The Growing Point | September 2019

Weed resistance at harvest and a comment on glyphosate

This time of year the focus is on getting the crop off the field before the snow flies. But it can also be great time to assess the efficacy of the weed management program that was used this year.  It’s worth making some observations and taking notes to help you develop your management program next year. Particularly from the swather or the combine you’ve got a great view where you can inspect the distribution of those weeds. And the distribution can be very important.

First things first, with the weather this year we are seeing a lot of second flush weeds. So if you’re noticing a fairly even distribution (as even as weeds are ever distributed) across the field there is a good chance that those weeds came up after in-crop applications but the crop was canopied so there was no real opportunity to control them.  There isn’t a lot that can be done about that unless you’ve jumped on board and are an early adopter of harvest weed seed control. Hopefully the weeds are far enough behind that they won’t set seeds at all and harvest will actually stop their seed production. But if there are distinct patches in the field, there’s a good chance something else is going on. Now if it’s a strip down the field where there’s been a sprayer miss, those aren’t the types of patches I’m talking about. I’m talking a patch in the field with no obvious explanation. 

These types of patches could be an indication that there may be herbicide resistance developing. If the patch is small – PULL IT!  If it’s not herbicide resistant you still stopped weed seed-bank inputs. If it is a resistant patch you may have stopped the population from establishing in that field.  If you have patches and you noticed after your in-crop applications that there were some areas where you didn’t get great herbicide efficacy, it’s probably a good time to consider having some herbicide resistance testing done, particularly if you have a dead plants right beside live plants through that patch.  Two weeds in Alberta where it definitely makes sense to have some testing done would be wild oat and cleavers. Group 2 resistant cleavers is definitely on the climb and can impact your broadleaf weed management decisions in most crops. With resistant wild oats it’s important to find out what herbicide products are left in your toolbox. Resistance can be to only specific structural groups within the group 1s or none of the group 1s may work anymore, and it’s a similar story with group 2s. It’s important to find out that resistance profile so that you don’t waste money next year on guessing which herbicide product may be effective. Kochia would be another weed that would be worth monitoring. With 3-way resistance documented in Alberta, knowing what tools are effective can be so critical.

What’s the process to have resistance testing done? Samples can be sent to the Saskatchewan Crop Protection Lab for resistance testing. They test weeds based on your requests: for example, you can have them check specific group one products or request all of the group 1 structural groups be checked to determine which products might work on your population. If you are having all the subgroups done, it does take quite a bit of seed (2000 seeds per subgroup sample). But even though group 1 herbicides are all grouped together, they interact differently with the enzyme so it’s important to understand what pattern of resistance your specific population has and what that means for managing populations moving forward. To have testing done at the Crop Protection Lab it costs $200 per subgroup sample – which is pretty cheap compared to a guess at what product will work over all your acres.

If you decide to go ahead with resistance testing, you need to collect mature seeds from your field in the fall and give them a chance to air dry before mailing them. If you send immature or non-dry seeds they can mold making testing impossible. So take the time to collect carefully and dry those seeds out. If your weeds have not yet produced seed, you need to decide how badly you want the resistance test done. If you think it is critical to your weed management program moving forward it may be worth leaving that patch of the field unharvested until the weeds mature (similar to leaving sloughs to pick up later).  Ambitious producers could also transplant weeds somewhere they would have an opportunity to mature (the garden at home?) until they can collect the seeds. Bear in mind that 2000 seeds per subgroup sample number though if you decide to transplant. More information on resistance testing as well as the forms and mailing address for the crop protection lab can be found here

A couple other quick comments about weed management at harvest timing.  Glyphosate is not a desiccant. Glyphosate should only be used pre-harvest if the goal is to manage weeds. Fall applications can be really effective for management of perennials, as control can be gained of the root system instead of just the top growth. If the goal of the application is to dry down crops, there are other true desiccant products that should be used. Be cautious of crop stages, especially where stages are variable and be cognizant of pre-harvest intervals after glyphosate application. Glyphosate is an incredibly useful weed management tool, especially in western Canada where we currently only have a single weed resistant (kochia). We need to use glyphosate judiciously so that we don’t lose such a valuable tool to MRL issues.

Wishing everyone a happy, healthy, safe, dry harvest.