Chemical Bush Control

ENCROACHER bush is controlled on about 128 000 hectares of Namibian farmland each year, or about 0,2% of Namibia’s land surface area.

The recent bush control survey amongst commercial farmers indicated that chemical methods are the most-often used means of control.

About two-thirds of all bush control activities involve chemicals, which are applied to about 84 000 hectares per year. The area treated each year is expected to increase rapidly as more and more farmers decide to do something against the bush encroachment problem, which can reduce grazing capacity of a farm to one-tenth of what it could be. Since chemicals that kill bush (“arboricides”) are potent plant toxins, it is vital that farmers know how to use them correctly.

The container of arboricide that a farmer buys in a shop contains at least two different chemical fractions: the stuff that actually kills plants (called “active ingredient”) and some other chemicals that have other functions than to kill plants (called “additives” or “adjuvants”). Between 20% and 50% of the total arboricide is active ingredient and the remainder, 50-80%, are various additives that keep the chemical mix stable, prevent deterioration in hot conditions or in the sun, “stick” the poison to leaves etc. but do not kill plants as such, i.e. they are called “inactive”.

The four most widely used active ingredients to control encroacher bush in Namibia are tebuthiurone, bromacil, picloram and triclopyr. The former two active ingredients interfere with the plant’s photosynthetic processes and cause repeated cycles of leaf drop and re-growth (“green flush”) until the plant’s carbohydrate reserves are depleted and it dies, literally of starvation. Tebuthiurone and bromacil are supposed to be insoluble in water and bind readily to soil particles. For these reasons, they are ideally suited to be applied to the soil under the canopy of targeted plants. Rain washes the soil-applied arboricide into the root zone, it gets absorbed by the target plant and kills it.

The active ingredients picloram and triclopyr are highly soluble in water and do not bind to soil particles. They should thus not be applied to soil but to the targeted plants directly, either to the stem or the leaves (or both). Foliar-applied arboricides mimic a plant growth hormone and stimulate the plant to grow itself to death, causing only one, terminal green flush.

A couple of things are important to know about active arboricide ingredients:

– They kill any and all plants applied to, including grasses. The trick with chemical bush control is thus to apply the arboricides only to those plants that you want to kill, and then little enough that all gets used up in the process and none remains afterwards to kill other plants as well.

– Apply too much of an arboricide and you sterilise the soil for years to come. No plants can grow there. This is the residual action of an arboricide and we are now finding out to our dismay that it usually lasts much longer than the label stated.

– Especially foliar-applied arboricides but increasingly soil-applied arboricides too leach into groundwater. There, they get distributed underground to who-knows-where and can kill trees in unexpected locations (mostly next to rivers or drainage lines), even 15-20 years after first having been applied and far away from their place of application (e.g. on the neighbour’s farm).

For the last two reasons, an active ingredient that was popular only 20 years ago (ethidimurone, sold as “Ustilan”) got banned, because it was too potent and polluting, especially near water (open or underground). Unfortunately, it appears that the solubility and residual effect of bromacil was understated and it may be going the same way as ethidimurone, but has not yet been banned.

Farmers are aised to apply this and other arboricides with extreme caution as inappropriate use can have devastating effects on the environment. Follow the label’s instructions religiously.

Source : The Namibian