Winning the Weed War in DSR: A Practical Herbicide Playbook

Poor and uneven crop establishment in DSR can often be traced to three specific, identifiable causes: phytotoxicity caused by pre-emergence herbicides applied under excessively wet soil conditions, often due to unexpected rainfall; seeds sown deeper than the optimal 2–3 cm depth; and field inundation caused by unpredictable monsoon rains during the critical early crop emergence stage. Of the three, herbicide choice is the one entirely within a farmer's control at the point of purchase — sowing depth is a matter of equipment and technique, and monsoon timing is simply unpredictable, but which herbicide goes into the sprayer, and when, is a decision made before either of those other risks even materializes. Understanding which herbicides tolerate wet conditions safely, and which don't, directly addresses this first and most controllable of the three causes.

To mitigate the risk of herbicide toxicity, on-station trials were conducted at the Patna CSISA Research Platform in 2018 and 2019, specifically simulating wet soil conditions through 5-cm flooding for 24 hours — a deliberate stress test designed to reproduce exactly the excess-moisture scenario an unlucky rain shower creates for a real farmer's field a day or two after sowing. The results were clear-cut: herbicides such as oxadiargyl and pretilachlor (used with a safener) demonstrated a low risk of phytotoxicity under these wet, excess-moisture conditions. Pendimethalin, by contrast — a commonly used herbicide — caused significant crop damage, reducing crop stand by 45% under the same wet conditions. That's a direct, field-tested answer to a question many DSR farmers face every monsoon: which pre-emergence herbicide is actually safe to apply when the soil is wetter than planned, and running the comparison across two separate years (2018 and 2019) rather than a single season gives the result more weight than a one-off trial would.

Pre-emergence herbicides all share the same basic purpose — stopping weeds before they emerge alongside the rice seedling — but they differ enough in commercial formulation, dose and safe application window that mixing them up is a genuine risk for a farmer working from memory rather than a label. Pretilachlor 50% EC (sold as Craze, Prolax, Hifit, Sofit and Preet Plus, among others) is applied at 1.0 L/ha within 0–3 days after sowing (DAS). Pendimethalin 30% EC (Dhanutop) is applied at 2.5 L/ha in the same 0–3 DAS window — though, as the Patna trials show, only where wet-soil risk is managed. Pyrazosulfuron-ethyl 10% WP (Saathi) is applied at a considerably lower 30 g/ha, within a slightly wider 0–5 DAS window, reflecting its different mode of action and potency at label rate.

For weeds that emerge despite pre-emergence treatment, post-emergence (PoE) options include bispyribac-sodium 10% SC (Nominee Gold, Adora) at 250 ml/ha, 15–25 DAS; ethoxysulfuron 15% WG (Sunrice) at 100 g/ha, 5–10 days earlier post-emergence (EPoE); fenoxaprop-p-ethyl 6.9% EC (Rice Star, Whip Super, Legacee) at 1,000 ml/ha, 10–15 DAS (EPoE); oxadiargyl (RAFAZ) at 1–1.25 L/ha, 14–18 days post-emergence; 2,4-D ethyl ester (HEERA-44) at 2.5 L/ha, 30–40 days post-emergence; and carfentrazone-ethyl 40% DF (Affinity) at 60 g/ha, 10–15 days after sowing.

Beyond single-active herbicides, a growing range of mixed products lets farmers target a wider weed spectrum in one pass. Triafamone 20% + Ethoxysulfuron 10% (Council Activ) is applied post-emergence at 225 g/ha, 8–12 days after transplanting or sowing (DAT/DAS). Florpyrauxifen-benzyl + Cyhalofop-butyl (Novalact) and Florpyrauxifen-benzyl + Penoxsulam (Novixid) are both applied post-emergence at 1.25 L/ha, 15–20 DAT/DAS. Pretilachlor + Florpyrauxifen-benzyl (Altessia) follows earlier, at 800 ml/acre, 5–10 days after transplanting. Metsulfuron Methyl 10% + Chlorimuron Ethyl 10% (Almix) is applied post-emergence at 20 g/ha, 20–25 days after sowing, while the same active-ingredient combination sold as Chlormax can be applied either pre- or post-emergence at the same 20 g/ha rate. Pretilachlor + Bensulfuron (sold as Bentila or Londax power) is a pre-emergence option, applied at 10 kg/ha within 3–5 days after transplanting.

With this range of pre-emergence, post-emergence and mixed products now available, the practical challenge for most DSR farmers has shifted from 'is there a herbicide that controls my weeds' to 'am I applying the right one, at the right time, for my field's actual moisture conditions' — which is exactly the phytotoxicity lesson the Patna trials were designed to answer.

A correct herbicide schedule is not a replacement for the other DSR practices covered in this series — it is what makes several of them work as intended. Herbicide-tolerant varieties such as CR Dhan 807 still rely on Imazethapyr being applied within its own correct 15–17 day window; drone-based spraying still needs the right active ingredient at the right dose to avoid the phytotoxicity seen at full-strength application; and even a well-executed vattar DSR or seed-priming programme can be undone by a wet-condition-sensitive herbicide applied at the wrong moment. Getting the herbicide schedule right is less a standalone technique than the connective layer underneath nearly every other advance this series has covered — which is also why a single mistake here, like an ill-timed pendimethalin application, can erase the benefit of everything else a farmer has done correctly.