By Dr. Brian de Villiers
We often stress the importance of getting enough spray droplets to all portions of the plant. However, we also need to ensure that more droplets stay on the leaf where they are deposited. Surfactants and oil adjuvants decrease the surface tension of the spray solution. This decreases droplet bounce and increases the retention of spray droplets on leaves. It’s a numbers game, and if more spray droplets are retained, more active ingredient will be available to work on or inside the plant. Retention is influenced by spray factors like droplet spectrum, but crop and weed leaves also play a massive role. Plant size and surface will be discussed in more detail.
Plant size
On large crops and weeds, it is important to retain as many droplets as possible on all portions of the plant canopy. Spray droplets must be retained on the upper, middle, and lower portions of the plant. In tree crops, droplets often also need to be retained on the inner part of the tree. The opposite is also true for very small plants. Herbicides are normally applied to young, small weeds, to ensure effective control. The problem is that small weeds offer a very limited target for the spray droplets. It is important that the droplets that do reach the target, do not bounce off again. The challenge of small weeds becomes even more important when the leaves have a vertical orientation and the potential for retention is reduced. This is one of the reasons why the label recommended surfactant or oil should always be used. It is important to have reduced droplet bounce to ensure retention and coverage on large and small plant targets.
Plant surface
Leaf wax and trichomes (hair) are present on most leaf surfaces. Furthermore, the surface of young leaves is different to older leaves. These factors all have an effect on retention ability. It is important that spray droplets do not run off because of the waxy layer on the leaf surface. Spray droplets that are retained by leaves, may be suspended away from the leaf surface by a thick layer of trichomes. Therefore, it is important to get the droplet retained on the actual leaf surface where it can do its job. The difference in retention between young and older leaves is also often evident. This is why it is important to use the correct surfactant or oil adjuvant to ensure retention, despite these plant surface factors.
Villa’s stance
One of the main functions of surfactants and oils is to increase retention on all plant sizes and surfaces. Unfortunately, the importance of these adjuvants is often underestimated, and they are not used at all, or the label recommended adjuvant is substituted with an unsuitable product. Please treat these adjuvants as an integral part of spray mixtures. They will ensure retention, coverage, and control. Contact your Villa agronomist for more information about adjuvants and water quality.
