Corn Yield Loss From Weed Competition in Conventional and Organic Cropping Systems.

Weeds are a major constraint on crop yield in organic systems where herbicide technologies are unavailable.  Naturally weedy and weed-free subplots were established within the full plots of the long-term Farming Systems Project at Beltsville, Maryland, USA, to determine the effect of weed competition on corn yields in conventional and organic cropping systems.  Weed abundance was determined by sampling above-ground biomass and by visually estimating percent of soil area covered by weeds.  Regression analysis showed a strong linear relation between biomass and cover in subplots, however, the slope of the biomass to cover relationship was dependent on species composition and annual weather conditions, thereby precluding derivation of a common relational coefficient across all years and species.  Corn yield loss to weed competition was determined by regression of the percentage yield difference between paired weedy and weed-free subplots against the weed cover in the corresponding weedy subplot.  Weed competition reduced corn grain yield in all years, however, the degree of yield reduction ranged from 4 to 76%.  This considerable variability was explained by rainfall whereby the highest yield loss occurred in years with below-average rainfall and the lowest yield loss occurred in years with above-average rainfall.  The relationship between corn yield loss and weed cover in subplots was used to estimate corn yield loss and weed-free yield in the full plots.  Estimation of weed-free yield demonstrated that yield differences between the conventional and organic systems were explained not only by weed competition but also by nitrogen availability.   Results suggested that, in years with above-average rainfall, nitrogen availability was more limiting to organic corn yield than weed competition, but that, in years with below-average rainfall, weed competition was more limiting than nitrogen availability.