Effects of Soybean Planting Dates and Rates On Weed Pressure and Yield.

Organic farmers have long held that delayed planting of soybeans leads to reduced weed pressures and improved yields. In research conducted by Milt Haar, Craig Sheaffer, Don Wyse, Paul Porter, Carmen Fernholz, and Matt Harbur in 2006, 2007, and 2008, several varieties of organic soybeans were planted at three University of Minnesota research sites. Soybeans were planted on May 15, June 1, and June 15 at all sites. Weed counts were taken before planting and during the growing season. Standard weed control practices of rotary hoeing and row cultivation were used. Yields were determined for all plots. Research indicated that delayed planting until June 15 reduced soybean yield in 7 of 8 site-years and weed numbers in 7 of 8 site-years. Delay of planting until June 1 increased or decreased yield in 4 of 8 site years and weed numbers in 4 of 8 site-years. Soybean varieties demonstrated minor differences in yield and ability to compete with weeds; and no interaction in response to planting date. Planting population had little effect on yield or weed population.