Johnson grass becomes poisonous to livestock primarily when the plant is young, stressed, or damaged, because those conditions cause it to release hydrogen cyanide (commonly called prussic acid) or accumulate dangerous levels of nitrates. A stand of mature, healthy Johnson grass growing in good conditions on a mild day may pose little immediate threat, but the same field after a hard frost, a drought, or a fresh mowing can turn lethal within hours. Understanding the specific triggers matters far more than simply knowing the plant is toxic, because most livestock losses from Johnson grass are preventable with timing and management.
How Johnson Grass Produces Prussic Acid
Johnson grass (Sorghum halepense) belongs to the sorghum family, and like other sorghums it manufactures a compound called dhurrin in its leaves and stems. Dhurrin itself is not directly toxic. It functions as a built-in defense chemical the plant uses against insects and herbivores. The danger comes when plant cells are ruptured: enzymes that are normally kept separate from dhurrin suddenly come into contact with it and break it down, releasing hydrogen cyanide gas in the process.1Europe PMC. Dhurrin in Sorghum: Biosynthesis, Regulation, Biological Function and Challenges for Animal Production That cell rupture can happen when an animal chews the plant, when frost damages tissue, or when wilting crushes cells internally. The result is the same: a burst of cyanide that the animal inhales or swallows.
Hydrogen cyanide works fast. It blocks cells from using oxygen, so even though the animal’s blood is fully oxygenated, tissues starve. A lethal dose can kill a cow in under an hour. The speed of onset is one reason prussic acid poisoning is so feared by ranchers: an animal that looked fine at morning feeding can be dead before lunch.
The Conditions That Make It Dangerous
Dhurrin concentrations in Johnson grass are not constant. They fluctuate dramatically depending on the plant’s age, growth stage, and environment. Several well-established conditions spike the risk.
- Young, short growth: Seedlings and fresh regrowth contain far more dhurrin per pound of tissue than mature plants. The general rule among livestock producers is to avoid grazing Johnson grass shorter than about 18 to 24 inches. New tillers sprouting from the plant’s aggressive rhizome system are especially concentrated.
- Frost: A killing frost or even a light freeze ruptures plant cells across the entire stand at once. This releases cyanide throughout the forage. Animals turned onto a frost-damaged Johnson grass pasture face one of the highest-risk scenarios.
- Drought stress: When the plant stops growing because of dry conditions, dhurrin accumulates in the leaves rather than being diluted by new growth. The first rain after a drought triggers a flush of young, high-dhurrin shoots, creating a second wave of danger.
- Wilting and trampling: Mechanical damage from mowing, hail, or heavy trampling crushes cells and liberates cyanide. Freshly cut Johnson grass is more hazardous than standing forage.
- High nitrogen fertility: Soils heavily fertilized with nitrogen or amended with manure tend to produce plants with higher dhurrin levels, because nitrogen availability fuels the biochemical pathway that builds dhurrin.1Europe PMC. Dhurrin in Sorghum: Biosynthesis, Regulation, Biological Function and Challenges for Animal Production
The common thread is that anything stressing the plant or interrupting its normal growth tends to raise cyanide potential. Healthy, tall, actively growing Johnson grass dilutes its dhurrin across a large volume of tissue, keeping concentrations below the danger threshold. Stunted, damaged, or immature plants pack that same chemistry into a much smaller package.
Nitrate Poisoning Is the Other Threat
Prussic acid gets most of the attention, but Johnson grass can also accumulate toxic levels of nitrates, and the two problems behave differently enough that managing for one does not automatically protect against the other.
Plants take up nitrate from the soil and normally convert it into amino acids and proteins as they grow. When growth stalls due to drought, cloudy weather, or cold temperatures, the plant keeps absorbing nitrate but cannot process it fast enough. It builds up in the stems, particularly in the lower third of the plant. When a cow eats that forage, microbes in the rumen convert nitrate to nitrite, and nitrite enters the bloodstream and reacts with hemoglobin, forming methemoglobin. Methemoglobin cannot carry oxygen, so the animal essentially suffocates from the inside out.2Netherlands Journal of Agricultural Science. Nitrate poisoning in cattle. 2. Changes in nitrite in rumen fluid and methemoglobin formation in blood after high nitrate intake
The critical difference between nitrate and prussic acid poisoning is what happens during hay making. Hydrogen cyanide is a gas, so it largely dissipates as Johnson grass is cut, dried, and baled. Properly cured hay is much safer from a prussic acid standpoint than fresh forage. Nitrates, however, are stable salts. They do not evaporate. Hay baled from drought-stressed Johnson grass retains every bit of its nitrate load and remains dangerous months later. This catches some producers off guard: they assume that because the hay is dry, it is safe.
Which Livestock Are Most Vulnerable
Ruminants, especially cattle, are the animals most commonly killed by Johnson grass. Their rumen biology makes them efficient at converting cyanogenic glycosides into free cyanide and at converting nitrate into the more harmful nitrite. Sheep are also susceptible, though they tend to be slightly more tolerant of cyanide on a per-pound basis than cattle. Goats, while still at risk, seem to handle moderate cyanide exposure somewhat better than cattle or sheep, possibly because of differences in rumen microbe populations and feeding behavior that lead them to take in a more varied diet rather than gorging on a single forage.
Horses face a different and less widely known problem. Chronic exposure to sorghum-family grasses, including Johnson grass, has been linked to a condition involving bladder dysfunction and hind-limb incoordination. Unlike acute prussic acid poisoning, this syndrome develops over weeks of steady grazing. The mechanism is not the same as cyanide toxicity; it appears to involve damage to nerves in the spinal cord and urinary tract. Horse owners are generally advised to keep horses off Johnson grass pastures entirely, not just during high-risk windows, because the chronic neurological risk does not depend on the same stress triggers that spike cyanide levels.
How to Recognize Poisoning
Prussic acid poisoning and nitrate poisoning look similar in their early stages because both involve oxygen deprivation, but there are differences a producer or veterinarian can spot.
With prussic acid, onset is rapid. Animals may show labored breathing, staggering, and collapse within minutes of grazing. The mucous membranes (gums, inner eyelids) often appear bright cherry-red, because blood is fully oxygenated but cells cannot use the oxygen. A characteristic bitter-almond smell may be noticeable on the animal’s breath or in rumen contents, though not everyone can detect this odor.
Nitrate poisoning tends to develop somewhat more slowly, over several hours, because the conversion from nitrate to nitrite in the rumen takes time. Mucous membranes turn a chocolate-brown color rather than cherry-red, reflecting the methemoglobin in the blood.2Netherlands Journal of Agricultural Science. Nitrate poisoning in cattle. 2. Changes in nitrite in rumen fluid and methemoglobin formation in blood after high nitrate intake Animals may show weakness, trembling, and frequent urination before progressing to collapse. Pregnant animals exposed to sublethal nitrate levels can abort even if the cow herself survives, making nitrate a particular concern in cow-calf operations.
Emergency treatment exists for both types. Prussic acid poisoning is treated with intravenous sodium thiosulfate (and sometimes sodium nitrite, somewhat confusingly), which helps the body detoxify cyanide. Nitrate poisoning is treated with methylene blue, which converts methemoglobin back to functional hemoglobin. In both cases, the treatment window is short. Having a veterinarian on call and knowing which toxin you are likely dealing with can mean the difference between losing one animal and losing several.
Practical Management to Avoid Losses
Most experienced producers in the southeastern United States and other regions where Johnson grass is common learn to coexist with it rather than trying to eliminate it entirely, since the plant’s deep rhizome system makes eradication nearly impossible without repeated herbicide applications. The goal becomes managing grazing and harvest timing so that animals never encounter the plant during its dangerous windows.
- Height rule: Do not turn cattle or sheep onto Johnson grass pastures until the forage is at least 18 to 24 inches tall. Taller plants have lower cyanide concentrations per bite.
- Post-frost waiting period: After a frost, keep livestock off Johnson grass for at least a week, and preferably until the damaged plant material has completely dried out. Fresh frost-killed tissue is the highest-risk forage you can encounter.
- Drought awareness: During prolonged dry spells, remove animals from Johnson grass pastures. When rain finally arrives, wait for the resulting regrowth to reach safe height before reintroducing livestock.
- Test hay and silage: If you harvest Johnson grass that was growing under any kind of stress, send a sample to a forage-testing lab for nitrate analysis before feeding it. Labs typically report results in parts per million of nitrate-nitrogen, and your extension agent can help interpret the numbers relative to your animals’ tolerance.
- Dilute the diet: Supplementing with other feeds before turning animals onto a Johnson grass pasture reduces the rate of cyanide intake. A rumen full of other forage slows the animal’s consumption of the risky grass and gives the liver more time to detoxify small amounts of cyanide.
- Don’t graze hungry animals: Cattle that have been held off feed and then released onto Johnson grass tend to eat rapidly and indiscriminately, taking in a large dose of cyanide in a short time. Always offer hay or another feed source before turnout.
Silage handling deserves its own note. Ensiling Johnson grass can reduce prussic acid levels because fermentation encourages the breakdown of dhurrin before animals eat it, and some of the released cyanide escapes as gas during the fermentation process. However, the reduction is not always complete, and poorly fermented silage may still contain hazardous levels. Nitrate levels in silage also drop somewhat during fermentation, but again, not reliably enough to skip testing.
How Drought and Climate Patterns Shift the Risk
Regions experiencing more frequent or prolonged droughts are seeing Johnson grass toxicity become a recurring management headache rather than an occasional crisis. In a stable, wet growing season, Johnson grass may never reach dangerous cyanide or nitrate levels because the plant keeps growing and diluting its stored toxins. In a season with alternating drought and rain, the plant cycles through stress and recovery repeatedly, each cycle producing a fresh flush of high-dhurrin regrowth. Producers in the southern Great Plains and the mid-South have reported that the unpredictability of these cycles has made rules of thumb harder to rely on.
Soil fertility plays into this as well. Fields with high residual nitrogen, whether from prior fertilizer applications, manure spreading, or legume rotations, tend to produce Johnson grass with elevated nitrate and dhurrin levels during stress. Reducing nitrogen inputs on fields dominated by Johnson grass is one management lever, though it comes at the cost of overall forage yield.
Johnson Grass Versus Other Sorghum-Family Forages
Sudan grass, sorghum-sudan hybrids, and forage sorghum all share the same dhurrin-based toxicity risk as Johnson grass, because they belong to the same genus. The management guidelines are broadly similar: avoid young growth, wait after frost, test under stress conditions. Where Johnson grass differs is its perennial nature. Sudan grass and sorghum hybrids are annuals planted deliberately and managed on a known schedule. Johnson grass is a perennial weed that spreads aggressively via rhizomes and shows up uninvited in pastures, hayfields, ditches, and fence lines. You cannot control when it starts growing or when it gets frosted the way you can with a planted annual.
That lack of control makes Johnson grass the more dangerous forage in practice. A producer planting sorghum-sudan knows the planting date, can predict roughly when the stand will reach safe grazing height, and can plan a frost-avoidance harvest. Johnson grass scattered through a bermudagrass pasture does not cooperate with any schedule. It may send up new tillers after a summer mowing when the rest of the pasture is safe, creating patches of short, high-cyanide grass amid otherwise benign forage. Some producers manage this by spot-spraying Johnson grass with herbicide in bermudagrass pastures, targeting eradication of the weed rather than trying to manage around it.
Breeding and the Future of Safer Sorghum Forage
Researchers have been working for years to develop sorghum varieties with reduced dhurrin content, aiming to make sorghum-family forages safer for livestock without sacrificing yield. The genetics of dhurrin production are reasonably well understood. A small number of genes control the enzymes that build dhurrin, and mutant lines with disrupted versions of those genes produce much less of the compound.1Europe PMC. Dhurrin in Sorghum: Biosynthesis, Regulation, Biological Function and Challenges for Animal Production The tradeoff is that dhurrin also protects the plant against insect feeding and possibly against some fungal diseases, so low-dhurrin varieties may need more pest management to survive in the field.
For Johnson grass specifically, breeding is not a practical tool because it is a weed, not a crop. Nobody plants it on purpose, so developing improved cultivars would not help. The breeding work matters more for Sudan grass and forage sorghum, where producers choose what seed to plant. If low-dhurrin commercial varieties become widely available, they could reduce the overall burden of cyanide poisoning in livestock operations that rely on warm-season annual grasses, even if the perennial Johnson grass problem persists in pastures and waste areas. In the meantime, the management playbook for Johnson grass remains the same: know what triggers toxicity, test when in doubt, and keep animals away from the plant during its most dangerous moments.