Herbicide Application
Field Crops in South Africa

© Louise Brodie
Poor control of weeds on the side of a field may increase the risk of weeds developing herbicide resistance.
For the best results, herbicides should be applied according to the instructions, with products with the same mode of action being rotated, with other products to prevent the development of herbicide resistance. Herbicides may either be applied to destroy weeds that are dormant and are yet to sprout, which is called pre-emergent herbicides, or to manage weeds that have already germinated, called post-emergent herbicides.

Pre-emergent Herbicide Application

The timing of pre-emergent herbicide applications depends on the type of tillage used. On farms where conventional tillage is used, herbicides should be applied soon after the land has been cultivated so that weeds do not get a chance to germinate, Gavin Matthews advises in his article Pre-emergent Herbicides remain important published in an edition of Grain SA’s Pula Imvula magazine.

When using no or minimum-tillage, a ‘burn down’ herbicide may be applied to kill all weeds that have emerged followed by a pre-emergent herbicide to inhibit any more weeds from germinating. Whether conventional tillage or minimum-tillage is used, herbicides should be applied during damp climatic conditions to allow the chemicals to be pulled into the ground to destroy weed seeds.

Post-emergent Herbicide Application

Post-emergent herbicides are contact herbicides, which means they have to be absorbed by the leaves of the weed to destroy the plant. In his article, Keep the enemy at bay, which appeared in Grain SA’s Pula/Imvula magazine, Gavin Matthews has the following tips for optimal use:

Firstly, the herbicide has to be applied on a windless day, to prevent spray drift that can result in poor contact between the herbicide and weeds.

Unlike pre-emergent herbicides that have to be applied during moist conditions, post-emergent chemicals need to be applied on sunny days, when weeds are growing actively, for good chemical absorbance. Once the weed has stopped growing, the herbicide will have no or very little impact.

Thirdly, the chemical should be used at the right water ration and droplet size. Generally, about 150 litres of water would be used per hectare in a fine mist spray to get better chemical coverage of the plant. The water ration and droplet size will, however, be dependent on the product, so ask your chemical supplier for application advice.

Fourthly, the herbicide should be applied at a constant speed to avoid an over application or wastage in areas where the speed was too slow or under application when the tractor was driving too fast. The sprayer should be correctly calibrated according to the speed of application, which will depend on the topography of the farm. Ten kilometres per hour is a good speed to work with and helps to ease calibration calculations.

Lastly, additives, such as stickers, may be used to improve the efficacy of the herbicide. A sticker may allow the herbicide to better stick to the leaves of weeds and also alleviate the negative impact of rain that falls after spraying. Again, chemical suppliers should be consulted over whether an additive is necessary and which ones to use with which herbicide.

Lastly, farmers should ensure they use the right herbicide, in other words, one that will destroy the targeted weed without negatively affecting crop production.

By Glenneis Kriel