Guidelines for weed management in paddy fields

Weeds compete for nutrients, light and are shelters for pests in paddy fields. Instructions on managing grass with clean seeds, preparing the soil thoroughly, leveling the field, watering early, weeding at the right time and using herbicides correctly.

Summary: Weeds in paddy fields not only reduce productivity because of competition for fertilizer, water and light. Grass is also a shelter for pests, rats and secondary hosts of many pests. Effective grass management must start with clean seeds, flat land, water management and only using herbicides at the right time and in the right group.

Applies to: paddy fields sown, hand transplanted, machine transplanted in the Mekong Delta and other rice growing areas.

Duration: From before sowing to after sowing 30-40 days; Monitor remaining grass until the end of the season to prevent seeding.

Difficulty level: Basic to moderate.

Estimated additional costs: 150-500 thousand VND per acre depending on weed level, treatment used and weeding effort.

Why must we manage grass early

Weeds compete with rice in four aspects:

• Light.
• Nutrition.
• Water.
• Growth space.

If the grass grows strongly in the first 20-40 days after sowing, the rice will have poor tillering and the field will be uneven. At the end of the season, the grass also produces seeds and increases grass pressure for the next crop.

Common grass group

Hoa Ban Grass

This group resembles rice plants, difficult to distinguish when young. Common examples:

• Abyss grass.
• Phoenix tail grass.
• Weedy rice, wild rice.

Hoa Ban grass is very competitive and difficult to control if allowed to grow large.

Grass

Commonly found in wet, low-lying fields:

• Strabismus
• Chac.
• Energy grass.

This group can grow strongly when the field surface is not flat and water management is uneven.

Broadleaf grass

Usually easier to recognize:

• Lettuce.
• Vegetables.
• Some species of aquatic grasses.

Each group of grasses has different treatment and treatment time. Wrong identification can easily lead to ineffective use of medication.

Grass prevention from seed stage

• Use certified rice varieties.
• Do not use seeds mixed with grass seeds or weedy rice.
• Sifting and cleaning seeds before soaking.
• Do not take seeds from fields with a lot of weedy rice.

If the seed has mixed grass seeds, soil preparation and herbicides will be difficult to completely save.

Prepare the land and level the fields

Careful soil preparation is the foundation for grass management:

• Plow and harrow and beat the mud thoroughly.
• Dry the soil if you have time.
• Leveling the field.
• Do not let high places dry and low places get water.

Flat field surface helps:

• Herbicides are evenly distributed.
• Grass mulch is better.
• Rice grows evenly, competes better with grass.

In uneven fields, there are often places where the treatment is too diluted and places where the treatment is too strong, which is both ineffective and can easily damage the rice.

Managing water to suppress weeds

After sowing, proper water management helps limit weeds:

• Once the rice has taken root, apply water to the field as early as appropriate.
• Maintain a shallow water level to smother the grass but do not suffocate the young rice.
• With sparsely sown fields, stable water helps rice compete with grass better.
• Do not let the field dry for too long because the grass grows quickly.

Water is the cheapest "weed treatment" if the field surface is flat.

Manual weeding at the right time

Combined with weeding when transplanting or checking fields:

• The period 15-18 days after sowing is a good time to pull out remaining weeds.
• Pluck early when the grass is still small and the roots are not deep.
• Collect grass from the field, do not bury grass with seeds.
• Cut the remaining grass flowers before they go to seed.

If the grass is allowed to bloom and drop seeds, the pressure on the grass will increase significantly in the following season.

Use herbal treatment properly

Pre-germination treatment

Usually used after early sowing, when the grass has not yet grown or has just sprouted:

• Active ingredient group such as Pretilachlor, Butachlor or suitable product according to the label.
• Need flat field surface.
• Needs enough moisture or water level according to instructions.
• Do not spray when the field is dry and cracked.

Pre-emergent treatment is most effective when the soil is well prepared.

Post-germination treatment

Use when the grass has grown:

• Active ingredients may include Pyrazosulfuron-ethyl, Cyhalofop-butyl or other groups depending on the type of grass.
• Must identify the herb group before choosing treatment.
• Use when the grass is still small.
• Follow the correct dose and time according to the label.

There is no one treatment that is good for all types of weeds in all stages.

Alternate mechanism of action

If a group of herbicides is used for many consecutive crops, the grass may become less sensitive or resistant. Should:

• Rotate active ingredient groups.
• Combining soil preparation, watering and weeding.
• Do not increase the dose arbitrarily when you find the drug is less effective.

Monitor grass during crop

• [ ] Proportion of grass growing 7 days after sowing.
• [ ] Main grass groups: Hoa Ban, Trac Lac, broad-leaved.
• [ ] Is the field surface flat?
• [ ] Can the water be kept evenly?
• [ ] Will the remaining grass bloom?
• [ ] Herbicides used, dose, time.

Early monitoring helps treat when the grass is still small, saving time and treatment.

Common mistakes

Using seeds mixed with grass: putting grass seeds into the field right from the beginning.

Uneven soil preparation: herbicide and weed suppressant are less effective.

Allow the grass to grow before spraying: many post-emergence chemicals lose their effectiveness as the grass ages.

Failure to identify herb group: wrong drug chosen.

Using the same drug repeatedly: increases the risk of drug resistance.

Let the grass bloom at the end of the season: self-sow grass seeds for the next crop.

Take notes

• Date of land preparation and leveling level.
• The variety used, does it contain grass seeds?
• Herbicides used: pre-emergence/post-emergence.
• The grass group appears the most.
• Grass survival rate after 15-18 days.
• Heavy grass area for early treatment of next crop.

Recording helps detect which fields are prone to resistant weeds or unsatisfactory fields.

References

• *Guidelines for weed management in paddy fields* — Hainong technical document.
• *Integrated pest management for rice* — Plant Protection Department.
• *Technical guide to land preparation for rice cultivation* — Mekong Delta Rice Institute.

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• Technical instructions for preparing land for rice cultivation
• Process of soaking and fermenting winter-spring rice seeds
• Instructions on current rice sowing techniques
• Rice price tracking and 30-day forecast