Control by Tillage and Persistence of Volunteer Sunflower and Annual Weeds 1

Volunteer sunflower ( Helianthus annuus L.) is a major problem in crop rotations which include sunflower. This research was conducted to determine if the problem persists beyond the crop following sunflower and to learn if tillage methods affect populations of volunteer sunflower and annual weeds. Sunflower fields were harvested with a combine in 1973 and 1974 at Rosemount, Minn. Volunteer sunflower and annual weed numbers/ha were determined following five primary tillage treatments in combination with May and June planting of pinto bean ( Phaseolus vulgaris L.) and a secondary preplanting rototiller treatment. Counts were made shortly after emergence in permanent 1‐m 2 quadrats through 1977. Plots were rogued to prevent sunflower or weeds from producing seed. The weed seed population in the soil continued to supply weed plants every year, and there was no reduction in the last years of the trials. But volunteer sunflower populations in the 2nd year dropped to 2% of the first year's populations and were nearly 0 in the third and fourth years. Three treatment combinations that eliminated volunteer sunflower in 1 year were: no tillage in October followed by disk, plow, rototill in the spring; disk in October followed by plow, rototill in the spring; and rototill in October and again in the spring. Volunteer sunflower plants in the 1st year after sunflower were most numerous following rototilling in October and least numerous following no tillage in October. Weed plants were most numerous following moldboard plowing in October. About 90% of the volunteer sunflower emerged by 11 to 13 June.