Managing weeds before cover crops
As farmers get back to considering the acres they didn't get planted this year, there are some things to take into consideration on how to manage weeds and rebuild soil structure.
Some of the prevented planting fields are just being looked at again as everyone gets caught up on spraying the crop fields and is now looping back to start managing their PP. In going back to those PP fields, the weeds have likely gotten big and herbicide options are getting limited. Here are a few thoughts on how to manage the weeds in those fields prior to seeding a cover crop.
If you are conventional tillage, tilling the field for weed management won’t be a major setback for your soil health. You will still be incorporating some weed seed, but once the weeds are knocked down, then plant a cover crop to compete with new weeds coming up. With the dry conditions in most of North Dakota, the cover crop may be slow to establish. Keep that in mind when selecting a mix. You will likely want some warm season grasses (millet, sorghum, sorghum x sudangrass) in the mix to establish quickly; mix in some cool seasons (oats, cereal rye, barley) for longevity of the mix throughout the fall. Smaller seeds establish better in dry conditions, so including some radish (generally only 2 pounds per acre) or flax (again, 2 pounds per acre or less) may be helpful. If you think the weed pressures are going to be a problem, just do a grass mix so you have herbicide options.
If you are in a no-till system and in eastern North Dakota, you will still need to knock back the weeds but you don’t want to destroy the soil structure you’ve built in your system. Glyphosate can still control many glyphosate-susceptible weeds, but resistant waterhemp and ragweed are in many of these fields and often too large for other herbicides. We’re thinking that using a field cultivator set at a maximum depth of 2 inches to make a couple passes in different directions could do the trick. Ideally you would use something at least as aggressive as a disk. We hate to have disturbance or do any kind of tillage on no-till, but we also don’t want the weeds to become too much of a problem, and herbicide on large weeds isn’t a solution. Vertical tillage will not work well for managing weeds, which is why we think passes with the field cultivator may be the best option. Just keep it shallow. Then immediately seed a cover crop mix back onto those fields to rebuild the aggregation in the surface. You can follow similar approaches as listed above for mixes, keeping in mind that cover crops with fibrous roots (like grasses) are the best for rebuilding soil aggregation.
If you are in a no-till system and in western North Dakota, conditions are extremely dry and a tillage pass of any sort to manage weeds won’t help. Plus, a tillage pass will further dry out the soil and limit any use of cover crops (if that’s what you want to do). So, maybe mowing the weeds is the best option for this scenario. Whether you plant a cover crop or not, decide that on moisture conditions.
These solutions aren’t perfect, but they may spark your thought process for how to manage these acres considering their circumstances. The weeds need to be managed, and the next step would be to think about how to build or re-build the system with cover crops.
One last tip: We usually say to wait until July 15 for seeding diverse mixes of cover crops (ones that include radish) to avoid the radish going to seed. Right now, by the time you get the weeds under control and get the seed ordered and to your shop, it may be close enough to July 15 to just go ahead and seed. At this point, catching a rain to get the cover crops growing will be the most important factor. For more information on PP and cover crops, visit the NDSU soil health webpage (ndsu.edu/soilhealth).
Abbey Wick is an Extension soil health specialist at North Dakota State University. Joe Ikley, NDSU Extension weed specialist, and Dave Franzen, NDSU soil specialist, contributed to this report.