Herbicide-Tolerant Rice: India's Home-Grown Answer to the Weed Problem

Weed management remains one of the most critical challenges in direct-seeded rice. Unlike transplanted rice, where standing water naturally suppresses weed growth, DSR relies on aerobic soil conditions that are far more conducive to weed proliferation. The shift to DSR has brought a proliferation of difficult-to-control weeds — including Ischaemum rugosum and Leptochloa chinensis — alongside the emergence of herbicide-resistant weed species and the spread of weedy rice (Oryza sativa f. spontanea), all of which threaten the sustainability and productivity of DSR systems.

Herbicide-tolerant (HT) rice varieties, bred to withstand a specific herbicide without damage, address this directly: they let farmers spray a targeted herbicide that kills weeds — including weedy rice, which looks nearly identical to the crop and can't be removed selectively any other way — without harming the crop itself. By facilitating the use of targeted herbicides, HT varieties provide a practical, effective solution that encourages DSR adoption specifically in regions where weed pressure has been the main constraint holding farmers back.

India's HT rice development began at ICAR-National Institute for Plant Biotechnology (NIPB) with an EMS-induced mutant of the variety N22, created in 2014. In 2017, a herbicide-tolerant mutant line from this population — 'HTM-N22', later named 'Robin' — was identified, showing tolerance to the herbicide Imazethapyr due to a mutation in the Acetohydroxy Acid Synthase (AHAS) gene, without major phytotoxic effects.

Robin became the donor line for a family of HT varieties bred for India's diverse agro-climatic conditions. IARI released herbicide-tolerant basmati varieties Pusa Basmati 1979 and Pusa Basmati 1985 in 2021 — India's first non-GM HT basmati varieties, developed by backcrossing popular varieties Pusa 1121 and Pusa 1509 with Robin. Pusa Basmati 1979 matures in 140–145 days and yields 4.5–5.0 t/ha, carrying the mu-Ali gene that lets farmers spray Imazethapyr without damaging the crop. Pusa Basmati 1985 matures faster, in 115–120 days, saving 3–4 irrigations and allowing timely sowing of the following rabi crop.

Separately, ICAR-National Rice Research Institute (NRRI), Cuttack, transferred the herbicide-tolerant trait from Robin to four other popular varieties: Sahbhagidhan (bred for uplands), Naveen (favourable uplands and irrigated conditions), Swarna Sub1, and Pooja (both suited to shallow lowlands).

Among the varieties bred from Robin, the HT near-isogenic line of Sahbhagidhan was released as CR Dhan 807 — India's first non-GM herbicide-tolerant variety in the non-basmati category, developed through marker-assisted backcross breeding that retained over 95% of Sahbhagidhan's genome. It matures in 125–130 days, yields 5.0–5.5 t/ha, is highly suitable for rainfed conditions, and features high grain weight with resilience against drought. CR Dhan 807 has been notified for cultivation in six states — Jharkhand, Odisha, Chhattisgarh, Gujarat, Andhra Pradesh, and Tamil Nadu — for rainfed direct-seeded upland conditions and zero-tillage resource conservation practices. A related near-isogenic line, CR Dhan 812, goes further still — an 8.0 t/ha, 140–145 day variety that combines herbicide tolerance with resistance to major pests including stem borers and leaf folders.

All HT rice varieties bred from the Robin mutant are marketed under the trademark RobiNOweed. For effective weed control under this system, Imazethapyr is applied at 100 g a.i./ha (2.5 ml of the 10% SL commercial formulation per litre of water) as a foliar spray, ideally within 15–17 days after seedling emergence or at the 2–3 leaf stage of weeds, with adequate soil moisture but no standing water — irrigation should be avoided for at least 48 hours after application. Beyond the ICAR-bred non-basmati and basmati lines, India's HT rice options now also include hybrids: SAVA 134, a Bacterial Leaf Blight-tolerant 'smart rice' hybrid with strong lodging resistance, and SAVA 127, a high-yielding (7.0–8.5 t/ha) hybrid popular in Punjab and Haryana for its early-to-medium maturity and tolerance to Blast and Bacterial Leaf Blight.

Globally, herbicide-tolerant rice has been developed through both transgenic (GM) and non-GM routes. The Liberty Link and Roundup Ready systems, developed by Aventis and Monsanto respectively, use transgenic methods and are used primarily in the United States; Liberty Link varieties such as LLRICE06 and LLRICE62 carry the pat gene derived from Streptomyces hygroscopicus and were approved for food and feed use in the US in 2000, Canada in 2006, and later Mexico, Russia, Australia, New Zealand and Honduras. In the non-GM category, Clearfield rice — developed jointly by Louisiana State University Agricultural Center and American Cyanamid-BASF through induced mutagenesis targeting rice's own ALS gene — was commercialized in the US in 2002 and later adopted widely in Malaysia and Vietnam. A separate transgenic system, Provisia rice, developed by Louisiana State University Agricultural Center and tolerant to the herbicide quizalofop, saw its first variety (PVL01) approved for commercial cultivation in 2017. In China, six Jietian rice varieties carrying a comparable resistance mutation were released through chemical mutagenesis using ethyl methane sulfonate, giving tolerance to imazethapyr.

India's Robin-derived varieties follow the same non-GM, mutation-breeding route as Clearfield rather than genetic engineering — a distinction that matters enormously in a country where GM crop cultivation faces strict regulatory and political constraints. There is also a structural reason India had to build its own lineage rather than license an existing one: virtually all GM and non-GM herbicide-tolerant genetic stocks developed globally are protected by patents and intellectual property rights, which restricts their availability to public-sector research institutes in countries like India and limits their use in national rice-breeding programmes. By achieving herbicide tolerance through induced mutation and conventional backcrossing on India's own popular varieties rather than importing patented transgenics, India's HT rice varieties can move through regulatory approval and into farmers' fields without either the GM debate or the IP licensing constraints that would otherwise stall them entirely.