Effect of seeding rate on weed suppression activity and yield of indica and tropical japonica rice cultivars

Authors: Gealy, David; Duke, Sara

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2017
Publication Date: 9/1/2017
Citation: Gealy, D.R., Duke, S.E. 2017. Effect of Seeding Rate on Weed Suppression Activity and Yield of Indica and Tropical Japonica Rice Cultivars. Weed Science 65(5):659-668.

Interpretive Summary: Weeds are prevalent in rice fields throughout the U.S. costing farmers millions of dollars every year. Most rice varieties are not good competitors against weeds, but our earlier research results have shown that certain varieties can reduce growth and vigor of barnyardgrass, one of the most troublesome and prevalent weeds of rice in the USA and throughout the world. These varieties appear to have an ability to fight off barnyardgrass in traditional, conventionally-managed rice fields as well as in those using more sustainable management approaches such as reduced rates of herbicide and irrigation water. As a continuation of this work, a two-year field experiment was conducted to learn whether a reduction in seeding rates (which would reduce seed costs) might affect how well these rice varieties suppress the growth and vigor of barnyardgrass. The varieties tested were a popular non-suppressive commercial variety, ‘Wells’, the weed-suppressive varieties, ‘PI312777’, ‘Rondo’, and ‘4612’, and the commercial hybrid variety, ‘XL723’. Although PI 312777 produced the most tillers (often present in varieties that are good weed competitors), XL 723 suppressed weeds the best, perhaps because its plants had already grown to be more massive (i.e. total weight) than those of all other varieties by the middle of the growing season. Compared with the other varieties, the yields of PI 312777 and 4612 fell only slightly at the lowest seeding rate, showing that these two varieties tolerated weeds at the low seeding rate nearly as well as at the high rate. This might be a desirable variety trait for farmers attempting to reduce their seeding rates. None of the rice varieties tested at the lowest seeding rate consistently suppressed weeds or produced high yields in weed-infested plots, indicating that moderate to high seeding rates are likely to be necessary for optimum natural suppression of weeds in reduced-input systems. Our results also indicated that varieties capable of producing large amounts of mid-season plant mass in weed-infested plots might be a key to finding better rice lines for weed suppression.

Technical Abstract: Weeds are ubiquitous and costly pests in U.S. rice production systems. Although most rice cultivars do not suppress weeds dramatically, certain indica cultivars and commercial hybrids have been shown to suppress barnyardgrass (Echinochloa crus-galli) in conventional as well as reduced-herbicide and reduced-irrigation management systems in the southern USA. Thus, a field study was undertaken to determine the degree to which reduced seeding rates would affect natural weed suppression of conventional and weed suppressive cultivars. The experimental design was a randomize split-plot block design with a factorial combination of five cultivars by three seeding rates (160, 320, and 480 seeds m-2) as the main plots, two weed levels (weed-free and weedy) as the sub plot, and four replications. Cultivars included a non-suppressive conventional long-grain, ‘Wells’, high-tillering, weed-suppressive cultivars ‘PI312777’, ‘4484-1693’ (‘Rondo’), and ‘4612’ from Asia, and the commercial hybrid ‘XL723’. PI 312777 produced the most tillers, whereas XL 723 exhibited the greatest midseason shoot biomass as well as the greatest weed suppression. Yields of PI 312777 and 4612 declined minimally with decreasing seeding rates when compared with the other cultivars, and thus, tolerated weeds at the low seeding rate nearly as well as at the high rate. This would be a desirable rice trait for natural weed suppression at reduced seeding rates. The difference between rice yields in weed-free and weedy plots was negatively correlated with mid-season rice shoot biomass in weedy plots (r= -0.5441), visual weed control ratings (r= -0.7907), and rice harvest height in weedy plots (r= -0.6909), and positively correlated with late-season barnyardgrass biomass in weedy plots (r= 0.5738). Overall, our results indicated that at the lowest seeding rate, none of the rice cultivars consistently suppressed weeds or produced high yields in the presence of uncontrolled weed pressure. Thus, moderate to high planting densities are likely to remain necessary for consistent weed suppression in reduced-input systems.