Using a grazing pressure index to predict cattle damage of regenerating tree seedlings

Abstract

This research investigated the potential for using cattle grazing pressure (AU Mg-1 ha-1) and stocking rate (Animal Unit Days ha-1) for predicting basal scarring and browsing of lodgepole pine (Pinus contorta Dougl.) seedlings on cutblocks in southern British Columbia from 1989 to 1992. Cattle browsing on lodgepole pine seedlings occurred almost exclusively during the first 2 years of grazing. Browsing increased (P < 0.05; r2 = 0.71) with increasing stocking rate only during the first year of grazing. Browsing increased with increasing grazing pressure in 1989 (P < 0.05; r2 = 0.38) and 1990 (P < 0.05; r2 = 0.39). Basal scarring peaked during the second year of grazing, but was correlated (P < 0.05; r2 = 0.79) with stocking rate only during the first year of grazing. Increasing grazing pressure was associated with higher (P < 0.05) basal scarring during all 4 years of the study, and likely better predicts trampling damage than does stocking rate, particularly during the first year of grazing. Basal scarring during 1989 generally 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 unquestionably influenced by many factors (e.g., pasture size, kind of grazing animal, forage species, tree height, water availability, topography, and weather patterns during the grazing period). Nonetheless, our work provides evidence that grazing pressure provides a useful index for predicting the potential for trampling damage of lodgepole pine seedlings by cattle.