Weeds are ubiquitous and economically damaging in southern U.S. rice systems. Barnyardgrass has consistently been one of the most prevalent and troublesome of these. Although most rice cultivars do not suppress weeds dramatically, certain Indica cultivars and commercial hybrids are known to suppress barnyardgrass aggressively in conventional, drill-seeded rice systems in the southern United States. A field study was conducted to determine the degree to which either reducing or increasing standard seeding rates would affect natural suppression of weeds by conventional inbred and weed-suppressive cultivars. Five cultivars were evaluated at three seeding rates (160 [low], 320 [medium; conventional recommendation for inbred cultivars], and 480 [high] seeds m⁻²) and two weed levels (weed-free and weedy). Cultivars included a conventional, non–weed suppressive long-grain, ‘Wells’; high-tillering weed-suppressive cultivars ‘PI312777,’ ‘Rondo,’ and ‘4612’ from Asia; and the commercial hybrid ‘XL723.’ Overall, PI 312777 produced the most tillers, whereas XL 723 exhibited the greatest midseason shoot biomass and the greatest weed suppression. Yields of PI 312777 and 4612, both of which are Indica cultivars considered to be good weed suppressors, changed minimally across all seeding rates when compared with the other cultivars and thus tolerated weeds at the low rate nearly as well as at the high rate. Such a tolerance to weeds might be useful in the maintenance of weed suppression at reduced rice-seeding rates and suggests that reduced seeding rates of PI 312777 and 4612 would be less risky for yield loss when compared with the other cultivars tested. Visual suppression ratings were positively correlated with rice yield within weed-infested plots, suggesting that yield performance under weed pressure might be a good indicator of weed-suppression ability of cultivars in these systems. In contrast with PI 312777 and 4612, yields of the conventional inbred cultivar and commercial hybrid appeared to benefit from the high seeding rate. Overall, moderate to high seeding rates are likely to be needed for consistent weed suppression for all of the cultivar types evaluated in this study.

Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv.; rice, Oryza sativa L.