Herbicides Pest Management Weed Management

Why Scout for Weeds after a Herbicide Application?

Kristen Callow, OMAFRA Weed Management Program Lead – Horticulture

Oh baby its cold outside … so why in the heck am I thinking about scouting for weeds.  Well, as you can imagine I think about weeds a lot.  I know how boring is that? For me it’s exciting … no comments please.

Weed escapes
Weed escapes

From my perspective it’s never too early to get organized and plan your priorities for the field season, including which crops you want to plant in which fields, along with what herbicides you are going to apply.  So why not start thinking about how and when you are going to scout your fields for weeds and what you are going to do if you find weed escapes. I truly believe that weed scouting is the only way to know for sure what weeds are present and will help you to determine how to manage your changing weed populations over time.  In the end, weed scouting will save you time and money over the years.  Believe it!  I know it seems tedious at the time you are doing it but it is absolutely necessary because it will save you money in the end and a lot of headaches.

Many factors can contribute to the presence of weeds after a herbicide application and later in the growing season. Scouting is the only way to know which weeds are present, and their patterns in the field can help to understand why they are present.

Recommended procedures for scouting weeds after a herbicide application:

  • scout 7 to 14 days after each herbicide application and near harvest,
  • identify and record the weed species present,
  • determine the distribution pattern of plants in the field. If possible, correlate with an application pattern or identify as random,
  • determine if the plants that are present survived a previously applied herbicide or emerged after the last herbicide application,
  • observe individual plant responses, especially if plants survived a herbicide application, and
  • look at previous field history to understand what changes may be occurring.

It is extremely important to investigate and rule out all other factors affecting herbicide performance before suspecting herbicide resistance. Past cultural, chemical, and mechanical weed management practices have all influenced the current weed community. These practices can also provide insight into the likelihood that weed populations may become herbicide-resistant. Some important information to consider includes:

  • number of herbicide mechanisms of action used across a rotation,
  • number and kinds of crops in rotation,
  • use of mechanical and cultural weed management practices, and
  • the presence of weed species, including density and distribution, over time.

The probability for herbicide-resistant weed populations to evolve is increased as diversity in weed management practices is reduced. This can occur when there is a reduction in the number of herbicide mechanisms of action used with or without simultaneous reductions in the use of mechanical and cultural management options.  In the early years of herbicide resistance evolution, herbicide-resistant weeds may be scattered as single plants across the field, or present in single or multiple, small patches.  In later years, after the herbicide-resistant weed population has grown, small to moderate, or large and dense patches of weeds may be present.

When only a few plants survive a herbicide application and during the early years of herbicide resistance, consider hand-removing them and making adjustments to future weed management strategies. Waiting until numerous dense weed patches evolve during the later years of herbicide resistance can contribute to profit losses because of reduced yields and increased input costs.

Keeping records on weed populations, including their density and distribution, will help you to note important changes that may be underway in the field, and then make the necessary adjustments to future management strategies.

In summary, please remember that scouting fields to determine the reasons for weed survival after a herbicide application is important;  field history, weed biology, environment, application parameters, crop cultural practices, and herbicide resistance are factors that can contribute to weeds surviving the application of a herbicide;  symptomology may differ between the observations of low-level and high-level resistance; and  confirming herbicide resistance early and removing herbicide-resistant weeds by hand can decrease the spread of herbicide-resistant weeds, thereby reducing the costs required to manage them.



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