08/11/2026
🌱 JOHNSONGRASS + HOT, DRY WEATHER = TIME TO THINK ABOUT NITRATES
If you have Johnsongrass growing around your place right now, this is a good time for a quick reminder about nitrate accumulation — especially as Oklahoma moves through the heat and drought stress of summer.
Johnsongrass can be a useful forage, but it is also one of the forage species with a high potential to accumulate nitrates. The plant itself isn't harmed by accumulating nitrate. The concern comes when livestock consume enough nitrate that it is converted to nitrite in the rumen, interfering with the blood's ability to carry oxygen.
The good news? Nitrate problems are manageable when we identify them and make informed management decisions. Testing is one of our best tools.
🌾 WHY DOES DROUGHT MATTER?
Under normal growing conditions, plants take up nitrate from the soil and use it to produce protein. When drought, high temperatures, frost or other stresses slow plant growth, nitrate can accumulate — particularly in the lower portion of the stem.
This is one reason drought-stressed Johnsongrass deserves some extra attention.
And here's an important Oklahoma twist: the risk doesn't necessarily end when it rains.
When drought-stressed plants receive rain and begin growing again, nitrate accumulation can remain elevated for a period of time. OSU recommends being especially cautious following a drought-ending rain and testing forage rather than assuming that a rainfall event has made it safe.
🧪 WHAT IF YOUR DIPHENYLAMINE TEST IS POSITIVE?
Your local County Extension office can perform a quick diphenylamine screening test on the forage in question: if your Johnsongrass turns positive, don't panic — but don't treat the quick test as a final answer, either.
The diphenylamine test tells us that nitrate is present, but it does NOT tell us how much nitrate is present. False positives can occur, which is why OSU recommends sending forage that screens positive to a laboratory for quantitative analysis.
➡️ The next step is a laboratory nitrate test through the OSU Soil, Water & Forage Analytical Laboratory (SWFAL) which is fielded by your local county extension office.
That test gives you an actual nitrate concentration (in ppm) that can be used to make a much better feeding or grazing decision.
Your local OSU Extension office can help you with sampling and submitting the forage.
🚜 AND HERE'S WHERE HAY MANAGEMENT REALLY MATTERS...
One of the biggest misconceptions about nitrate is that it will “go away” when you make hay.
It won't.
Unlike prussic acid, nitrate levels generally do not decrease appreciably when forage is cut, dried in the sun, or baled. That means a Johnsongrass field that tests high before cutting doesn't become safe simply because you turned it into hay.
In fact, nitrate can be especially important to consider when making hay because nitrate isn't distributed evenly throughout a plant or field.
📌 Nitrate concentrations tend to be higher in the lower portions of the stem.
📌 “Hot spots” can occur within a field, meaning one area may test considerably higher than another.
📌 With large round bales, forage from a high-nitrate area can become concentrated into a single bale.
📌 A positive test is a reason to evaluate the forage before cutting, baling, grazing, or feeding it, not a reason to simply move forward with harvest and deal with it later.
If you're standing in a drought-stressed Johnsongrass field that has screened positive, don't assume, “I'll just cut it and the problem will be gone.” The nitrate is still there after harvest.
💧 DON'T FORGET THE WATER!
There is another piece of the nitrate puzzle that can become important during hot, dry Oklahoma summers:
Drinking water.
Stock ponds can contain nitrate, depending on the watershed, runoff, manure or fertilizer sources, and other environmental factors. During drought and summer conditions, water quality can become another factor worth evaluating.
Why does that matter?
Because livestock don't get nitrate exposure from just one source.
If cattle are consuming nitrate-containing forage AND nitrate-containing water, the nitrate contribution from both sources needs to be considered when evaluating the animal's overall exposure.
OSU's livestock water test includes nitrate-nitrogen along with other important water-quality parameters. If you're concerned about a pond or other livestock water source, your County Extension office can help you submit a representative water sample for testing.
🐄 SO WHAT DO THE NUMBERS MEAN?
Forage nitrate results are reported in ppm nitrate (NO₃) on a dry-matter basis. OSU's general interpretation is:
✅ 0–3,000 ppm: Generally safe for all cattle
⚠️ 3,000–5,000 ppm: Generally safe for non-pregnant beef cattle, but there can be some risk to breeding performance and early-term pregnancy
⚠️ 5,000–10,000 ppm: Some risk for all cattle; potential for reproductive problems, weak calves, reduced growth or milk production
🚨 Over 10,000 ppm: Potentially toxic; can cause acute toxicity, abortions and death
These are general guidelines — not a substitute for looking at the animal, the forage, the rest of the ration, water source, stage of pregnancy, and expected intake. This is why having an actual laboratory result is so valuable.
🌾 WHAT SHOULD YOU DO WITH HIGH-NITRATE HAY?
Don't automatically assume that every bale has to be thrown away.
Management depends on how high the nitrate concentration is and how the forage will be used. OSU recommendations include options such as diluting higher-nitrate forage with low-nitrate feed, limiting the amount of high-nitrate hay in the total diet, sorting hay by nitrate concentration, and considering the class and stage of production of the cattle consuming it.
However, extremely high-nitrate forage may not be safely managed through dilution alone. OSU identifies extremely high concentrations — around 25,000 ppm — as presenting a very high risk where disposal may be the safest option.
And remember: a nitrate test is only representative of the field area or group of bales from which the sample was collected. Proper sampling is extremely important.
🌤️ A LITTLE TESTING NOW CAN PREVENT A BIG PROBLEM LATER
Johnsongrass isn't automatically “bad.” Nitrate isn't automatically a problem. And drought doesn't mean every field is toxic.
But hot, dry weather + stressed forage + susceptible plant species = a good reason to test rather than guess.
If you're grazing Johnsongrass, cutting it for hay, or planning to feed Johnsongrass hay later this year, take a few minutes to evaluate the situation.
Have a question about nitrate testing, forage sampling, livestock water testing, or what to do with a positive screening test?
📍 Contact your local OSU Extension office. We can help you determine the appropriate next step and get samples submitted for laboratory analysis when needed.
👉 When in doubt, test it out! A little time spent testing can provide a lot more confidence in the decisions you make for your herd. 🐄🌱
