How to control weeds

Weed control is important to prevent losses in yield and production costs, and to preserve good grain quality. Specifically, weeds

• decrease yields by direct competition for sunlight, nutrients, and water
• increase production costs e.g., higher labor or input costs
• reduce grain quality and price
  For example, weed seeds in grain can cause the buyer price to be reduced.

Weed management should be practiced during specific stages of rice production:

During land preparation

Control of weeds during land preparation is crucial to reduce the amount of weed pressure in the field. Land preparation should start 3−4 weeks before planting. Plowing destroys weeds and remaining stubble from the previous crop. Weeds should be allowed to grow before the next cultivation. In addition, a level field helps retain a constant water level that controls weeds.

 For wet seeded rice

• Plow and harrow several times before planting. Depending on weed population, three or more operations can be done.
• Allow weeds to emerge for at least 2 weeks then kill by another shallow tillage. This reduces the number of weed seeds in the soil, and greatly reduces weeds for the subsequent crop.

 For dry seeded rice

• Allow weeds to emerge within 1−2 weeks, then kill them with either a non-selective herbicide or by light cultivation. 
• Spray herbicides, and perform manual and/or mechanical weeding.

In the nursery

During early crop growth

Weed control is critical after planting until the canopy closes. Control methods vary depending on the rice ecosystem and planting method:

 For transplanted rice

• Apply pre-emergence herbicide (e.g., pretilachlor or butachlor at 2−3 DAT)
• If grass weeds are the main weed problem, apply early post-emergence herbicide 

• Do not allow soil surface to dry after transplanting. Keep the soil moist to saturated. Dry soil reduces the performance of pre-emergence herbicides. 
• Maintain a 5−7 cm water depth to prevent germination of weeds until 7−10 days before harvest.
• If herbicides have not been applied, or if weeds are emerging, you may use push weeder to control weed seedlings that are at 3−4 leaf stages. Irrigate one day later to prevent buried and uprooted weeds from recovering.

• • Maintain shallow flooding 7−10 DAT, drain the field, then push the weeder down the row to bury emerged weed seedlings.
  • Leave the field saturated for 2 days to keep the buried weed seedling in the mud layer then flood the field up to 5 cm of water.

• Handweed as needed until the canopy closes.

 For wet seeded rice (broadcast or drum seeded)

• Apply pre-emergence herbicide (e.g., pretilachlor + fenclorim 2−3 DAS)
• If grass weeds are the main weed problem, apply early post-emergence herbicide 

• Do not allow soil surface to dry after seeding. Flush irrigate as needed to keep the soil moist to saturated. A dried soil surface will reduce the performance of pre-emergence herbicides. Irrigating more than 10 days after seeding encourages more weed growth and deeper water level is needed to control weeds.
• If herbicides have not been applied, or if weeds are emerging, you may use push weeder in a row-seeded crop to control weed seedlings that are at 3−4 leaf stages. Irrigate one day later to prevent buried and uprooted weeds from recovering.
• Maintain a 5−7 cm water depth to prevent germination of weeds until 7−10 days before harvest.
• Handweed as needed until the canopy closes.

 For dry seeded rice

• A weed-free field is essential for early vigor in a dry-seeded rice crop. Be sure to follow steps for weed control during land preparation to avoid yield loss to weeds.

• Apply pre-emergence herbicide (e.g., oxadiazon or pendimethalin) onto a moist soil 2−3 DAS. If the seed is sown on dry soil, flush irrigate the field first then spray the herbicide.

Pre-emergence herbicides should not be applied in standing water to avoid toxicity.

Read: Knockdown and pre-emergence herbicide in DSR (pdf) from Direct Seeded Rice (DSR) in the Eastern Gangetic Plains of India

• Handweed as needed until the canopy closes.