A handbbok on Weed Control in Rice.pdf

Granular herbicide application
Most farmers apply granular herbicides
by broadcasting. Whatever the method
used, safety and uniformity of application
are important. Unlike liquid formulations,
where the amount of water used
is not critical as long as the sprayer has
been calibrated to deliver a certain rate,
granular formulations are not further
diluted with a carrier. Thus the formulated
product is constant and the
amount applied is determined by the
recommended application rate per
hectare. It is important to follow this
calibration method:
1. Determine the amount of formulated
product per hectare using the formula
on page 59.
Example: To apply 2.0 kg ai/ha of
Machete with 5% ai, multiply 2 kg by
100, then divide by 5 = 40 kg/ha.
2. Measure and mark out a 5-m × 5-m
area and determine the amount of
granules required for the area. The
amount of butachlor required for
25 m2 is
25
× 40 kg = 0.1 kg = 100 g
10,000
3. Walk at a comfortable pace and
practice applying the herbicide several
times, until you achieve a 100 g/25 m2 application rate.
When the granules are to be mixed in
a carrier such as sand, use a different
calibration method, as follows:
1. Measure and mark out a length of 100
m.
2. Weigh enough herbicide to cover this
area. Walk at a comfortable pace and
practice applying herbicide uniformly
over this area.
3. Reweigh the herbicide remaining.
The difference will give the amount of
herbicide applied.
4. Measure the swath width.
62 Weed control handbook
To calculate the application rate, use
the following formula:
Granules applied
Application rate
in kg/ha =
in g per 100 m
Swath width
in cm
× 10
Example: If the weight of granules
used over a 100-m length with a swath
diameter of 40 cm is 120 g, then the
application rate is
120 × 10
40
= 30 kg/ha
Granular herbicides, although about
twice as costly as nongranular ones,
can be applied once by hand, in asso-
ciation with fertilizer. They can also be
applied using any one of a number of
mechanical spreaders. In small plots,
granules can be applied by shaking
them from a bottle with perforated lid
(IRRI, unpublished circular).
Safe use of herbicides
Improper herbicide use can harm
crops, humans, wildlife, domestic
animals, and the environment. There
are two types of herbicide toxicity:
acute (a single oral dose) and chronic
(a sublethal dose repeated over time).
LD 50 and LC 50
LD 50 is the expression for a single dose
that, when taken orally, kills 50% of a
group of test animal. It is usually
expressed in milligrams of herbicide
per kilogram of body weight of test
animal. The higher the herbicide toxi-
city, the lower the LD 50 . For example,
the LD 50 of paraquat is 150 mg/kg; that
of butachlor is 2,000 mg/kg.
We emphasize that the LD 50 is NOT
the safe dose level—50% of test ani-
mals die at that dose.
LC 50 is the concentration required to
kill 50% of the test organisms in an
environment (usually water). The LC 50
[96 h] for carp is 0.32 mg/liter.
Indiscriminate herbicide use in
irrigated rice can adversely affect wild-
life, humans, and the environment.
Improper herbicide use will contami-
nate local water bodies. Inappropriate
herbicide use or use of herbicides
highly toxic to fish will kill or contami-
nate the fish in that water, which will
affect the fish consumers.
Safe handling of herbicides
Handling an undiluted herbicide is
more dangerous than handling the
diluted product. Herbicides can enter
the human body through the skin,
mouth, nose, and eyes. Absorption
through skin is common and can occur
from chemical spilling and splashing,
and from drifting of spray. Absorption
through the nose and mouth can occur
from inhaling spray droplets, vapors,
or chemical dust. Safety tips for herbi-
cide use are as follows:
Mix herbicides or other pesticides in
the open air, never in enclosed
places with inadequate ventilation.
Read the label on the herbicide
container and make sure any special
instructions are understood.
Leaks from a badly maintained
sprayer may increase contact
between the herbicide and the skin,
permitting the herbicide to enter the
body. Tighten leaking sprayer parts
and check seals and washers to
avoid leakage.
Wear protective clothing. Special
protective clothing is heavy and
expensive, and uncomfortable when
used in the tropics. For small-scale
farmers who use herbicides occa-
sionally, a complete set of protective
clothing may not be necessary. All