Simple Scouting Hacks

Scouting weeds prior to applying herbicide is incredibly important for weed identification, staging and assessing densities to fine-tune herbicide selection. Different tools can be used for scouting, ranging from boots on the ground to machinery in the field to drones in the air. The next step is to scout after the herbicide application to make sure that a certain level of success has been achieved. Usually waiting two or three weeks after a herbicide application will allow for a good assessment of the effectiveness of the treatment. And while the adage of “scout early, scout often” is true, when it comes to assessing weed control, there is such a thing as being “too early” but also a huge danger in being “too late.”

First let’s tackle the concept of too late. While herbicides are typically very effective in controlling weeds, it’s essential to double-check for any weeds that might have escaped. This allows for other weed control actions to take place that will prevent or reduce seed set, especially important with herbicide-resistant, tough-to-control or prolific seed-producing weeds. Preventing seed set is preventing future problems. If the scouting operation is left too late, there may not be any good options for preventing seed set.

There are at least three possible timings that could be considered “too late.” Those timings include:

• once the weeds reach a certain growth stage where there are no effective herbicides;

• when the weeds have already begun to set seed; or,

• when weeds that were controlled effectively have turned to dust and new flushes of weeds have germinated so there is no way to know if the weed control action had any effect at all.

It isn’t a specific date in the calendar, nor is it a specific number of days after seeding or spraying. Too late might be something like fungicide timing, when there are no herbicide options, or no way to mow or till out the weed patches without causing significant damage to the remaining crop. For that reason, going early is far better than too late, which is almost impossible to recover from.

How is it possible to scout “too early” after weed control strategies have been executed? The primary challenge is allowing the weed control action to take effect and then being able to assess the efficacy. With tillage, it would mean waiting to see if the weeds died or re-rooted, while mowing would be monitoring to determine when to mow again, in order to prevent seed set. With herbicides, symptoms will usually take 10 to 14 days to appear, although it could take longer in certain growing environments. The trick with herbicide applications is being able to recognize the symptoms (or lack thereof) and make a decision on whether to take further action.

Assessing herbicide efficacy is challenging, especially if the weed hasn’t completely died. There are many resources that can help with this step including agronomists (retail or independent), regional and provincial weed specialists, and representatives from the chemical manufacturers. These experts are fantastic at providing information, but it may be a challenge to get them to the field exactly when you want it scouted. In that case, since there haven’t been many new herbicide modes of action introduced in the past decade or two, there are still many relevant publications with pictures of herbicide symptoms to help with assessments.

Most contact herbicides will impact on leaf material in a matter of hours or days, sometimes bleaching the leaf white or turning them yellow before the plant turns brown and dies, and the challenge is assessing if there will be any regrowth. With systemic herbicides, symptoms may take longer to appear even though the weed is no longer growing. Some systemic herbicides impact at the growing point, such as Group 2 herbicides, while others cause plants to grow in unusual ways (Group 4 herbicides). Older leaves may stay green for days or weeks with some herbicides, which can be a source of confusion when scouting. In addition, herbicide mixtures may show a combination of different symptoms at different times under different growing conditions.

If the weed is still green, the final step is to determine if it is actively growing. There are three ways to determine this. First is assessing the growing point. For instance, Group 1 herbicides will cause the growing point to pinch off and turn brown and will easily pull out. If the growing point is not helpful, then digging up the plant and assessing the roots may be an option. If the roots are thickened, pruned or lack root hairs, that is an indication that the plant is no longer actively growing.

If the first two tests are inconclusive, then the easiest tool might be a can of spray paint! Spray paint has been used to assess how fast a corn crop is growing, or the variation in the germination of a crop, so it should also be able to help assess growth of the crop and weeds after a herbicide application to decide on efficacy. Spray painting the growing point of the crop and then of suspect weeds will allow you to return to the area and assess the growth of both. If the crop has put on new leaves, but the weeds have not, then the herbicide has been effective. If the weeds are continuing to grow, then it’s time to develop another strategy for reducing their impact. Scouting early gives you more time to make those plans.

Regardless of which scouting techniques are ultimately employed, the old adage of “scout early, scout often” rings true. The danger of scouting too late is that there may be no way to remedy the situation. The downside to scouting too early is that you might underestimate the effectiveness of the treatment and take action when it is not needed. The upside to scouting too early is that you can always check back!

Tom Wolf, PhD, P.Ag.

Tom Wolf grew up on a grain farm in southern Manitoba. He obtained his BSA and M.Sc. (Plant Science) at the University of Manitoba and his PhD (Agronomy) at Ohio State University. Tom was a research scientist with Agriculture & Agri-Food Canada for 17 years before forming AgriMetrix, an agricultural research company that he now operates in Saskatoon. He specializes in spray drift, pesticide efficacy and sprayer tank cleanout, and conducts research and training on these topics throughout Canada. Tom sits on the Board of the Saskatchewan Soil Conservation Association, is an active member of the American Society of Agricultural and Biological Engineers and is a member and past president of the Canadian Weed Science Society.