Nahh, If roundup dries on reasonably tender vegetation(less than 12") you have it.(may take slightly longer than two hours in some circumstances, usually less) That is with the low rate(1 qt/acre), without extra surfactant or liquid ammonium sulfate to help it. The only field I had any problems with this year got sprayed only 30 minutes ahead of a hard rain, and still had good control of the grasses, just some sweetgums and milkweed that didnt die.