A handbbok on Weed Control in Rice.pdf

achieved by applying herbicides to the
soil surface, by band application, or by
using shields. Selectivity of soilapplied
herbicides may be lost if the
herbicides leach through the soil and
come into contact with rice roots and
shoots.
Plant growth stage. In general, plants
are most susceptible to herbicides at
the seedling stage. As plants grow,
they become less susceptible. In direct
seeded rice, there is no difference in
growth stage between rice and weeds,
and selectivity cannot be achieved
through plant growth stage. In trans-
planted rice, however, differences in
growth stage and position of growing
points of rice and weeds can be used to
achieve selectivity. Plants growing
under drought conditions are less
affected by herbicides than plants
growing with normal soil moisture.
Biological factors of selectivity
Biological factors of herbicide selectiv-
ity include differences in morphology,
physiology, and metabolism among
plant species. Leaf surfaces that are
waxy, smooth, or densely hairy are
wetted less readily by aqueous sprays
than are surfaces that are less waxy or
moderately hairy. Vertical leaves retain
less spray than do horizontal leaves.
Once absorbed by a plant cell, the
herbicide may be immobilized within
the cell; that also contributes to herbi-
cide selectivity. Selectivity among
plant species may be achieved when
some plant species are able to detoxify
a particular herbicide, while others are
unable to do so and are killed. For
example, rice plants are 40 times more
tolerant of propanil than Echinochloa
50 Weed control handbook
crus-galli. This is due to differences in
plant enzyme levels. Rice plants have a
high level of aryl acylamidase, an
enzyme that hydrolyzes propanil to
nonphytotoxic 3,4-dichloroaniline and
propionic acid. E. crus-galli has a low
level of this enzyme and is unable to
hydrolyze propanil; therefore it is
easily killed. This detoxification
process can be inhibited by organo-
phosphorus and carbamate insecti-
cides.
Herbicide movement
in plants
Herbicides must enter the plant before
their toxic effect can be induced.
Herbicides applied to leaf surfaces and
buds penetrate the plant by diffusion.
Higher temperatures increase the rate
of penetration. Herbicide absorption
takes place in the guard cells of the
stomata and through the cuticle.
In the soil, herbicides move in the
soil solution to the seed and roots, or
are intercepted by the root tips. Herbi-
cides may penetrate the walls of root
epidermal cells by mass flow.
Once in plants, herbicides move via
the phloem and the xylem systems to
cells and tissues remote from the site of
uptake. The phloem conveys sugar
from the green tissues of the plant
(where sugar is manufactured) to
storage tissues. Very young leaves do
not export sugar, so herbicides applied
to them remain there. If the transport of
sugar is restricted, as when plants are
under low light intensity, redistribu-
tion of herbicides will not occur. As a
result, the general recommendation for
many translocated herbicides is that
the weeds should be in active growth.
Glyphosate and MCPA are examples
of herbicides translocated in the
phloem.
The xylem is the plant system
through which water and dissolved
mineral nutrients pass to the leaves.
Herbicides taken up by the root move
along this stream. Herbicide uptake is
therefore affected by factors affecting
transpiration rate, such as light, temperature,
wind speed, humidity, and
soil moisture. The triazine herbicides
are good examples of herbicides transported
up in the system.
For a more detailed discussion of the
principles of herbicide action, refer to
Weed science principles by W.P. Ander-
son (1983).
Timing of herbicide
application
Weeds should be removed from rice as
early as possible. Thus, herbicides
should be applied during early crop
growth stages. The time to apply a
herbicide depends on the properties of
the herbicide and the target weeds,
weather, and cultural practices. Herbi-
cides can be applied at several periods
before and during the crop growing
period. In general, herbicides are
applied at preplanting, preemergence,
or postemergence.
Preplanting herbicide application
A preplanting herbicide application is
made before the rice crop is sown. This
application helps in land preparation
in a minimum tillage cropping system.
Translocated foliar herbicides (such as
glyphosate) kill perennial broadleaf
weeds and grasses found in the fallow
vegetation. Where annual weeds
predominate, paraquat is adequate.
Where volatile preplanting herbicides
are used, they must be incorporated
into the soil before planting, to avoid
damage to the rice crop.