Studies of weed and crop competition can be used to predict yield losses from weed presence and to determine optimum levels or periods of weed control. However, competition is a complex phenomenon that is governed by various biological, environmental, and proximity factors. The factors of proximity include plant density, species proportion, and spatial arrangement among individuals. Several experimental methods have been developed that attach different levels of importance to proximity factors. These methods are described, and the advantages and disadvantages of each are discussed. Density, proportion, and arrangement of plants influence the outcome of competition experiments and should be incorporated into studies of crop-weed interference, since differing estimates for the effects of weeds on crop productivity can be obtained, depending upon the experimental method used.

Density and spatial interference of barnyardgrass with ‘Lemont’ and ‘Newbonnet’ rice cultivars was investigated in 1987 and 1988. Barnyardgrass reduced grain yield of Lemont, a semidwarf cultivar, more than Newbonnet, a short-statured cultivar. Linear regression indicated that season-long interference of barnyardgrass at 1 to 20 plants m–2 reduced grain yields of Lemont and Newbonnet by 301 and 257 kg ha–1 per barnyardgrass plant, respectively. Grain yields of Lemont and Newbonnet were reduced by barnyardgrass densities to 40 plants m–2 but were not affected further at densities ≥40 plants m–2. Barnyardgrass reduced straw yield of Lemont more than of Newbonnet. Although barnyardgrass culms m–2 and straw yield increased as weed density increased, the weed produced more culms per plant at 2 and 5 m–2 than at 20 to 80 m–2. In a spatial interference experiment, Lemont grain yield reduction averaged 21% when plants were grown within 0 to 25 cm of a barnyardgrass plant group (group = 4 plants per 140 cm2). However, rice yields were not reduced when barnyardgrass plant groups were 25 to 50 or 50 to 100 cm away. Additional treatments included two barnyardgrass groups spaced 20, 40, 80, or 100 cm apart When Lemont grew between barnyardgrass groups spaced 20 or 40 cm apart, grain yields were reduced 40 and 27%, respectively, compared with weed-free rice. However, Lemont yield was not reduced when rice plants grew between weed plant groups spaced 100 cm apart.