Technical Guidance on Chemical Weed Control in Paddy Fields

Chemical weed control in paddy fields is a key step in achieving high and stable rice yields. There are three main types of weeds in the rice fields of our county: grassy weeds such as barnyardgrass, sprangletop, and crabgrass; broadleaf weeds such as monochoria, pondweed, and Rotala indica; and sedge family weeds. In recent years, because rice planting methods in our county have become diverse, the weed spectrum in paddy fields has undergone considerable change: first, some malignant weeds in paddy fields are on the rise. In addition to barnyardgrass, which remains widespread and abundant, sprangletop and Cyperus serotinus have shown a clear upward trend; second, weed occurrence and damage in the mid-to-late rice season have intensified, with an obvious second weed emergence peak during field drying, mostly broadleaf weeds such as monochoria, Sagittaria pygmaea, and Lindernia procumbens. Each township should adopt corresponding control measures according to different cultivation methods and field weed spectra, grasp the optimal application timing, and effectively carry out weed control in paddy fields.

I. Chemical weed control in transplanted fields

1. Hand-transplanted fields: 5-7 days after transplanting, once the seedlings have established, combine the application of green-turning fertilizer and dimehypo for controlling the striped stem borer, and apply 30 g per mu of 20% acetochlor-bensulfuron wettable powder, maintaining a shallow water layer for 5-7 days after application.

2. Machine-transplanted fields: 3-5 days after machine transplanting, apply 100 g per mu of 30% bensulfuron-pretilachlor wettable powder by the medicine-soil or medicine-fertilizer method, maintaining water for 5-7 days after application.

3. Seedling-throwing fields: 3-7 days after throwing, combine with flooding to establish the seedlings, and apply 100 g per mu of 30% pretilachlor-bensulfuron wettable powder or 120-150 g of 30% butachlor-bensulfuron wettable powder by the medicine-soil or medicine-fertilizer method, maintaining water for 5-7 days after application. If supplementary treatment is needed, around the 3-leaf stage of barnyardgrass, spray 50 g per mu of 36% quinclorac-bensulfuron wettable powder without a water layer, then flood 1-2 days after application and maintain water for 3-5 days. A benzoic acid mixture (110 mL of MCPA aqueous solution + 100-150 mL per mu of 48% bentazon aqueous solution) diluted in 30 kg of water can also be sprayed to control residual sedges and broadleaf weeds.

II. Chemical weed control in direct-seeded fields

The weed control technology for direct-seeded rice fields adopts the principle of "one pre-emergence sealing, two post-emergence kills, three supplementary treatments." "One sealing" refers to pre-emergence treatment; "two kills" is a supplement to the pre-emergence treatment, using post-emergence stem and leaf treatment; "three supplements" means that, on the basis of the first two steps, whether to apply herbicide is decided according to the occurrence and species of weeds.

(I) Pre-emergence treatment: About 2-3 days after rice sowing and irrigation, once the seedbed surface is exposed above the water and there is no standing water on the surface, spray with pretilachlor-bensulfuron herbicides such as 30% Zhiboning at 100-120 g. After application, keep water in the furrows and the paddy field moist, and promptly open level-water gaps to avoid waterlogging of seedlings during heavy rain, which could cause herbicide damage.

(II) Post-emergence sealing and killing: For fields where the early weed control effect is unsatisfactory and weeds are abundant, supplementary control can be carried out at the 3-leaf stage of the seedlings. For fields with mixed grassy weeds, broadleaf weeds, and sedge family weeds, the application timing must be strictly controlled after rice reaches the 3-leaf 1-heart stage. At the same time, drain the standing water from the field and apply 80-100 mL per mu of 10% metamifop emulsifiable concentrate (Hanqiuhao) at the 3-5 leaf stage of weeds, diluted in 30-45 kg of water, and spray evenly with a manual sprayer. Irrigate two days after application, maintain a 3-5 cm water layer for more than 5 days, and then proceed with normal field management. Alternatively, spray evenly with 60 mL per mu of 2.5% Daoxie (penoxsulam suspension concentrate) or quinclorac-bensulfuron herbicides, then re-flood after 24 hours and maintain a shallow water layer for 2-3 days. For fields with mixed broadleaf weeds and sedge family weeds, apply 20 g per mu of 10% pyrazosulfuron-ethyl or 20 g of 10% bensulfuron-methyl wettable powder, broadcast with fine soil or fertilizer, and maintain a shallow water layer for 3-5 days. For fields with mixed sprangletop and barnyardgrass, at the 3-5 leaf stage of rice seedlings (4-5 leaf stage of weeds), spray 60-70 mL per mu of 10% Qianjin. Drain the water before application, re-flood one day after application, and maintain a shallow water layer for 3-5 days. For fields with mixed sprangletop and broadleaf weeds, either use 40-60 mL per mu of 2.5% penoxsulam oil suspension concentrate alone, or one week after applying Qianjin, use 20 g per mu of 10% bensulfuron-methyl wettable powder, both diluted in 30 kg of water for spraying.

(III) Supplementary treatment: Based on the effect after "one sealing and two kills," mainly to control monochoria and sedge family weeds, apply 20 g per mu of 10% bensulfuron-methyl wettable powder mixed with fertilizer or sand and broadcast. For perennial rhizomatous malignant weeds that reproduce by rootstocks, carry out manual pulling.

III. Precautions

1. Fields must be level, and the water layer must never submerge the rice heart leaf to prevent herbicide damage; if the field leaks water, replenish it promptly; the dosage must be accurate, and the medicine must be mixed evenly; when preparing medicine-soil (fertilizer), first mix the powder evenly with a small amount of fine soil, then add it to the fine soil (or fertilizer) and mix, ensuring the medicine-soil or (medicine-fertilizer) is mixed evenly; appropriately increase or decrease the dosage according to the size of the weeds in the field.