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N[ATERIALS AND METHODS Suppression treatments included Curtail (3 qt/A), Escort (1 oz1A), sequential treatments of Roundup (1 qt + 1 qt/A), and two mowings. Curtail, Escort, and the first Roundup treatment were applied in spring when Russian knapweed was in the bud growth stage. The first mowing treatment was done at the bud growth stage and repeated 8 weeks later at which time the second Roundup treatment was applied; Russian knapweed was in a vegetative growth stage at the second mowing and Roundup treatments. A no suppression treatment control v.~as included. Both sites were rototilled in September, 1993 and the Boysen Reservoir site was planted in October and the 1\4ead site in November, 1993. Perennial grasses at both sites included 'Hyvar' crested v.~heatgrass, 'Bozoisky' Russian wildrye, streambank wheatgrass, thickspike wheatgrass, and anon- seeded control. Percent cover data were taken in each plot in May and June in 1993 at A4ead and Boysen Reservoir, respectively, to establish baseline plant populations and again at the same time in ] 994 and 1995 to determine the effects of treatments. Plant biomass was harvested from each plot in August, 1994 and ]995 at which time percent control was estimated visually in each plot. RESULTS Mearl, Co/orarlo: In 1995, Russian knapweed cover was lowest from the Curtail suppression treatments and 33% less than from other suppression treatments and 30% less than the control (Table 1). Similarly, the least Russian knapweed biomass was harvested from the Curtail treatments (51, 62, 55, and 43% Less than in the control, Escort, Roundup, and i mowing treatments, respectively). An average of 61 % of Russian knapweed was controlled by Curtail while all other suppression treatments controlled less than 10% of Russian knapweed. Russian wildrye and crested wheatgrass established better than the other grass species at 1`4ead. Approximately 2.4 and 5 times more crested wheatgrass was harvested compared to streambank wheatgrass and thickspike wheatgrass, respectively, and 3.6 and 1.8 times more Russian wildrye than streambank or thickspike wheatgrasses (Table 2). However, only Russian ~~ildrye produced greater cover than other seeded grasses at Mead. A significant replication effect was observed at Mead for Russian knapweed cover, biomass, and control, and seeded grass cover and biomass. For 1, example, there was approximately ]0 to ] 1 times more seeded grass biomass harvested in reps 1 and 2 compared to 3 and 4 (Table 3). Seeded grasses emerged early in reps 3 and 4 in 1994 (approximately early February) and a hard freeze killed emerged grasses in these reps. Seeded Brasses had not emerged in reps 1 and 2 before the freeze occurred. Consequently, seeded grasses established better and Russian knapweed .vas controlled better in reps 1 and 2 than in reps 3 and 4. Russian knapweed cover was 61 and 55% in reps 3 and 4, but 31 and 24% in reps 1 and 2 (data not shown). T~'o treatment by seeded grass interaction Has observed at Tlead. _ -6-