Even though field conditions may be suitable to begin field work, temperatures may not be ideal for effective weed control. The best advice is to delay burndown until warm temperatures occur and remain warm. We really probably cannot wait for temps to warm before rolling with corn preemergence herbicides, so what do we do?
1) Residual herbicides are not generally affected by the cold temps, as long as application is still possible at above freezing temps.
2) If freezing temps are seen, we really need to wait a while for active weed growth to resume assuring a good uptake of burndown herbicides.
3) Burndown, which is really a post-emergence herbicide program, does like warmer conditions. There is potential for reduced herbicide performance of products applied during this cold period
4) Early cold applications may reduce the control of biennial weeds, perennial weed, or larger weeds that have survived the winter.
5) Herbicide activity will be slow and show delayed development of visual injury symptoms.
6) Where feasible, keep to the upper rate ranges, including herbicides like 2, 4-D ester, to improve performance. 2, 4-D ester is preferred as it increases performance vs. amine formulations.
7) Corn pre herbicides that contain atrazine are also a plus, as the atrazine will continue to provide burndown and residual control after the conditions warm.
8) Follow all label directions for additives and adjuvants. Be sure to use Ammonium Sulfate (AMS) where recommended. AMS aids in absoroption of most herbicides, especially glyphosate.