I've had several questions on how long to wait following the frost event this past weekend before making a postemergence application. The answer depends on several factors, including 1) stage of corn at frost injury, 2) vigor of corn at time of frost, 3) weather following the frost, and 4) margin of crop safety with herbicides being applied.
Crop tolerance to most herbicides is based on the ability of the crop to rapidly metabolize the herbicide. Metabolizing the herbicide requires energy, and this is where the frost injury comes into play. About the V1-V2 stage corn switches from relying on energy stored in seed reserves to solar energy (photosynthesis). Frost eliminated the solar collectors (leaves), thus the plant has to convert back to utilizing stored energy at a time when there are very few reserves left. There may not be sufficient reserves to both generate new leaves and degrade the herbicide, resulting in greatly reduced herbicide tolerance.
So how long should you wait? The answer is the longer the better. Ideally I'd say you would want to wait until at least one new leaf has fully developed (collar visible). Most frost damaged plants probably haven't accomplished this yet. Unfortunately, with the forecasted period of extended rain people would like to spray now. In this situation I think fields should be evaluated for the severity of the weed infestation and what products are being applied. If weeds are just emerging and you think you would be able to successfully control them in 7 to 10 days, then I think holding off would be a wise decision. However, if there are some large, difficult to control weeds present (e.g. giant ragweed), then maybe it's a better decision to take a chance on experiencing crop injury. Also consider the margin of crop safety of the herbicides being applied. Group 27 herbicides (Armezon, Callisto, Impact, Laudis, etc.) are less likely to cause yield impacting injury than a group 4 (2,4-D; dicamba) or group 2 product.
For some fields there won't be a good solution and choosing between the lesser of two evils may be the only option. This is another reason why all fields should be treated with preemergence herbicides. Preemergence herbicides provide greater flexibility in these situations by providing the crop a head start on weeds, therefore providing a larger window of opportunity to apply the postemergence product.