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.

Research was conducted to evaluate the weed suppression potential of ‘Rondo’ (4484-1693; PI 657830), a sister line (4484-1665), and other indica rice lines against barnyardgrass in field plots in Stuttgart, AR, using minimal herbicide inputs in two separate 3-yr experiments. Under weed pressure, Rondo and the sister line (4484-1665) generally produced yields that were comparable to those of weed-suppressive indica standards and approximately 50% greater than those of the least-suppressive commercial cultivars, such as ‘Kaybonnet’, ‘Katy’, and ‘Lemont’. Rice yield under weed pressure was correlated with weed-free yield and harvest height. Indica lines tended to produce more tillers than did the commercial cultivars. Tillering potential under weed-free conditions was not correlated with weed suppression or yield loss; however, tillering under weed pressure was strongly correlated with weed suppression and biomass, and yield and yield loss under the weed densities in these experiments. Rondo is presently being used for commercial organic rice production in Texas, in part due to its high yield potential and ability to suppress or tolerate rice pests, including weeds. Our results suggest that the weed-suppressive ability of Rondo and the other indica lines evaluated in these experiments is superior to that of many commercial cultivars.

In a 4-yr field study, “weed suppressive” rice cultivars provided 30%greater control of barnyardgrass and sustained 44% less yield loss (relativeto weed-free) compared to “nonsuppressive” tropical japonica rice cultivars. 13C analysis revealed that rice root mass predominatedvertically and laterally within the soil profile of plots infested withbarnyardgrass. Among all cultivars, rice roots accounted for 75 to 90% ofthe total root mass in samples, and this was most concentrated in thesurface 5 cm of soil in the row. Barnyardgrass roots were most prevalent inthe surface 5 cm between rows where they accounted for 30% of total rootmass. Overall, barnyardgrass root mass was about twice as high innonsuppressive rice compared to suppressive rice. Weed suppression byindica/tropical japonica rice crosses generally was intermediate betweenthat 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 weedyplots. Control ratings in weedy plots also were negatively correlated withrice percent height reduction (r = −0.415) and % yield loss (r = −0.747)relative to weed-free plots, and with barnyardgrass root mass as a percentof total root mass (r = −0.612). Control ratings were positively correlatedwith rice yield under weed pressure (r = 0.429) but were correlated withrice root mass in-rows only (r = −0.322). Clearly, rice root mass could nothave been the major cause of the differences in barnyardgrass controlbetween cultivars.

Assessing belowground plant interference in rice has been difficult in thepast because intertwined weed and crop roots cannot be readily separated. A 13C discrimination method has been developed to assessdistribution of intermixed roots of barnyardgrass and rice in field soils,but the suitability of this approach for other rice weeds is not known. 13C depletion levels in roots and leaves of rice were comparedwith those of 10 troublesome weed species grown in monoculture in thegreenhouse or field. Included were C4 tropical grasses:barnyardgrass, bearded sprangletop, Amazon sprangletop, broadleafsignalgrass, fall panicum, and large crabgrass; C4 sedge, yellownutsedge; and C3 species: red rice, gooseweed, and redstem. Riceroot δ13C levels averaged ∼ −28‰, indicating that these roots arehighly 13C-depleted. Root δ13C levels ranged from −12‰to −17‰ among the tropical grasses, and were −10‰ in yellow nutsedge,indicating that these species were less 13C depleted than rice,and were C4 plants suitable for 13C discriminationstudies with rice. Among the C4 species, bearded sprangletop andyellow nutsedge were most and least 13C depleted, respectively. δ13C levels in shoot and root tissue of pot-grown plantsaveraged 6% greater for C4 plants and 9% greater for rice in thefield than in the greenhouse. In pots, shoots of rice typically wereslightly more 13C depleted than roots. A reverse trend was seenin most C4 species, particularly for broadleaf signalgrass andplants sampled from field plots. Corrections derived from inputs includingthe total mass, carbon mass, carbon fraction, and δ13C levels ofroots and soil increased greatly the accuracy of root mass estimates andincreased slightly the accuracy of root δ13C estimates (∼ 0.6 to0.9%) in samples containing soil. Similar corrective equations were derivedfor mixtures of rice and C4 weed roots and soil, and are proposedas a labor-saving option in 13C discrimination root studies.

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.