A handbbok on Weed Control in Rice.pdf

Runoff
Runoff is one of the main pathways of
herbicide loss from flooded ricefields.
Herbicides are usually transported as
solutes in soil water; their movement
will depend on solubility and adsorp-
tive capacity. In runoff water, however,
herbicides are transported both as
solutes and on soil sediment particles.
An irrigation system needs to be well-
regulated to retain herbicides within
the system. Irrigation drainage and
overflow should be avoided, to reduce
herbicide losses and to protect down-
stream water from pollution.
Volatilization
All herbicides are volatile (have a
tendency to change from a solid or
liquid to a gaseous state). Volatility,
however, varies among herbicides and
increases with higher temperature.
Volatile herbicides should be mechani-
cally incorporated into the soil to avoid
excessive chemical loss.
Volatilization from the floodwater
into the atmosphere is an important
route of herbicide loss from lowland
ricefields. The volatilization rate
depends on water evaporation rate,
water depth, water solubility, and
vapor pressure of the herbicide.
Volatilization loss of herbicides from a
shallow, warm water flooded field can
be highly significant. Volatilization
from the soil surface of upland rice
may be much greater than that from
the floodwater of lowland rice.
Thiocarbamate herbicides are volatile
in floodwater.
52 Weed control handbook
Photodegradation
Photochemical and biochemical
degradation of herbicides govern their
fate in a flooded environment. For
example, the high pH of the water
induces hydrolysis of carboxylic esters.
Some herbicides adsorb ultraviolet
radiation. The presence of humic acids
in floodwater may also induce photo-
chemical degradation of herbicides
that do not adsorb ultraviolet radiation
(Yaron et al 1985).
Persistence
Herbicide persistence is the length of
time a herbicide remains active in the
soil. Persistence depends on the
amount of herbicide applied, the rate at
which it is broken down, properties of
the particular herbicide, and leaching.
Herbicides with long persistence keep
a crop weed-free for a longer time than
do herbicides with short persistence.
Persistence of a herbicide beyond the
rice-growing season, however, is
undesirable because other crops
sensitive to that herbicide cannot be
grown on the same land for some time.
Effect of environment on
herbicidal activity
Several environmental factors affect
the success of weed control by soil or
foliar-applied herbicides. Temperature,
relative humidity, soil moisture, and
wind are important.
Temperature
Absorption and translocation of herbi-
cides increase as temperature
increases, and selectivity can be
changed by differences in absorption
and translocation. For example, if
propanil is applied to rice when the
temperature is above 38 °C, phytotoxic-
ity may occur. At high temperatures,
simetryn will cause injury, even in
japonica rice, as a result of higher
absorption through the roots. In
general, the rate at which herbicide
degrades increases with increasing
temperature.
Relative humidity
At high relative humidity, leaf stomata
are open, which increases absorption of
herbicide into the leaf. Evaporation of
herbicides from leaf surfaces is slowed
at high relative humidity. Slow evapo-
ration lengthens the time the herbicide
can enter the plant.
Soil moisture
Soil moisture affects herbicide effec-
tiveness by influencing the amount of
herbicide in the soil solution and the
depth of herbicide movement in the
soil profile. When a residual herbicide
is applied to a dry soil, it relies on soil
moisture (from rain or from irrigation)
to move it to the root zone. Inadequate
movement is a common cause of the
failure of herbicides in upland fields.
Wind
Wind adversely affects the absorption
of foliar-applied herbicides by increas-
ing the evaporation of spray droplets
and the volatilization of herbicide
residue from the leaf surfaces.
Properties of herbicides
When the probable behavior of a
herbicide can be predicted from its
properties, that information can be
used to design safer and more effective
application. Some properties of herbi-
cides that affect their biological activity