Paraquat for rice
In Asia alone, more than two billion people obtain over 60% of their calories from rice. It is the most rapidly growing source of food in Africa and is critical to food security. Long grain rices are typically of the indica race and include the fragrant Jasmine rice from Thailand and Basmati rice from India. Short grain rice, typically japonica, is usually more sticky than long grain and is favoured in Japan. Saki rice is grown in Japan to make rice wine, and in Indonesia there are red and black grained varieties. About 80% of the world's rice is grown by smallholders in these places. In Asia, women are often responsible for rice farming as men have moved to work in the cities.
Efficient and productive rice-based production systems are essential for economic development and for improved quality of life for much of the world's population. Plant breeding, crop protection, water management and fertilization have increased productivity and reduced the costs of production.
Weeds can reduce the yield and quality of rice by competing with the crop for light, nutrients and space; and their seeds can contaminate the harvested grain. Some of the most widespread and aggressive weeds are the Echinochloa species (barnyardgrass). Infestations of only ten of these weeds per square metre have been recorded to reduce yields by 25%. Also difficult to control is red rice which is the same species as commercial varieties. Cyperus rotundus is a sedge which has been called ‘the world’s worst weed’.
Rice fact file
- 1 billion people have jobs associated with rice
- 685 million tonnes were harvested in 2009
- 85% of rice is grown in paddies
- 4 crops per year can be grown in some areas
- 2 main types of growing system: flooded (paddy) and dry (upland)
Using paraquat for weed control enables savings in time, cost and labor by reducing or eliminating the need for plowing in preparing fields for rice.
As a broad spectrum herbicide, paraquat allowed the development of no-tillage (‘no-till’) systems. These systems do not rely on plow to control weeds. Not disturbing the soil helps prevent erosion and maintains a healthy soil. Recent work suggests that the production of methane (a greenhouse gas more damaging than carbon dioxide) from plant material decaying in anaerobic conditions after burial is significantly reduced under no-till.
Controlling weeds can cost a lot of money, time and effort. Competitive weeds must not be permitted to set seed as this will contaminate the land for years to come. Apart from directly affecting the crop, weeds can block irrigation channels making water management difficult. Allowing weeds to grow unchecked in and around rice fields (ie on paddy bunds) can attract insect pests and rodents, and act as hosts for diseases. However, a managed vegetative cover is important to give structure and stability to the soil, helping to prevent erosion, and providing a habitat for beneficial flora and fauna.
Herbicides like paraquat and glyphosate have no residual activity in the soil and do not affect the rice crop. Glyphosate gives good control of perennial weeds, but its intensive use can lead to adverse changes in the weed flora towards more aggressive broadleaved weeds, and even cases of weed resistance to glyphosate are arising. Paraquat can provide the alternative means of effective and sustainable weed control.