It is getting late early with respect to burndown in Tennessee. This is particularly true for corn acres where we are well behind the burndown pace of most Tennessee springs. Typically by late March we have a good bit of corn ground burned down and often have a few acres planted.
The 10 day forecast calls for a good chance of rain 7 out of those 10 days. Regardless of whether the forecast is accurate corn burndown and planting will have to occur fairly close together.
The biggest concerns are glyphosate-resistant ryegrass and, in a couple counties, glyphosate-resistant Johnsongrass. If ryegrass is not controlled prior to corn emergence there are no good herbicide control options. The standard practice to control ryegrass before corn planting is to add 12 to 16 oz/A of Select Max in with glyphosate.
However, the plant back for corn following that rate is 30 days. There is a special section on the Select Max label to apply 6 oz/A of Select Max 6 days ahead of corn planting. That rate will likely not be enough to provide consistent control well established ryegrass.
The other option is Gramoxone. One application of Gramoxone mixed with one qt/A of atrazine can control small ryegrass. However, for well-established ryegrass a sequential application of Gramoxone applied about 10 days apart has provided 90% control in research.
With respect to Johnsongrass, a burndown application of Select Max plus glyphosate is the best way to start to manage it. Then for regrowth Steadfast Q or Capreno would be the best POST applied options.
Fortunately, for most other weeds, late burndown in corn can be very effective and safe to the crop. The main reason for this is that atrazine is a great burndown addition to Gramoxone. Gramoxone alone can struggle on some grassy weeds and even a few broadleaf weeds like henbit, particularly at rates below 48 oz/A. However, adding a quart of atrazine greatly increases the consistency of control.
Lastly, the best rule of thumb to get the best control possible from any Gramoxone application is to apply it early to mid-morning or late afternoon to evening. It is one of the few herbicides that does not work well when applied in the middle of the day. Therefore, application early or late in the day or during overcast conditions greatly improves the performance of Gramoxone.