Johnson grass is a fast-growing, warm-season forage that can produce excellent hay for livestock when managed correctly. While often considered a weed in many agricultural settings, its high tonnage yield makes it a valuable feed source in regions where it thrives. The primary goal when preparing this grass for hay is to neutralize its potential toxicity, transforming the plant from a risk into a safe and nutritious feed. This process, known as curing, relies on careful timing and thorough drying to ensure the final product is safe for consumption.
Understanding the Prussic Acid Toxicity Risk
The concern surrounding Johnson grass stems from its ability to accumulate compounds called cyanogenic glycosides, which are naturally present in the plant’s tissues. These compounds are generally harmless until the plant cells are ruptured, which can happen when the plant is chewed, wilted, or damaged. Once the cell structure breaks down, an enzyme within the plant, called emulsion, rapidly converts the cyanogenic glycosides into hydrocyanic acid, more commonly known as prussic acid or cyanide.
This hydrocyanic acid is a potent, rapidly acting poison that inhibits an animal’s ability to utilize oxygen at the cellular level, essentially causing death by internal suffocation. The concentration of these toxic compounds increases when the plant is under stress from environmental conditions. Stressors such as drought, frost, wilting, or herbicide application can significantly elevate the concentration. The highest levels of toxicity are found in the leaves of young, rapidly growing plants and in new regrowth, where the concentration of glycosides is greatest.
Optimal Harvesting Timing and Preparation
Properly timing the harvest is the first line of defense against toxicity and is essential for maximizing the forage’s quality. Johnson grass should be cut when the plant is mature but before the seed heads fully develop, ideally between the boot stage and the early heading stage. Cutting at this stage balances good nutritional value with a lower risk of prussic acid accumulation, which naturally decreases as the plant ages.
Avoid cutting the grass immediately after a severe environmental stress event, such as a drought-breaking rain or a frost. If the grass has been subjected to a killing frost, wait at least seven to fourteen days before cutting to allow accumulated toxins to dissipate. The toxicity risk is highest in the young, stunted regrowth that follows a stress period, so the plant must be allowed to recover and mature. Successful curing requires multiple days of clear, hot weather, as detoxification depends entirely on rapid and complete drying.
The Curing Process for Detoxification
Curing hay removes toxicity from Johnson grass because hydrocyanic acid is a gas that dissipates as the plant material dries. The cut grass must be spread out in a wide, thin layer, known as a swath, immediately after mowing to maximize exposure to the sun and air. This wide swath management allows for rapid and uniform moisture loss, accelerating the volatilization of the prussic acid.
Repeatedly turning, or tedding, the hay is necessary to ensure every part of the swath dries thoroughly and evenly, preventing moisture pockets that can harbor the toxin. Since the thick stems dry slower than the leaves, proper conditioning and turning are necessary to prevent the baled hay from retaining toxic levels of cyanide. Curing must continue until the forage reaches a moisture content below the safe threshold, typically between 15% and 18% for large bales. This thorough drying period allows the volatile prussic acid to safely escape into the atmosphere, making the hay safe for feeding.
Post-Harvest Safety Checks and Storage
Once the hay is thoroughly cured and baled, final safety measures should be taken, particularly if the grass was cut during marginal or high-risk conditions. High-risk batches, such as hay harvested shortly after a drought or frost, should be professionally tested for prussic acid levels before being fed to livestock. While curing significantly reduces the toxin, testing provides confirmation of safety.
Proper storage is necessary to maintain the safety and quality of the cured hay. Bales must be kept dry and well-ventilated to prevent moisture re-absorption, which can lead to mold development and spontaneous heating. Heating above 150°F can lead to spontaneous combustion, and lower temperatures can cause “browning,” reducing the hay’s protein digestibility. For hay cut under questionable conditions, wait a minimum of 90 days after baling before feeding, allowing any residual prussic acid to fully dissipate during storage.