Elucidating the apparent maize tolerance to weed competition in long-term organically managed systems

In a long-term cropping systems trial comparing organically and conventionally managed systems, organic maize production sustained crop yields equal to conventional methods despite higher weed levels. In 2005 and 2006, an experiment nested within the trial was conducted to gain insight into this apparent crop tolerance to weed competition. Density of mixed weed species was experimentally manipulated to achieve a broad range of weed infestation levels. Under standard management conditions, all cropping systems produced equivalent maize yields, even though weedy plant biomass in the organic treatments was between fourfold and sevenfold greater than in the conventionally managed maize. Increased yield capacity, evidenced when plots were maintained weed-free, and enhanced crop competitiveness, were the main pillars of this apparent crop tolerance to weed competition in the organic systems. Increased soil resource availability and a faster relative crop growth rate in the organic systems probably contributed to these factors, which play an important role in buffering crop fitness during years of less than ideal weed control. Simultaneously, the experiment illustrated the poor efficacy of mechanical weed management in the organic systems, which is the main reason organic maize did not out-yield conventional maize under standard management conditions