Author: Mike Cowbrough

  • Product Failure or Sprayer Contamination? A Case Study

    Product Failure or Sprayer Contamination? A Case Study

    It’s unfortunate, but every year we encounter a few instances of mysterious crop injury following an application. Was it a product formulation issue? Was it a mismatch between genetics and chemistry? Was it a stressed crop? Or was it a contaminated sprayer? Sometimes we never find out. Sometimes we do.

    The Investigation

    In this case study, Roundup WeatherMAX was applied to Pioneer P23Z58E (Enlist) soybean and the applicator noticed significant necrosis around 5 days after application (Figure 1). They suspected an issue with the variety and/or the product.

    Figure 1 – Damaged soybean,

    In response, we compared three spray solutions (Figure 2):

    1. Herbicide from the grower mixed with clean water
    2. Herbicide from an independent source mixed with clean water, and
    3. Spray solution collected directly from the grower’s sprayer.
    Figure 2 – A closer look at all 3 solutions. This bottle on the left (solution from sprayer) is clearly different from the other two, suggesting some manner of contamination. Significant or not, the freshly-mixed WeatherMAX from the grower (middle) appears to be a deeper blue colour than that from the independent source (right).

    The sprayer sample contained visible sediment (Figure 3) and differed in appearance from the freshly mixed solutions, suggesting potential contamination.

    Figure 3 – Sediment in sprayer solution sanple.

    The treatments were applied to two soybean varieties to determine whether varietal sensitivity was involved.

    What the Bioassay Showed

    Soybeans treated with either source of freshly mixed herbicide showed no injury (Figures 4 and 5). In contrast, soybeans treated with spray solution collected from the sprayer developed significant leaf necrosis (Figure 6). Similar responses occurred across two soybean varieties, ruling out both product quality and varietal sensitivity as causes of the injury.

    Figure 4 – There was no injury where the grower’s unused herbicide was mixed with deionized water and sprayed 9 days earlier.
    Figure 5 – The independently-sourced herbicide mixed with deionized water gave no crop injury.
    Figure 5 – Significant leaf necrosis from the bottle containing spray solution collected from the grower’s sprayer.

    Conclusion

    Field bioassays are valuable diagnostic tools that sprayer operators can use themselves to determine the cause of unexpected results. In this case, the investigation demonstrated that crop injury was caused by residual contamination within the sprayer rather than by the herbicide formulation or soybean genetics.

    Visible sediment or unusual spray solution appearance may indicate contamination. Even when a sprayer has been cleaned, residues can remain in plumbing, filters, and other components if cleanout procedures are not thorough. Effective sprayer hygiene is essential when switching between products.

    Thorough sprayer cleanout remains one of the most important practices for preventing avoidable crop injury and maintaining confidence in pesticide performance. The grower’s final comment following the assay was “we did it to ourselves”.

  • Ragweed Control

    Ragweed Control

    When a grower asks two specialists the same question, they can receive skewed (or even contradictory) answers. In this case, however, we were all on the same wave length. The following exchange took place between an Ontario grower with a nozzle/weed question and Jason Deveau (nozzles) and Mike Cowbrough (weeds).

    Grower: Can you please help me? I have been spraying fomesafen (trade name Reflex) in soybean with an AI 11005 nozzle at 20 gallons per acre for years. However, for the last two years we have not been getting ragweed control. We get a quick burn, but then the ragweed regrows. The agrichemical company recommended going back to a TeeJet XR flat fan nozzle to give “excellent coverage for a contract herbicide”. Is there a nozzle choice that gives coverage but better drift control than an XR nozzle?

    Nozzle Specialist: PLEASE don’t use an XR nozzle for herbicide applications. Unless you’re using a 06 (grey), XR’s create a Fine or Very Fine spray quality, which is irresponsible for any herbicide. Drift is not my only concern with an XR; Small droplets are so easily influenced by temperature and wind that you’d be surprised how few will even reach the target.

    20 gallons seems like a relatively high herbicide volume per acre, and even with a Coarse droplet size you should be getting excellent coverage. Let’s ask Mike if yours is a common problem. It could be timing, or some form of resistance, which would trump nozzle selection.

    Weed Specialist: I have included an excerpt from the Problem Weed Book where I summarized common ragweed efficacy with post-emergence soybean herbicides. You will note that over the course of nine trials, there is quite a variation in control of common ragweed with fomesafen (Reflex).

    Now – looking at the table above, you might ask yourself “Why wouldn’t I use FirstRate? It seems to be more consistent at controlling ragweed than Reflex.” Yes, but there are a lot of common ragweed populations in Ontario that are resistant to FirstRate. There are no known populations of ragweed in Ontario that are resistant to fomesafen. Globally, there are instances of PPO (Reflex) resistant ragweed populations, so it would be wise to rule that out. In the future I can coordinate the testing of your common ragweed for herbicide resistance.

    In my experience, there are three factors that influence control of ragweed with fomesafen:

    Factor # 1 – Plant staging

    We see trouble when applications are made to common ragweed beyond the 6-8 leaf stage of growth. Since fomesafen is a contact herbicide, the more advanced the plant, the more growing points that need to be “burnt off” and the greater likelihood of re-growth from those that were not sufficiently damaged. Typically things look great one week after application, but within two or three the plant has branched out and started to regrow.

    Factor # 2 – Water volume

    If using AI nozzles, increased water volumes (20 gal/ac) have been demonstrated to provide better control of common ragweed with fomesafen than lower water volumes (10 gal/acre). Citation is here. The following graph shows the influence of water volume on control of velvetleaf with fomesafen, the same trend exists with common ragweed.

    Factor # 3 – Temperature (Time of day)

    While this has not been proven in the literature, I’ve observed it under field conditions. When applications are made in the heat of the day, control is better than when applications are made in the evening. Generally, ambient air temperature and post-emergence weed control are related and the response can be significant with certain weed species and herbicides.

    I agree that we shouldn’t be using the XR nozzle. I wonder if the agrichemical company recommended it based in part on the citation I provided under Factor # 2, where the authors found that water volume did not influence the Reflex’s control of common ragweed using flat fan nozzles.

    Fomesafen is about the best active we have for common ragweed in soybeans and edible beans, but it can be very inconsistent. If you have a decent soil applied herbicide down in soybean (e.g. Triactor, Bifecta, Lorox), that will go a long way to lower the population density of common ragweed and make it easier to target (and control) smaller plants with fomesafen.

    Grower: Thanks for the information. I did not know that fomesafen had such a range in ragweed control. In OMAFRA Publication 75 reflex has the highest rating. A majority of the time I use reflex to control escaped ragweed from the pre emerge program. Our 20 g/ac volume seems to help control, but not enough to get above 90%. I’ll avoid the XR and look to time-of-day to help with control.

    Weed Specialist: The ratings in Publication 75 are challenging to develop because they are based on ideal conditions, but life rarely is. For example, the maximum labelled stage for common ragweed to be adequately controlled by Reflex is 4-leaf. The ragweed in the following photo is actually pushing the 6-leaf stage.

    This stage can be difficult to target when ragweed can push out two new leaves every four-five days. Even with glyphosate, the rate of glyphosate needs to be increased 100% to maintain control should the ragweed exceeds 10 cm in height. Rate increase is not an option for Reflex (unless you’re a fan of really brown soybeans).

    Best of luck with your applications. Ragweed is always the Achilles heel in our IP soybean weed control trials.