Are You Staging Your Corn Correctly?

Submitted by Anne Kirk, Cereal Specialist, Manitoba Agriculture

For more information see Manitoba Corn Growers newsletter for Growth Stage Information for Herbicide Application

When applying post-emergent herbicides, proper corn growth staging is extremely important. Herbicide labels may refer to plant height, crop growth stage, or both when listing crop stage timing. Farmers and agronomists need to accurately stage corn plants to ensure that herbicides are being applied at the correct stage. Some common methods of determining corn growth stage are listed below.

Figure 1: Corn Staging Diagram

Figure 2: Live Plant Corn Staging


Corn Height Method  Measure from the soil surface to the highest point of the arch of the uppermost leaf whose tip is pointing down.  Don’t measure to the “highest point” on the plant, which is often the tip of the next emerging leaf above.  Refer to Figure 1/2 on how to correctly determine the height of a corn plant. Corn height varies due to growing and crop management conditions, and is not the most accurate way to stage corn.

Leaf Over Method Count the number of leaves, starting from the lowest (the coleoptile leaf with a rounded tip) to the last leaf that is arched over (tip pointing down). Younger leaves that are standing straight up are not counted.  In Figure 1/2, the corn plant would be at the 4 leaf stage using the leaf over method.

Leaf Tip Method – Count all leaves, including any leaf tips that have emerged from the whorl at the top of the plant. In Figure 1/2, the corn plant would be at the 6 leaf stage using the leaf tip method.

Leaf Collar Method (V-stage) –   Count the number of leaves with visible collars, starting from the lowest (the coleoptile leaf with a rounded tip) and ending with the uppermost leaf with a visible leaf collar. This method is the most common staging system and involves dividing the plant development into vegetative (V) and reproductive (R) stages.  The leaf collar method is generally also the easiest to use, and related better to the physiological stage of the plant and therefore to the effects of herbicides.  In Figure 1/2, the corn plant would be at 3 leaf stage (V3) using the leaf collar method.

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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:


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‘Is my crop too stressed for herbicide application?’ has been a common question this week. As mentioned in an earlier post, recent wet weather has coincided with the window for post-emergent herbicide application in many crops.

The ability of a crop to ‘tolerate’ a herbicide application depends on its capability to metabolize or compartmentalize the active ingredient before it causes plant injury. This is the basis for herbicide selectivity.  Excess soil moisture reduces oxygen availability to the crop, which affects physiological processes like metabolism.  As such, water-stressed crops may not be able to effectively metabolize herbicides, resulting in crop injury.

What can you do reduce the risk of crop injury from herbicide application?

  • Check for new growth as an indicator that the crop has resumed physiological processes like photosynthesis and metabolism. If you don’t see any new growth in a stressed crop, wait 24 hours and re-assess.
  • Consider your herbicide choice. Some herbicides are more likely to result in injury to stressed crops than others. Group 2 herbicides, especially the more residual products, are an example. However, the risk of crop injury can also vary among chemistries within a herbicide group. For instance, pinoxaden may be a safer group 1 on stressed wheat than fenoxaprop.
  • What’s in your mix? Increasing the number of different products in your tank can overwhelm a stressed crop’s metabolic capabilities. Tank mixes that cause antagonism generally increase crop safety but also have decreased efficacy on weeds. Avoid using products that ‘heat up’ a tank mix, which can increase the risk of crop injury. Talk to your chem rep if you’re unsure; they may recommend different tank mixes or separate passes.
  • Wait until the end of the day. Applying herbicides in the evening can reduce their impact on a stressed crop, although research has shown that later-in-the-day herbicide applications can also be less efficacious. The trade off may be worth it since daytime temperatures over 27°C can add additional stress to the plants and can increase the activity of certain herbicides.
  • Check the forecast for rain. Trying to get a herbicide application on before a forecasted rain works for healthy crops but may not be the best strategy for stressed crops as addition rain may compound the problem. Besides additional stress, shallow, stressed crops roots can be impacted by herbicides moving into the root zone as well.
  • And finally, compare the risk of potential crop injury (i.e. how stressed is the crop, what proportion of the field is stressed, etc.) to the risk of yield loss due to weed pressure. If the stressed crop is limited to a few low spots in the field, it’s likely worth risking a few acres of injury to protect yield from weed competition. However, if most of the field is water-stressed, it might be worth moving on to a different field and returning after a day or two.

Submitted by Jeanette Gaultier (Weeds Specialist) and Ingrid Kristjanson (FPE Moris), Manitoba Agriculture.

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Wet field conditions across the province have many questioning the timeliness of their post-emergent herbicide application. Unfortunately, options are limited.  The stage of the crop and/or weeds dictate whether you  1) spray now and deal with the ruts later, or 2) hold off a few days in hopes of dryer weather.

Consider the following when prioritizing fields for herbicide application:

  • Crop and weed stage are critical. Applying herbicide(s) outside of the crop and/or weed stage indicated on the product label can result in crop injury and decreased herbicide efficacy. Be sure to check the condition of your crop before spraying, since stressed crops may be more susceptible to herbicide injury. The window for in-crop herbicide application varies by product and crop. Crops with relatively few herbicide options, like field peas, may have a small window of opportunity.
  • Crop competitiveness. The critical weed-free period indicates when and for how a crop needs to be kept weed-free to minimize yield loss. In general, the more competitive the crop, the shorter the critical weed free period. Therefore, your pre-seed burnoff, pre-plant or pre-emergent herbicide application may carry you further with competitive crops (e.g. cereals) compared to crops like soybean, corn or flax.
Picture3*Current University of Manitoba/Manitoba Pulse & Soybean Growers project to refine the critical weed-free period for soybean grown in MB.

  • Economic thresholds (ET). Thresholds can help determine if you need to apply an in-crop herbicide based on the density of a particular weed. For example, 6 to 16 plants/m2 of wild oat can result in less than 5% yield loss in spring wheat (actual ET depends on weed and crop staging; refer to page38 of the 2016 Guide to Field Crop Protection). Similar ETs exist for select grassy weeds in wheat, barley, canola, and flax, for kochia and biennial wormwood in sunflower and for volunteer canola in soybean (see below). The downside to weed ETs is that they are species specific and they don’t consider weed seed return to the weed seedbank.


A few other spray tips:

  • Don’t ‘save time’ by skimping on sprayer clean. Refer to the product label & page 15 of the Guide to Field Crop Protection for clean out instructions.
  • Check out SPRAYcast ( for a 3-day forecast of optimal spray times.

Happy herbiciding!

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

2016 Guide to Field Crop Protection:


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Avoid Herbicide Injury in 2013

Cool periods in May and June 2012 played havoc with some herbicide applications, leading to injury in normally tolerant crops.  Examples from last season include group 1, 2 and/or 4 injury in wheat and barley and group 2, 9 and/or 10 injury in canola. 

Crop tolerance to certain herbicides or herbicide groups allows for selective, post-emergent weed control.  Tolerance is achieved by metabolism or deactivation of herbicide molecules within the plant.  The process of deactivation requires that plants are actively growing and not stressed.

All crops have a minimum temperature above which plants are able to carry out normal functions for growth (e.g. photosynthesis, metabolism, etc.).  This temperature is often referred to as ‘base temperature’ and is used to calculate heat unit accumulation (e.g. GDDs) for individual crops (Table 1).  Base temperature is not the same as freezing point.

Table 1. Base Temperatures for Select Manitoba Crops

Crop Base Temperature (°C)*
Canola 0 – 5
Cereals 0 – 4.5
Corn 6 – 10
Flax 4
Potato 5 – 7
Soybean, beans 5 – 8
Sunflower 5 – 8

It’s important to note that, while we use common base temperatures for certain crops, actual base temperature depends on crop variety, growth stage and cold-hardening.

Given the above, herbicides should be applied when day time temperature is ≥ 10°C to ensure optimal crop tolerance.  This will also result in better weed control.  MAFRI also recommends producers:

  • Aim for day time temperatures of ≥ 10°C on the day of application and for 2 days after application
  • Apply herbicides earlier in the day if night time temperature is expected to be below 10°C
  • Refer to the product label for specific instructions regarding herbicide application and environmental conditions

Submitted by:  Jeanette Gaultier, MAFRI Pesticides – Minor Use & Regulatory Specialist

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