Because of the simultaneous emergence of weeds and rice in direct-seeded rice (DSR), weeds are more problematic in DSR than in transplanted rice systems. In Pakistan, yield losses due to weeds in dry-seeded systems are recorded at 27-80%; however, in extreme cases, it results in complete crop failure up to 100%. However, The continuous injudicious use of herbicides has effects on environmental pollution, especially water pollution, human health, animals, and marine life, risks to beneficial organisms, adjacent crops, and the evolution of herbicide-resistant weeds. Therefore, there is a need to develop sustainable integrated weed management strategies, including diverse agronomic approaches to suppress weeds and minimize herbicide use.
Because of the simultaneous emergence of weeds and rice in direct-seeded rice (DSR), weeds are more problematic in DSR than in transplanted rice systems. Weeds compete with the crop for moisture, nutrients, light, and space resulting in poor crop growth and requiring a higher level of input resources, especially nutrients and moisture.
In Pakistan, yield losses due to weeds in dry-seeded systems are recorded at 27-80%; however, in extreme cases, it results in complete crop failure up to 100%. Hence, judicious weed management is crucial in attaining food security in developing countries of Asia.
Weeds in DSR can be managed through several methods; however, manual and chemical methods are most commonly practiced. Manual weeding is very common in South Asian countries, but nowadays it is not an economical and feasible option because of labor scarcity and high wages. In the current scenario, the only feasible and economical method of weed control in rice is the use of chemicals.
Chemical weed control using herbicides saves time and labor. Several pre-and post-emergence herbicides have been reported for effective weed control in DSR, however, many studies reported that pre-emergence herbicides must be applied to stop the simultaneous emergence of weeds.
Pre-emergence herbicides need to be applied to soil but in Pakistan farmers usually, don’t practice this. Pre-emergence herbicides (oxadiazon, pendimethalin, etc) are usually applied 3 days after sowing (DAS) of rice, preferably immediately after planting and before the emergence of weeds and crops. These herbicides are usually cell division inhibitors and are no longer effective beyond the first leaf stage.
Early post-emergence herbicides (e.g., butachlor, propanil, thiobencarb) are applied at the 2–4 leaf stages. Late post-emergence herbicides (e.g., bispyribac-sodium, azimsulfuron, fenoxaprop, ethoxysulfuron, 2,4-D) are usually applied on foliar and the application time ranges from 14–28 DAS.
The major rice establishment system in Pakistan is transplanted rice where a farmer uses early post-emergence herbicides which require standing water in the field for 5-7 days after their use but it is not possible in the DSR systems.
In DSR, weeds began to emerge simultaneously with rice seedlings and farmers do not try to control them with pre-emergence herbicides to avoid any phytotoxicity to rice seedlings. Research showed that there is severe phytotoxicity to rice seedlings if there is rain or irrigation immediately after the application of pre-emergence herbicides.
These weeds need to be controlled by using late post-emergence herbicides. Since the DSR field gets infested by complex and diverse weed species, no single herbicide is competent to control all weed species. Therefore, a combination of herbicide (sequential applications or tank mixtures) or a broad-spectrum herbicide along with other cultural practices is essential for effective control of all groups of weeds such as sedges, broadleaved, and grasses.
The continuous injudicious use of herbicides has effects on environmental pollution, especially water pollution, human health, animals, and marine life, risks to beneficial organisms, adjacent crops; and the evolution of herbicide-resistant weeds. Currently, there are 403 documented cases (species by the site of action) of herbicide resistance among 218 weed species of 66 crops in 61 countries worldwide.
The negative effects of herbicide application on the environment may limit the use of herbicide options available for farmers in the future. Therefore, there is a need to develop sustainable integrated weed management strategies, including diverse agronomic approaches to suppress weeds and minimize herbicide use.
The present study was therefore designed to evaluate the late-post-emergence herbicides alone and in combination with manual weeding to find out the most appropriate weed control method with less use of herbicides in controlling weeds of DSR.
The current study suggests that various weed control options are available to control weeds economically in DSR systems. Mechanized DSR is a new system in Asia, including Pakistan, and the availability of different weed management options will help in promoting this system, as weeds are the main constraint in this system.
Our study inferred that bispyribac sodium or ethoxysulfuon herbicides can control annual grasses, sedges, and broadleaf weed species. In this regard, post-emergence herbicide should be used once the weed seedlings have emerged in the field. Subsequent weed flushes can be controlled by manual weeding.
Before the dissemination of DSR systems across the country, there is a need to develop integrated weed management strategies involving the use of a stale seedbed, high seeding rates, weed-competitive cultivars, narrow row spacing, and economical herbicide combinations for the effective control of weeds.
Read the study:
Awan TH, Saleem, Usama MU, Hussain S, Ahmed S, Ali, HH. (2019). Integrated weeds management in dry-seeded basmati rice. The Journal of Animal & Plant Sciences, 29(5): p. 1299
