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.

In a 4-yr field study, “weed suppressive” rice cultivars provided 30% greater control of barnyardgrass and sustained 44% less yield loss (relative to weed-free) compared to “nonsuppressive” tropical japonica rice cultivars. 13C analysis revealed that rice root mass predominated vertically and laterally within the soil profile of plots infested with barnyardgrass. Among all cultivars, rice roots accounted for 75 to 90% of the total root mass in samples, and this was most concentrated in the surface 5 cm of soil in the row. Barnyardgrass roots were most prevalent in the surface 5 cm between rows where they accounted for 30% of total root mass. Overall, barnyardgrass root mass was about twice as high in nonsuppressive rice compared to suppressive rice. Weed suppression by indica/tropical japonica rice crosses generally was intermediate between that of the other two rice groups. At the 0- to 5-cm depth, between-rows, barnyardgrass root mass was correlated negatively with rice height (r = −0.424), yield (r = −0.306), and weed control ratings (r = −0.524) in weedy plots. Control ratings in weedy plots also were negatively correlated with rice percent height reduction (r = −0.415) and % yield loss (r = −0.747) relative to weed-free plots, and with barnyardgrass root mass as a percent of total root mass (r = −0.612). Control ratings were positively correlated with rice yield under weed pressure (r = 0.429) but were correlated with rice root mass in-rows only (r = −0.322). Clearly, rice root mass could not have been the major cause of the differences in barnyardgrass control between cultivars.

Field studies were conducted to compare the barnyardgrass suppression by four U.S. (‘Starbonnet’, ‘Kaybonnet’, ‘Lemont’, and ‘Cypress’) and three highly competitive, high-yielding Asian cultivars (‘PI 312777′, ‘Guichao’, and ‘Teqing’). The economic consequence of applying less than the recommended propanil rates to these cultivars was also evaluated. Grain yields increased, and barnyardgrass biomass decreased with increasing propanil rates. With or without propanil, the Asian rice cultivars consistently suppressed barnyardgrass more and consequently produced higher grain yields than did U.S. cultivars. The economic benefit derived from propanil application was less for Asian than for U.S. cultivars. Asian cultivars produced higher rough rice yields, resulting in higher net returns (not adjusted for milling) than did the commercial cultivars, but this advantage was usually reduced when adjusting for their lower milling yields. These results suggest that growing weed-suppressive Asian rice cultivars in conjunction with reduced herbicide rates could be an effective and economical weed management strategy for rice in the southern United States. However, first, their plant type and grain quality characteristics must be improved.