Peter Gambara

ONE of the contributors of low maize yields in Zimbabwe is the inability to control weeds in the crop. 

Generally, farmers need to keep their maize fields clean of weeds for the first six to eight weeks, in order to get a good yield from the crop. However, many farmers fail to control weeds, resulting in a poor crop. Uncontrolled weeds in maize can contribute 50 percent or more to yield loss. In addition, when weeds are not controlled and are allowed to develop seeds, the farmer will have created a weed seed bank for the next seven years.

Two ways of controlling weeds in a maize crop are weeding manually or using herbicides to control the seeds. Manual or hand weeding is normally slow and cannot be relied upon by commercial farmers. Hired workers also tend to leave some weeds uncut and simply place soil on top of the weeds. Those weeds will eventually recover and affect the maize crop negatively. 

Small-scale farmers usually to complement hand weeding with the use of cultivators that are used to disturb the weeds in between lines. The farmers will then come in and remove the remaining weeds between the maize crops or near the maize crops where the cultivator could not reach the weeds.

Using herbicides has an advantage over manual weed control of weeds, due to the ease of application. The cost of labour nowadays is quite prohibitive and it can be a back-breaking exercise for a farmer to weed the fields by himself/herself. 

Hired labour may demand more money where the weed infestation is high or if the weeds have overgrown. Herbicides are chemicals that are applied to kill weed seed before it germinates or to inhibit the growth of already germinated weeds. It is critical for farmers to understand the nomenclature of weeds. 

Herbicides are normally categorised as being either selective or non-selective. Selective herbicides are designed to kill specifics weeds, while at the same time leaving other weeds. 

A herbicide label will list the target weeds that the chemical can kill. Non-selective herbicides on the other hand, kill almost all plants.

There are over 25 herbicides that are registered for use in maize in Zimbabwe. Weeds are broadly classified either as broad leaves of grasses and specific herbicides are manufactured for specific types of weeds. 

The weed killers can be applied before they germinate (pre-emergence) or after they have germinated (post emergence). Some farmers tend to confuse the pre-emergence and post emergence terms to mean before or after the emergence of the maize. 

Pre-emergent herbicides are usually applied soon after planting the maize so as to kill the seed that is in the soil by stopping the formation of new root cells of the weeds. 

This will ensure that the maize emerges in a clean environment without any competition for fertilisers/nutrients, water, sunlight and air. A maize crop that emerges among weeds is usually thin and longish due to competition to reach more sunlight.

Some of the most common selective pre-emergent herbicides in maize include Metalachlor, Atrazine, MCPA and Terbutryn. It is possible to mix most herbicides and apply them together. However, farmers should always read the label to see if the intended herbicides are compatible.

Metalachlor can be mixed at a rate of 1, 5 litres per hectare, with Atrazine at the rate of 3 to 5 litres per hectare in 200 litres of water and applied to a hectare. 

Metalachlor can be mixed at the same rate and mixed with MCPA at 3-5 litres per hectare or Terbutryn at 2, 5 litres per hectare and mixed in 200 litres of water and sprayed onto one hectare. One can also use Acetochlor, Relay, Dual Magnum, Rush or Lasso in place of the Metalachlor.

However, a warning should be sounded here about the use of Atrazine as it should only be used by those farmers who grow maize repeatedly on the same piece of land. 

Atrazine has a residual effect on broadleaf crops for a period of up to 18 months after use. 

Once one has used Atrazine, he/she cannot grow broadleaf crops like soyabean, tobacco, cowpeas or cucurbits (mananga) on that same piece of land the next season. 

Atrazine will control most broadleaves as well as rapoko grass, while Metalochlor, Dual Magnum and Acetochlor and Rush will control most annual grasses. Another herbicide that can control grasses is Frontier Optima. Atrazine and MCPA can also be used as post emergent herbicides to control most broadleaves.

As indicated above, farmers have the option to control weeds, either pre-emergent or post emergent. 

Where farmers have to control weeds after they have emerged, it is important to point out that the weeds should be controlled before they reach the 3-leaf stage. 

Once they pass a certain stage, they may be be difficult to control. Other herbicides available for the control of post emergent weeds include Nicosulfuron to control Shamva grass and wheat re-growth, Halosulfuron to control yellow nutsedge (nutgrass) and Stella Star to control broadleaves and grasses. 

Some farmers use Stellar Star as a “clean it all” post emergent herbicide. However, farmers are advised to check their costs first before making such decisions, as Stella Star is expensive compared to other herbicides.

Non-selective herbicides like Glyphosate or Paraquat may be used to control perennial grasses like Katambora grass (tsangadzi) or in cases where a farmer has practised minimum tillage, to kill all the green weeds in the field. 

It is important to mention that such herbicides are non selective, if applied to a field where the maize has germinated, the maize will be killed as well. 

Farmers intending to use it, should therefore time the application, when the maize has not yet started emerging from the soil. Maize exposed to Glyphosate may be stunted in growth.

It was pointed out above that most herbicides are mixed with 200 litres of water before spraying. 

However, most farmers get confused with mixing rates per boom sprayer or a knapsack sprayer. 

Most boom sprayers have 400 to 600 litre capacity. One should, therefore, add 3 litres Metalachlor and 6-10 litres Atrazine to a 400-litre boom sprayer and 4, 5 litres and 9-15 litres to a 600-litre boom sprayer.

Trying to mix these herbicides per 16-litre knapsack sprayer proves to be more difficult for farmers and the best is to mix these herbicides in a 200-litre drum and then have your workers draw the mixture from the drum. 

It is recommended to stir the mixture every time one is refilling the knapsack sprayer from the drum, just in case one of the chemicals has settled at the bottom of the drum. 

One worker is capable of spraying two hectares per day or 2 x 200 litre drums per day. 

However, it is always important to supervise workers tasked with either using a boom sprayer or knapsack sprayers. If unsupervised, workers may cheat, so that they can finish their daily work (mugwazo) early. 

They will, therefore, be very fast or throw away from mixed herbicides as they get to the edges of the field, when no one is seeing them.

It is important to stress that herbicides should preferably be applied in a wet environment. In the case of pre-emergent herbicides, the moisture is required for the chemicals to sink into the top one and half centimetres of the soil to reach the weed seed. 

And in the case of post-emergent herbicides, the moisture is essential for the chemicals to stick to the plants, so that they are absorbed by the plants. 

Lastly, herbicides are chemicals that pose hazards to health if not properly used. Therefore, farmers should always read labels, before using herbicides. The quantities used differ with your type of soil or on whether one is using a pre-emergent or post emergent herbicide. For example, Atrazine is used at a rate of 3 litres per hectare in sandy soils, but the rate should be increased in red clay soils. 

One should also clean and check the sprayer before use. Farmers have been known to have destroyed a whole crop because they used an uncleaned sprayer that was previously used to spray glyphosate. 

There are also special flat fan nozzles that should be used with herbicides. Some farmers make the mistake of using nozzles meant for pesticides in applying herbicides. 

Farmers are also encouraged to provide protective clothing to workers who handle or spray herbicides. Poisoning due to improper handling of chemicals sometimes happens very slowly. 

Peter Gambara is an agricultural economist and consultant based in Harare.

Farm workers will only realise in their old age that they exposed themselves to dangerous chemicals. Therefore, farmers should provide proper masks and clothing that protects the whole body from harmful chemicals.