HERBICIDES in Asian rice - IRRI books - International Rice ...

One of the main difficulties associated with the use of farm chemicals is ensuring
that there is no pollution of water, for any of its many uses. Caution must be used
when applying herbicides in floodwater to avoid movement of the herbicide into
groundwater and to keep treated water from contaminating other water as it drains
from treated fields (Bayer 1991). It is highly dangerous to introduce any chemical
into waterways without first determining whether it could have any ill effects on human
and animal health. III effects may be direct, in that they affect the quality of
domestic water supplies, be distasteful to livestock or wildlife, or create a hazard for
irrigated crops. They may be indirect, in that decomposition of dead plant material
causes a problem.
The need for effective protection against water pollution is acute and urgent for
protecting the welfare of rural people living in remote villages and using water from
irrigation canals for drinking, bathing, and other household purposes (Cheam 1974).
In Malaysia, 51.3% of farmer respondents said that they had experienced symptoms
associated with pesticide poisoning. The highest incidence (24.8%) was due to herbicides,
primarily 2,4-D and paraquat. Headaches and dizziness were the most frequently
mentioned symptoms. However, only 12.8% said they had sought medical treatment
(Ho et al 1990).
Current concerns over the environmental impact of pesticides may lead to a reduction
in the number of rice herbicides available to the farmer and to social and
financial pressures to reduce herbicide use. Although there are many ways the safety
and effectiveness of existing products can be improved, concerns such as persistence,
leachability, and adverse toxicology can only be overcome through product replacement.
Industrial research aimed at discovering new products will be an important key
to meeting the challenges of crop protection (Mehta 1992).
Injudicious use of herbicides
A major concern about pesticide use in rice is injudicious and unsafe use (Rola and
Pingali 1993). In Sabah and Sarawak, Malaysia, paraquat is applied either alone or in
combination with 2,4-D to control weeds in transplanted rice. In upland rice in northern
Thailand, paraquat is applied after seeding but before crop emergence to control
emerging weeds. In Malaysia, paraquat is recommended as a preplanting treatment in
both transplanted and direct-seeded rice (N.K. Ho, 1993, Muda Agricultural Development
Authority, pers. commun.).
The recommended way to apply herbicides is with knapsack sprayers (the most
widely used method) or as granules. Farmers, however, have been known to mix liquid
herbicides with sand, fertilizer (usually urea), or a mixture of fertilizer and insecticide;
to apply tank mixtures of herbicides and insecticides; to apply liquid herbicides
with mist blowers; and to pour undiluted herbicides directly into floodwater.
Although herbicides that control weeds effectively in all rice cultures are available,
farmers need guidance on how to optimize their use. Application methods that save
labor, reduce exposure hazards, and avoid environmental pollution are needed.
Economic and technological influences on herbicide use 237