The decision on application method for herbicides boils down to two main factors: (a) target type and (b) mode of action. In general, It’s easier for sprays to stick to broadleaf plants on account of their comparatively larger leaf size and better wettability compared to grassy plants. There are exceptions, of course – at the cotyledon stage, broadleaf plants can be very small and a finer spray may be needed. Water sensitive paper is a very useful tool to make that assessment. Imagine if a tiny cotyledon could fit between deposits – that could be a miss!
Some weeds are also more difficult to wet, and those may also need a finer spray or a better surfactant for proper leaf contact. An easy test is to apply plain water to the leaf with a spray bottle. If the water beads off or the droplets remain perched on top in discrete spheres, the surface is considered hard to wet. Most grassy weeds are considered hard to wet, while most broadleaf weeds are easy to wet, but there are exceptions in each case.
Grassy weeds are as a rule both smaller, more vertically oriented, and almost without exception more difficult to wet than broadleaf species. All these factors call for finer sprays for effective targeting and spray retention.
There are about thirty mode of action (MOA) groups among the herbicides with about ten accounting for the majority in Canadian prairie agriculture. It’s probably an over-simplification to categorize them into just two groups – systemic and contact. But that grouping goes a long way to making an application decision.
Contact products (MOA Group 5, 6, 10, 14, 22, 27) must form a deposit that provides good coverage. Droplets need to be closely spaced and cover a significant proportion of the surface area because their effects occurs under the droplet, and doesn’t spread far from there. One way to generate more droplets is to reduce droplet diameter, another is to add more water. A reasonable combination of both is ideal.
Systemic products (MOA Group 1, 2, 4, 9) will translocate within the plant to their site of action after uptake. As a result, coverage is less important as long as sufficient dose is presented to the plant. In practice, this means coarser sprays and/or less water may be acceptable.
When two factors are combined, either in a tank mix or a weed spectrum, the more limiting rules. Application of a tank mix or product that is active on both broadleaf and grass plants will be governed by the limitation placed on grass targets. A tank mix comprised of both systemic and contact products is governed by the limitations placed on contact products.
A factor we should also consider is soil activity and the presence of residue. Studies have shown that soil-active products are relatively insensitive to droplet size. But if they have to travel through a layer of trash to get to the soil surface, more application volume is the best tool.
The spray qualities listed in the table are those identified by nozzle manufacturers in their catalogues, websites, or apps. Note that Wilger also offers traditional VMD measurements on their site, allowing users to be a bit more specific if necessary.