Use of 13C Isotope Discrimination Analysis to Quantify Distribution of Barnyardgrass and Rice Roots in a Four-Year Study of Weed-Suppressive Rice

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. C-13 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.