A handbbok on Weed Control in Rice.pdf

area less than 1 ha or for a tankful. The
amount of water needed to spray a
given area can be calculated as
If the area to be sprayed is 1,000 m 2 and
the sprayer output is 200 liters/ha
(from the calibration described earlier),
then the amount of water required to
spray this area is :
When the area to be sprayed is de-
termined on the basis of sprayer
capacity, area can be calculated as
If a knapsack sprayer has a capacity
of 15 liters and the sprayer output is
200 liters/ha, then the area covered by
one full tank (15 liters) is
The total amount of formulated
product to be used in the examples
above can be calculated as
60 Weed control handbook
To summarize, the steps required to
arrive at the amount of herbicide and
water to be sprayed on a given area are
1. Determine sprayer output per
hectare (calibrate the sprayer).
2. Determine the quantity of formu-
lated product required per hectare
(from label or other recommendations).
3. Use the size of the area to be
sprayed or the sprayer capacity
(whichever is smaller) to determine the
amount of water needed.
4. Calculate the formulated product
needed for the quantity of water.
Appendix B gives calculated dosage
rates per hectare for different formula-
tions, so that you can cross check your
own calculations.
Calculation of dosage in acid
equivalents of salts and esters
Herbicide dosage recommendations
computed in kilograms of active
ingredient per hectare refer to the
unaltered chemical molecule. But in
herbicide molecules that are acids, the
acidic portion is normally transformed
to a salt or ester, to improve such
characteristics as solubility in water or
oil and foliar penetration. In general,
the parent acid portion of the herbicide
molecule remains as the herbicidally
active portion, while the ester or salt
attached to the parent acid satisfies the
functions for increased solubility or
increased penetration. The acid equiva-
lent of a salt or ester form of a herbi-
cide, therefore, is that portion of the
molecule representing the original acid
form of the molecule. The dosage rec-
ommendation for such herbicides is
given as kilograms of acid equivalent
of the active ingredient per hectare.
The recommended dosage of herbi-
cides applied as salts or esters is often
based on the herbicidal portion of the
salt or ester molecule, and excludes the
herbicidally inactive portion
(Anderson 1983).
The product labels of most commercial
herbicide formulations containing
salts and esters specify the amount of
acid equivalent present in the formulation.
The acid equivalent is equal to the
difference in weight, expressed as percent,
between the parent acid molecule
(minus a value of 1, representing the
loss of the H + ) and that of the salt or
ester molecule. It is always less than
l00%, and is calculated by the
following formula:
Molecular weight (mol/wt) of
acid
Example: The molecular weight of
2,4-D is 221. The molecular weight of
its isopropyl ester is 263. The acid
equivalent of the isopropyl ester of
2,4-D is determined as
Field techniques for
using herbicides
Proper field techniques are important
to get good weed control from herbi-
cides. Read all labels before using a
herbicide, and follow the directions. To
kill the weeds and not the rice, using
the correct dosage is essential. Contact
herbicides work best when they are
applied in a high volume of water.