Properly cured Johnsongrass hay can be a safe and reasonably nutritious feed, but the curing process only eliminates one of the two toxicity risks the plant carries. Johnsongrass, like other members of the sorghum family, produces cyanogenic glycosides that release prussic acid (hydrogen cyanide) and can also accumulate dangerous levels of nitrates. Drying the grass into hay effectively drives off the prussic acid, but nitrates remain in dried forage at roughly the same concentration as when the plant was cut. Understanding which hazard you’ve solved and which you haven’t is the difference between safe hay and a dead animal.
The Two Toxins in Johnsongrass
Johnsongrass produces a compound called dhurrin, a cyanogenic glycoside stored in the plant’s cells. When plant tissue is damaged by chewing, freezing, wilting, or crushing, enzymes break dhurrin down and release hydrogen cyanide gas, commonly called prussic acid. This is the toxin that can kill cattle within minutes of grazing fresh, stressed Johnsongrass. Young regrowth, drought-stunted plants, and frost-damaged stands are the most dangerous because they concentrate dhurrin in a smaller amount of tissue.
The second toxin is nitrate. Plants absorb nitrate from the soil as part of normal nitrogen metabolism, but under certain conditions they take up more than they can convert into protein. Heavy nitrogen fertilization, drought stress, cloudy weather, and herbicide damage can all cause nitrate to pile up in the lower stems and leaves. When livestock eat high-nitrate forage, gut bacteria convert nitrate to nitrite, which interferes with the blood’s ability to carry oxygen. Cattle that consume too much can suffocate internally, turning the classic chocolate-brown color in their blood and mucous membranes.
These two toxins behave very differently during hay curing, and that difference is the crux of the whole safety question.
Why Drying Eliminates Prussic Acid
Hydrogen cyanide is a volatile gas. Once plant cells break down during cutting and wilting, the enzymatic reaction that produces HCN from dhurrin proceeds rapidly, and the gas escapes into the air. As Johnsongrass dries in the field over several days, the moisture that would otherwise trap HCN evaporates, carrying the gas with it. By the time the hay reaches a stable, well-cured state at around 15 to 18 percent moisture, virtually all the prussic acid has dissipated.
This is why properly made Johnsongrass hay is widely considered safe from cyanide poisoning. The key word is “properly.” If hay is baled too wet or rained on and then baled before it dries again, pockets of moisture can trap residual HCN or create conditions where the enzymatic breakdown continues in a sealed environment. Silage and haylage present a more complicated picture because the fermentation environment is anaerobic and wet, though research has shown that ensiling can still produce acceptable Johnsongrass forage when done correctly.
Why Nitrates Stick Around in Hay
Unlike prussic acid, nitrate is a stable, non-volatile compound. It does not evaporate during drying. Whatever concentration of nitrate was in the plant at the moment you cut it will still be in the bale six months later. This is the part that catches people off guard: they’ve heard that curing Johnsongrass makes it safe, and they assume that means all the toxins are gone. It doesn’t. It means the cyanide is gone. The nitrate stays.
Nitrate levels in Johnsongrass are highest in the lower third of the stem. If you cut the hay higher, leaving more stubble, you leave the most nitrate-dense tissue in the field instead of in the windrow. Raising the cutter bar to six or eight inches is one of the simplest and most effective ways to reduce the nitrate load in your bales. This does mean you sacrifice some tonnage per acre, but the trade-off is a safer product.
Ensiling, by contrast, can actually reduce nitrate levels. Fermentation bacteria convert some nitrate into other nitrogen compounds and gases during the ensiling process. Research on Johnsongrass silage has shown that it can be successfully ensiled and that the process does alter the forage’s chemistry beyond just drying it.
When to Cut for Safety and Quality
Timing your harvest is the single most important management decision for both nutritive value and toxin risk. Younger Johnsongrass has higher crude protein and lower fiber, but it also has higher concentrations of dhurrin per unit of tissue. As the plant matures past the boot stage and into flowering and seed development, protein drops and fiber increases, while dhurrin concentrations decline somewhat on a whole-plant basis simply because the plant has more structural tissue diluting the toxin.
Research comparing Johnsongrass harvested at multiple maturity stages found that forage cut at the three-week regrowth and boot stages had the highest crude protein, the highest total digestible nutrients, and the lowest fiber content. Once the plants reached the flower and dough stages, protein dropped significantly and fiber increased as lignified structural tissue accumulated.1PubMed Central. The impact of maturity stages on yield, quality, and nutritive value of ensiled Johnsongrass [Sorghum halepense (L.) Pers] These results held across multiple harvests, confirming a pattern well known in other sorghum-family forages: waiting too long to cut sacrifices feed quality.
But there’s a tension. Cutting young, leafy Johnsongrass gives you the best feed value, yet that young tissue carries the most cyanide potential. The practical sweet spot for hay is the late boot to early heading stage. At that point, the plant has enough biomass to justify the harvest, protein is still reasonable, and the cyanide risk, while present in the fresh plant, will dissipate during a normal field cure. If the stand has been stressed by drought or a recent frost, delay cutting until the tissue has had time to grow past the stress event, or at least test the standing forage before committing to a harvest.
Proper Curing Technique
The mechanics of curing Johnsongrass hay are not dramatically different from curing any warm-season grass, but a few details matter more because of the toxicity question.
- Cut in good weather: You need at least three to four consecutive days of warm, dry conditions. Johnsongrass stems are thick and hold moisture longer than finer grasses. Rain on a drying windrow resets the clock and can promote mold, which creates its own set of problems.
- Condition the stems: Running the forage through a mower-conditioner that crimps or cracks the stems speeds drying dramatically. Faster drying means faster HCN release and less time for mold to develop.
- Ted or rake as needed: Flipping and spreading the windrow exposes more surface area to sun and wind. With Johnsongrass’s heavy stems, this step is often necessary rather than optional.
- Raise the cutting height: Six to eight inches of stubble leaves the most nitrate-laden tissue in the field. It also benefits the plant’s rhizome reserves for regrowth.
- Bale at proper moisture: For small square bales, aim for 15 to 18 percent moisture. For large round bales, closer to 14 to 16 percent, since the dense core takes longer to equilibrate. A bale that feels right on the outside can still be dangerously wet in the middle.
If the hay will be stored outside, wrapping or covering it protects against re-wetting, which is relevant not just for general quality but because damp hay can develop secondary mold toxins. Prussic acid will not re-form in properly cured dry hay, but wet storage creates a whole different set of feed safety problems.
Testing Before You Feed
Even if you’ve done everything right during harvest and curing, testing the finished hay is the only way to know what you actually have. A standard forage analysis from a lab will tell you the crude protein, fiber fractions, and energy content. If you have any reason to suspect nitrate accumulation, request a nitrate test specifically. Most basic forage panels do not include nitrate unless you ask.
The general guidelines used across most U.S. extension services classify nitrate levels below about 0.5 percent (on a dry matter basis, sometimes expressed as 5,000 ppm nitrate-nitrogen or its equivalent in potassium nitrate) as generally safe for all classes of livestock. Levels between 0.5 and 1.0 percent are considered potentially dangerous if the hay makes up the entire diet, but can often be fed safely when diluted with low-nitrate feeds. Above 1.0 percent, the hay is genuinely hazardous and should either be blended carefully at a fraction of the total ration or discarded. Pregnant cattle are especially sensitive to nitrate toxicity, so err on the conservative side for cow-calf operations.
Prussic acid testing on cured hay is rarely necessary because the compound has already volatilized by the time the hay is baled. If you’re making silage or haylage from Johnsongrass, though, the situation is different, and a cyanide test on the fermented product is worth considering, especially if the standing crop was young or stressed at harvest.
Horses and Johnsongrass
Horses deserve a separate mention because they face a risk from sorghum-type grasses that cattle do not. In addition to the cyanide and nitrate hazards shared with ruminants, horses grazing sorghum-family plants have been reported to develop a neurological condition involving bladder dysfunction and hind-limb incoordination. This syndrome has been associated with grazing pastures dominated by sorghum species and is thought to involve chronic low-level cyanide exposure that damages specific nerves over time.
The condition has been documented in horses grazing fresh sorghum-family forages rather than eating cured hay, and the risk appears tied to prolonged exposure to the living plant rather than to properly dried forage. Still, many horse owners and equine nutritionists take a conservative stance and avoid feeding Johnsongrass hay to horses altogether, preferring bermudagrass, timothy, or orchardgrass. If you do choose to feed Johnsongrass hay to horses, make sure it is fully cured, was not harvested from a stressed or very young stand, and has been tested for nitrates.
Managing the Stand for Better Hay
Johnsongrass is one of those plants that provokes a love-hate relationship among producers. In many states it is officially listed as a noxious weed, yet in other regions it is deliberately maintained as a forage crop, particularly on marginal land where it thrives without much help. If you’re working with a Johnsongrass stand rather than fighting it, a few management practices improve both yield and safety.
Fertility management matters less than you might expect for crude protein in Johnsongrass. Research comparing commercial fertilizer, broiler litter, and interseeded white clover found that forage dry matter yield and foliar concentrations of crude protein and cell wall constituents were not significantly different among fertilizer treatments.2Applied and Environmental Soil Science. Productivity and Nutritive Quality of Johnsongrass (Sorghum halepense) as Influenced by Commercial Fertilizer, Broiler Litter, and Interseeded White Clover (Trifolium repens) Johnsongrass is an aggressive nutrient scavenger with a deep, extensive rhizome system, and it tends to produce similar forage quality regardless of whether the nitrogen comes from synthetic fertilizer or organic sources. What heavy nitrogen application does do, however, is increase the risk of nitrate accumulation, especially if a dry spell follows the fertilizer application. Moderate nitrogen rates are safer than pushing for maximum tonnage.
Harvest frequency is another lever. Taking two or three cuttings per season rather than one late cut keeps the plants in a younger, more digestible growth phase and prevents seed production that would spread the stand further. Research confirms that Johnsongrass ensiled or harvested before the flower stage delivers substantially better nutritive value than forage allowed to reach the dough stage, and this pattern held across both first and second cuttings in a season.1PubMed Central. The impact of maturity stages on yield, quality, and nutritive value of ensiled Johnsongrass [Sorghum halepense (L.) Pers] Frequent cutting also depletes the carbohydrate reserves in the rhizomes over time, which is something to be aware of if you want to maintain the stand long-term rather than exhaust it.
Conditions That Spike Toxin Levels
Several environmental events can cause a Johnsongrass stand that was perfectly fine last week to become dangerous this week. Knowing these triggers lets you avoid cutting at the worst possible time.
- Frost: A light frost ruptures plant cells, triggering rapid enzymatic release of HCN from dhurrin. Fresh Johnsongrass within a few days of frost is extremely dangerous to graze. Wait at least a week after a killing frost and until the tissue is completely dead and dry before considering harvest.
- Drought followed by rain: During drought, the plant concentrates dhurrin in its reduced leaf area. When rain arrives and new growth flushes out, that young tissue is loaded with cyanide potential. The first flush of regrowth after a drought break is the riskiest forage of the season.
- Herbicide damage: Sublethal herbicide exposure stresses the plant and can increase both cyanide and nitrate levels. If a neighboring field was sprayed and drift hit your Johnsongrass stand, do not harvest until the plants have fully recovered or died back.
- Heavy nitrogen and cloudy weather: High soil nitrogen combined with low sunlight slows the plant’s ability to convert nitrate into amino acids. The result is nitrate stacking up in the stems faster than it gets metabolized.
None of these situations make it impossible to harvest Johnsongrass for hay. They simply mean that hay cut under these conditions carries a higher nitrate load, or was higher in dhurrin at the time of cutting and needs a longer, more thorough field cure. When in doubt, test the standing forage with a quick diphenylamine nitrate spot test before committing to a harvest, and give the windrow an extra day of drying before baling.
Ensiling as an Alternative to Dry Hay
If your operation is set up for silage or baleage, ensiling Johnsongrass offers some advantages over dry hay. The fermentation process can reduce nitrate levels because anaerobic bacteria convert some nitrate into gaseous nitrogen compounds that escape. Prussic acid, being volatile, also tends to dissipate during the early stages of ensiling as the chopped forage heats and gases off.
Research on Johnsongrass silage has shown that the crop can be successfully ensiled, though nutritive value is strongly tied to the maturity stage at harvest. Silage made from three-week regrowth and boot-stage Johnsongrass had substantially higher in-vitro digestibility than silage made from flower- or dough-stage plants.1PubMed Central. The impact of maturity stages on yield, quality, and nutritive value of ensiled Johnsongrass [Sorghum halepense (L.) Pers] The practical message is the same as for hay: cut earlier rather than later if you want feed quality, and do not let the crop get past heading before you harvest.
One caution with silage is that the initial gas-off period after chopping and packing can release concentrated nitrogen dioxide fumes (the silo gas problem), especially from high-nitrate forage. These fumes are acutely toxic to humans. Stay away from newly filled silos or freshly sealed baleage for at least two to three weeks, and ventilate any enclosed structure thoroughly before entering.
Keeping Johnsongrass in Check
Because Johnsongrass spreads aggressively by both seed and rhizome, managing it for hay often means balancing its usefulness as forage against its tendency to invade cropland. The rhizome network stores large amounts of carbohydrates that fuel regrowth after cutting or herbicide treatment. Research on rhizome carbohydrate reserves found that reducing total nonstructural carbohydrates by about 60 percent or more prevented regrowth from treated rhizomes.3BioOne (Weed Technology). Johnsongrass Control, Total Nonstructural Carbohydrates in Rhizomes, and Regrowth After Application of Herbicides Used in Herbicide-Resistant Corn (Zea mays) Frequent cutting for hay can gradually deplete these reserves, which is useful if you want to weaken the stand on field borders or transition areas, but counterproductive if you want a persistent forage stand. If you’re deliberately maintaining a Johnsongrass hay field, allow the plants occasional recovery time between cuttings so the rhizomes can recharge, and prevent seed production by cutting before the heads mature and shatter.