New shoots produced after frost can be especially hazardous. Prussic acid content appears to be higher in plants grown in soils high in nitrogen and low in phosphorus. When fertilizing forage ground, use split nitrogen applications when the total amount exceeds 60 lbs of N per acre to decrease the risk of prussic acid toxicity. Herbicides, such as 2,4-D, can increase prussic acid concentrations in forage for several weeks after application. Plan grazing rotations accordingly.
Drought increases the chance for high levels of prussic acid in plants.
This may be because the plants have not been able to mature and contain mostly leaves which are higher in prussic acid. In general, any stress condition which retards plant growth can result in higher than normal levels of prussic acid. Frost or freezing causes plant cells to rupture, allowing prussic acid to be released. Do not graze frosted summer annuals until regrowth of shoots is 15 to 18 inches tall, or until several days after the entire plant and shoots are killed by subsequent frost. During the forage analysis the plant cells are broken, allowing the cyanogenic compound to mix with the plant enzymes which release the prussic acid.