With fall herbicide applications, a growing trend is to include one or more residual herbicides in the tank mix. For farmers in lower rainfall areas, this provides more time to get enough moisture to activate the herbicide. Some residual herbicides can also add to the burndown of emerged weeds. Examples of residual herbicides with burndown activity include Sharpen, Valor, Authority/Spartan, and atrazine. Others like the Group 15’s (Zidua, Dual, Harness, Outlook, etc.) and the yellows (Trifluralin, Prowl, etc.) have good residual but no control of emerged weeds.
If you’re using a fall residual herbicide, check the rotational options for your upcoming crop. For example, atrazine use in the fall limits your potential crop choices in the spring to corn or sorghum. For that reason and for environmental concerns, we prefer to see you use something other than atrazine.
Valor herbicide is the most popular option for soybean growers. With a low price, burndown activity, long residual weed control, and safety to a wide number of crops, it’s a tough option to beat, but Authority or Spartan would be fairly similar. With Valor, we recommend the 4 ounce use rate in the fall ahead of soybeans, but if you want to use Valor this fall and plant something other than soybeans in the spring, you may only be able to apply 2 ounces per acre, so check the label to be sure for your area and crop rotation.
Fall herbicide applications can be a valuable tool in your weed control arsenal. Stick to fields that are not subject to flooding. Target fields you will be able to plant early in the spring as fall herbicides may wear out sooner than spring-applied products. Finally, leave a check strip here and there to see how fall programs work for you. I’m confident you’ll like what you see. In dry areas, there is often a huge advantage to fall spraying because the herbicides will be fully activated and effective immediately in the spring, which leads to better early-season weed control.
One final note – killing weeds in the fall can take away host crops for soybean cyst nematodes and other pests, so for this reason and all those I listed above, we encourage you to at least consider a fall herbicide application, especially on your weediest fields or where winter annuals, perennials, or biennials are a major issue.