Weeds are among the main biological constraints to realizing attainable rice yield potential and significantly reducing profitability. In Bangladesh, the climate and edaphic conditions are highly favorable for weed growth. The types of weed management practices employed can directly influence the weed control cost and farm income.Adequate knowledge on the safe handling of herbicides is lacking among most smallholder farmers in Bangladesh. It is therefore important to identify herbicides that are less toxic, pose less risk to human health, are relatively safe for the environment, and are effective for weed control.
In Bangladesh and much of tropical and subtropical Asia, rice is predominantly established under wet-tilled or “puddled” soil conditions followed by transplanting of seedlings (PTR). Rice is cultivated throughout the year with three distinct growing seasons popularly known as pre-monsoon or ‘Aus’ rice grown from April to August, monsoon or ‘Aman’ rice from June to November, and winter or ‘Boro’ rice from December to May, covering 9%, 49%, and 42% of total rice area, respectively.
Weeds are among the main biological constraints to realizing attainable rice yield potential. They also significantly reduce profitability. Worldwide, it is estimated that the yield losses due to weeds in rice under farmers’ current weed management practices are about 10%. In tropical Asia, yield losses due to weeds in lowland rice range from 10% to 20%. In Bangladesh, the climate and edaphic conditions are highly favorable for weed growth.
This can lead to significant yield losses without adequate weed management. In the absence of weed control, rice yield losses due to weeds ranged from 15% to 40% in PTR and 40% to 100% in direct-seeded rice. Yield losses due to weeds are reported higher in the Aman season than the Boro season, with losses ranging from 30% to 40% in Aman and 22% to 36% in the Boro season. The types of weed management practices employed can directly influence the weed control cost and farm income. In tropical and subtropical Asia including Bangladesh, manual weeding using hand-pulling or uprooting using a ‘Niri’ hoe has traditionally been the most common practice of weed control in rice.
Although manual weeding can be effective, because of the rising scarcity of labor at critical times, weeding can either be delayed or insufficient, resulting in increased yield losses. Bangladesh is also in a phase of economic transition and rapid growth of non-farm employment options; the resulting scarcity of labor is driving increasing rural labor costs, making manual weed control progressively less attractive. Herbicide-based weed control is consequently becoming more popular, as it can reduce overall costs by minimizing costly labor. In the last three decades, the use of herbicides in Bangladesh has increased 37-fold. Yet although herbicides can effectively control rice weeds, sole dependence on chemical control measures poses both environmental and economic risks.
The former include the evolution of herbicide resistance in weeds and negative effects on non-target organisms, whereas the latter include additional costs involved in controlling new weed species that may result from shifts in weed flora with use of chemical control methods. These factors warrant integrated approaches to manage weeds while reducing the environmental hazards associated with herbicides, and high costs associated with manual weeding. Estimates indicate that farmers spend about USD 100 to USD 300/hectare, which is about 10% to 20% of total production cost for controlling weeds in rice fields. Hence, herbicide-based integrated weed management (IWM )could be an effective strategy to reduce weed control costs, reduce yield gap, and increase yield and profits from PTR production.
IWM can be defined as the integration of more than one approach involving cultural, physical, biological, and chemical methods. It consists of both chemical and nonchemical approaches and focuses on keeping weed populations below a certain threshold level by optimizing control measures in a strategic and holistic way. Herbicides are used as a last resort in IWM, although where they are required, they should be used in an integrated management approach, such as integration of soil-active preemergence and postemergence herbicides, rotation of herbicides with different modes of action (MOAs), or mixing of herbicides with different MOAs with best application practices.
For the PTR system in tropical and subtropical Asia, several cultural practices including appropriate land preparation by puddling and leveling, uniform crop establishment as assured by transplanting method, early flooding, and transplanting larger seedlings with potential to develop groundcover faster constitute an integral part of IWM. Along with these practices, the use of safer and effective herbicides with manual or mechanical weeding could provide both economical and environmental advantages where weed control is constrained in PTR.
Adequate knowledge on the safe handling of herbicides is lacking among most smallholder farmers in Bangladesh. It is therefore important to identify herbicides that are less toxic, pose less risk to human health, are relatively safe for the environment, and are effective for weed control. Based on 12 pesticide risk analysis factors, the US Agency for International Development (USAID) has issued the Pesticide Evaluation Report and Safer-Use Action Plan (PERSUAP) and listed safer pesticides including herbicides for use in Bangladesh.
In this study, we evaluated herbicides that are approved in the PERSUAP in different combinations with different MOAs, applied as preemergence, postemergence, or both pre- and postemergence, or used as mixtures by integrating manual and mechanical weeding to identify effective, affordable, and safer options for weed control in PTR as compared to farmers’ current weed management practices.
Although several researchers have evaluated the performance of herbicides for their effects on weed control and grain yield as compared to weedy checks in PTR under on-station settings, there have been comparatively few assessments of herbicide-based IWM options under on-farm conditions that consider not only weed control, but also yield, weed control cost, net income, and labor use.
Therefore, the specific objective of this study was to evaluate the performance of different herbicide-based IWM practices with respect to yield, economics, and labor use as compared to current farmers’ weed management practices in PTR under diverse environmental and on-farm conditions.
We hypothesized that integration of preemergence or postemergence herbicides with manual and mechanical weeding or the integration of both preemergence followed by postemergence herbicides with manual weeding would control weeds better in transplanted Aman and Boro rice, and thereby increase rice yield and reduce weed control cost as compared to farmers’ current weed management practices.
In summary, our study provides evidence that the use of less toxic herbicide-based IWM options with sequential application of the preemergence herbicide mefenacet plus bensulfuron-methyl and postemergence application of bispyribac-sodium or penoxsulam fb one hand-weeding can be cost-effective and profitable alternatives to the predominant use by farmers of pretilachlor as preemergence followed by two hand-weedings, all without compromising on rice yields.
The nonchemical option with mechanical weeding fb one hand-weeding can also be an alternative to FP; this treatment generated similar yields and profits for farmers who have no access to, or have no knowledge and skills in safely using the herbicides. The integration of nonchemical methods such as manual or mechanical weeding with the rotational use of effective yet lower toxicity herbicides is critical in delaying and/or preventing evolution of herbicide resistance while also mitigating the potential environmental trade-offs associated with weed control in intensive rice production systems in South Asia.
This work provides an important contribution toward the identification of cost-effective mechanical weeding options that can be used in isolation or in combination with safer molecules with different MOAs to effectively control complex weed flora in transplanted rice, while also achieving reduced herbicide and labor use to mitigate weed-inflicted yield and profit-loss risks.
Read the study:
Ahmed S, Kumar V, Alam M, DewanM, Bhuiyan K, Miajy A, et al. (2021) Integrated weed management in transplanted rice: Options for addressing labor constraints and improving farmers’ income in Bangladesh. Weed Technology, 35(5), 697-709.