There are two ways that Johnson grass can be toxic to cattle. One is when it has been stressed due to drought and gets a rain. The new regrowth has prussic acid (the white substance emntioned in an earlier thread). The other situation is when you get a frost and it bites the johnsongrass back and you get quick regrowth. It is ok after about 10 days.
Prevention: If plants have been injured defer grazing until they are recovered from injury. After a hard freeze, or severe drought, avoid grazing for approximately 1 week. After a rain or irrigation on drought stressed fields wait at least 2 weeks after plants begin to grow before grazing.
Very much a concern lately after some decent rains after a prolonged dry period. After our recent rainfall windfall, I waited about 16 days after green-up to turn in on some haygrazer. Also made sure it didn't show signs of heat stress before I turned in. Did not have any sign of problems from either nitrates or prussic acid. But it definitely can cause some problems if things are wrong.
You can cut forages that show prussic acid present because it will dissipate as it cures. Need to let it cure for 5-7 days. Nitrates do not dissipate. All you can do their is dilute.
You can have forages tested for nitrates and prussic acid.
with the cool wet weather we have been having I would say the Johnson grass SHOULD be ok
I just cut some for hay today and I am not concerned with the nitrates as we have had plenty of rain and cool weather this summer
In the fall I wait til we have a hard freeze to turn cows in on it
johnson grass can be real toxic after a rain.an cause the cows to bloat bad an die.but it can be treated.best to hold them out for 4 days after it rains till it drys.same with the frost.but i cant answer for drought stress.