TILLAGE EFFECTS ON WEED SEED RETURN AND SEEDBANK COMPOSITION

Weed seed return and seedbank composition, with particular reference t o common lambsquarters, were studied in four tillage systems establish ed on a site near Fingal, Ontario, The tillage treatments were moldboa rd plow chisel plow ridge-till, and no-till, The cropping system was a corn-soybean rotation. Tillage effects on weed population composition were assessed after all weed control measures had been implemented, M ore than 60% of the weed seedbank was concentrated in the upper 5 cm o f soil in chisel plow and no-till, The seedbank of the moldboard plow system was more uniformly distributed over depth and larger than the o ther systems, Common lambsquarters comprised more than 50% of the seed bank in all systems except ridge-till, but only dominated the abovegro und weed population in chisel plow Seedbank populations of common lamb squarters with moldboard plowing were greater than those with ridge-ti ll and no-till, and chisel plow seedbank populations were greater than those in ridge-till, Chisel and moldboard plow systems generally had higher aboveground plant populations of common lambsquarters than the other two systems, Seed production per plant by common lambsquarters w as equivalent among the four systems, but estimated seed production pe r unit area was higher in moldboard plow and chisel plow systems than in the other systems, Populations of common lambsquarters and similar species may produce more seeds and persist in moldboard plow and chise l plow systems; these weeds may produce fewer seeds per unit area and be easier to manage in no-till and ridge-till systems.