Johnson grass (Sorghum halepense) is a warm-season perennial grass common across the southern United States. Originally introduced as a forage crop, it is now known as an aggressive, invasive species that competes heavily with cultivated plants. This dual identity makes its use as livestock feed controversial, as it offers substantial forage quality but carries significant risk. The central question is whether this abundant grass can be reliably transformed into high-quality, safe hay.
Identifying the Johnson Grass Plant
The plant is characterized by its tall, coarse growth habit, often reaching heights between five and eight feet. A distinctive feature is the prominent, white mid-vein that runs down the center of each leaf blade, which helps distinguish it from other coarse grasses.
As an aggressive perennial, Johnson grass spreads by seed and an extensive system of white, scaly rhizomes (thick, underground stems). These rhizomes allow the plant to establish dense, persistent clumps, making it difficult to eradicate once established. Its ability to rapidly recover and spread is why it is classified as a noxious weed in many agricultural regions.
Assessing Nutritional Quality for Livestock
When harvested at the appropriate stage, Johnson grass provides a nutritional profile competitive with other warm-season forages like Bermuda grass. High-quality hay can contain crude protein (CP) levels ranging from 10% to 14%. This protein content is comparable to many grass hays used for maintenance rations in cattle.
Its Total Digestible Nutrients (TDN) value, a measure of energy, falls between 50% and 60% when the plant is in its vegetative stage. Nutritional value declines rapidly as the plant matures and develops its seed head, causing the fiber content to increase significantly. To capture higher nutrient density, the grass must be cut before it reaches full maturity.
Understanding the Potential Toxicity Risks
The main concern regarding Johnson grass hay involves two toxicity risks: prussic acid and nitrate accumulation. Prussic acid (hydrocyanic acid, or HCN) is formed when the plant’s naturally occurring cyanogenic glucosides are broken down. This breakdown happens rapidly when the plant is stressed by conditions like drought, frost, or trampling.
Once ingested, prussic acid inhibits the animal’s ability to utilize oxygen at the cellular level, causing internal suffocation. Symptoms of poisoning, such as staggering and difficult breathing, can appear quickly, often leading to death within minutes or hours. This risk is highest in young, short regrowth and after a killing frost.
The second danger is the accumulation of nitrates, inorganic compounds absorbed from the soil. Nitrate levels become toxic when the plant takes up large amounts of nitrogen fertilizer but cannot process it due to environmental factors like cloudy weather or severe drought. When consumed, nitrates are converted to nitrites in the rumen, which bind to hemoglobin and prevent red blood cells from carrying oxygen.
Unlike prussic acid, which dissipates as the forage dries, elevated nitrate concentrations are maintained in cured hay. Therefore, hay grown under conditions of high nitrogen fertilization or severe stress should always be tested for nitrate levels prior to feeding.
Optimal Harvesting and Curing Methods
To maximize nutritional value and mitigate toxicity, Johnson grass should be harvested at the boot stage, just before the seed head emerges. Cutting the grass when it is 30 to 40 inches tall ensures high crude protein and energy while keeping fiber manageable. The recommended cutting height is approximately six inches, which leaves enough residual plant material for rapid, healthy regrowth.
The curing process is a defense against prussic acid toxicity, as the toxin is volatile and dissipates as the forage dries completely. Hay must be thoroughly cured and dried to a safe moisture level (15% or less) before baling to ensure the prussic acid has evaporated. Improperly cured or wet hay retains the toxin and poses a danger.
Since nitrate levels do not decrease during the curing process, forage testing is necessary if the grass was grown under high-risk conditions, such as heavy nitrogen application. Submitting a forage sample for nitrate analysis provides an objective measure of safety. While prussic acid is rarely a concern in properly dried hay, a nitrate test confirms the safety of the baled product.