The rice production method, dry direct seeding (DSR), is the alternate option for the successful cultivation of aus rice where irrigation water is scarce and the wage rate of agricultural labor is high. However, the success of DSR mostly depends on efficient and economic weed management. The objectives of the current study are to evaluate the yield performance and weed competitiveness of aus rice cultivars including hybrid and inbred at different seeding rates and weeding regimes in DSR seeded systems in Bangladesh.
The common rice production method in South Asia (SA) is transplanting the rice seedling into puddled soil. Puddling and transplanting operations consumed a huge amount of water, approximately 30% of the total water requirement for rice cultivation of a season. The alternative rice production method, which is dry direct seeding (DSR), is becoming popular in SA countries where irrigation water is scarce and the wage rate of agricultural labor is high. Moreover, due to the development of better technology (e.g., mechanized line sowing seed drill, mechanical weeder, and improved herbicides) in the last decades, DSR has become more contented than the transplanted rice.
Bangladesh is the fourth largest rice-producing country in the world and second in SA, and around 67% of the country’s cultivated land area is used for rice production. Rice grows here throughout the year and there are three distinct growing seasons popularly known as aus (April–August), aman (June–December), and boro (November–May), covered by 9%, 49%, and 42% area, respectively.
Rice in aus and aman are considered mainly rainfed but significant partial irrigations are needed depending on rainfall variability and land conditions. Boro is completely irrigated rice and more than 80% of irrigated areas depend on groundwater. Sources of groundwater are decreasing over time due to unregulated use, leading to a scarcity of irrigation water across a large part of the country at the end of the boro season. Irrigation cost in Bangladesh is increasing over time, therefore, the profitability of boro rice is decreasing.
In the wet season, aman rice is the main crop in the field and there are only a few options for another non-rice crop during this time due to frequent and heavy rains and waterlogging conditions. With the increasing population, Bangladesh also needs to increase the production of other non-rice crops such as pulses, oilseed, vegetables, and spices. Those crops are mostly grown during the dry season in Bangladesh and this is also the boro rice cultivation period.
If we examine the rice production statistics in Bangladesh from 1970 to 2018, we find a decreasing trend in aus areas from 1970 to 2010 and thereafter, an increasing trend. Aman areas from 1970 to 2018 were almost static and boro areas had an increasing trend from 1970 to 2010 and thereafter almost static. Increasing the cultivation of non-rice crops during the dry season (boro) would save groundwater as well as the production of other nutritious crops for the growing population, but this could reduce the total volume of rice production (national production).
In this situation, to minimize rice shortages, aus yield and area need to be increased. Considering the important contribution of aus rice to Bangladesh’s rice production levels, the government is also trying to increase the aus rice areas. Aus season usually starts at the end of a couple of months of dry spell and just before the start of the wet season; therefore, it is difficult for farmers to manage this large volume of water for puddling and transplanting, and this is one of the main reasons that many areas suitable for aus remain fallow.
Therefore, DSR is the alternate option for the successful cultivation of aus rice where irrigation water is scarce and the wage rate of agricultural labor is high. However, the success of DSR mostly depends on efficient and economic weed management. Weed infestation in DSR systems is usually higher than the puddled transplanted rice; because of this, there is no size advantage of rice plants at the time of emergence, which is usually present in transplanted systems
A DSR field remains mostly aerobic as opposed to the continuous standing water in puddled and transplanted rice, another important reason why weed grows more in DSR systems. Weed management currently is almost herbicide-dependent and sequential application of pre-and post-emergence herbicides or tank mixtures of different post-emergence herbicides was found effective in controlling weed in DSR systems.
However, weed management currently is not cost-effective without herbicides and injudicious use of herbicides may adversely affect the environment and help to develop resistant weed biotypes . Therefore, weed management in DSR needs integration of different practices such as preventive measures, cultural methods, mechanical and manual weeding, etc., that could reduce farmers’ dependence on herbicides and offer the best sustainable weed management.
Competitive rice cultivar (CV) is a component of integrated weed management (IWM) and an attractive option to suppress weed growth where there is no need to incur any additional costs. Competitive CV suppresses weeds by its quick canopy coverage or CV may be more capable of reducing the ability of a weed species through competition for limited resources, or cultivars may produce chemical exudates that reduce the growth of weeds.
Although rice is generally a weak competitor against weeds, the identification of superior weed competitive rice cultivars could play an important role in reducing herbicide load in the agro-ecosystem by reducing its use and enhancing the performance of herbicides.
Seed rate (SR) is an important factor in DSR crop establishment which determines the adequate plant population for optimum yield. In the DSR system, low plant population and high gaps encourage the growth of weeds. On the other hand, high plant density produces less productive tillers, greater chance of disease and insect infestation, increases crop lodging, and requires higher nitrogen application.
High SR is encouraging in DSR system for compensating for poor crop establishment in where weed problem is higher, the chance of bird and rodent damage, and seed rotting due to too much rain shortly after sowing, or seedling death due to submergence. Therefore, there will likely be an interaction between cultivar choice and seed rate on the yield of rice, especially in dryseeded conditions.
A few similar types of studies were conducted in SA, Southeast Asian countries and in Bangladesh to evaluate the weed competitiveness of boro rice cultivars; however, no single work so far in aus rice in DSR conditions. The objectives of the current study are to evaluate the yield performance and weed competitiveness of aus rice cultivars including hybrid and inbred at different SR and weeding regimes in DSR seeded systems.
Weed infestations significantly reduced the grain yield in all the cultivars and even in partial-weedy conditions, the yield was 20–55% lower compared with the weed-free conditions, indicating the importance of weed management in DSR systems. Compared to the inbred cultivars, the hybrid showed more weed competitiveness even at the low SR, indicating that the cultivation of hybrids may be considered an important component of IWM in this system.
Increasing rice seeding rates may help to suppress weed growth to some extent and reduce losses to weeds, however, other problems associated with high SR are requiring higher nitrogen fertilizer, lodging problems, rat and insect damage, and high prune to disease infestation. The evaluation of hybrids in DSR confirms possession of weed-competitive traits and provides farmers with a wider choice of options when cultivating DSR. The results of our study indicate that, in Bangladesh, farmers can use a seeding rate of 20 kg/ha for hybrid and 40 kg/ha for an inbred cultivar in the situation of relatively low weed pressure.
However, in the situation of high weed pressure and the absence of effective weed control options, farmers should use relatively higher seeding rates of 40 kg/ha in hybrid and 80 kg/ha in inbred to reduce yield losses due to weed infestation.
Read the study:
Ahmed S, Alam MJ, Hossain A, Islam, AKMM, Awan, TH, Soufan W, Qahtan AA, Okla, MK, El Sabagh A. (2021) Interactive effect of weeding regimes, rice cultivars, and seeding rates influence the rice-weed competition under dry direct-seeded sondition. Sustainability 13: 317