What About Weeds?Despite organic farming's enrichment of the soil, weed problems during the 9-year study were enough to lower corn and soybean—but not wheat—yields below those of no-till crops.
But in another long-term experiment begun in 1996, Teasdale learned that adding more kinds of crops to the organic rotation helped control weeds.
"Weeds tend to adapt to crops whose growth timetable creates conditions favorable to weed growth," Teasdale says.
Planting the same summer annual crop year after year allows weeds suited to that growth cycle to keep maturing and adding their seeds to the soil. In organic systems, Teasdale showed that rotating diverse crops markedly lowers the numbers of weed seeds lying dormant in soil.
In an ongoing experiment called the "Farming Systems Project," Teasdale and ARS soil scientist Michel Cavigelli showed that after 10 years, corn yields were higher in diverse organic rotations that included a perennial legume.
"This is one of a few studies that consider the effects of rotation length and crop complexity on organic grain yields," Teasdale says.
John R. Teasdale is with the USDA–ARS Sustainable Agricultural Systems Laboratory, Bldg. 001, Room 245B, Beltsville, MD 20705; phone (301) 504-5504, fax (301) 504-6491.