Chop and feed as needed. Dried fodders and hays. Test stems and stalks. If dark blue-black color rapidly develops, check the leaves. If leaves are also very positive, make a quantitative analysis before feeding the forage or feed it cautiously.
Test stems of plants.
If stems test positive, then test leaves. If both are positive, excessive nitrates are present and quantitative analysis is needed along with cautious feeding. Sudan, Sudex, etc., are more likely to be a problem than cool season grasses. You may occasionally find that grasses in old feedlots or well fertilized pastures are a problem Mixed ground feed. Grains would be free of nitrates as measured by this reagent.