Integrated management of weeds in direct-seeded rice in Cambodia

Weeds are a bigger problem for dry direct seeding rice (DDSR) compared to transplanting because they germinate and emerge at the same time as the rice and are therefore more competitive under DDSR conditions. Weed competition in direct-seeded rice systems can result in rice yield losses of up to 50% and there is considerable scope to reduce the yield gap in direct-seeded rice systems with integrated weed management strategies.

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Agriculture accounts for 22% of Cambodia’s Gross Domestic Product (GDP), and rice is by far the most important commodity, making up around half of the agricultural GDP. The total area of agricultural production in Cambodia was 4 million hectares (ha) in 2016 and of this, there was 3 million ha (75%) of rice. Cambodia produced 10.7 million tons of paddy in 2018 and a domestic surplus of 5.6 million tons (54%)

In recent years, there has been significant out-migration of labor from rural areas in Cambodia and this has resulted in rapid mechanization to maintain production and productivity. In addition, because of the labor shortage, transplanting rice has been replaced by hand broadcasting at seeding rates of 80–300 kg per ha in Battambang Province.

Eighty-eight percent of farmers keep their own seeds for sowing and these seeds contain an average of 482 weed seeds per kilogram. Only 18% of rice farmers now practice hand-weeding and 100% of farmers use herbicides. Farmers use high seeding rates to compensate for poor seed quality, and poor crop emergence, and to compensate for granivory by rodents, birds, and insects.

The use of high seeding rates also suppresses weed competition but this seed is likely to be heavily contaminated with weed seeds, thus potentially exacerbating the weed problem. The use of high seeding rates also suppresses weed competition but this seed is likely to be heavily contaminated with weed seeds, thus potentially exacerbating the weed problem. Significant shifts in weed species’ compositions have occurred after the shift from transplanting to direct-seeded rice in other Asian countries.

In areas of Cambodia where direct seeding has been practiced for a number of years, grass weeds such as Echinochloa spp. and Leptochloa chinensis have become more prevalent and are proving difficult to control in direct-seeded rice.

Direct seeding is also associated with reduced access to water management for weed control. Aquatic weeds such as Monochoria vaginalis and Marsilea minuta were more prevalent in transplanted rice systems. Submerged and floating weed species have become less prevalent with the shift to direct seeding of rice.

Weeds are a bigger problem for dry direct-seeded rice (DDSR) compared to transplanting because they germinate and emerge at the same time as the rice and are therefore more competitive under DDSR conditions.

Cambodian farmers rely primarily on post-emergence herbicides for weed control and have not adopted pre-emergence herbicides because these usually require machine planting to enable the placement of the rice seed below the herbicide layer. The use of a seed drill opens up the opportunity to use post-sowing pre-emergence herbicides for DDSR.

Farmer interviews suggest that there is a lack of farmer knowledge on the use of correct herbicides, application rates, time of application, and correct method of application of herbicides. The majority of farmers (75%) rely on the advice of input dealers for the choice of herbicide. Other sources of information for the surveyed farmers are other farmers (46%), chemical companies (8%), and the herbicide label (19%).

The objectives of this work were to:

Determine the costs and benefits of establishment methods and seeding rates for weed management in rice and to provide recommendations for best-practice rice seeding methods and seeding rates.

Evaluate pre-emergence herbicides in drill-planted dry direct-seeded rice as an alternative to high seeding rates to improve weed suppression in rice.

Outline an integrated weed management (IWM) strategy for DDSR applicable to rice production systems in North-West Cambodia.

Integrated Pest Management (IPM) has been successfully demonstrated for rice in Cambodia since 1993. However, its dissemination and sustained adoption among farmers have not met similar success. There is evidence that mutual socio-technical dependencies predispose farmers to be locked into pesticide use in preference to the adoption of IPM.

In the case of weed management in rice, farmers rely predominantly on input sellers for advice and there is a strong bias for dependence on post-emergence herbicides despite the options available for the integrated management of weeds in direct-seeded rice include the stale seedbed technique; crop residue mulching; reduced or no-tillage; use of weed-free crop seed; high seeding rates; improved herbicide application and timing; machinery hygiene.

Therefore, biotic and abiotic factors other than the applied treatments are likely to have been limiting. It is possible that, under higher-yielding conditions, higher seeding rates could give economic returns for suppressing weeds.

However, when the cost of seed was taken into account, increasing the seeding rate above 100 kg/ha in a weed-free crop could not be justified. Similarly, increasing the seeding rate to 200 kg/ha could only be marginally justified in a weedy crop. However, the net benefit for the non-weedy treatment at 150 kg/ ha was USD 733/ha compared to the weedy treatment at 200 kg/ha (USD 644) giving a shortfall of USD 89.

This suggests that the money spent on extra seed might be better spent on alternative weed control methods, including herbicide use.

After a marginal analysis of costs and returns, this study found that all broadcast seeding rates were dominated both with and without weeds and that the drum seeder at 80 kg/ha was the most cost-effective treatment with and without weeds in a wet-seeded direct seeding situation.

This study suggests that increasing the seeding rate alone is not a cost-effective method to reduce paddy yield losses from weeds and that an integrated approach to weed management should be adopted.  Study results are consistent with the need to integrate herbicide use with other weed management strategies, such as increased seeding rates.

Furthermore, hand-weeding at the crop flowering stage will reduce the recharge of the weed seed bank and weed seed contamination of the harvested paddy.

Self-sown rice is a significant problem that could be reduced by the stale seedbed technique, reduced tillage, and crop residue mulching. In North-West Cambodia, there is an exploitable yield gap of 1.3 tons/ha in wet-season direct-seeded rice.

Weed competition in direct-seeded rice systems can result in rice yield losses of up to 50% and there is considerable scope to reduce the yield gap in direct-seeded rice systems with integrated weed management (IWM) strategies.

We propose an IWM strategy for direct-seeded rice (DSR) in North-West Cambodia. The elements of the strategy are:

Machinery, especially combine harvesters, should be cleaned before movement from field to field and farm to farm. Machines should be adjusted, if possible, to reduce the return of rice seed to the field.

The stale seedbed technique keeps self-sown rice and weed seeds on the soil surface where numbers can be more easily reduced by germination after rain or eaten by granivores. Emerged weeds are controlled using pre-sowing non-selective herbicides before sowing.

Crop residue mulching reduces weed emergence and keeps weeds at the soil surface where they can be more easily controlled.

Avoiding inversion tillage (disc ploughing) reduces the chance of burying weed seeds which acquire dark-induced dormancy. Subsequent cultivation brings buried seeds to the surface where they can germinate and emerge with the crop. Rotary cultivation can reduce the number of dormant weed seeds being brought to the soil surface.

Seed kept for sowing should be cleaned to remove weed seed contaminants including those of weedy rice (Oryza sativa f. spontanea).

Seeding rates can be reduced with drum and drill seeding and this enables the weed-free seed to be purchased from reputable seed producers.

For DDSR, our work suggests that pre-emergence herbicides can improve weed control in combination with post-emergence herbicides (if required) and optimal seeding rates.

Tall weeds such as Echinochloa crus galli and weedy rice can be hand-rogued as the rice begins to flower. These weeds mature around one week earlier than short-duration rice and this practice reduces the contamination of the paddy sample and reduces the recharge of the weed seed bank.

Read the full study:
Martin R, Som B, Janiya J, Rien R, Yous S, Chhun S, and Korn C. (2020) Integrated Management of Weeds in Direct-Seeded Rice in Cambodia. Agronomy 2020, 10, 1557.