USING A GRAZING PRESSURE INDEX TO PREDICT CATTLE DAMAGE OF REGENERATING TREE SEEDLINGS

This research investigated the potential for using cattle grazing pres sure (AU Mg-1 ha(-1)) and stocking rate (Animal Unit Days ha(-1)) for predicting basal scarring and browsing of lodgepole pine (Pinus contor ta Dougl.) seedlings on cutblocks in southern British Columbia from 19 89 to 1992, Cattle browsing on lodgepole pine seedlings occurred almos t exclusively during the first 2 years of grazing. Browsing increased (P < 0.05; r(2)= 0.71) with increasing stocking rate only during the f irst year of grazing. Browsing increased with increasing grazing press ure in 1989 (P < 0.05; r(2)= 0.38) and 1990 (P < 0.05; r(2) = 0.39), B asal scarring peaked during the second year of grazing, but was correl ated (P < 0.05; r(2) = 0.79) with stocking rate only during the first year of grazing. Increasing grazing pressure was associated with highe r (P < 0.05) basal scarring during all 4 years of the study, and likel y better predicts trampling damage than does stocking rate, particular ly during the first year of grazing. Basal scarring during 1989 genera lly increased to > 10% of sample trees when grazing pressure exceeded 12.0 AU Mg-1 ha(-1). This threshold grazing pressure value of 12.0 AU Mg-1 ha(-1), however, cannot likely be extrapolated directly to other sites. Grazing pressure values and associated basal scarring are unque stionably influenced by many factors (e,g,, pasture size, kind of graz ing animal, forage species, tree height, water availability, topograph y, and weather patterns during the grazing period). Nonetheless, our w ork provides evidence that grazing pressure provides a useful index fo r predicting the potential for trampling damage of lodgepole pine seed lings by cattle.