Four Reasons to Monitor Herbicide Efficacy


The in-crop spraying season is just around the corner for a number of growers in Western Canada. Spraying at the proper time allows your crop to use nutrients and moisture without having to compete against various weeds, as well as allows your crop to metabolize herbicides more readily. What I want to focus on now is the importance of checking herbicide efficacy.

I know a lot of growers who apply generic clodinafop or fenoxaprop product and generic Buctril M year after year in their cereal crops and never really go back after they spray to get a better idea of what really occurred after they sprayed. This is why a lot of them spray the same herbicide year after year, assuming it did the job they wanted. A lot of the time that is the case, but sometimes they are giving up yield.

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There are a number of things to check for when doing herbicide efficacy checks. The first thing I like to look at is the crop safety. I like to look at the newest leaf coming out as well as the older leaves and general colour and look of the crop. If you run into issues it could mean you had a rough tank mix, improper timing or sprayed when the conditions weren’t optimal. This is also why if you have proper notes you can check back and confirm staging was proper, tank mix was fine, but your records show that the temperatures dipped down to 5 degrees the day after you sprayed which can alter a products safety on the crop. This way you know for the future how your crop reacts to certain products under specific conditions.

Next thing to look for is weed control. I like to put percentages on things to gauge an idea of if control was where it should be or if there was something that went wrong. The way to do this is to do random counts throughout the field on the problem weeds and determine just how well the product worked by looking at how many plants of a specific species are dying and how many are simply hurt and will regrow. You can keep these records for the future, and next time you have a wild buckwheat problem you can note that Benchmark (for example) was the product over the past few years that gave you the best wild buckwheat control. The best control is an easy decision in years to come if you have a field with a wild buckwheat problem.

Checking out the efficacy also allows you to find funky patterns of weeds getting out of control. This could mean there was an issue with heavy, heavy weed pressure and the coverage wasn’t there, which may be something to note for next year, adding a reminder to increase water volume. Secondly, it could mean you have ran into resistance issues. If you sprayed with a Group 1 grassy herbicide and you are noting very specific patches across the field where wild oats didn’t die it may be a good time to take samples and send them away for a resistance test. Lastly, funky patterns can also show if you had certain misses through out the field which could mean your GPS was potentially off, causing some misses.

Weed control can also be compromised when you are looking at products that prefer to be in solutions that aren’t a high pH. This means that there are some pesticides that can be antagonized by waters with a pH above 7. High pH can cause issue with products such as glyphosate or Group 1 dim herbicides, for example, then you can expect some diminished weed control. This will be noticeable as weeds will be sick looking, and regrow, almost like they had been sprayed with a cut rate of the product. If you haven’t had your water tested and note scenarios like this out in your fields then it may be a lead for you to look into.

Noting conditions your products were sprayed under, keeping records and then checking herbicide efficacy can be an effective way to decide on what products to use in future years. You can also determine what conditions they thrive in or what conditions they are not as strong in. Taking some extra time to check efficacy seven to 21 days after spraying can save you some headaches later on in the growing season.

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