Experiments were conducted from 1973 through 1975 on Lucedale sandy loam to determine the influence of in-row cotton (Gossypium hirsutum L. ‘Stoneville 213’) densities on the competitiveness of low-level infestations of sicklepod (Cassia obtusifolia L.) and pigweed (Amaranthus spp.). Weeds were established at densities of 0, 4, 12, and 32 weeds per 15 m of row and allowed to compete the entire season with cotton grown at densities of 5, 10, or 20 plants/m of row corresponding to 47000, 94000 and 187000 cotton plants/ha. Conventional cultural practices were employed in these experiments. Cotton yields were inversely related to weed density; however, the density of cotton did not influence the competitive effect of sicklepod or pigweed. Pigweed or sicklepod dry weed weight was reduced when competing with 187000 cotton plants/ha.

Four greenhouse experiments were conducted to study the effects of plant residues and extracts of yellow nutsedge (Cyperus esculentus L.) plant residues on the growth of corn (Zea mays L.) and soybeans [Glycine max (L.) Merr.]. At equal concentrations, tuber residues reduced the dry weight of corn and soybeans more than foliage residues. As the concentration increased, growth decreased, affecting soybeans more than corn. Soybean growth was significantly reduced by the addition of tuber extracts. At a constant residue concentration, increasing the percentage of sand in the soil mixture reduced the growth of corn and soybeans. Growth inhibition was greatest when tuber residues were in contact with the corn or soybean seed. We conclude that extracts and residues of yellow nutsedge have an allelopathic effect on corn and soybeans under greenhouse conditions.

The involvement of allelopathy in various crop–weed competition studies has been suggested by several authors, but its significance has been demonstrated with varying success. It is extremely difficult to unambiguously demonstrate allelopathy in nature because of the complexity of plant interference and its relationship to soil chemistry. However, with an increased understanding of the chemical processes occurring in the agroecosystem, genetic mapping of quantitative traits, and the ability to identify allelochemicals, an effort should now be directed toward understanding the mechanisms for allelopathy, as well as trying to optimize an allelopathic effect to produce more competitive crops. The approach used for rice allelopathy research can be used as a general framework for understanding how genetically encoded traits affect the competitive ability of plants. This framework requires the involvement of a range of scientists from multidisciplinary research areas with the overall objective of optimizing competitive ability in crops. Such research efforts could reduce dependency on herbicides and thus increase the sustainability of weed management practices. This paper aims to illustrate the importance of allelopathy for crop competitive ability and to identify a framework suitable for result-oriented collaborative research toward breeding for competitive ability in crops.