Competitive N uptake between rice and weedy rice

Weedy rice, red rice (Oryza sativa L.), is a serious problem in rice-producing areas of the southern U.S. and various regions worldwide. It competes for production inputs, increases weed control cost, reduces yield and grain quality, and could eliminate economic returns. Research was conducted to compare: (i) the relative efficiencies of rice and weedy rice in recovering fertilizer N under a competitive condition, (ii) the accumulation and partitioning of N by rice and weedy rice, and (iii) the N use efficiency of rice and weedy rice. Experiments were conducted in 1999 and 2000 at the Rice Research and Extension Center, Stuttgart, Arkansas, USA. Experimental units were arranged in a split–split plot design with N rate (0, 5, 10, 15, and 20 g m−2) as mainplot, plant type (rice and weedy rice) as subplot, and type of shoot tissue (lower leaf, leaf sheath, top leaf, and panicle) as the sub-subplot. ‘Drew’ rice and strawhull weedy rice were drill-seeded in alternate rows. At 26–27 days after planting, metal collars were driven into the soil to establish two microplots per treatment. Urea labeled with 15N was applied to dry soil in the microplots immediately before permanent flood. Data were collected at panicle initiation (PI) and 2 weeks after heading (WAH). Regardless of growth stage, Drew rice and weedy rice produced equivalent shoot biomass when no fertilizer N was added. At PI, both plant types showed a quadratic response to fertilizer N with respect to biomass production. However, where the response to N was positive, weedy rice accumulated 42 g shoot biomass g−1 of added N whereas shoot biomass accumulation for Drew was 33 g g−1 of added N. Overall, weedy rice accumulated more biomass than rice upon addition of N fertilizer. The difference in shoot biomass production between Drew and weedy rice became larger as the growing season progressed. At 2 WAH, weedy rice continued to produce more biomass at the highest N rate, but Drew did not respond substantially to added N. Weedy rice had more culm biomass (55% of total) than Drew (48%). Lower leaves constituted 16–19% of total shoot biomass, flag leaf 6%, and panicles comprised the rest for both plant types. In general, weedy rice accumulated more N, responded to higher N rates, accumulated more N in the panicles, and had a higher N use efficiency (for biomass production) than Drew rice. Weedy rice can accumulate 63% of fertilizer N, 2 WAH. We conclude that weedy rice responds to higher N rates, takes up more N, and has higher N use efficiency for biomass production than rice. Whenever a rice field is heavily infested with weedy rice, controlling weedy rice should be the priority. Otherwise, fertilizer N application (which is more expensive than weedy rice control) is not profitable.