Start Clean and Stay Clean by Scouting Fields This Spring

By Meaghan Anderson and Wesley Everman

Rain and wind has impeded getting fieldwork done this spring, but it looks like the weather forecast is heading for drier conditions. This will allow farmers to get the remaining corn and soybean fields planted across Iowa prior to when yield potential starts to drop off in mid-May. The latest USDA Crop Progress Report estimates that 34% of our corn is planted and 25% of the soybean crop is planted. 

The forecast appears to have nearly ideal wind speeds to make effective, on-target herbicide applications in the coming days. Before spraying pre-plant or preemergence herbicides, consider scouting fields before spraying in order to:

1. Determine if the crop hasn’t emerged before a herbicide application goes on if the field was planted already; and
2. Make sure that your planned herbicide program can manage any weeds that have emerged.

In corn, many herbicides are labeled for both PRE and early POST application timings. One important caveat is that once the corn starts to emerge, most labels prohibit the inclusion of UAN as a carrier for POST applications due to the risk of damaging young corn plants. You can read more on that topic in the article “UAN and herbicide applications on emerged corn.”

In soybean, restrictions are stricter and the risk of injury is very high if many PRE herbicide products are applied to emerging or even soybean that are beginning to create cracks in the planted row. I’ve already received images this spring of the burning injury from HG 14 products that washed onto newly emerged seedlings (Figure 1); minor injury from HG 14 products is not unexpected, but off-label applications may result in severe, stand-reducing injury. Read the article “Look before you leap: Cautions for delayed preemergence applications in soybean” for more information.

In addition to the risks of crop injury, be aware of the presence of emerged weeds that might not be controlled by the planned herbicide program. I have observed numerous summer annual weed species emerge in the past several weeks – giant ragweed, common lambsquarters, velvetleaf, ivyleaf morningglory, and several others. Most importantly, agronomists in Iowa have noticed waterhemp emerging as well (Figure 2). While a standard burndown herbicide application with glyphosate may manage a variety of other species well, it is unlikely to control emerged waterhemp. Depending on the planned crop and whether it has been planted yet, numerous burndown herbicide options are available that are still effective on waterhemp like paraquat, 2,4-D, glufosinate, and saflufenacil.

Prior to spraying, scout fields and check herbicide labels to determine whether your planned herbicide will be sufficient to manage emerged weeds, avoid unnecessary crop injury, and adjust accordingly to make sure your fields start clean and stay clean this summer.