High nitrates in rye forage can be a concern after the plants have been stressed by a freeze. This is especially true if the field had a spring nitrogen fertilizer application one to two weeks prior to the freeze. Although the rye may be damaged and stunted by the freeze, growth will resume and it may be very rapid over the next couple of weeks after a freeze. With this growth, nitrates that had accumulated will quickly be used up and/or diluted through the plant. One could send a sample in for a nitrate test, but keep in mind that the tested nitrate level will be a lot lower with one week's additional growth.
Prussic acid will not be a problem.
With the interruption of plant growth, grass tetany could be a concern. Consider supplementing the cattle with magnesium oxide.
