Relative Contributions of Allelopathy and Competitive Traits to the Weed Suppressive Ability of Winter Wheat Lines Against Italian Ryegrass

Allelopathy and competitive ability have been identified as independent factors contributing to the weed suppressive ability of crop cultivars; however, it is not clear whether these factors have equal influence on weed suppression outcomes of winter wheat (Triticum aestivum L.) lines in the field. Fifty-eight winter wheat lines adapted to the southeastern United States were screened for allelopathic activity against Italian ryegrass (Lolium perenne L. ssp. multiflorum [Lam.] Husnot) in an agar-based seedling bioassay. Eight strongly and weakly allelopathic lines were identified and evaluated for weed suppressive ability and grain yield tolerance in a replicated field experiment conducted in North Carolina. Significant genotypic differences in weed suppressive ability were found in three of four study environments, while genotypic differences in yield tolerance were identified in all environments. Although the allelopathic activity of genotypes varied in the seedling bioassay, no correlations between allelopathy and weed suppressive ability or grain yield tolerance were observed. Weed suppressive ability was correlated with competitive traits, including vigor and erect growth habit during tillering (Zadoks GS 29), high leaf area index (LAI) at stem extension (GS 31), plant height at tillering and stem extension (GS 29, 31), grain yield in weedy conditions, and grain yield tolerance. Therefore, breeders in the southeastern United States should focus their efforts on improving competitive traits within adapted germplasm rather than selecting for cultivars with high allelopathic activity to achieve maximum gains in weed suppressive ability against Italian ryegrass.