Wheat harvest started last week and will be wide open this week of May 21^st.  Double-crop soybean planting will begin in earnest. Typically good stands of wheat can shade the soil which helps reduce emergence of weeds. This can make a wheat/double-crop soybean rotation a good rotation to reduce Palmer amaranth seed bank.

On the other hand, in thin stands of wheat or in drowned out areas there is often considerable weed pressure. The two weeds most common in these situations are horseweed and Palmer amaranth. As you all are aware, we have struggled trying to burndown horseweed and Palmer amaranth this spring.  It is typically even more difficult to control them when most of the weed material we want to spray has been cut off with wheat harvest.   Moreover, in the many very dry areas of Tennessee growers will often want to be conservative on how much they will invest in herbicide for a soybean crop that may make very little unless the weather turns around.

There are two main management strategies to control these cut-off weeds.  First is to use a tankmix that has a good probability of controling the weeds and second is to apply these tank mixtures in the most effective way possible. In my mind there are three herbicide mixtures that have the best probability of removing horseweed and Palmer pigweed following wheat harvest.

First is a combination of paraquat and metribuzin.   Be sure to utilize the higher rates of the paraquat product which would be 48 oz of Gramoxone SL or 32 oz/A of a generic paraquat.  The metribuzin rate should be 4 to 6 oz/A.  Include a non-ionic surfactant at quart per 100 gallons of mix, or crop oil concentrate at 1 gallon per 100 gallons of spray mix.  This combination should be very effective on Palmer amaranth (>97%) and provide good control of horseweed and giant ragweed (85 to 95%). The advantages to this combination are that it is in the mid-level price range and the metribuzin will provide some relatively inexpensive residual control. The disadvantage is the trouble some are having with tankmixes of paraquat products clabbering up in spray tanks.  In order to reduce this problem be sure to add the surfactant first then acidify the water in the tank with some sort of AMS product (most often the surfactant and AMS are a premix), then the metribuzin and last the paraquat product. 

Liberty at 29 oz/A rate is another option.  Tankmix of metribuzin will also help with control and provide some residual of Palmer amaranth.  Research we just evaluated this week would suggest that AMS should not be added to Liberty as it reduces Palmer control. This combination should be very effective on horseweed and giant ragweed (>97%) and provide good control of Palmer amaranth (>95%). The advantage to this tankmix is that with the warm temperatures it will be the most efficacious on Palmer amaranth and horseweed of the tankmixes listed.  The disadvantage is that it is the most expensive program listed. Moreover many will likely rather use their Liberty in crop and not as a burndown. 

The third tankmixture is 5 oz of Verdict or 1 oz of Sharpen tank-mixed with 32 oz of glyphosate.  This combination will provide good control of horseweed and small pigweed (90 to 97%).  Be sure to apply it with the MSO and surfactant as directed by the label.  The advantage to this tank-mix is that it is fairly economically priced compared to the others.  Moreover it will provide some limited residual of Palmer amaranth.  

The most effective way to apply all three of the tankmixes listed above is to ensure good coverage.  This is even more important on weeds that are cut in half.  Thorough spray coverage is essential for optimal performance.  Ground application requires a minimum of 15 gal. water/acre, and preferably 20 gals. Moreover, be sure to utilize the adjuvants as directed by the label. Finally, with the very dry soil conditions soybeans will often not emerge quickly. In these cases it might be good to wait a day or two to apply the burndown and allow the weeds to regrow a bit to have some more surface area to spray.