Johnson grass is genuinely dangerous for horses. It contains compounds that can release cyanide in the digestive tract, and prolonged grazing on it has been linked to irreversible neurological and bladder damage. The risk isn’t constant, though. Certain weather conditions, growth stages, and how the grass is processed all determine whether a particular stand of Johnson grass will actually harm your horse.
Two Ways Johnson Grass Poisons Horses
Johnson grass carries a compound called dhurrin, a cyanogenic glycoside that’s harmless on its own. When the plant is damaged, whether by chewing, wilting, or frost, enzymes break dhurrin down and release hydrogen cyanide (HCN), commonly called prussic acid. Once absorbed into the bloodstream, cyanide doesn’t stop red blood cells from carrying oxygen. Instead, it blocks cells throughout the body from actually using that oxygen. The horse essentially suffocates at the cellular level, even though its blood is fully oxygenated. Animals that die from prussic acid poisoning have characteristically bright red blood for this reason.
Acute cyanide poisoning can kill within minutes to hours. Symptoms include labored breathing, trembling, staggering, and collapse. The lethal threshold is well established: forage containing more than 220 parts per million (ppm) of HCN on a wet-weight basis is considered very dangerous. Below 100 ppm is generally safe for grazing. For dried forage, anything above 750 ppm is hazardous, 500 to 750 ppm is suspect, and below 500 ppm is considered safe.
Chronic Damage From Long-Term Grazing
Even if a horse doesn’t get a large enough single dose to cause acute poisoning, weeks of steady grazing on Johnson grass or related sorghum forages can cause a condition called cystitis-ataxia syndrome. This involves progressive nerve damage that affects the bladder and the spinal cord. Horses lose coordination in their hind legs, dribble urine, and develop chronic bladder infections from incomplete emptying. The damage to the nervous system is often permanent. Horses diagnosed with this condition rarely recover fully, making prevention far more important than treatment.
Nitrate Risk Is Lower in Horses, but Not Zero
Johnson grass also accumulates nitrates, especially during drought. In ruminants like cattle, gut bacteria convert nitrate to nitrite, which interferes with the blood’s ability to carry oxygen. Horses process forage differently. Because nitrate conversion doesn’t happen until the hindgut, there are no confirmed reports of horses developing nitrate poisoning directly from grazing sorghum-type forages. That said, horses can still be poisoned if nitrates in forage or water have already been converted to nitrite by environmental bacteria before the horse consumes them. Poorly stored hay or contaminated water can create this scenario.
Symptoms of nitrate or nitrite poisoning include colic, frequent urination, and diarrhea, progressing to difficulty breathing, rapid heart rate, muscle tremors, seizures, and blue or brown discoloration of the gums.
When Johnson Grass Becomes Most Toxic
The cyanide content of Johnson grass isn’t static. It spikes under specific conditions, and knowing these patterns is the most practical thing you can do to protect your horse.
- Drought stress: When rain stops and the plant is under stress, prussic acid concentrates in the leaves, especially in young growth. After a drought breaks and rain returns, new shoots are particularly dangerous.
- Frost: A killing frost ruptures plant cells, mixing the enzymes and dhurrin that are normally kept separate. Standing Johnson grass killed by frost should be avoided for at least two weeks.
- New growth and regrowth: Prussic acid is highest in young, rapidly growing tissue. Short plants and fresh regrowth after mowing, grazing, or herbicide damage carry the greatest risk. Wait until forage has grown to at least 2 feet tall before allowing any grazing.
- Herbicide damage: Spraying Johnson grass with herbicide causes the same cell damage as frost, releasing cyanide. Horses should not graze treated areas until well after the plants have died and dried.
After a good rain following drought or frost, wait at least five days, and ideally two weeks, before letting horses near areas where Johnson grass grows.
Is Johnson Grass Hay Safe?
Properly cured Johnson grass hay is significantly safer than fresh or wilted forage. Prussic acid is a gas, and it dissipates as the plant dries. Most of the HCN escapes within about 48 hours of cutting. Texas A&M AgriLife Extension recommends allowing hay to cure for three to four days to be safe. If hay is baled while still damp or before curing is complete, it can retain enough cyanide to poison livestock.
The key word is “properly.” If you’re buying hay and you suspect it contains Johnson grass, ask how long it was cured and whether it was baled under dry conditions. Hay that was rained on during curing and then baled quickly is a risk. When in doubt, you can have hay tested for HCN levels before feeding it.
Removing Johnson Grass From Horse Pastures
Because even low-level chronic exposure can cause irreversible nerve damage in horses, the safest approach is to eliminate Johnson grass from pastures entirely. This is easier said than done. Johnson grass spreads aggressively through both seeds and underground rhizomes, and eradication takes persistence over multiple growing seasons.
In bermudagrass or bahiagrass pastures, selective herbicides containing sulfosulfuron (applied when Johnson grass is 18 inches to bloom height) can target it without killing the desirable forage. Another option is nicosulfuron plus metsulfuron, applied within 7 to 10 days after a hay cutting in bermudagrass pastures. For tall fescue pastures, there are no selective herbicides that kill Johnson grass without also killing the fescue. In those situations, spot-treating with glyphosate using a ropewick or wiper applicator (which touches only the taller Johnson grass and spares shorter fescue) is the main option, though it requires a good height difference between the two grasses and at least two passes for adequate coverage.
A dormant-season application of pendimethalin can prevent Johnson grass seedlings from establishing, but it won’t affect existing plants growing from rhizomes. Combining pre-emergence control with post-emergence treatment during the growing season gives the best long-term results. After any herbicide application, keep horses off the treated area. The dying grass releases cyanide just as frost-damaged grass does.
Practical Steps to Keep Horses Safe
If you can’t eliminate Johnson grass immediately, manage the risk by never grazing horses on pastures where Johnson grass is shorter than 2 feet. Avoid turnout after frost, drought-breaking rains, or any event that damages the plants. Inspect hay for Johnson grass before buying, and confirm it was properly field-cured for at least three to four days. If your pastures have scattered Johnson grass and your horse has been grazing there for weeks without issue, don’t assume the risk has passed. Chronic cystitis-ataxia syndrome develops silently over weeks before symptoms appear, and by the time you notice hind-end wobbling or urine dribbling, the damage is likely done.
For horse owners, the simplest rule is this: treat Johnson grass as a weed to be removed, not a forage to be managed. The margin of safety is too narrow and the consequences of chronic exposure too severe to gamble on it as a feed source.