Sulfonylurea herbicides were used extensively for weed control on Conservation Reserve Program (CRP) seedings, constituting over 98% of the residual herbicides applied from 1986–1990 in southeastern Colorado. Differences in species establishment were observed in CRP fields treated with sulfonylurea herbicides, suggesting that soils and climatic variation alone did not fully account for this establishment pattern. Impacts of 2 commonly used sulfonylurea herbicides and 2 auxin herbicides on establishment, inter-specific seedling competition and physiological response under CRP field conditions were evaluated. Seeded species were blue grama [Bouteloua gracilis (Willd). Ex Kunth) Lag. ex Griffiths], sideoats grama [Bouteloua curtipendula (Michx.) Torr.], western wheatgrass [Pascopyrum smith; (Rydb.) A. Love], switchgrass [Panicum virgatum L.], and sand dropseed [Sporobolus cryptandrus (Torr.) Gray]. Sulfonylurea herbicide application increased sideoats grama cover and live standing crop as much as 43% over auxin herbicide and mowing treatments, whereas switchgrass and western wheatgrass were reduced up to 71% by sulfonylurea treatment. Switchgrass cover was reduced by application of either sulfonylurea or auxin herbicides. Blue grama and sand dropseed were least affected by herbicide treatment. Auxin herbicide treatment resulted in 70% increases in plant diversity for seeded species and total plant community over sulfonylurea treatment, primarily attributable to increased frequency of annual forbs. Seral stage was more advanced under sulfonylurea treatment, however, because of increased frequency, cover and live standing crop of perennial forbs, grasses, and half-shrubs.
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Vol. 57 • No. 2