CA Series Entry 4: Herbicide application

Simon, measuring the amount of spray.

I know what you may be thinking – herbicide application in conservation agriculture? Correct. We are not talking about organic farming. CA allows the use of synthetic chemical fertilizers, fungicides, pesticides and herbicides. What matters in CA is again, precision. In today’s entry, this means, measured dilution of herbicide in water and correct amount of spray per area.

This was a highly detailed session where we looked at every component of knapsack herbicide spraying. While every knapsack design is slightly different they all do the same thing and have interchangeable nozzles that affect the spray pattern. For herbicide application the best nozzle is known as a deflector or reflex nozzle. These are unfortunately not readily available in Ghana. The project I work with had to order them from South Africa. The deflector nozzle sprays in a line and has a swath (spray range) of either 1.2 or 2.4 meters. The number also indicates liters sprayed per minute.

This Matab can carry up to twelve litres.

Manual spraying has so many factors. I really wonder how accurate farmers are with application. For example, some factors are:
-nozzle (what spray pattern does it emit and what is its swath?)
-pressure – is the farmer pumping to maintain a constant pressure in the knapsack chamber or does he/she only pump when he/she notices a pressure drop while spraying?)
-height of nozzle from ground (will affect swath)
-spraying technique – is the farmer holding the spray steady and in front or waving it around him/her as he walks? Does he continuously pump as he walks or stops walking and then pumps before continuing again.
-walking speed – the faster he walks, the less spray an area will get (potentially underdosing), the slower he walks the more spray an area will get (potentially overdosing)

With all this in mind, we looked at how to reduce the margins of errors for each of the variables using a deflector spray.

First we calibrated the spray, making sure its swath was what it said it was and that it released the amount of liquid it said it did. We practiced with water alone and every farmer who invests in a knapsack should do these checks too. With a measuring tape we verified the swath and sprayed into a measuring cup for 30 seconds (double that would be the liters per minute). To get the correct swath for the nozzle in use (either 1.2 or 2.4 meters) the nozzle must be 60cm from the ground. A trick to keep that 60cm is to tie a string to the nozzle with a light weight at the other end to hold the string taut. This way the farmer would know if the nozzle was dipping low or pulled up while walking and spraying. By spraying into a cup, we also practiced pumping and maintaining as constant a pressure as possible. This is important as the pressure will affect how much spray is released, thus affecting dosage per unit of time and area.

Once it has been confirmed the nozzle is functioning as it is supposed to and the farmer has ‘calibrated him/herself’ on pump consistency, the farmer must then calibrate their walking speed. The aim is to walk one meter per second. So, with a knapsack, a timer and measuring tape the farmer can do this. So, if one second equals one meter, the ideal time to practice to should be between 30 to 60 seconds. As distance is what the farmer is trying to perfect, he/she must practice to time, so measurement should be done after walking and spraying. Here, the farmer is also practicing walking and spraying at the same time. Doing so many things at the same time will require concentration and the farmer may speed up, or slow down without realizing it. The first time practicing, it is good not to know what the chosen measure of distance looks like because it will affect the farmer’s walking speed if they know they have five or 20 seconds left on the spot and they can see the ‘finish line.’

For correct herbicide application, math needs to be done. To be accurate these factors need to be known:
-land size
-nozzle swath
-nozzle spray release per minute
-knapsack carrying capacity
-herbicide recommendation (ex. 8 liters per ha) – and then converted into what is needed for an acre.

In practice, most smallholder farmers does not know the true size of his/her land (usually measured using a person’s stride) and in northern Ghana, most smallholder farmers have at the most only a few year’s worth of formal education. If the farmer is a woman, most likely she cannot read or write at all. It is this part of the herbicide application process that many farmers get wrong and is likely the main cause of over or under dosing. The project I am linked with works on this problem by training lead farmers in the communities they are in to be local herbicide suppliers and most importantly, teach fellow farmers to how to mix and apply, including wearing appropriate clothing and footwear. A step in the right direction, no doubt.


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