FEATURE ARTICLES
Understanding the cause of weed escapes
The key is not to assume what is to blame
Posted: November 4, 2008
With so much at stake, it's easy to assume the worst. When we discover patches of weeds still standing after a herbicide application, we often jump to conclusions. However, an agronomist in Alberta says weed "escapes" can be caused by numerous factors and it's important to sort through them before deciding on a plan of action.
Emile deMilliano, a manager of agronomic services with Viterra, has three key tips when dealing with weed escapes: 1.) Don't jump to the conclusion that resistance is the cause, but don't rule it out, either, 2.) Walk the field to assess possible reasons for inadequate control, and 3.) Do so as soon as possible. "When it comes to diagnosing the causes of weed escapes, 'probably' isn't good enough," he says.
Here are some questions deMilliano says producers should ask themselves when diagnosing weed escapes.
What are my expectations? One hundred percent control 100 percent of the time is hard to achieve given the many variables that can be involved. "If you have really intense weed pressure, even with 95 percent control the other five percent may still look rough because there's such a huge weed population," says deMilliano.
What do my records say? Producers should review their records to see if a factor in the application or mixing process could have played a role in inconsistent herbicide efficacy, he says. Patterns of misses and "carcasses" can be good visual indicators that something went wrong.
Did Mother Nature play a role? Environmental factors such as cold weather and rainfall are common culprits behind weed escapes, says deMilliano. Herbicides vary in terms of rain fastness, or in other words the window of time after application that the product is vulnerable to being washed away by rainfall. "A six-hour rain fastness is a long time. At times, you have to be a pretty good weather forecaster to second-guess that," he says.
What stage are the plants at? In many cases, what appear to be escapes are actually weeds that were not yet up when the field was sprayed. Try to determine what stage the weeds are at now versus their staging when they were sprayed, says deMilliano. "Were they up when they were being sprayed or is this a second flush? It's quite common to spray and a few days later have a new flush of weeds emerge."
Is the product a factor? It's important to understand the herbicide used, says deMilliano. Does the product exert a high level of control or is it expected to only provide suppression? How long does it take for the product to show results? This information can typically be found on the product label, in provincial crop protection guides, or by calling the retailer or manufacturer.
Is a second application worth it? There are three reasons producers would make a second application, says deMilliano: impact on yield, impact on seed bank, and pride. "The impact on yield can generate immediate short term benefits," he says. "Avoiding an increase in the seed bank has longer term implications and pride is something that is difficult to measure."
Given suspicions of a possible resistance issue, growers may want to take a close look at their herbicide selection for the second pass over the field, says deMilliano. "It may warrant using a herbicide from a different group if crop and weed staging allow it."
Do I have a resistance problem? Finally, if you have any suspicions whatsoever that there is herbicide resistance in your field, follow up. "Check your herbicide records closely and submit some seed for resistance testing at the end of the season," says deMilliano.
Author: Jeff Melchior
Sponsored by: AgTech Centre
