Adverse conditions are the true test for adjuvants

By Dr Brian de Villiers

January 2026
 
In our interactions with stakeholders we sometimes learn that growers had replaced an adjuvant registered for use with a particular crop protection product (CPP) with an alternative product, and that the resulting control of the CPP was still acceptable. These instances may create the impression that all adjuvants are the same, and that registered
adjuvants can be replaced with any alternative product. This is a dangerous assumption, because under optimal growing conditions there may be little for the adjuvants to remedy. Where we really see significant differences between adjuvants is when production conditions or water quality are sub-optimal. It could be that one whole season could present difficult conditions, or different farms or water sources may show marked differences within one season. This month we will discuss why it is a huge risk to replace the registered adjuvant with another product.

Water quality

pH, buffering capacity, and dissolved cations all play a role in water quality. If the antagonistic cation levels in spray water are low, water conditioners don’t have much to remedy and there may not be a huge difference between adjuvants. However, another water source could have very different cation levels and ratios. This will have a significant influence on efficacy and the difference in performance between adjuvants. The same principle applies to pH and buffering capacity. A replacement buffer may be as effective as the registered product when the buffering capacity is acceptable. However, there will be a huge difference in pH control when the buffering capacity is extremely high or low. Although the differences between water conditioners are exposed by poor water quality, we often also see a visible difference even with good quality spray water. Therefore, the choice of water conditioner is crucial for acceptable control.

Weather conditions

Various weather conditions have an impact on the performance of CPPs. Humidity is one of these conditions because it influences the drying time of spray droplet residuals. Low humidity and a faster drying time decrease the absorption period for systemic herbicides such as glyphosate. This is why certain adjuvants contain humectants to extend the drying-off period of spray droplets. The problem is that not all adjuvants contain the same chemistry or amount of humectant. When conditions are optimal, the adjuvant with a lower humectant content may have a similar effect to the registered product because the humidity levels are high enough for adequate herbicide absorption. However, when the humidity levels drop, the difference between these adjuvants will be exposed. This could even occur on the same day!

Villa’s stance

Always use the registered adjuvant. Don’t assume that if a product is just as effective as the registered adjuvant, that this will be the case for all weather conditions or water sources. The incorrect adjuvant can even give variable CPP control on the same day. It is simply not worth the risk. Contact your Villa agronomist for more information about adjuvants and water quality.

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