World’s rice demand is projected to increase by 50% from 1997 to 2050 to keep pace with population growth .Compared to other field crops, rice is most widely grown under irrigated condition which accounts for about 50% of the total amount of water diverted for irrigation, which in itself accounts for 80% of the amount of fresh water diverted. This is due to the high unproductive water losses by evaporation, surface run-off, and percolation. Producing one kilogram of unprocessed rice grain under irrigation is estimated to use between 1500 and 5000 L of water, depending on the local climate, soil type and rice variety . This amount is about twice or even more than wheat or maize. However, declining water availability threatens the sustainability of traditional flood-irrigated rice ecosystems . In Asia, it is predicted that 17 million hectares of irrigated rice areas may have “physical water scarcity” and 22 million hectares areas may be subject to “economic water scarcity” by 2025 .
In Asia, where about 60% of the world’s population lives, food security is challenged by increasing food demand and threatened by declining water availability.It is, therefore, no longer feasible to flood rice fields for better crop establishment and weed control . Among different water-saving approaches, aerobic rice cultivation has come up with a huge success in different parts of the world.Growing rice in non-saturated and non-puddled aerobic soil is a promising water-wise technique of rice cultivation under the context of ever-mounting water scarcity. Growing rice under aerobic conditions requires 36% 41%less water than under the conventional method . In response to the labour and water shortage problem, cannabis drying racks some alternative rice production methods were suggested by researchers worldwide including alternate wetting and drying , system of rice intensification, and raised bed for saturated soil culture. Compared with flooded rice, aerobic rice had lower production cost, higher water productivity, and a comparable outcome.Weeds are a major yield limiting factor in rice production,and the literature reporting yield losses is numerous. Globally, actual rice yield losses due to pests have been estimated at 40%, of which weeds have the highest loss potential.
The worldwide estimated rice yield loss due to weeds is around 10%of the total production . Yield reduction due to weeds is more critical in direct seeded rice than in transplanted rice. In dry seeded aerobic rice,relative yield loss caused by weeds is 50% 91% , while in transplanted rice,yield loss has been estimated to be only 13% . Among the rice ecosystems,yield losses are the highest in aerobic rice . In extreme cases, weed infestation may cause complete failure of aerobic rice . Weeds persist by adapting to cultural practices, and every cultural practice influences the competitive ability of both the crop and weed resulting complex interactions.Cultural approaches play significant role to determine the competitiveness of a crop with weeds for above ground and below ground resources and hence might influence weed management . Integrated weed management strategies offer several options, but risks of developing resistant weed biotypes ,and environmental hazard resulting from herbicides from chemical control , and labor-intensive manual weeding methods demand aneco-friendly and less labor-dependent weed management system for sustainable aerobic rice production.As observed by many researchers, the performance of herbicides can be enhanced if crop varieties with higher weed competitiveness are used especially in herbicide-dominant systems.
Weed competitiveness comprises two components:weed suppressive ability—the ability to lessen weed growth through competition, and weed tolerance—the capability of maintaining potential yields in the presence of weeds . The potentiality of using weed competitive variety in integrated weed management has been documented in many crops including rice. The deployment of weed competitive variety is not only eco-friendly but also a very cost effective tool for integrated weed management. Considering the high vulnerability of aerobic rice to weeds, development of weed competitive aerobic rice variety has been suggested by many researchers .No work has so far been done to assess the ability of the huge pool of Bangladeshi rice germplasms to wrestle weeds under aerobic soil conditions. In this backdrop, the present study was undertaken to study the variation in weed competitiveness among selected high yielding rice varieties and to recognize agronomic traits conferring weed competitiveness of rice grown under aerobicsoil conditions.