My bulls just finished an area of johnson grass and they had no problems. Are you sure it was the johnson grass? I love feeding it because it grows well and the love it. In fact if you want to get rid of it, just let the cows on it all the time and they will eat every sprig that pops up. Johnson grass has to go to seed to keep growing.
Johnson grass can have prussic acid if it is stressed - caused by freezing and drought.
Most cattle disease is caused by lack of nutrition - bangs disease, etc. There is a great book by Pat Coleby from "Down Under" and sold through www.acresusa.com
about natural care and feeding of cows.
Our cows have free choice salt chunks from Redman UT that has about 70 trace minerals (The salt stuff from Mortons at your feed store is junk from junk science) The way the salt world works is that companies like Morton buy salt and remove all the trace minerals and then sell the minerals and keep a few that they determine is necessary. This applies to humans as well. Did you ever realize why you have to buy salt with iodine added - it was taken out for profit selling it else where and had to be added for thyroid problems. This is the same thing with Sea Salt in health food store, all the trace minerals are removed.
Our cows also have free choice kelp meal because of amino acids and trace minerals. And this is also why we are now fertilizing with ocean water becasue it has 92 trace minerals, amino acids, enzymes, and beneficial bacteria. Fish from the sea never have cancer but fresh water fish and people have all kinds of tumors and cancers. I don't think that is a dumb luck. I think cancer, heart disease, and other degenerative diseases are caused by the lack of trace minerals - hence the lack of healthy foods through you local grocery stores. 100 years ago the cancer rate was 3 % and today it is about 38 or 39 %.
This is a long way from johnson grass, but I haven't had any problems and I know our nutrition is the best it can be and getting better. I know we are parasite free and we use no toxic chemicals to lower their resistance.
Did you look for other causes?