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Project description

Using Subterranean Clover with Sheep Grazing and Mowing to Control Yellow Starthistle in Pastures. (96CC021)
Program UC IPM competitive research grants program
W.A. Williams, Agronomy and Range Science, UC Davis
D.W. Pratt, Cooperative Extension, Solano County
Host/habitat Pastures
Pest Yellow Starthistle
Discipline Weed Science
Cultural Controls
Start year (duration)  1996 (One Year)
Objectives Develop an effective long-term control program for yellow starthistle using integrated approaches of subterranean clover plantings, mowing, and sheep grazing in pastures and ungrazed areas.
Final report Our previous research demonstrated that controlled grazing and timed mowing are useful methods for reducing yellow starthistle infestations without herbicides. However, to obtain long-term control, we hypothesized that competitive vegetation should be established. We tested this over a four-year period with two experiments using combinations of subclover seeding, phosphorus fertilizer, sheep grazing, and mowing in grazed and ungrazed paddocks. In the pasture experiment we tested: 1) subclover seeding with controlled grazing (short duration, high-intensity) and timed mowing, 2) grazing and mowing with no subclover seeding, and a 3) control, with no subclover, grazing or mowing. The seeded (once in Oct. 1993) paddocks were grazed three times between early February and late May, and mowed in early July each year. The unseeded paddocks were grazed in late March and late May and mowed once in early July. The treatments in the ungrazed experiment were: 1) subclover seeding with mowing, 2) mowing with no subclover seeding, and a 3) control, with no subclover or mowing. The seeded plots were mowed two times, once in February to enhance subclover growth, and a second time in mid-June during yellow starthistle's early flowering stage. In the unseeded plots we mowed only once, also in yellow starthistle's early flowering stage.

Our results indicate that there were weed control benefits from planting subclover as a competitive plant, but the tested varieties declined substantially after four years. The best control came from subclover seeding with phosphorus fertilizer and mowing. The grazed treatment was less effective because the vegetative canopy was removed during a critical period. Maintaining a dense spring canopy of vegetation is an essential ingredient for optimal yellow starthistle control. Yellow starthistle responds to the increased sunlight from spring defoliation by producing plants that are more prostrate and with growing buds closer to the ground, making plants less susceptible to control with defoliation by later spring grazing and mowing.

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