Pest, Weeds, Agri-Crop

      Weeds are plants that grow in places where they are not wanted. Weeds reduce rice yields  by competing with rice plants for sunlight, moisture, space, and soil nutrients. They also serve as alternate hosts of insect pests and diseases. They are more serious in upland and direct seeded wet land than in transplanted irrigated rice paddies.

The types of weeds commonly found in rice fields are:

1.   Grasses. These are monocotyledonous plants which have long narrow leaves, usually flat leaves with parallel veins and round hollow stem.

2.   Sedges. These are weeds with triangular stem, long narrow leaves and modified rhizomes for storage and propagation.

3.   Broadleaves. These are dicotyledonous plants with netted veined leaves.

Methods of Weed Control

          In all cases, all kinds of weeds must be controlled before they could compete with the plants. It is therefore more economical to make use of the cultural and biological methods before resorting to  chemicals.

1. Physical/mechanical method. Weeds can be controlled by means of:
1. hand pulling or hand weeding
2. use of mechanical weeders or rotary weeder possible when transplanted, using the straight row planting and mechanical seeder in direct row planting
3. efficient use of irrigation water which prevents the emergence of all kinds of weeds
2. Cultural method. This involves good land preparation and flooding during land preparation to remove weed seeds, closer crop spacing,  and use of varieties that are taller and will produce more tillers early in the season.
3. Chemical Control. Herbicides may be used to kill weeds or suppress their growth.

          Chemical control is cheaper than hand weeding and can be used in all rice environments. However, the continued use of the same herbicides leads to build-up of perennial weeds which are difficult to control with herbicide. This can be prevented by hand or mechanical weeding periodically. Herbicides are applied either pre-emergence (before weeds germinate) or post-emergence (after weeds have germinated) in granular or sprayable form.

          Pre-emergence application of granular or sprayable herbicides controls most annual weeds. This is applied 3-5 DAT in a water depth of 3-5 centimeters.

          Post-emergence herbicides control annual broadleaf weeds and sedges. This is applied 20-25 DAT. Drain water from the paddy before spraying. Reflood the paddy the following day to a depth of 5-7 cm. to suppress late germinating weeds.

         In spraying herbicides always follow the recommended rate of application  so as not to incur damage to the growing plant.

         

4. Preventive Method.  Before sowing the seeds make use of big container in soaking to allow the weed seeds to float and discard. During the land preparation for the seed bed just after leveling the bed, flood and allow the water to flow out to discard the floating seeds. Use  certified seeds, wire mesh in canal, and remove weeds before seeds mature.

Pointers for Effective Herbicide Application

1. Apply herbicides at the right time.
1. Pre-planting – apply herbicide after land preparation before the crop is planted.
2. Pre-emergence – apply herbicide before the emergence of the crop or weeds.
3. Post-emergence – herbicide is applied after emergence of the crop or weeds.
2. Use the right dosage of  chemicals. Weak or strong dosage of chemicals may not be useful to the plant; therefore, follow the recommended dosage.
3. Spray during fair weather. Do not spray when the wind is strong nor during rainy weather. Spray early in the morning or late in the afternoon.
4. Distribute spray solution accurately.

Preparing Herbicide Sprays

          All granular herbicides can be used directly but herbicides in the form of wettable powder (WP), as well as emulsifiable  concentrates (EC) should be mixed with water or other diluting liquids such as oil before using. If you mix them properly, your herbicide will work effectively. If any common container is used in the preparation, label it properly and keep it away from children.

1. Prepare the required amount of herbicide. Remember that too much herbicide may injure the crop and too little herbicide may not control weeds effectively.
2. Pour ¼ of the required water into the container. Your container should contain more than what the sprayer tank can contain. Never apply the herbicide ahead of the water because the wettable powder tends to float and many emulsifiable concentrates are acidic.
3. Add the herbicide to the water. If you are using wettable powder, thoroughly mix it with small amount of water before pouring into the water container. This makes the dispersion of the powder in a large amount of water easier.
4. Mix the herbicide in the water container using a bamboo stirrer, not your hand.
5. Add the remaining water to complete the dilution.