Proper weed management

Weeds can reduce rice yields by competing for moisture, nutrients and light. Delays in weeding can cost a great deal to the farmer in terms of reduced yields. This is normally the case where farmers establish large fields of rice at the same time and lack enough labour to weed them. Weed control is necessary to prevent yield loss, maintain purity of harvested grain and prevent clogging of irrigation channels. Use of synthetic herbicides to control weeds is not allowed in organic farming because of their negative residual effect in the ecosystem.

Generally weed management in both upland and lowland systems is targeted on the proper control of weed multiplication. This means any intervention to control weeds should be carried out before the weeds produce seeds.

Keeping the fields covered with green manure cover crops, whenever there is no rice crop, helps to reduce weeds. Well-managed green manure legumes planted during the season of no rice crop also provides good opportunities for controlling weeds. Legumes like Stylosanthes guianensis, Canavalia ensiformis, or Mucuna spp grow aggressively and establish a thick biomass which kills most weeds. This biomass, combined with rice straws left in the field after harvesting, will provide good mulching material to protect the soil and later increase soil organic matter when ploughed or dug into the soil during land preparation.

There are other agronomic methods mentioned below that can be used in controlling weeds in rainfed and irrigated lowland systems.

Discussion: Weed management in rice fields

Ask the participants whether they experience any major weed problems. List the local names of major weed species and identify the local weed control strategies that are being used.

Weed management in rainfed systems

Timely land preparation - A reduction in weeks is achieved by waiting for moist conditions so that any weed remnants from last season germinate. They should then be dug or ploughed under before planting begins.
Establishment of adequate plant population - Sowing/planting rice seeds/seedlings close enough so that adequate plant population is maintained with little or no space left for weeds to grow.
Selection of suitable rice varieties - Rice varieties such as NERICA can compete effectively with weeds compared to others. NERICA varieties possess early vigour during the vegetative growth phase and this is a potentially useful trait for weed competitiveness.
Timing of weeding - Manual weeding, hand pulling or using a hand hoe is a common, efficient method of weed control, especially if timed when weed pressure is still minimal in the field. For example, one weeding within 15 to 21 days after sowing, followed by a second weeding at panicle initiation stage (about 42 to 50 days after sowing) is sufficient for upland rice to grow well. To manage this weeding routine, farmers should scatter the planting of rice fields in a mosaic pattern to avoid being overwhelmed by the weeding requirements.

Weed management in irrigated lowland systems

Water management - With good water management, especially in flooded systems, many weed species can be well-managed since they will not germinate or survive under flooded conditions. But flooding will be more effective if fields are levelled well. Levelling can be done by removal and deposition of surface soil from high- to low-lying areas mechanically or manually with rakes. It makes the water depth uniform and facilitates rapid flow into and from the field. A continuous water level of 2-5 cm can be maintained for some time to kill weeds under the flooded conditions.

Timing of weeding - Although manual weeding can be very labour intensive in flooded rice systems, it is also effective, especially when it is done at the right time. For example, farmers may start weeding either with a hoe or a mechanical rotary weeder (if available) about 10 to 12 days after transplanting, followed with hand weeding immediately or within one week, and complete the weeding sessions within 20 to 30 days after transplanting. The weeds should then be buried in between the plants to rot and provide additional nutrients. In case of very hardy grasses such as Cyperus spp, which cannot be suppressed this way, they should be left to dry in the sun, and when dead, buried or composted.