With the extremely low cost of glyphosate (sold as Roundup, Touchdown, etc.), it's easy to be tempted to use this herbicide, and this herbicide only, as your burndown herbicide of choice on your no-till corn and soybean acres. However, there are many reasons you should consider adding another herbicide tank-mix partner to your burndown glyphosate application. First and foremost, the addition of a herbicide other than glyphosate adds another herbicide mode of action onto the acre, which is critical in the management of glyphosate-resistant weeds that are present like giant ragweed and marestail (also called horseweed). Both of these weed species have already emerged this spring and are present in many Missouri corn and soybean fields right now (Figure 1). As I have indicated in previous newsletter articles and presentations, typically more than half of our giant ragweed population that is going to emerge for the season has done so by the time of our typical soybean planting dates in Missouri. This is to an even greater degree true for marestail as well; typically about 75% or more of the population of this weed has emerged before we plant soybeans in Missouri.
The best way to deal with either of these weeds is to add an effective herbicide tank-mix partner to your glyphosate burndown like 2, 4-D, Clarity, or Sharpen. These tank-mix partners allow flexibility in planting either corn or soybeans, although you must be careful to follow the preplant intervals for both 2, 4-D and Clarity. Although it seems to be a standard practice in corn to apply a residual, pre-emergence herbicide that has activity on your most problematic weed species, it is much less common in soybeans. For fields that will be planted to soybeans, you may also want to consider the addition of a herbicide that provides both burndown AND residual control of these weed species. Any of the herbicides that contain chlorimuron (Classic, Envive, Valor XLT, etc.), cloransulam (Authority First, FirstRate, Sonic), or saflufenacil (Sharpen, Optill, Verdict) generally provide good burndown control of giant ragweed and marestail, and provide some residual activity on these species as well. For a complete listing of corn and soybean herbicides that are effective on these weeds and others, consult the 2011 Missouri Pest Management Guide (MU Ext. Publ. M171), http://weedscience.missouri.edu/publications/m00171.pdf.
In addition to any glyphosate-resistant weeds that may be present in your fields, another reason to consider using a herbicide other than just glyphosate in your burndown is to achieve better control of the winter annual weed species that are currently present. Some of the most common winter annual weeds that we encounter across the state are henbit, chickweed, and field pennycress, and if you didn't make a fall herbicide application last year, it is likely that your corn or soybean fields are covered up with these weeds right now. In fact, henbit is blooming in most no-till corn and soybean fields throughout the state as I have seen many "fields of purple" in my travels lately. Although we tend to get away with burndown herbicide applications that are made to henbit that is blooming, it is not an ideal time to make a herbicide application to this species. Several states in the southern U.S. have already experienced problems with burning down henbit earlier this spring. I don't know if we will experience similar problems or not but we should be aware of the possibility and one way to decrease the chances of this occurring is to add a herbicide tank-mix partner to your glyphosate burndown.
Lastly, there are some winter annual weeds — like field pansy (Figure 3) — that just simply are not controlled well with applications of glyphosate alone. For these species, you must add an additional herbicide tank-mix partner that is effective on the weed in question in order to achieve complete burndown weed control in an effort to start with a clean field at corn or soybean planting.
REVISED: October 2, 2015