RELATIONSHIPS AMONG IDAHO FESCUE DEFOLIATION, SOIL-WATER, AND SPOTTEDKNAPWEED EMERGENCE AND GROWTH

Developing rangeland management strategies to minimize spotted knapwee d (Centaurea maculosa Lam.) invasion will require understanding the ef fects of intensity, frequency, and season of grazing on weed establish ment. We studied the effects of hand-clipping 2-year-old Idaho fescue (Festuca idahoensis Elmer) plants 0, 30, 60, or 90%, 1, 2, or 3 times (14-day intervals) on spotted knapweed seedling emergence and growth i n pots watered with 150 ml once weekly or 50 ml 3 times weekly. Pots w ere seeded with 5,000 spotted knapweed seeds m(-2), replicated twice, and placed in a growth chamber in a completely randomized design. The experiment was repeated once. Plants were harvested after 50 days. Tre atment effects on soil moisture, Idaho fescue and spotted knapweed sho ot and root weight, and leaf area were compared using analysis of vari ance and regression analysis. At final harvest, Idaho fescue shoot wei ght and leaf area decreased with increasing defoliation level and freq uency. Idaho fescue root weight was not affected by any treatment. A s ingle Idaho fescue defoliation at 30% and 90% increased spotted knapwe ed weight and numbers per pot respectively, over those pots with undef oliated plants. The level of defoliation necessary to enhance spotted knapweed numbers was lower as defoliation frequency increased. As defo liation level and frequency increased, soil water content increased re sulting in a corresponding increase in spotted knapweed emergence and growth.