Sites, mowing, 2,4-D, and seasons affect bitterbrush values

Abstract

This study was conducted in a sagebrush-bitterbrush vegetation type in southcentral Wyoming to compare the effects of mowing and 2,4-D application on shrub cover and bitterbrush (Purshia tridentata Pursh) use, preference values, browse values, and residual twig weight after summer/fall browsing by cattle and winter big game use. Areas 3 to 6 ha in size were mowed, sprayed with 2,4-D, or left untreated on 4 different sites. Total shrub cover was reduced from 38% (53% mountain big sagebrush (Artemisia tridentata spp. vaseyana), 37% bitterbrush) to 23% (38% sagebrush, 52% bitterbrush) by mowing, and to 17% (18% sagebrush, 64% bitterbrush) by herbicide application. Twig weight use was greater on mowed areas, on the more productive sites, and during summer cattle browsing when leaves were present than during winter browsing by big game. Frequency of use was inversely related to bitterbrush availability, although the relationship was confounded by different snow depths on different sites. Utilization values based on basal diameter were more sensitive to site and treatment effects than length and weight utilization values, but were influenced by differences in twig morphology and did not reflect differences in twig use, preference values, browse values, or browsing residual weights. Mowing produced the greatest preference values and browsing residual weights for cattle browsing and the greatest browse values for both cattle and big game. Preference values, browse values, and browsing residual weight appear to be useful indicators of site, treatment, and browsing effects.