Quick answer: Two different toxins threaten cattle fed annual and stressed forages, and they behave in opposite ways. Prussic acid (cyanide) affects the sorghum family (sorghum-sudangrass, sudangrass, johnsongrass) and is worst in young growth and after frost — but it gases off as the forage cures and ferments, so waiting before feeding fixes it. Nitrates affect drought-stressed, heavily-fertilized crops (drought corn above all, plus small grains and sorghums) — and they do not gas off. Dry hay keeps its full nitrate load forever, and ensiling only cuts it part way. The single most important rule: with prussic acid, time heals; with nitrates, you must test. When in doubt on either, test the forage and call your vet or extension office before feeding.
If you bale summer annuals, cover crops, or a drought-salvaged crop, you need to understand these two before you feed. They're often confused, the management is different for each, and getting it wrong kills cattle fast. This is the reference our crop-specific guides — sorghum-sudangrass, drought corn, oats, cereal rye, and wheat — all point back to.
The one-minute version: opposite toxins
| Prussic acid (cyanide / HCN) | Nitrates (NO₃) | |
|---|---|---|
| Which crops | Sorghum family only: sorghum-sudangrass, sudangrass, forage/grain sorghum, johnsongrass, shattercane | Many: drought corn, small grains (oats, wheat, rye, barley), sorghums, pigweed/kochia and other weeds, brassicas |
| Worst in | Young, fast growth; new regrowth; leaves; right after frost | Lower third of the stalk; stems; after drought + heavy N, especially just after a drought-breaking rain or frost |
| Does it gas off? | Yes — dissipates as forage cures and ferments | No — stable; dry hay keeps its full load |
| Does waiting help? | Yes — time is the cure | Only partly, and only via fermentation — not in dry hay |
| How it kills | Blocks oxygen use; bright cherry-red blood; minutes | Blocks blood's oxygen carrying; brown/chocolate blood; abortions at lower doses |
That "does it gas off?" row is the whole game. The folk wisdom "let it sit and it'll be fine" is true for prussic acid and dangerously false for nitrates in dry hay. Keep reading — confusing the two is exactly how cattle die.
Prussic acid (cyanide): a sorghum-family problem
Prussic acid is hydrogen cyanide. It forms in the sorghum family when plant cells rupture — which is why frost and young, fast growth are the triggers. It is not a concern in corn, small grains, or legumes; among summer annuals, that's why some operators sidestep it entirely:
"Alternative is millet. No prussic acid problems."
— Bob ncmo, NE Illinois · AgTalk thread 1102145
The danger windows for sorghums:
- Young or short growth — don't graze or green-chop sorghum-sudangrass under about 18–24 inches; the youngest tissue is the most toxic.
- New regrowth after cutting or grazing, and regrowth after a drought breaks.
- Right after a frost — frost ruptures cells and spikes free cyanide. Don't graze frosted sorghums for at least 5–7 days, and don't bale them the same day.
Why baling and wrapping make it safer
Here's the good news: prussic acid is volatile. It dissipates as forage dries and cures, and it gases off during silage fermentation. So putting frosted sorghum up as baleage and waiting before you feed is exactly the right move — which is precisely the standard field advice:
"No problems. Just give it time to ferment before feeding." … "We'd been told it's fine — just wait like 60 or 90 days after harvesting before you feed it."
— lgn98868 & nydirtfarmer, on frosted sudangrass · AgTalk thread 811538
Practical rule: wrap frosted or questionable sorghum as baleage, then let it ferment several weeks (a month or more is conservative and common) before feeding. Dry sorghum hay also loses much of its prussic acid over several weeks of storage. If you must feed sooner, test for cyanide first. (Wrapping wet, frosted sorghum works even when it looks dead — operators report it ensiles fine and "you'd be surprised how dry stuff can be and still ensile" with plenty of film; see our sorghum-sudangrass guide.)
Nitrates: the drought-and-fertilizer problem
Nitrates are taken up from the soil and normally converted into protein by a healthy, growing plant. When stress stops growth, nitrate backs up and accumulates in the stalk faster than the plant can use it. The forum's plain-language summary is exactly right:
"Nitrates are only a concern when excessive fertilizer is applied, or if the crop is under some sort of stress — like a severe drought or a hard killing frost."
— Hagie pilot, Ontario · AgTalk thread 1079385
It takes both a nitrogen load (heavy fertilizer or manure) and a stress that halts growth: drought, frost, hail, prolonged cloudy weather, or herbicide damage. Drought corn is the textbook case — and it's a killer:
"A local farmer decided to chop drought corn and, while waiting on a plugged silo pipe, fed some to 70 steers on feed. Came back to find 7 with their feet in the air. Tested high on nitrates."
— Agtopper, western Kentucky · AgTalk thread 1068682
Why nitrates are the more treacherous of the two
Unlike prussic acid, nitrate does not gas off. Two consequences you must internalize:
- Dry hay keeps its entire nitrate load — indefinitely. Curing does nothing. High-nitrate hay is just as dangerous a year later. This is the deadliest misconception in forage safety: people assume "old hay is safe hay," and for nitrates that's false.
- Ensiling reduces nitrate only partway. Fermentation in a wrapped bale, tube, or bunker typically lowers nitrate on the order of a third to a half over the first several weeks — helpful, but not a fix. You still must test the ensiled feed.
Management that actually lowers nitrate risk:
- Raise the cutting height. Nitrate concentrates in the lower third of the stalk; cutting 8–12 inches higher leaves the worst of it in the field.
- Wait after a drought-breaking rain. Nitrate spikes for several days after rain hits a stressed crop, then the plant metabolizes it down over roughly a week of good growth. Don't rush to cut in that window.
- Ensile rather than dry-bale or green-chop a high-risk crop — partial reduction beats none, and fresh green-chop is the most dangerous form.
- Dilute and adapt. Blend high-nitrate forage with clean feed, introduce cattle gradually, and keep energy in the ration — your nutritionist can set safe inclusion rates from the test number.
A safety note for people, not just cattle: ensiling high-nitrate forage releases nitrogen dioxide — "silo gas" — a heavy, toxic, sometimes brownish gas that can injure or kill in enclosed silos and bunkers in the first days after filling. Stay out of enclosed silage structures during early fermentation and ventilate well. Wrapped bales and tubes outdoors are far lower risk, but the gas is real.
Testing: cheap insurance you should actually buy
Both toxins are easy to test for, and the cost is trivial against a dead animal. Because you'll often be quality-testing salvaged or annual forage to value and ration it anyway, add the safety test while you're at it:
"If you're doing a sample anyway, be sure to pay the little bit extra and get nitrates tested too, even if you don't think it'll be an issue."
— tjdub · AgTalk thread 801435
- Nitrate: a forage lab gives you a number; there are also quick qualitative field tests. Results are reported different ways (nitrate vs nitrate-nitrogen, wet vs dry basis), so let the lab interpret its own number rather than comparing thresholds across sources. As a rough orientation only, many labs treat low nitrate as generally safe, a middle band as "limit-feed / dilute," and high levels as dangerous — but follow your lab's and vet's interpretation, especially for pregnant cattle, where lower levels can cause abortions.
- Prussic acid: test fresh or recently-harvested sorghums if you need to feed before they've had time to dissipate; if you can simply wait several weeks on wrapped or cured forage, that's usually the easier path.
How net wrap and film fit in
Good preservation is part of forage safety. Tight, well-sealed baleage ferments cleanly — which is what drives prussic acid off and gives you a stable, testable feed. Loose, under-wrapped, or air-channeled bales mold and spoil instead of fermenting, leaving you with neither the safety benefit nor good feed. Use net wrap as the inner layer to hold dense, wet annual bales in shape, finish with enough silage film (6–8 layers), and wrap promptly. The mechanics of a clean wet bale are covered in our first-time baleage guide.
The bottom line
- Know which toxin you're dealing with. Sorghum family → think prussic acid. Drought-stressed or heavily-fertilized anything → think nitrates. Sorghums after a drought-and-frost can have both.
- Prussic acid: wait it out. Wrap, ferment, and let several weeks pass; time and fermentation drive it off.
- Nitrates: test, don't wait. Dry hay never loses nitrate; ensiling only helps partway. Raise cutting height, delay after rain, ensile, dilute — and test before feeding.
- When in doubt, test and call your vet or extension office. Both toxins kill fast and both are preventable.
Frequently asked questions
What's the difference between prussic acid and nitrate poisoning?
They're different toxins with opposite behavior. Prussic acid (cyanide) forms in the sorghum family, is worst in young growth and after frost, and gases off as forage cures and ferments — so waiting before feeding fixes it. Nitrates accumulate in drought-stressed, heavily-fertilized crops (drought corn especially), concentrate in the lower stalk, and do not gas off — dry hay keeps its full nitrate load and ensiling only reduces it part way. The practical rule: time cures prussic acid; only testing protects you from nitrates.
Is frosted sorghum-sudangrass safe to feed?
Not immediately. Frost ruptures cells and spikes prussic acid, so don't graze frosted sorghums for at least 5–7 days and don't bale them the same day. The safe path is to wrap frosted sorghum as baleage and let it ferment several weeks — a month or more is conservative and common — before feeding, because prussic acid dissipates during curing and fermentation. If frost was combined with heavy nitrogen, also test for nitrates, which don't gas off.
Does dry hay lose its nitrates over time?
No. This is the most dangerous misconception in forage safety. Nitrate is stable, so dry hay keeps essentially its full nitrate load indefinitely — high-nitrate hay is just as dangerous a year later. Only fermentation (ensiling) reduces nitrate, and only by roughly a third to a half. If you have high-nitrate forage, dry-baling and "letting it sit" does not make it safe; you must test it and manage feeding accordingly.
Does wrapping or ensiling remove nitrates?
Partly. Fermentation in a wrapped bale, tube, or bunker typically lowers nitrate levels on the order of a third to a half over the first several weeks — better than dry hay, which loses none, but not a complete fix. Always test ensiled high-risk forage before feeding. Also be aware that ensiling high-nitrate forage produces toxic nitrogen-dioxide "silo gas," which is a hazard to people in enclosed silos during early fermentation.
Which crops carry prussic acid, and which carry nitrates?
Prussic acid is limited to the sorghum family: sorghum-sudangrass, sudangrass, forage and grain sorghum, johnsongrass, and shattercane. Corn, small grains, millet, and legumes do not carry it. Nitrates are much broader — drought corn is the classic, but stressed small grains (oats, wheat, rye, barley), sorghums, brassicas, and many weeds (pigweed, kochia, lambsquarter) can all accumulate nitrates when heavily fertilized and stressed. Sorghums hit by both drought and frost can carry both toxins at once.
The XES Netting team manufactures bale net wrap for round balers and writes these guides so forage operators can find clear, source-cited answers. Every farmer quote in this post is verbatim with a link to the original AgTalk thread. This is general educational information, not veterinary advice. Prussic acid and nitrates can kill cattle quickly — always test questionable forage through a qualified lab and consult your veterinarian or extension office before feeding.
Featured photo: Hay bales on ranch meadow by U.S. Fish and Wildlife Service, public domain, via Wikimedia Commons.
