Wheat harvest was in full swing last week in Tennessee. Double-crop soybean planting will begin in earnest. Typically good stands of wheat can shade the soil, which helps reduce weed emergence. This wheat/double-crop soybean rotation can be good to reduce the 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. Burndowning down horseweed and Palmer amaranth this spring was a struggle. It is typically even more difficult to control them when most of the weed material need to spray has been cut off with wheat harvest. Moreover, in the many very dry areas of Tennessee, growers often want to conserve how much they 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:
1) Use a tankmix that has a good probability of controlling the weeds.
2) Apply these tank mixtures in the most effective way possible.
There are three herbicide mixtures that have the best probability of removing horseweed and Palmer pigweed following wheat harvest:
1) 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 2 pts 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%). This combination is in the mid-level price range and the metribuzin will provide some relatively inexpensive residual control. But some have trouble with these tankmixes of paraquat products clabbering up in spray tanks. 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.
2) Liberty at 29 oz/A rate. Tankmix of metribuzin will help with control and provide some residual of Palmer amaranth. Recent research suggests 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. But it is the most expensive program listed. Moreover many will likely rather use their Liberty in crop and not as a burndown.
3) Use 5 oz of Verdict or 1 oz of Sharpen tank-mixed with 32 oz of glyphosate. This combination provides 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. This tank-mix is fairly economical compared to the others. It will provide some limited residual of Palmer amaranth, too.
Thorough spray coverage is essential for all three options, especially on weeds that are cut in half. Ground application requires a minimum of 15 gal. water/acre, and preferably 20 gals. Be sure to utilize the adjuvants as directed by the label.
(Information provided by Larry Steckel, a University of Tennessee Extension Weed Specialist)