A push for direct-seeded rice and zero-tillage wheat, using a new non-GM herbicide-tolerance technology, has the potential to reduce the environmental footprint of India’s two major cereal crops.

A farmer in his direct-seeded rice field in Haryana’s Jind district. (Express Photo by Harish Damodaran)

Cultivating rice using less water and not burning the leftover straw after harvesting, and wheat without any ploughing and land preparation, has been a holy grail for agricultural scientists and policymakers concerned over the ecological footprint of the two cereal grain crops.

But there’s been some breakthrough of late, with the breeding of varieties/hybrids that can “tolerate” the application of a herbicide – Imazethapyr – to control weeds and grasses that affect the growth of crop plants by competing with them for nutrients, water and sunlight.

Such breeding efforts are happening both in the public and private sector.

The current kharif season has seen the commercial planting of two basmati varieties (Pusa Basmati 1979 and Pusa Basmati 1985) and two non-basmati rice hybrids (Sava 134 and Sava 127), developed by the Indian Agricultural Research Institute (IARI) and Savannah Seeds Pvt. Ltd respectively. These contain a mutated acetolactate synthase (ALS) gene that enables farmers to spray Imazethapyr for controlling weeds in rice, such as Echinochloa colona (commonly called jangli chawal), Cyperus rotundus (motha) and Trianthema portulacastrum (patthar-chatta).

In the coming rabi (spring-winter) season, Mahyco Pvt. Ltd is expected to launch its wheat varieties, Goal and Mukut, which are also amenable to Imazethapyr application for controlling Phalaris minor (gulli danda), Chenopodium album (bathua) and other such major weeds. The Jalna (Maharashtra)-based company and Savannah Seeds – the South Asian subsidiary of the Alvin (Texas)-headquartered RiceTec Inc. – have even formed a joint venture to take their Imazethapyr-tolerant ‘FullPage’ direct seeded rice (DSR) and ‘FreeHit’ zero-tillage (ZT) wheat technologies to farmers for making this cropping system “more climate-smart and sustainable”.

Weed control now

Farmers flood their paddy fields and do 3-4 ploughings for wheat mainly to control weeds.

Normal rice cultivation involves preparing nurseries, where the paddy seeds are raised into young plants that are uprooted and re-planted around 30 days later in the main field. The field in which the seedlings are transplanted is first “puddled” or tilled in standing water to break up the clods and churn the soil to make it soft.

For the first 2-3 weeks after transplanting, the field has to be irrigated every 1-2 days to maintain a water depth of 4-5 cm. This is necessary to prevent weed growth during the crop’s early stage. Water acts like a natural herbicide, preventing the weed seeds from germinating and killing the already emerged seedlings. Farmers continue giving water once a week in the remaining 105-110 days – out of the crop’s total 155-160 days duration from seed to grain.

In all, the traditional puddling-cum-transplantation route requires up to 30 irrigations, each consuming over 200,000 litres of water per acre. Puddling alone consumes water equivalent to three irrigations. Besides, there is the labour cost of transplanting, at Rs 4,000-5,000/acre.

In wheat, farmers not only burn the stubble from the previously-harvested paddy crop. They also plough the field – initially twice using a harrow or cultivator, followed by an irrigation and either one more ploughing with a rotavator or two with harrow/cultivator. All this, before sowing the wheat seeds, is primarily for weed management.

Herbicide-tolerant solutions

DSR and ZT wheat basically replace water and repeated field ploughings with a chemical herbicide (Imazethapyr) to take care of weeds.

DSR dispenses with the need for any paddy nursery, puddling, transplanting and flooding of fields. The paddy seeds can be sown directly, just like wheat. Only laser leveling of the land, once before sowing and costing Rs 1,200/acre or so, is recommended to ensure uniform placement of seeds and fertiliser as well as distribution of water. Overall, there is roughly 30% saving of water, apart from labour in transplanting and weed management, and fuel used for puddling.

Mahyco claims that ‘FreeHit’ ZT technology makes it possible to sow wheat directly – without any paddy stubble burning or even land preparation. Farmers can use a tractor-mounted Super Seeder machine with rotavator that cuts the standing stubble and mixes it in the soil. Alternatively, they can use a Happy Seeder without rotavator that only cuts and leaves the crop residue on the field. Either way, sowing is done with no tillage, saving both cost and time. Imazethapyr is to be sprayed along with Metribuzin, a selective herbicide already used in wheat, when the crop is about 25 days old.

Are these Imazethapyr-tolerant rice and wheat hybrids/varieties genetically modified (GM) crops?

No, they just have an ALS gene that has undergone mutation. This gene is already present in rice and wheat – not introduced from soil bacteria or other unrelated species, as with Monsanto’s ‘Bollgard’ Bt cotton and the still-to-be-approved GM hybrid mustard bred by scientists at Delhi University’s Centre for Genetic Manipulation of Crop Plants.

The ALS gene codes for an enzyme (protein) that helps synthesise essential amino acids for plant growth and development. Imazethapyr sprayed on normal paddy and wheat binds itself to the ALS enzymes, inhibiting their production of the amino acids. The herbicide will, then, kill the crop along with the weeds, as it cannot distinguish between the two.

The IARI, Savannah Seeds and Mahyco-bred varieties/hybrids contain a mutated ALS gene, whose DNA sequence – the order of its chemical bases or “letters” that spell out the instructions to a cell’s protein-making machinery – has been altered using a chemical mutant or radiation. As a result, the ALS enzymes no longer have binding sites for Imazethapyr and the amino acid synthesis isn’t inhibited. The plants can now “tolerate” the herbicide, which only kills the weeds.

But aren’t farmers already growing rice through DSR?

DSR cultivation at present is based on two herbicides: one “pre-emergent” (Pendimethalin, applied within 24 hours of sowing) and the other “post-emergent” (Bispyribac-sodium, after 20-25 days). But these aren’t effective against all weeds. Imazethapyr is a broad-spectrum herbicide with a wider weed-control range. It is also safer, as the ALS gene isn’t present in humans and mammals and the chemical will not bind itself to them.

IARI has distributed 200 quintals of seeds of its two Imazethapyr-tolerant basmati varieties for planting by farmers this season. At 8 kg/acre, these can cover 2,500 acres. Savannah Seeds’ rice hybrids have been planted in 15,000 acres across Punjab, Haryana, western Uttar Pradesh, Madhya Pradesh and Chhattisgarh.

The adoption of DSR and ZT – conferring benefits of reduced water and fuel consumption, plus no environmental pollution crop residue burning – is likely to be better with herbicide-tolerant technologies. And not being GM may help too.

Published Date: August 6, 2024