A:   Like other grasses in the sorghum family, Johnsongrass can accumulate nitrates and develop prussic acid. The nitrates are usually not a problem in the open pasture situation. The prussic acid problem can form right after a frost breaks plant cell membranes. This breakage allows the chemicals that form prussic acid to mix together and release this poisonous compound rapidly. Livestock eating frozen sorghum can get a sudden, high dose of prussic acid and potentially die. Fortunately, prussic acid soon turns into a gas and disappears into the air. So wait 3 to 5 days after a freeze before grazing the Johnsongrass. The other thing to watch out for is how hard the freeze was. Ideally, there would be a hard freeze that completely stops plant growth for the season. Sometimes there can be just a light frost that only affects the tops of the leaves and then some time later harder freezes that end plant growth.