The increasing use of synthetic chemicals for pest control in rice hasbecome an overwhelming economical border, and more important, it could posea serious threat of the environment. In addition, the extensive use ofsynthetic herbicides has been the cause for the evolution ofherbicide-resistant barnyardgrass worldwide. This weed species is the mostcompetitive weed in rice after the red rice. Thus, this study was conductedto examine the combination effects of aqueous sunflower leaf extracts withlower rate of pretilachlor on barnyardgrass emergence and growth in Marang(sandy loam) and Seberang (silt loamy) soil series under glasshouseconditions. Interestingly, the ED95 values (rate that causes 95%inhibition) of pretilachlor for emergence and shoot fresh weight (SFW) of barnyardgrass were reduced by 79 and 82%,respectively, when being mixed with sunflower leaf extracts in Marangseries. In contrast, the addition of sunflower leaf extracts increased ED95 value of pretilachlor in Seberang series. Rice seedlingsat 4 and 8 d after sowing (DAS) were found to be tolerant to this mixturetreatment. However, root growth of rice seedlings were inhibited at 0 and 2DAS. These results suggest that sunflower leaf extracts have potential toreduce rate of pretilachlor for inhibiting emergence and growth ofbarnyardgrass without injuring rice seedlings in rice fields depending onsoil types and growth stage of rice.

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.