We have been getting lots of inquiries lately about fields that didn’t get planted to a corn or soybean crop, and have lots of weeds growing in them. With the challenging weather we have had, we didn’t need any more challenges this year. Common sense tells us that something should be done to these fields to prevent the weeds from going to seed and increase weed control costs for many years to come. Keep in mind that weeds like foxtail and giant ragweed produce hundreds of seeds on each plant, and pigweeds, like waterhemp, redroot/smooth pigweed, and Palmer amaranth, can produce thousands and even hundreds of thousands of seeds per plant. Allowing them to go to seed is not a good idea since all weeds have built-in dormancy mechanisms, which result in large fractions of seeds germinating in flushes across multiple growing seasons, even as long as 10 years down the road.
Figure 1. Regrowth of grasses and ragweeds after failed herbicide application to large weeds. (Photo Credit: Marcelo Zimmer)
Figure 2. Failed herbicide application to field infested with horseweed (marestail). (Photo Credit: Marcelo Zimmer).
So what can we do on prevented planting acres? Here are options to consider and are listed in no particular order. You will have to decide which one fits your operation the best.
- Plant a cover crop and hope that establishment and growth of the cover crop suppresses weeds. You will need to control the existing weeds first to allow proper seed to soil contact for establishment of the cover crop. However, since we are now into mid summer, getting adequate rainfall for germination may be a challenge in some areas of the state. Never thought I would say that at the beginning of this growing season!
- Plant a grass cover crop like cereal rye or wheat and use low cost herbicides like 2,4-D or dicamba to control broadleaf weeds. One of the benefits of planting a grass cover crop is that it allows you to use a broadleaf herbicide to control broadleaf weeds in the cover crop. If you use a cover crop mixture that contains grass and broadleaf species, you will not have the option of spraying herbicides to kill the weeds that come up in these mixed stands.
- Till the ground to kill the weeds. Most likely you will have to do this more than once since additional flushes of weeds will emerge when rainfall events occur. Keep in mind that is important to uproot the large weeds so they die. Large weeds can re root themselves if soil moisture is present, continue growing and produce seed. Tillage operations must be thorough to kill the large established weeds.
- Spray broad spectrum herbicide programs to kill the weeds. Most likely this would need to be done more than once if there is not a crop growing on the ground to provide weed suppression by shading. A couple of options include a) glyphosate + 2,4-D or dicamba, b) glufosinate + 2,4-D or dicamba + clethodim if tough grasses are present, c) gramoxone + sencor + 2,4-D or dicamba. We realize on these prevented planting acres, that most folks will attempt to keep costs down. However it’s important to recognize that in many fields, the weeds have gotten relatively large, and we are in a weather pattern that is now causing the weeds to harden off and be more tolerant to herbicides. So for this reason higher rates of herbicides we’ll be required to achieve complete control.
- Mow down fields infested with large weeds, especially tough to control weeds like horseweed (marestail), giant ragweed, and the pigweeds. Spray the regrowth with broadleaf herbicides like 2,4-D and dicamba to finish them off. Large weeds won’t be easily controlled with herbicide applications only (Figures 1 and 2) and mowing big plants will help reduce weed biomass and plant stored reserves. Make sure the weeds are actively growing before spraying herbicides.
In summary, I wish we could wave our magic wand and make the weeds go away. But that’s not likely to happen anytime soon and it’s going to require effort on your part to keep hammering away at the weeds, minimizing seed bank inputs in order to keep weed control costs in the future at reasonable levels.
