Current Crop Topics

The latest information on crop management and agronomy issues in field and horticultural crops in Manitoba.


General Agronomy

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 2002 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.


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 31, 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. 
For more information on resistant weeds and weed management, visit the Manitoba Agriculture website:



Waterhemp Now A Manitoba Weed

Can you identify the plants in the two pots below?
The plants on the right are redroot pigweed (Amaranthus retroflexus); the plants on the left are waterhemp (Amaranthus tuberculatus).
Unfortunately, the need to distinguish between these closely related weed species has become a reality for Manitoba producers and agronomists since waterhemp was found in the province in the fall of 2016.  Suspect plant specimen collected from a soybean field in the RM of Taché was verified as waterhemp by staff with the Agriculture and Agri-Food Canada Collection of Vascular plants in Ottawa.  Waterhemp occurs in neighbouring states and provinces, including Minnesota, North Dakota and Ontario.
Both species thrive in agricultural fields where they compete with crops for nutrients, moisture and light.  Waterhemp has no hairs on its stem or leaves, which can be used to distinguish it from redroot pigweed when plants are small.  The lack of hairs give waterhemp leaves a ‘glossy’ look unlike that of the ‘dull’ green leaves of redroot pigweed.  Also, waterhemp leaves are lanceolate in shape (longer than they are wide) compared to the more ovate leaves of redroot pigweed.  Colour is not a reliable identifying characteristic since both species can be green, red or variations of the two colours.
Mature waterhemp plants tend to be more branched than redroot pigweed.  And unlike redroot pigweed, which has male and female flowers on the same plant, waterhemp has separate male and female plants.  Waterhemp inflorescence are long, slender and vary in colour compared with the compact, prickly inflorescence of redroot pigweed.  Like most pigweeds, waterhemp is a prolific seed producer with up to a million seeds per plant (under ideal conditions).
Waterhemp populations resistant to group 2, group 9 (glyphosate) and group 2+9 exist throughout the US, including Minnesota and North Dakota, and in Ontario.  Seed from one of the plants found in Manitoba have been sent to Ontario for resistance testing.
Information on waterhemp will be added to Manitoba Agriculture’s weeds webpage and will be included at the Weed Seedling Identification Day (hosted by the Manitoba Weed Supervisors Association).  Manitoba Agriculture staff will conduct a waterhemp surveillance program in and around the RM of Taché in 2017.
Additional information on waterhemp is available at: (excluding herbicide recommendations).
Photo: Waterhemp in Manitoba, Tone Ag Consulting  



Fall is the most effective time to manage certain weed species.  The recommendations seem clear-cut:  winter annuals = fall herbicide application/tillage; annuals = no fall management.  But figuring out the life cycle of the weeds in your field this fall is the catch...
Bromes, cleavers, chickweed, night-flowering catchfly, narrow-leaved hawk’s-beard, shepherd’s-purse and stinkweed are all facultative winter annuals, meaning that they can germinate in either the fall or the spring depending on environmental conditions.  These weeds are often best managed in the fall, if populations warrant it.  In general, waiting until about this time of year maximizes fall-germinating flushes of winter annuals.  If using a herbicide, consider weed stage and the weather forecast, prior to application.
The problem is, given the right conditions – like the long falls and mild winters we’ve had the last few years –several of our annual weed species can also successfully overwinter:
Biennial wormwood – Despite its name, biennial wormwood behaves like an annual in agricultural fields.  When scouting, estimate the average growth stage of biennial wormwood populations in a field.  If the majority of the plants have already set seed, a fall herbicide application won’t help.  An application may be worthwhile only if there is a large flush of biennial wormwood that haven’t set seed and are less than ~3 inches tall.  Herbicide tank-mixes containing glyphosate + group 4 are more effective than glyphosate + group 2 on this weed.
Round-leaved mallow (RLM) – This annual weed can act as either a winter annual or a short-lived perennial, although it is more sensitive to freezing than our common winter annuals.  Mild winters in 2015 & 2016 provided the right conditions for RLM to overwinter, allowing it to become (even more) problematic in certain fields over the last few growing seasons.  Long range forecasters are predicting a harsh winter across the prairies this year, which should control RLM.  However, if you have little faith in forecasts and decide to apply a herbicide, glyphosate mixed with either Distinct or DyVel DSp has activity on this weed.
Stork’s bill – Like biennial wormwood, stork’s bill tends to be predominantly an annual in Manitoba.  If this is a problem weed for you, scout affected fields to determine average weed stage.  Again, if most of your stork’s bill has set seed you’re better off working on a plan for next year.  Stork’s bill, especially larger plants, is relatively tolerant of many herbicides.  If you decide to apply a herbicide because of stork’s bill this fall, glyphosate + group 2 or glyphosate + group 2 + group 4 on weeds up to the 4 to 6 leaf stage is probably your best bet. 
Information on more weeds and their life cycles is available at: 



Soil Fumigants

Soil fumigants are pesticides that form a gas when applied to soil. Due to the toxicity of soil fumigants and the potential for gases to move from the soil to the air, soil fumigants are classified as restricted use pesticides.

Prior to using soil fumigants read the entire product label. More information on the below requirements for use can be found on the product label.

Requirements for the use of soil fumigants include:
  • Soil fumigation licence: Soil fumigants are only to be used by individuals holding a soil fumigation licence. This restriction applies to all handlers of soil fumigants. Soil fumigation certification is currently not available in Manitoba. Users of soil fumigants are required to certify in Ontario and apply for a licence in Manitoba. More information on certification and licensing can be found below.
  • A Fumigation Management Plan (FMP) for all soil fumigation applications: A FMP must be developed prior to the start of any application. Instructions for the preparation of a FMP are required to be on the product label. To create a FMP follow label instructions or use the template found at: 
  • Mandatory Good Agricultural Practices (GAPs): GAPs improve the safety and efficacy of soil fumigation and must be followed during all fumigant applications. These application practices include monitoring of weather conditions, soil preparation, monitoring of soil temperature and moisture, and proper soil sealing.
  • Notification: The applicator must verbally warn workers of the application. Fumigant Application signs must be posted on all entrances to the application block.
  • Restrictions for workers re-entering treated areas: Only workers involved in soil fumigant application are allowed in the treated area following application. See label for duration of restriction.
  • Buffer zone: A buffer zone is an area established around the perimeter of the treated area where entry by anyone other than fumigant handlers is prohibited. A buffer zone must be established for every fumigant application. Buffer zone distances are based on factors such as application rate, area treated, and the application method. See product label for information on determining buffer zone distances.

Steps to obtaining a soil fumigation licence:

  1. Complete the Application for Pesticide Certification
    • The application form is available on the Ontario Pesticide Training and Certification (OPT&C) website under Exterminator Training:
    • The application fee is $165 per person and includes the core manual, one module manual, and the examination.
    • A soil fumigation licence requires certification in Agriculture Core and Fumigation Soil. On the application form indicate that you are applying for certification in Fumigation – Soil.
    • Indicate the location and date of the exam under Examination Information:
      • Location: Carman, Manitoba
      • Date: June 29, 2016
  2. Read Agriculture Core and Fumigation Soil manuals
  3. Write the exam
    • The exam is scheduled for 9:00 - 11:30 am on June 29, 2016 at the MAFRD GO Centre in Carman, Manitoba (65 3rd Avenue NE, Carman, MB)
  4. Obtain general liability and pesticide drift insurance
    • Individuals applying for a Pesticide Applicator Licence are required to have general liability and pesticide drift insurance. Individual or company policies are acceptable.
  5. Apply for a licence

For more information on soil fumigation certification and licensing contact:

Anne Kirk, Pesticide Minor Use and Regulatory
Manitoba Agriculture, Food and Rural Development
Phone: (204) 745-5663 , Email: