It is extremely difficult to achieve long-term management of Canada thistle and other perennials during drought because plant growth ceases and herbicides rarely get to the root systems. Drought can slow down activity of Roundup. Instead of 4 to 7 days, Roundup may take 10 or more days to burn down plants. Addition of ammonium sulfate to Roundup is more important when drought conditions exist. Increase the rate to manage drought stressed weeds.
Antagonism is much more common when trying the manage drought stressed annual grasses.
Due to the relative cost of wild oat herbicides it is unlikely that reductions in yield loss will cover the cost of wild oat management this year. Because of this, spot spray wild oat herbicides or use Roundup as a cheaper spot spray alternative to manage the most heavily infested areas. Achieve does not work well under drought conditions. Delay application until after rainfall to improve activity. Make sure to add ammonium sulfate to the Achieve spray solution.