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Highlight: Atrazine was applied at 2 kg/ha for three consecutive years on shortgrass range in northeastern Colorado. The atrazine controlled all annual plant species, greatly reduced frequency of occurrence of cool-season perennial grasses, and increased drought survival of warm-season perennial grasses and two warmseason perennial forbs. Other species varied in their susceptibility to atrazine. The species frequency method of vegetation sampling used in this study provided reliable data for 27 of the approximately 100 species encountered on this range.

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Following massive nitrogen (N) fertilization, at rates of 224, 448, and 672 kg N/ha applied in April 1969, nitrate accumulation by species and plant groups on mixed-grass prairie was measured for 3 years. All species and plant groups accumulated Nitrate-N in direct relation to rates of applied N. Two annual forbs accumulated nitrate-N above the 2000 ppm level, which is considered toxic to livestock. In 1970, the first year of residual effect, slimleaf goosefoot contained nitrate-N levels two to three times higher than the potentially toxic level, and in 19 71 greenflower pepperweed contained nitrate-N levels slightly above the potentially toxic level. The use of massive rates of N as a range improvement practice should be used with caution unless potentially toxic species are con trolled.