After a hard freeze how much Johnson grass will kill a cow? And is there any other grass that is toxic after a freeze? 

A: Johnsongrass can accumulate nitrates and develop prussic acid (cyanogenic glucosides). The nitrates are usually not a problem in the open pasture situation. The prussic acid problem can form right after a freeze breaks plant cell membranes. This breakage allows the chemicals that form prussic acid to mix together and release this poisonous compound rapidly. Livestock eating johnsongrass right after the freeze 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.

Because the concentration of prussic acid can vary in the plant, It is difficult to say how much must be consumed before it is lethal. Johnsongrass and other grasses in the sorghum family (sudangrass and hybrds, etc.) can potentially have the prussic acid problem. Although it is very rare, there are several other grasses such as bermudagrass, arrowgrass, indiangrass and several broadleaf plants and shrubs that could potentially have this problem as well.