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Smart weed control can soften the blow of fertiliser price spike

hammer smashing piggy bank


Numerous studies over the past few decades have confirmed that weeds are greedy feeders with an appetite for nitrogen specifically. In the same way that maize plants respond to the application of nutrients, weeds grow better when they have access to extra nitrogen. Most important is the fact that weeds outcompete maize plants, leaving less nutrients available to the crop. The significance of this was highlighted by a study that showed that a 1% rise in the total weed cover can decrease maize yield by 68kg/ha.


Farmers cannot afford for weeds to benefit most from costly fertilisers. Therefore, weed control has to start early in the weed’s growth cycle. This is, firstly, because competition from weeds during early stages of maize development significantly decreases yield, and, secondly, because bigger weeds are more difficult to control. In addition, the weeds continuously compete for the fertiliser intended for the crop. The best way to optimise and protect maize yield is by limiting the time weeds have to interfere with maize plants by applying herbicides early on. In fact, any damage (lack of nutrients, water, sunlight, etc. due to weed competition) that a maize plant suffers at the 3-leaf stage –– is irreversible and will influence yield. Other critical stages for weed control are the 5-leaf stage, tasselling and the reproductive stage.

Yield Graph showing affect of fertilizer on weeds

Weed management and fertiliser application are two of the most critical management factors influencing maize yield. When producers make the most of both these resources, they will not only maximise their maize yield, but also optimise the return on every rand spent on inputs.


Per Hectare, N,P & K uptake by a moderate weed population 

Weeks After Emergence N P K
2 1.4 0.1 1.1
3 3.3 0.3 2.1
4 21.6 2.2 17.7
5 27.8 2.4 21.8
6 93.7 5.4 57.2
7 94.8 5.5 78.4
13 122.9 11.2 109.1








PC Nel & JG EHlers, Department of Plant Production, University of Pretoria

During the first 13 weeks of the crop, the weeds use the equivalent of 370 mm/ha rain