By Larry Steckel, Extension Weed Specialist
In years past when all our Palmer amaranth was PPO-susceptible, many used Verdict or Sharpen tankmixed with glyphosate and MSO as a very successful burndown for horseweed and Palmer amaranth. That mix still works well on horseweed, but I have received reports that Verdict and Sharpen are not burning down all the Palmer amaranth in some fields.
Indeed, I have seen this at Jackson in some of our research. I would suspect that the difference this year is that the Palmer that has escaped that burndown is PPO-Resistant.
In one recent study we examined burning down 2-inch Palmer amaranth that turned out to be a population that is about 20% PPO-resistant. What we found is that Sharpen or Verdict tankmixed with glyphosate and MSO provided about 76% control. When we added 4 ounces of metribuzin in with those tankmixes, the Palmer amaranth burndown improved to 96%.
Gramoxone has always been the most effective burndown option for Palmer amaranth. This is even more the situation now with the PPO-resistant Palmer amaranth showing up. However, there are at times fields where the primary weed at planting is horseweed and in those cases Gramoxone is not the best option. The Verdict + glyphosate + metribuzin would most likely be the best fit.