Accurate spraying
Using low dosages of contact herbicides on small weeds with high
efficacy demands an accurate spraying technique. All weeds must be hit
and the leaves have to be covered equally. The dosing determined from
dosage tables must be applied on the field homogeneously. How to achieve
this spraying result with a nap sack lance sprayer?
Method of spraying 
The most easy way to spray accurate dosages and have an even
distribution of small droplets is to use the lance sprayer and spray
straight tracks of 60-100 cm width. The flow of the nozzle should be
constant and calibrated, as well as the walking speed and working width
(Height of the nozzle above the ground).
Equipment
To have a constant flow of herbicide, the operator must either pump very regularly or use a constant pressure valve.
Constant
pressure valve
To obtain a steady flow from the sprayer, the flow of the nozzle must be
constant. This means the pressure at the nozzle must be constant.
Constant pressure can be achieved by constant pumping of the operator,
but this will be difficult in practice. Using a constant pressure valve
ensures that pressure at the nozzle is always the same. This valve opens
only at a certain pressure and keeps pressure stable.
Flat tip nozzles
For
an equal spread of small droplets, a flat tip nozzle is the best choice.
Normal flat tip nozzles (like the Teejet XR) yield a decreasing amount
of fluid from the centre to the outside, so overlap of the tracks should
be 50% to have an equal dosage all over the field. This reduces working
width and spraying capacity, the advantage is that if the overlap is not
exact, there will be no large differences in evenness.
E-type nozzles have an even distribution of almost total working width.
So the overlap can be reduced to a minimum, like several centimetres to
have an equal dosage all over the field. In a small area of overlap the
dosage will be a little higher.
In soybean and maize the E-type nozzle seems the most suitable.
Calibrating flow
Every nozzle has it’s own yield at a certain pressure. It can be
different from the basic flow tables provided by the manufacturer. So,
before spraying the nozzles should be calibrated and the yield must be
determined. Collect the amount of fluid sprayed in 30 or 60 seconds.
Repeat this several times and calculate the average flow (in ml/min).
Working width of sprayer
Next the working width of the sprayer has to be determined (trackwidth).
The width depends on the distance of the nozzle from the ground and the
spraying angle of the nozzle. Spraying angles differ from 80 – 120
degrees. When the angle is wide, the nozzle can be held low, and the
spraying pattern is less sensitive to the wind. The working width should
be fitted to the width of the plots on the fields. When the distance
between the rows of the crop is 70 cm, the working width can best be set
at 75 cm. In this way the tracks are easy to be recognised in the field.
With 5 cm overlap on both sites you can be sure all weeds are covered
with herbicide. The working height of the nozzle should be determined
and remembered or marked on the trouser of the operator.
Walking speed
The dosage applied depends on the correct and constant walking speed of
the operator. Using the result of calibrating the flow and the working
width, and the dosage of water per hectare and the walking speed can
easily be calculated.
K= flow of nozzle (ltr./min)
A= width of track (m), B=length of track(m), C=amount of water per
hectare to be used
Amount of water on one track (ltr): A x B x
( C / 10.000 ) = D
Time to spray track (seconds per track) = D / ( K / 60 ) =
A x B x C x 60 / 10000 / K
(example: at field of 100 m length (B), track width of
0.7 meter (A), 300 ltr water/ha
(C) and a nozzle flow of 0,65 ltr/min (K), the operator needs 0.7 x 100
x 300 x 60 / 10.000 / 0.65 = 194 seconds to spray one track. At this
speed, dosage will be applied as in the dosage table)
Spraying experience
So the right walking speed is essential to achieve right dosing and
equal spreading in the field The calculated walking speed should also be
tested in the field, before spraying. When used several times, the
operator will have more experience and maintaining a constant walking
speed will give little problem. Right walking speed should be tested
always once before doing the spraying itself.
Evenness of spraying and spraying pattern
The
evenness of spraying and equal size of droplets can be checked on dry
concrete or using water sensitive paper. The droplets should not be to
large, to prevent leaking of herbicide from the leaves of the weeds.
On the other hand, the droplets should not be to small, to prevent
drifting of the liquid by the wind to a track or field area nearby.