All the cover crops planted last year has led to many calls on management this year. One of the most common calls is do herbicides applied to heavy and, on occasion, sometimes still very green cover get to the soil so they can provide residual Palmer amaranth control?
We, along with other universities, have done some research on integrating PRE applied herbicides in with cover crops for Palmer amaranth control. In general the more water soluble white herbicides like atrazine in corn, Cotoran in cotton, and metribuzin (Tricor, Sencor) in soybean, will readily move to the soil after a rain and help provide Palmer amaranth control.
Dr. Tim Grey (et al.) from the University of Georgia shared some of their data at a recent weed conference. Their studies showed that herbicides like fomesafen (Reflex, etc.) was very good at providing residual control of Palmer amaranth in heavy cover crop residue. Other herbicides like Dual Magnum was somewhat intermediate. Prowl showed the most impact of the cover hindering the herbicide residual control.
There is still a good deal of research that needs to be conducted on this question. However, that does not stop me from offering an educated guess. This means that judging by their relative water solubility, herbicides that would likely work well in heavy covers would be Zidua, Sharpen, Verdict and Valor. Herbicides that may have the most issue providing residual control in cover crops would be Brake FX, dicamba and Caparol.