Gleni,
Don't panic about the Johnson grass. It, along with many other grasses, can develop excessive nitrates or Prussic acid-- if conditions are right. Johnson grass is used as a live grazing feed and for baling. It is not routinely toxic. It's fairly easy to control in a yard. Frequent mowing, keeping the grass short enough that it doesn't seed, will knock it back and other grasses will take over. If you dig out the roots are fairly large and easy to identify... Nothing like Bermuda grass!
David R
