winter.