I often get calls this time of year with questions on burning down horseweed or henbit before corn planting that will likely start in 30 to 40 days. My response is that horseweed and/or henbit are much easier to control with the herbicides available in a corn burndown program than they are in soybean or cotton. Herbicides like dicamba, 2,4-D, and Verdict which are all available for soybean burndown can be used at higher rates if needed in corn. Moreover, tankmixing in some atrazine can be a great help with knocking henbit out and negating the need to up the rates of the other herbicides. Moreover even if some horseweed is up with the corn, there are still many good options to control it. Halex GT or Capreno plus atrazine or Status come to mind very quickly as great option to control escaped horseweed POST in corn.
The much larger concern for burndown before corn planting is ryegrass. Though I was a fan of the mild winter, it has helped ryegrass become well established in many fields and this will likely make it much more difficult to control than what was the case last year.
In trying to managing GR ryegrass, it is best to do it as early as practical and utilize a clethodim and glyphosate tankmix. This tankmix needs to be applied at least 30 days ahead of corn planting to use a clethodim rate that has any chance of controlling the weed. Talking rates of clethodim can be confusing as there are 1 lb, 2 lb and 3 lb/gallon clethodim products available. Select Max is the clethodim trade name most are familiar with and it is a 1 lb/ gallon formulation product. So using Select Max as an example, I have had best results with 12 oz/A of it tankmixed with 32 oz/A of Roundup Powermax.
If corn planting is likely inside of 30 days, then two sequential shots of Gramoxone at 40 to 48 oz/A spaced about 10 days apart will typically do a reasonable job. Another often asked question is what about using clethodim for ryegrass within 30 days of corn planting instead of two shots of Gramoxone.
There is a supplemental label for Select Max to be used for burndown before corn planting. Select Max may be applied at 6 oz/A and corn can be planted 6 days after application. Many folks went this route in their corn burndown last year and it worked more consistently than glyphosate alone on ryegrass and poa. However, we had a cold winter last year and the ryegrass was not as well established as it is this year so that lower clethodim rate may not perform as well this spring.
Using the higher clethodim rate 30 days ahead of planting or the sequential Gramoxone application will likely provide the best results on the more robust ryegrass this spring.