Applying crop and weed competitive dynamics for weed management in soybean and peanut

Demand for organic food products has consistently increased for more than 20 years. Demand for organic grain has been particularly high, leading to price premiums of over double the conventional price. The largest obstacle to organic soybean production is weed management. The first investigation aimed at improving weed management in organic soybean tested the effectiveness of pre-plant rotary hoeing to reduce the need for multiple post-plant rotary hoeing. Pre-plant rotary hoe treatments included a weekly rotary hoeing four weeks before planting, two weeks before planting and none. Post-plant rotary hoe treatments consisted of zero, one, two, three, and four post-plant rotary hoe uses. Weed control was increased with pre-plant rotary hoeing at Plymouth in 2006 and 2007 but this effect disappeared with the first post-plant rotary hoeing. Multiple post-plant rotary hoe uses decreased soybean plant populations, decreased soybean canopy height, lowered soybean pod position and decreased soybean yield.;In another experiment, the effect of soybean population on weed control was investigated. This research was conducted in 2006 and 2007 to investigate seeding rates of 185,000; 309,000; 432,000; and 556,000 live seeds/ha. All rates were planted on 76 cm row spacing in organic and conventional weed management systems. Increased soybean seeding rates reduced weed ratings at 3 of the 5 sites. Increased soybean seeding rates also resulted in higher yield at 3 of the 4 sites. Maximum economic returns for organic treatments were achieved with the highest seeding rate in all sites.;In a separate experiment, the effect of soybean genotype on weed suppression was investigated. Twenty seven genotypes were chosen based on varying seed sizes, leaf shape, and height. Genotypes were compared in weedy and weed free conditions. Canopy traits and percent ground cover estimates were measured in weed free plots. Soybean and weed biomass has harvested at 7 weeks after emergence. Differences in weed biomass were detected between genotypes in both years. Optimum models from multiple regression showed seed size to be the most significant trait measured in overall genotype competitive ability in both years.;In an additional experiment, the influence of soybean seed size within a genotype was investigated. Three popular soybean varieties: Hutcheson, NC-Roy, and NC-Raleigh were separated into four or five seed size classes. Seed sizes ranged from 10 to 20 g/100 seed. Each seed size class was grown in weedy and weed free conditions at Kinston, NC in 2007 and 2008 and at Plymouth, NC in 2008. The effect of soybean seed size on increased soybean biomass was detected in all environments when grown in competition with weeds. In the two environments with higher weed population densities, planting larger soybean seed reduced weed biomass at 7 weeks after emergence (R2=0.42 and R 2=0.54 in Kinston 2007 and Kinston 2008 respectively).;A study in peanut production systems was conducted to define interactions of three levels of weed management (clethodim applied postemergence, cultivation and hand removal of weeds, clethodim and appropriate broadleaf herbicides applied postemergence), three levels of planting pattern (single rows spaced 91 cm apart, standard twin rows spaced 20 cm apart on 91-cm centers, narrow twin rows consisting of twin rows spaced 20 cm apart on 46-cm centers), and two levels of cultivar (NC 12C and VA 98R) on weed control, peanut yield, and estimated economic return. Cultivar and planting pattern had only minor effects on weed control and interactions of these treatment factors seldom occurred. Weed control with cultivation and hand removal was similar to weed management with grass and broadleaf herbicides. Pod yield did not differ among treatments when these broadleaf weeds were dominant, but did differ when Texas panicum was dominant.