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

Factors that reduce toxic risk
Physical factors. A significant physical factor that reduces the impact of a herbicide is
dissolution, the ability of a chemical to dissolve in water and then undergo further
dilution. To some degree, all pesticides are soluble in water, even those that may be
labeled “insoluble.” Over time, herbicides applied to flooded ricefields leach out of
their formulation carrier and dissolve. The addition of more water to a herbicidetreated
field, through rainfall or irrigation, dilutes the solution, which reduces toxic
risk. The dissolved herbicide is absorbed into plants, adsorbed or bound to sediment,
carried away by runoff, or volatilized into the atmosphere.
Volatilization is the most important route for the dissipation of many herbicides.
The chemicals evaporate from fieldwater and, to a lesser extent, from damp soil. The
rate of volatilization is governed primarily by Henry’s Law: H = P/S, where H is the
Henry’s Law constant, P the vapor pressure of the chemical, and S the aqueous solubility
of the chemical (Lyman et al 1990). Water depth, temperature, and windspeed
also affect volatilization. Volatilization can be quite rapid (Table 2). The half-life (time
required for an initial concentration to decrease by half) of the herbicide molinate is 2
d, close to that predicted by Henry’s Law. This short half-life indicates that the dissipation
of molinate is due primarily to its volatility. An estimated 75-85% of molinate
applied as a granular formulation to a flooded ricefield volatilizes into the atmosphere
(Crosby 1983). The odor of volatilizing molinate can be detected kilometers away
from recently treated ricefields.
Applying herbicides to ricefield soil before the field is flooded inhibits dissolution
and volatilization, and guarantees increased persistence of the chemicals. Although
this is not necessarily detrimental—preflood application will tend to decrease
the amount of herbicide needed for a given level of weed control—it does emphasize
the fact that a large part of any herbicide treatment is dissipated.
Chemical/biochemical factors. Herbicides are chemicals that can combine with
the environmental reactants oxygen, water, and natural organic matter. For example,
Table 2. Environmental properties of rice herbicides. a
Herbicide
Half-life b Relative Solubility Soil
(h) volatility c (ppm) binding d
BCF e
Fenoxaprop ethyl 3 0.003 1 6800 1056
Bensulfuron methyl 12