Can Cattle Graze Frosted Sorghum Safely? Prussic Acid Risk After Frost

By Dr Duncan Houston

Frosted sorghum may look like valuable standing feed. Under the wrong conditions, however, it can release enough hydrogen cyanide to cause staggering, seizures, collapse and death within minutes.

The most dangerous mistake is using one simple rule for every frosted paddock. A mature crop after a true killing freeze is different from a stand touched by several light frosts and then producing fresh green tillers. The plant species, maturity, frost severity, new growth, nitrogen history and intended use all change the risk.

The other major concern is nitrate. Frost-stressed sorghum can present a cyanide risk, a nitrate risk or both, and the two toxins behave very differently during haymaking and ensiling.

Quick Answer

Do not graze sorghum, sorghum-sudangrass, sudangrass or Johnson grass immediately after frost.

After a true killing freeze, a conservative working rule is to wait at least seven days and until the entire stand is dead and field-cured. Some extension programmes recommend seven to ten days. After light or repeated frosts, keep cattle out until the plant is completely killed or any new regrowth has reached at least 18 to 24 inches. Test both prussic acid and nitrate whenever the crop was young, drought-stressed, heavily fertilised or producing fresh tillers. (extension.okstate.edu)

Cattle showing rapid breathing, salivation, tremors, staggering, convulsions or sudden collapse require immediate veterinary antidotal treatment. Do not wait for a laboratory result before calling. (MSD Veterinary Manual)

Frosted Forage at a Glance

Situation	Practical response
Sorghum-family forage during or immediately after frost	Remove cattle and prevent access
One light, non-killing frost	Keep cattle out because surviving plants may produce highly toxic shoots
Repeated light frosts	Treat every new frost as a new risk event
True killing freeze	Wait at least seven days, preferably seven to ten days, and until all tissue is dead and dry
New green tillers after frost	Do not graze until regrowth is at least 18 to 24 inches tall or is completely killed
Immature crop before the freeze	Use extra caution and test before grazing
Properly fermented silage	Generally the lowest cyanide-risk use, but feed only after adequate fermentation and test if the crop was highly stressed
Properly cured hay	Usually lower risk, but stressed or immature sorghum should still be tested
Pearl millet	Does not usually present a prussic acid risk, but nitrate toxicity remains possible
Any sudden signs after turnout	Remove unaffected cattle calmly and call the veterinarian immediately

The safest interval is not determined by the calendar alone. The cattle should remain out until the crop has actually died, no hazardous regrowth is present and the forage has been tested when conditions are uncertain. (UNL Beef)

What Is Prussic Acid Poisoning?

Prussic acid is another name for hydrogen cyanide, or HCN.

Sorghum plants contain a cyanogenic glycoside called dhurrin. In an intact plant cell, dhurrin and the enzymes capable of breaking it down are largely separated. Frost, chewing, crushing, wilting or chopping disrupts those cells, allowing dhurrin to be converted rapidly into hydrogen cyanide. (KSRE Bookstore)

Rumen microorganisms can also release cyanide from cyanogenic plant material. This is one reason cattle and other ruminants are particularly vulnerable.

Once absorbed, cyanide inhibits cellular respiration. Oxygen may still be present in the blood, but the tissues cannot use it properly. The animal effectively develops cellular asphyxiation despite having oxygenated blood. (MSD Veterinary Manual)

Does Frost Create Cyanide?

Not exactly.

Frost does two clinically important things:

1. It damages cell membranes and allows rapid release of cyanide from existing cyanogenic compounds.

2. If the frost does not kill the plant completely, the stress may stimulate fresh tillers or regrowth with very high dhurrin concentrations.

Young leaves, growing points and new shoots usually have the greatest cyanide potential. A paddock that looked mature before a light frost may become hazardous again when cattle selectively graze the bright green regrowth. (KSRE Bookstore)

Which Forages Can Cause Prussic Acid Poisoning?

The most important cattle forages include:

• Grain sorghum

• Forage sorghum

• Sorghum-sudangrass hybrids

• Sudangrass

• Johnson grass

• Shattercane

• Volunteer sorghum within another crop or pasture

Other cyanogenic plants include chokecherry, wild black cherry, elderberry, Indiangrass, flax and certain other species. Wilted cherry leaves falling into a paddock can cause the same rapid cyanide emergency as stressed sorghum. (KSRE Bookstore)

Which Sorghum Types Carry the Greatest Risk?

The exact risk varies between cultivars, but the general order is:

1. Grain sorghum and some forage sorghums, higher risk

2. Sorghum-sudangrass hybrids, intermediate risk

3. Sudangrass, generally lower but still capable of toxicity

Lower-HCN cultivars reduce risk but do not remove the effects of immaturity, frost, drought or excessive nitrogen fertilisation. (KSRE Bookstore)

What About Pearl Millet?

Pearl millet and foxtail millet do not normally produce dangerous prussic acid concentrations.

They can still accumulate toxic nitrate, particularly after drought, frost or heavy nitrogen fertilisation. “No cyanide risk” does not mean “automatically safe forage.” (UGA Extension)

Which Plant Parts Are Most Dangerous?

Prussic acid risk is highest in:

• Young leaves

• The upper leaves

• Growing points

• New tillers

• Regrowth after grazing or cutting

• Regrowth following drought-breaking rain

• Regrowth following a light frost

Cattle often select the leafiest and youngest material first, which can expose them to a much higher cyanide concentration than a whole-plant average suggests. (KSRE Bookstore)

This is different from nitrate accumulation, which is usually greatest in the lower stem.

What Conditions Increase Cyanide Risk?

Young or Short Plants

Sorghum-family forages should generally reach at least 18 to 24 inches before grazing under normal growing conditions.

Very young plants and short regrowth can contain substantially more dhurrin than mature forage. Some forage sorghum and sorghum-sudangrass programmes use a 24-inch minimum. (KSRE Bookstore)

Drought Stress

Drought slows plant growth while cyanogenic compounds may remain concentrated.

The period following drought can be particularly dangerous. Rain may stimulate attractive, rapidly growing shoots that cattle consume eagerly, while those shoots contain high cyanide potential. (University of Missouri Extension)

Frost or Freeze

Cell rupture after frost releases cyanide rapidly.

Repeated light frosts may be more difficult to manage than one complete killing freeze because different sections of the plant may be injured on different nights while other sections continue growing. (UNL Beef)

Excessive Nitrogen

Heavy nitrogen fertilisation can increase both cyanide and nitrate risk.

Soil and forage management should be based on testing rather than applying nitrogen alone to a stressed sorghum stand. (KSRE Bookstore)

Hungry or Unadapted Cattle

Hungry cattle consume large amounts quickly and may select young leaves and shoots aggressively.

Provide safe hay before turnout and never use a suspect sorghum paddock as the first feed offered after transport, yarding or a prolonged period without feed. (extension.okstate.edu)

Light Frost Versus Killing Freeze

This distinction changes the management decision.

After a Light Frost

A light frost may damage only the upper leaves while leaving the lower plant and growing points alive.

This creates two problems:

• Cyanide may be released from damaged tissue.

• New tillers may emerge and contain extremely high dhurrin concentrations.

Keep cattle out. If another frost occurs, restart the waiting period. If regrowth develops, do not graze until it reaches at least 18 to 24 inches or until a later killing freeze completely terminates the plant. (UNL Beef)

After a Killing Freeze

A killing freeze stops plant growth completely.

Recommendations vary somewhat between regions, commonly ranging from five to ten days. A conservative practical rule is:

• Wait at least seven days.

• Prefer seven to ten days when conditions have been marginal.

• Confirm that all plants are dead and field-cured.

• Inspect closely for surviving tillers or pockets of green regrowth.

• Test if the crop was immature, drought-stressed or heavily fertilised.

Penn State recommends waiting at least seven to ten days after a killing frost, while K-State advises waiting seven to ten days and resuming only after the stand has been entirely dead for about a week. (Penn State Extension)

What if the Crop Was Too Short Before the Freeze?

A crop that was already too immature to graze safely before the frost should not automatically become safe seven days afterwards.

Use a longer waiting period and laboratory testing. Some extension guidance recommends at least two weeks for forage that was immature before the killing freeze. (UGA Extension)

Prussic Acid Versus Nitrate Poisoning

These two forage toxicities are frequently confused.

They may occur in the same field, but one test result does not predict the other.

Feature	Prussic acid or cyanide	Nitrate or nitrite
Main plant location	Young leaves, growing points and regrowth	Lower stems and stalk bases
Main mechanism	Prevents cells from using oxygen	Converts haemoglobin to methaemoglobin, reducing oxygen transport
Typical onset	Often within minutes	Commonly develops over tens of minutes to several hours
Blood colour	May remain bright cherry red	Often chocolate brown
Effect of haymaking	Usually reduces risk, but high-risk hay may still require testing	Nitrate remains in hay
Effect of ensiling	Often reduces cyanide substantially	May reduce nitrate, but does not guarantee safety
Test needed	Hydrogen cyanide or cyanide potential	Nitrate or nitrate-nitrogen
Antidote	Sodium nitrite and sodium thiosulfate, or another veterinary cyanide protocol	Methylene blue under veterinary direction

Frosted sorghum should often be tested for both cyanide and nitrate because the concentrations do not necessarily rise or fall together. (UNL Beef)

Do Not Diagnose From Blood Colour Alone

Bright red blood supports cyanide poisoning, while chocolate-brown blood supports nitrate poisoning.

Colour can change after death, exposure to air and sample deterioration. It is a useful clue, not a substitute for history, forage testing and veterinary toxicology. (KSRE Bookstore)

What Are the Signs of Prussic Acid Poisoning?

Clinical signs may begin within 15 to 30 minutes of eating toxic forage. Death may occur shortly afterwards. (University of Missouri Extension)

Possible signs include:

• Sudden anxiety or restlessness

• Rapid breathing

• Laboured breathing

• Rapid heart rate

• Excessive salivation

• Foaming around the mouth or nose

• Muscle trembling

• Weakness

• Staggering

• Incoordination

• Repeated urination or defecation

• Convulsions

• Collapse

• Sudden death

The mucous membranes may appear unusually bright red because oxygen remains within the blood while the tissues cannot use it. This sign is not always obvious in the live animal. (extension.okstate.edu)

Can Cattle Look Normal Before Collapsing?

Yes.

A group may begin grazing normally, then several cattle may suddenly become breathless, stagger or collapse. Some animals are found dead with little observed warning.

In one Missouri incident, more than 70 adult cattle died after access to drought-stressed pasture containing lush Johnson grass regrowth. Several animals briefly staggered or lay down before dying, while others were simply found dead. (University of Missouri Extension)

How Worried Should You Be?

Lower Risk

The forage:

• Is not a cyanogenic species

• Is mature and unstressed

• Has not recently been frosted

• Contains no short regrowth

• Has tested below the laboratory’s safe threshold

• Is being introduced to well-fed cattle

Action: Graze with normal monitoring while continuing to manage nitrate and other forage risks.

Moderate Risk

The forage:

• Is a mature sorghum-family crop

• Experienced a confirmed killing freeze more than seven days ago

• Is fully dead and dry

• Has no visible green tillers

• Was not heavily drought-stressed

Action: Testing remains sensible, particularly if the crop was heavily fertilised. Feed hay before turnout and monitor the group closely.

High Risk

The forage:

• Was frosted within the previous seven days

• Experienced several light frosts

• Contains young green tillers

• Was under 18 to 24 inches tall

• Is drought-stressed or recently regrowing

• Received substantial nitrogen

• Will be grazed by hungry cattle

• Has not been tested

Action: Do not graze or green-chop. Fence cattle out and arrange cyanide and nitrate testing.

Critical

One or more cattle show:

• Sudden rapid breathing

• Salivation

• Trembling

• Staggering

• Convulsions

• Collapse

• Sudden death after pasture entry

Action: Remove unaffected cattle from the feed source calmly and call the veterinarian immediately. Do not wait for testing before initiating the emergency response.

When Is This an Emergency?

Treat suspected prussic acid poisoning as an immediate emergency.

Call the veterinarian when cattle develop any of the following after eating sorghum, Johnson grass or another cyanogenic plant:

• Sudden respiratory distress

• Excessive salivation

• Muscle tremors

• Weakness or staggering

• Convulsions

• Collapse

• Multiple unexpected deaths

The antidote must be administered before prolonged oxygen deprivation causes irreversible injury. Many animals die before veterinary treatment can begin, but cattle treated promptly can recover rapidly. (MSD Veterinary Manual)

What To Do Right Now During a Suspected Poisoning

1. Stop Access to the Forage

Remove unaffected cattle calmly from the paddock or feed.

Do not chase, stress or force collapsing animals to walk long distances. Excitement and muscular activity increase oxygen demand.

2. Call the Veterinarian Immediately

Tell the veterinarian:

• Which forage was being grazed

• When cattle entered it

• Whether frost, drought or recent regrowth occurred

• How many animals are affected

• Which signs are present

• Whether any cattle have died

• Whether nitrate is also suspected

Specifically mention suspected cyanide or prussic acid poisoning so the appropriate antidote can be prepared before arrival.

3. Do Not Wait for Laboratory Confirmation

Treatment is selected from the history and rapid clinical pattern.

Forage, rumen contents and other diagnostic samples can be collected once the emergency response has begun.

4. Keep Affected Cattle Quiet

Reduce handling, noise and unnecessary movement.

Place recumbent cattle in sternal position if this can be done safely, but do not delay antidotal treatment.

5. Preserve the Forage

Prevent other livestock from entering the area.

After treatment has begun, collect samples using the laboratory’s instructions. Cyanide is volatile, and poorly handled samples can produce a falsely reassuring result. (extension.okstate.edu)

How Is Cyanide Poisoning Treated?

The traditional veterinary antidote is:

1. Intravenous sodium nitrite

2. Followed by intravenous sodium thiosulfate

Sodium nitrite creates methaemoglobin that temporarily binds cyanide, while sodium thiosulfate provides sulfur that helps convert cyanide into thiocyanate for excretion. (MSD Veterinary Manual)

Additional veterinary options may include:

• Sodium thiosulfate alone in selected uncertain cases

• Hydroxocobalamin with sodium thiosulfate

• Oxygen and supportive treatment

• Repeat antidote treatment where signs recur and laboratory monitoring supports it

These treatments require veterinary direction. Sodium nitrite deliberately induces methaemoglobinaemia and can be dangerous if the diagnosis is wrong, particularly when nitrate poisoning is actually present. (MSD Veterinary Manual)

Is Treatment Usually Too Late?

Not always.

Treatment frequently cannot begin quickly enough because the poisoning progresses so rapidly. When the veterinarian reaches the animal early and administers the antidote promptly, dramatic recovery is possible.

Animals still alive one to two hours after the onset of visible signs generally have a better chance of recovery. (KSRE Bookstore)

A farm with recurrent exposure risk should establish an emergency cyanide protocol with its attending veterinarian before frost season. Having a protocol is useful. Improvising sodium nitrite dosing beside a seizuring cow is not.

How Should Frosted Forage Be Tested?

Contact the Laboratory Before Sampling

Prussic acid can escape from cut forage as a gas.

Laboratories may require a particular container, preservation method, shipping interval or sample type. Always obtain the laboratory’s current submission instructions first. (extension.okstate.edu)

Collect a Representative Sample

For a grazing paddock:

• Sample several locations

• Include areas with different growth and frost exposure

• Include the leaves and young growth cattle are likely to select

• Sample visible regrowth separately

• Keep samples airtight

• Submit them promptly

For nitrate testing, include representative stem material and follow the laboratory’s specific protocol because nitrate is often concentrated near the base.

Test Each Meaningful Risk Event

A result from before the frost does not establish safety after the frost.

Retest when:

• Another frost affects previously living tissue

• New tillers emerge

• Drought ends with rain

• A different paddock is used

• The crop is harvested into hay or silage

• The forage changes substantially before feeding

Quick Tests Versus Quantitative Tests

Field test paper may indicate whether cyanide is present, but it does not provide a dependable feeding concentration.

A quantitative laboratory result is more useful for deciding whether forage can be grazed, diluted or rejected. Quick tests can also lose sensitivity through poor storage, light exposure or incorrect handling. (extension.okstate.edu)

How Are Prussic Acid Test Results Interpreted?

Laboratories may report results on a wet-weight or dry-matter basis. The units must be checked before making a feeding decision.

A commonly used dry-matter interpretation is:

HCN concentration	General interpretation
Below 500 ppm	Generally considered safe
500 to 750 ppm	Potentially toxic; should not be the sole feed
Above 750 ppm	Dangerous and should not be fed without specific professional management
Above 1,000 ppm	Extremely dangerous in commonly used extension tables

Different laboratories use slightly different cut-offs. Follow the interpreting laboratory’s reference range and involve the herd veterinarian or nutritionist whenever the result is above 500 ppm. (KSRE Bookstore)

Testing reduces uncertainty, but a single sample cannot perfectly represent selective grazing across an uneven paddock.

Is Frosted Sorghum Safe as Hay?

Proper curing generally reduces free hydrogen cyanide because HCN is volatile.

However, “make it into hay” is not a universal safety guarantee. Recent K-State and Purdue work found that dhurrin, the cyanogenic precursor, remained stable in sorghum hay for at least two months, and cattle rumen enzymes may still release cyanide from it. K-State therefore recommends making sorghum hay only when the crop was mature and not significantly cold- or drought-stressed, and testing the finished hay when there is doubt. (KSRE Bookstore)

The Practical Hay Rule

Hay is generally safer when:

• The crop was more than 18 to 24 inches tall

• It was not severely drought-stressed

• It was not harvested immediately after frost

• It was completely cured

• The final hay was tested when initial risk was high

Nitrate remains in dried hay. A crop made safer for cyanide through curing may still be dangerous because of nitrate. (UNL Beef)

Is Ensiling Safer?

Silage is generally considered the lowest-risk use of sorghum when cyanide potential is a concern.

Chopping and fermentation commonly reduce dhurrin and cyanide potential by 50% or more. Sorghum silage should be allowed to ferment properly, generally for at least three to four weeks, before feeding. High-risk silage should still be tested after fermentation. (KSRE Bookstore)

Ensiling may also reduce nitrate concentrations, but it does not guarantee that high-nitrate forage becomes safe. Test the finished feed before using it.

What About Green Chop?

Green chopping can reduce selective grazing because cattle receive stems and leaves together. It may also allow some HCN to escape during chopping and handling.

It is still less safe than properly fermented silage, particularly when the crop has just been frosted, is short or contains high-risk regrowth. Do not green-chop obviously suspect forage as an improvised way of making it safe. Test first. (KSRE Bookstore)

Why Sentinel Cattle Are Not a Safe Test

Some older recommendations suggest placing one or two cattle into a paddock and watching them before releasing the herd.

I would not use this as a safety test.

Cyanide poisoning can kill a healthy adult cow within minutes, and visible clinical signs may leave almost no treatment window. Laboratory forage testing is the appropriate way to assess uncertainty, not sacrificing the bravest cow to agricultural quality control. (MSD Veterinary Manual)

Once properly tested forage is being introduced, close observation of the whole group remains sensible. That is monitoring, not deliberate diagnostic exposure.

What Else Can Cause Sudden Collapse After Turnout?

Possible problem	Useful clues
Nitrate or nitrite poisoning	Chocolate-brown blood, rapid breathing, weakness and abortions with lower chronic exposure
Grass tetany	Nervousness, hypersensitivity, stiffness and seizures, usually on lush high-potassium pasture
Urea or ammonia toxicity	Salivation, abdominal pain, bloat, rapid breathing and possible ammonia odour
Organophosphate poisoning	Salivation, diarrhoea, small pupils, tremors and respiratory distress
Acute bloat	Marked left abdominal distension and respiratory compression
Polioencephalomalacia	Blindness, head pressing, star-gazing and seizures
Poisonous plants	Exposure-specific signs, often neurological or gastrointestinal
Lightning or electrocution	Several animals found dead in one location with little premonitory illness
Acute respiratory disease	Fever, respiratory effort, coughing and less direct association with one forage entry
Anthrax in endemic regions	Sudden death, bleeding from body openings and incomplete rigor mortis

Do not open an unexplained sudden-death carcass before discussing anthrax risk with the veterinarian where the disease is regionally possible.

What To Do After a Frost Event

1. Identify the Plants

Check the seed tag and inspect for:

• Forage sorghum

• Grain sorghum

• Sorghum-sudangrass

• Sudangrass

• Johnson grass

• Volunteer sorghum

• Pearl millet

• Oats or other nitrate-accumulating species

A mixed cover crop is not automatically safe because most of its species are harmless. Cattle may selectively eat the toxic component.

2. Determine Whether the Frost Was Killing

Look for:

• Completely dead leaves and stems

• Continued green tissue

• Surviving lower nodes

• New green tillers

• Different frost severity across the field

• Protected areas near trees or slopes

When uncertainty remains, manage it as a non-killing frost.

3. Fence Cattle Out

Do this before inspecting the paddock extensively. Cattle have a remarkable ability to locate the one green tiller you were hoping they would ignore.

4. Start the Waiting Period

After a killing freeze, wait at least seven days and until all plants are dead and cured.

After another frost, restart the count for newly affected living tissue.

5. Inspect for Regrowth

Do not graze new shoots until they reach at least 18 to 24 inches or are fully killed by a later freeze. (UNL Beef)

6. Test for Both Toxins

Request:

• Hydrogen cyanide or cyanide potential

• Nitrate or nitrate-nitrogen

Do not assume one safe result predicts the other.

7. Decide How the Forage Will Be Used

Depending on the results and crop condition, options may include:

• Delayed grazing

• Proper hay curing followed by testing

• Ensiling and testing after fermentation

• Dilution within a professionally formulated ration

• Complete rejection of the forage

8. Feed Safe Hay Before Turnout

Never release hungry cattle onto sorghum-family forage, even after the waiting period.

9. Build an Emergency Plan

The written protocol should identify:

• The attending veterinarian

• Where antidotal supplies are held

• Who is trained to recognise the signs

• How unaffected cattle will be removed

• Which laboratory accepts urgent samples

• How paddocks will be closed immediately

Common Frosted-Forage Mistakes

Using a Universal Four-Day Rule

Waiting times vary with frost severity, plant maturity and regrowth. Seven days and complete plant death is a safer minimum after a killing freeze, with seven to ten days preferred in many programmes.

Treating Light Frost Like a Killing Freeze

A light frost may leave the plant alive and stimulate highly toxic new shoots.

Forgetting To Restart the Clock

Each new frost that damages living tissue creates a new risk period.

Grazing Green Regrowth

New tillers are often the most cyanogenic part of the stand.

Testing Only for Nitrate

Cyanide and nitrate concentrations do not reliably track together.

Assuming Hay Is Automatically Safe

Proper curing reduces free HCN, but high-risk sorghum hay may retain cyanogenic potential and should be tested.

Turning Hungry Cattle Onto the Paddock

Rapid intake substantially increases exposure.

Using Sentinel Cows

Clinical disease can develop too quickly for this to be considered a safe test.

Relying on Blood Colour or Almond Smell

Neither is a sufficiently dependable field diagnosis.

Assuming Treatment Never Works

Immediate antidotal therapy can be successful. The problem is getting treatment into the animal before irreversible oxygen deprivation occurs.

How Can Prussic Acid Poisoning Be Prevented?

Choose Lower-Risk Forages

Where appropriate, consider:

• Pearl millet

• Lower-HCN sudangrass cultivars

• Lower-dhurrin sorghum hybrids

• Alternative cool-season or warm-season forage species

The chosen crop must still suit the soil, climate, animal class and nitrate-management plan.

Graze at the Correct Height

Do not graze sorghum-family forage below approximately 18 to 24 inches under ordinary conditions.

Maintain adequate residual height and prevent cattle from returning too soon to short regrowth. (KSRE Bookstore)

Fertilise According to Soil Tests

Avoid excessive nitrogen and maintain appropriate phosphorus, potassium and soil pH.

Overfertilisation may increase both cyanide and nitrate risk. (extension.okstate.edu)

Monitor Drought and Frost

Record:

• Frost dates

• Whether each event was killing

• Rain after drought

• Regrowth dates

• Nitrogen applications

• Plant height

• Grazing and harvest dates

• Laboratory results

Control Johnson Grass and Volunteer Sorghum

Small areas along fences, lanes and water points can expose cattle even when the main crop is safe.

Inspect Trees Around Paddocks

Prevent cattle from accessing wilted cherry, chokecherry or related leaves following storms, pruning or frost.

Test Before Taking the Risk

Testing is particularly justified when:

• The crop is visibly stressed

• Plants are immature

• New shoots are present

• Nitrogen applications were high

• Several frost events occurred

• The forage will be the main feed

• The herd includes pregnant, high-value or otherwise vulnerable cattle

Frequently Asked Questions

How long should cattle stay off sorghum after frost?

After a killing freeze, keep cattle out for at least seven days and until the entire plant is dead and dry. Seven to ten days is a more conservative interval. After light or repeated frost, do not graze until the crop is fully killed or new growth reaches at least 18 to 24 inches. (Penn State Extension)

Is frosted sorghum safe once it is made into hay?

Proper curing usually reduces hydrogen cyanide substantially, but severely stressed or immature sorghum hay should be tested. Nitrate does not disappear during haymaking and must be assessed separately. (KSRE Bookstore)

Can forage be safe for nitrate but dangerous for prussic acid?

Yes. The two toxins accumulate in different plant tissues and do not reliably correlate. Test both when sorghum-family forage has been drought- or frost-stressed.

Does pearl millet cause prussic acid poisoning?

Pearl millet does not normally accumulate dangerous prussic acid. It can accumulate nitrate and still requires appropriate testing after stress. (UGA Extension)

Can cattle recover from prussic acid poisoning?

Yes, when veterinary antidotal treatment is administered immediately. Many animals die before treatment is possible because the onset is so rapid. Animals that remain alive one to two hours after signs begin generally have a better prognosis. (KSRE Bookstore)

Final Thoughts

Frost-damaged sorghum can be used safely, but only when the plant, frost event and intended feeding method are evaluated properly.

The most important points are:

1. Sorghum-family forage can release hydrogen cyanide after frost.

2. Cattle may collapse and die within minutes.

3. Young leaves, tillers and regrowth carry the greatest risk.

4. Light frost may be more complicated than one complete killing freeze.

5. Each additional frost can restart the waiting period.

6. After a killing freeze, wait at least seven days and until the stand is completely dead and cured.

7. Seven to ten days is the more conservative interval.

8. Do not graze regrowth until it is 18 to 24 inches tall or completely killed.

9. Never turn hungry cattle into suspect forage.

10. Test both prussic acid and nitrate.

11. Cyanide risk and nitrate risk do not necessarily rise together.

12. Proper curing usually reduces HCN, but high-risk hay should still be tested.

13. Nitrate remains in hay.

14. Properly fermented silage is usually the safest sorghum use.

15. Do not use sentinel cattle as a toxicity test.

16. Rapid breathing, salivation, tremors, staggering or collapse is an emergency.

17. Sodium nitrite and sodium thiosulfate can save affected cattle when given immediately.

18. The best emergency antidote remains prevention before the gate is opened.

The mistake that causes the largest losses is seeing dead-looking forage after a frost and assuming dead-looking means chemically safe. What matters is whether every plant is actually dead, whether hazardous regrowth is present and whether testing supports the grazing decision.

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ASK A VET™ can help organise frost dates, forage species, plant height, laboratory results and grazing records while producers work directly with their attending livestock veterinarian and forage adviser. Cattle with rapid breathing, salivation, tremors, staggering, seizures or collapse still require immediate hands-on veterinary treatment.