A handbbok on Weed Control in Rice.pdf

Chapter 4
Principles of herbicide use
Herbicides are chemical substances or
cultured biological organisms that kill
or suppress plant growth by affecting
one or more of the processes—cell
division, tissue development,
chlorophyll formation, photosynthesis,
respiration, nitrogen metabolism,
enzyme activity—that are vital to plant
survival. In general, herbicides applied
at high rates kill all plants. At low rates,
some herbicides kill some plants
without damaging other plants.
Herbicides with such an ability are said
to be selective. Use of selective
herbicides for weed control has be-
come popular among rice farmers
because of increasing labor costs for
hand or mechanical weeding.
Types of herbicides
Herbicides are commonly referred to as
contact or translocated.
Contact herbicides
Contact herbicides control weeds by
killing the tissues in direct contact with
the herbicide. They are normally
applied to leaves and stems. Because
they affect only the plant parts they
come into contact with, they are less
effective on perennial weeds than on
annual weeds. Thorough coverage of
the plant is essential for contact herbi-
cides to be effective. A contact herbi-
cide may be selective, such as oxyfluor-
fen and propanil, or nonselective, such
as paraquat.
Translocated herbicides
Translocated (systemic) herbicides
move from the point where the
herbicide comes into contact with the
plant to other plant parts. Systemic
herbicides may be applied to stems and
leaves or to the soil (those applied to
soil are known as residual herbicides).
Butachlor, 2,4-D, and glyphosate are
examples of translocated herbicides.
Glyphosate is a systemic herbicide
when applied to stems and leaves, but
it has no activity when applied to the
soil. Translocated herbicides may be
either selective or nonselective.
Herbicide selectivity
Herbicide selectivity is very important
in crop production. Through selectiv-
ity, it is possible to use a herbicide to
kill a grassy weed, such as Echinochloa
crus-galli, in a grassy crop such as rice.
In practice, to avoid killing the rice
plants and for good weed control,
selective herbicides should be used at
recommended rates. Reduced rates
result in poor weed control. Nonselec-
tive herbicides, such as paraquat, are
harmful to rice even at low rates.
Herbicide selectivity can also be
achieved during application, by
directing the spray away from the crop
or by using protective shields. Chemi-
cal antidotes may be used to prevent
the herbicide from killing a susceptible
plant. For example, pretilachlor can
only be used in direct seeded rice in
conjunction with an antidote (e.g.,
fenclorim).
Physical factors of selectivity
To be effective, herbicides must come
into contact with the target plant and
be retained on its surfaces (root or
shoot) long enough and in amounts
large enough to kill the plant. The
physical factors of selectivity are those
that affect contact between the herbicide
applied and the plant surfaces and
the retention of the herbicides. Plants
absorb soil-applied herbicides through
roots and shoots of seedlings pushing
upward through the soil. Selectivity is
affected by herbicide dosage, formulation,
and placement and by the plant
growth stage.
Herbicide dosage. The amount of a
herbicide absorbed by rice is critical for
selectivity. Most herbicides are
nonselective at high application rates.
Herbicide formulation. Selectivity may
be achieved through application of
herbicides as granular formulations.
The granules that would be harmful to
rice if retained on its leaves bounce off
and fall to the soil.
Herbicide placement. To be effective,
herbicides must first enter the plant.
Selectivity based on placement is
achieved by preventing herbicides
from coming into contact with sites of
entry into the rice plant. This is
Herbicide use 49