From University of Nebraska-Lincoln Extension specialist Bruce Anderson:
After disaster strikes, replanting a grain crop may be nearly impossible due to herbicide carryover or the late planting date. As a result, annual emergency forage crops might be your only choice.
Unfortunately, previous herbicide use can cause problems with forages, also. Many pre-emerge herbicides for corn and milo will injure pearl millet and foxtail millet. But, sudangrass, forage sorghum, and sorghum-sudan hybrids will tolerate moderate levels of atrazine; and safened seed can be used if several other herbicides have been applied. These sorghums also tolerate most herbicides labeled for use with grain sorghum. Another possible emergency forage crop is short-season corn as silage or as late season pasture, especially if corn herbicides eliminate other possibilities.
Soybean herbicides that have residual soil activity can cause even bigger problems for replanting to forages. All summer grasses are sensitive to most soybean herbicides. Sunflowers for silage and replanted soybeans for hay or silage are among the few alternatives compatible with soybean herbicide carryover.
Even when you find out that an annual forage will grow, sometimes you may not be allowed to feed it legally. Many row crop herbicides have specific restrictions or at least lack approval for use with forages. So check out your options closely and carefully before making your selection.
Nobody likes to replant, but if you must, select a forage that is compatible with your herbicides and livestock.