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

esponsible for injury to several crops. To avoid herbicide drift onto nontarget crops,
regulatory agencies in the rice states restrict the use of aerial and ground applications
of rice herbicides. In Arkansas, for example, phenoxy herbicides cannot be applied
within 6 km of cotton, virtually eliminating their use on 70% of Arkansas rice hectarage.
In 1969, use of propanil was restricted to less than 10% of the rice hectarage in California
(Fig. 7) because of long-range drift and excessive damage to deciduous orchards
(Barbe and Hillis 1969, Elmore et al 1970). Drift from herbicides other than
the phenoxys and propanil has been far less extensive, but all herbicides have the
potential to cause damage to other crops. In the southern United States, drift from
applications of propanil, quinclorac, and triclopyr have injured nontarget crops.
Various adjuvants have been used for drift control, with mixed results. Ground
applications generally drift much less than aerial applications. On small hectarages
where herbicides can be applied by backpack sprayers or ground-driven applicators,
drift can be minimized. More selective herbicides and improvements in application
technology are needed to solve the problem of rice herbicide drift and injury to sensitive
crops.
Water quality
Fish kills in the agricultural drains of California rice production areas in the 1970s
and off-tastes in potable water in 1981-82 were attributed to the herbicides molinate
and thiobencarb, respectively. The fish kills were found only in agricultural drains
downstream from rice production areas, not in the Sacramento River or its natural
tributaries. While only nongame fish species of carp Cyprinus carpio were affected,
concerns were raised immediately about the implications for highly valued anadramous
species, such as salmon Salmo salar, bass Perca fluviatilis, and shad Alasa sapidissima,
that spawn in the river systems, as well as for the health of aquatic species in general.
In Asia, where rice-fish cropping systems are common, herbicides will need to be
tested and monitored to protect these important food production systems.
At nearly the same time as the fish kills caused by molinate, a sulfoxide metabolite
of thiobencarb was implicated as causing an off-taste in the municipal drinking
water of the City of Sacramento (Cornacchia et al 1984). The metabolite was not
detected in municipal drinking water, but the herbicide itself was found in the Sacramento
River at the intake of the treatment facility. The appearance of the herbicide
and the off-taste water followed peak thiobencarb use in rice. A taste panel evaluation
of metabolite-spiked water samples provided additional evidence that thiobencarb
was responsible. Thiobencarb use subsequently was restricted to less than 25% of
California rice hectarage (Fig. 7). Current state regulations set very low residue limits
for molinate (10 ppb), thiobencarb (1.5 ppb), and other pesticides in public waters.
Through the early 1980s, California rice growers continued to use conventional,
flow-through irrigation systems developed before the advent of herbicides and other
pesticides. Little thought was given to the consequence of applying herbicides in a
ricefield during the time tailwater was flowing from the field. When downstream
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