Corn Concerns & Curiosities

 A number of things are showing up in the corn patch, now that corn is actively growing and farmers/agronomists are scouting for emergence, growth and weeds.

  • Uneven emergence
  • Herbicide Injury
  • Scorthed Leaves
  • Wilted/Discoloured Corn
  • Sand-blasted Corn
  • Grey Corn Leaves
  • Yellow/Twited Corn Leaves

To see pictures and find out what is causing them see corn-concerns-curiosities (PDF 1.88MB)

Have a follow-up question?

Don’t Overlook Group 2 Herbicide Resistance

You’ve probably read about media dubbed ‘superweeds’ like glyphosate-resistant palmer amaranth and giant ragweed. Glyphosate-resistant weeds often earn this distinction because they are viewed as a greater management hardship for producers than weeds resistant to other herbicide mechanisms of actions (MOAs).  And maybe rightly so.  Farmers dealing with glyphosate resistant weeds elsewhere in the world have been reduced to tillage and hand rouging for weed control in some crops.

But, while glyphosate use dominates the Roundup Ready corn, soybean and/or cotton rotation in the US, group 2 herbicides play an (equally?) important role in our more diversified cropping system. For example, group 2 herbicides are used in crops like alfalfa, corn, dry beans, field pea, potato, soybean, sunflower, and in Clearfield and other group 2-tolerant crops.  These herbicides are also a popular choice for group 1-resistant grassy weed control in cereals.

The point of this article isn’t to downplay the importance of glyphosate resistance but to elevate consideration of group 2 resistance. In Manitoba, over 10 weed species are known to have biotypes resistant to group 2 herbicides.  And herbicide-resistant weed surveys led by AAFC indicate that the prevalence of certain species is increasing (Figure 1).

Figure 1: Prevalence of group 2 resistance in Manitoba in 2003 and 2008 as a percent of the weed species population surveyed (Beckie et al).


The following practices can help reduce the risk of developing herbicide resistant weeds and/or managing existing resistant weed populations:

  • Diversifying your crop rotation;
  • Using multiple herbicide MOAs effective on target weeds (e.g. herbicide ‘layering’, tank mixing);
  • Practising good basic agronomy (variety selection, seeding rates, etc.);
  • Judicial use of tillage.

If you suspect group 2 resistance in a weed species on your farm, it’s best to verify this by herbicide-resistance testing. Unfortunately there‘s no quick method – seed from the suspect population needs to be allowed to mature and collected.  Samples can be submitted to AgQuest for testing in Manitoba.

In my opinion, knowing if you have group 2 resistance and assessing your risk factors is worth it. Because while glyphosate resistance is grabbing headlines, group 2 resistance may be quietly growing in your fields.

Have a follow-up question?

Testing Weeds for Herbicide-Resistance

Do you have weeds that survived this year’s herbicide application(s)? Since there are many factors that can contribute to weed escapes, consider:

  • The distribution of escaped weeds. Herbicide-resistant weeds tend to occur in patches as opposed to geometric patterns (e.g. spray miss) or throughout the field (e.g. tolerant weeds).
  • Possibility of reduced herbicide efficacy. 2016 was a challenging year for weed management due to untimely and excessive rainfall. In many cases, weeds escaped because of herbicide application timing with respect to weed growth stage, limited herbicide choices because of crop growth stage (when producers finally could get on their fields) and product rainfastness.
  • Weed species. Annual weed species, like wild oat, green foxtail, cleavers, kochia, hemp-nettle, smartweeds, ragweeds and wild mustard, may be more likely to develop resistance compared with other weed species. Because the development of herbicide-resistance is based on chance, resistant weed patches are typically a single species, as opposed to non-resistant weed escapes, which may affect multiple weed species.

Suspect weed escapes can be confirmed as resistant or susceptible by herbicide-resistance testing. For most weeds, dry, mature seed is required for the analysis.  Although more is better, many labs require at least 100 g of small weed seeds (e.g. cleavers) and 200-250 g of large weed seeds (e.g. wild oat).  Weed seed samples should be submitted by December 31st, 2016 to either:

For suspected glyphosate-resistant kochia, a genetic-based tissue test is also available from the Pest Surveillance Initiative: In this case, about 5 to 10 g of green plant tissue (e.g. leaves and stems from plant tips) is needed for the analysis. Samples should be placed on ice and shipped immediately after collection. The advantage of the genetic test (vs. seed analysis) for kochia is the ability to determine resistance in-season.


Submitted by: Jeanette Gaultier, Provincial Weed Specialist, Manitoba Agriculture

For more information on resistant weeds and weed management, visit the Manitoba Agriculture website:


Have a follow-up question?

Getting What You Expect Out of Your Herbicide – Part 1: Cold Spring Weather

Submitted by Anastasia Kubinec, MAFRD Oilseed Crop Specialist

As seeding and spraying collide, for pre-seed burn off and winter cereals in-crop applications, here are some tips for early applications to get the best performance out of your herbicides. Part 1 is only covering the pre-seed herbicide options and the types of products that would be used on winter cereals.

  • Spray when weeds are actively growing
  • Spray when the temperatures are at least 150C
  • Use the water volume stated on the label
  • Use the surfactant or adjuvant included or recommended
  • Watch and follow the rain free period

Some of the herbicides that work well under cool conditions are:

  • Most Group 4 herbicides
  • Most of the residual herbicides of any group
  • “Fop” group 1 herbicides (ex. Horizon)
  • Carfentrazone (ex. CleanStart, Aim)

Frost and Spraying

Actual temperature and duration of the frost will impact plant growth, how well the herbicide will work and crop safety. For most Group 1 and Group 2 herbicides, it is better to wait at least 48 hours before application to allow weeds and the crop to recoup and start actively growing.

For pre-seed glyphosate, after a light frost (0 to -2°C), spraying could resume 24 hours later, but temperatures need to go up. Avoid spraying until the temperature is above 8°C….the closer it is to 150C and above, the better the control. If a harder frost occurs, avoid spraying for 48 hours and assess the damage on the target weeds. You want to see that plants are still green and actively growing, and daytime temperatures are warming up.

Frost occurring within 24 hours after spraying can also impact control.

Have a follow-up question?

Reminder on Herbicide Tank Mixing Order

Here is a reminder on Tank Mixing Order. 

  1. Dry flowables or dispersible granules
  2. Wettable powders or other solid formulations
  3. Liquid suspension concentrates
  4. Liquid soluble concentrates
  5. Emulsifiable concentrates
  6. Surfactants or adjuvants
  7. Anti-drift or anti-foaming additives

Thank-you to Ingrid Kristjanson, FPA in Morris for the reminder!

Also see more informaiton on mixing order in the Guide to Field Crop Protection 2015 on pages 14 & 15

Have a follow-up question?


Submitted by Jeanette Gaultier, MAFRI Pesticide Specialist

We cautioned last week about herbicide application in cool temperatures. Since the 2013 growing season has not been setting records for heat, you may be stuck trying to balance optimal temperatures with recommended herbicide application timings. 

Crop injury in some situations may be unavoidable since both temperature and incorrect crop staging affect crop tolerance to herbicides.  The good news is that you may not need to be concerned if you see symptoms in your crop post application since not all injury contributes to yield loss.  

  • Growth Setback – Stunting of cereal crops treated with certain group 2 herbicides (e.g. Everest, Simplicity) or with Avenge is possible but generally will not affect yield.  Improper timing or cool temperatures during group 1 herbicide application can also cause setback.  Do not worry about yield penalties though, unless stunting is severe or other growth setbacks, such as reduced tillering or stem/head kinking, are present.
  • Chlorosis –Pale plants and chlorosis are indicators of reduced photosynthesis, a process necessary for crop growth.  However, slight chlorosis of leaves and growing points will not lead to decreased yields if plants are able to ‘grow out’ of the stress early in the season.  For example, beans treated with Basagran + group 1 herbicide may have both green and yellow leaves on the same plant without affecting growth.  Cool temperature application of group 2 herbicides and/or glyphosate may also cause slight chlorosis, that crops can grow out, depending on the severity.  Do not worry unless symptoms are severe; plants are more chlorotic than not or deformed growth tips, necrosis are also present.

The above are generalizations and yield penalties from herbicide injury in specific crop situations can vary depending on the severity of injury symptoms, crop growth stage as well as other factors that may be contributing to crop stress.

If you are concerned about your crop or want a second opinion, crop samples can be submitted to your local GO office or the Crop Diagnostic Centre.  MAFRI staff are able to provide visual assessment of crop injury only; you may be referred to a private lab for herbicide residue or nutrient testing if assessment of the symptoms is inconclusive. 





Have a follow-up question?

My flax did not flower and looks like the growing point is dead, could it be herbicide injury?

It very well could be herbicide injury or potentially environmental stress damage.  Applying herbicides especially those with a broadleaf control component under very hot and humid conditions can damage flax.  The growing point may be damaged even more if spraying occurred after flax is 6 inches or taller.  Spraying flax when the temperature has not exceeded or is not going to exceed 28C within a 48 hour timeline around the herbicide application will help reduce the injury to flax.  Also, only spraying products that are registered on flax will help reduce injury as they have been tested and approved to provide acceptable weed control without causing unacceptable crop injury.

Have a follow-up question?