Competition between maize and barnyard grass (Echinochloa crus-galli) at different moisture regimes.
Competition between a natural stand of barnyard grass (Echinochloa crus-galli) and a maize crop was studied during two years. The combined biomass of maize and barnyard grass increased with soil water availability. Maize density varied little and its yield was determined by individual plant size. Maize shoot weight decreased with increase in root biomass of barnyard grass (which is a product of plant number per area and individual root weight) and decreasing soil water availability. Barnyard grass density was greatest when the water capacity of soils was low. Root weight and the proportion of dry matter allocated to roots (root:shoot ratio) was correlated with individual size of barnyard grass plants. Biomass of barnyard grass plants decreased but root:shoot ratio increased with decreasing soil water availability. Root competition was the major factor decreasing maize biomass while aboveground competition was of minor importance. With similar aboveground biomass, dense stands of barnyard grass consisting of small plants with numerous roots caused a greater decrease in maize yield than sparse stands consisting of large plants with fewer roots.