Horse owners and managers often encounter Johnson grass, scientifically known as Sorghum halepense, while managing pastures. This perennial grass is notorious for its aggressive growth and ability to thrive in difficult conditions. The central question remains whether this widespread plant provides any nutritional benefit or poses a significant health risk to equines.
Toxicological Concerns of Johnson Grass
The primary danger associated with Johnson grass for horses stems from its cyanogenic glycoside content, specifically dhurrin. Under normal growth conditions, the plant contains low levels of these compounds. However, when the plant is stressed—through drought, frost, or mechanical damage like frostbite—the glycosides convert into prussic acid, which is highly toxic.
Symptoms of Cyanide Poisoning
Horses consuming large quantities of fresh Johnson grass, particularly young shoots or stressed plants, can suffer from prussic acid poisoning. Symptoms manifest rapidly and include difficulty breathing, dilated pupils, tremors, staggering, and bright red mucous membranes. In severe cases, this can lead to seizures and sudden death due to respiratory failure.
Nutritional Value and Digestibility
Beyond toxicity, the nutritional profile of Johnson grass is generally poor for horses compared to traditional forage. While it is high in total digestible nutrients when young, its coarse, fibrous stems develop quickly.
Low Protein Content: As the plant matures, protein levels drop significantly, offering little nutritional value.
High Nitrate Accumulation: Under heavy fertilization or environmental stress, Johnson grass can accumulate dangerous levels of nitrates, which can cause abortion in pregnant mares and severe digestive upset in other horses.
Prussic Acid Risk: The risk of cyanide poisoning remains a constant concern, requiring careful management.
Management Strategies for Pasture
Given the risks, Johnson grass is generally considered a weed in equine pastures rather than a viable forage option. Effective management focuses on eradication or strict containment.
Mechanical and Chemical Control
Physical removal through repeated mowing or deep tillage can reduce stands, but rhizomes make complete eradication difficult. Herbicides labeled for use in pastures, such as glyphosate or specific grass-selective chemicals, are often necessary for full control. Any chemical application must be followed by a strict withdrawal period to ensure chemical residues do not contaminate hay or enter the horse’s system.
Hay Considerations
If Johnson grass is present in a hay field, it requires diligent handling. Baling should occur at the boot stage, before seed heads emerge, to minimize the coarse stem content. However, the cyanide risk does not disappear when dried; the compounds remain present in the dried forage.
Testing and Monitoring
Hay containing Johnson grass should be tested for nitrates and prussic acid before feeding. Soaking hay can reduce nitrate levels but is ineffective against prussic acid. Horses should be introduced to this hay cautiously, if at all, and never fed as a primary forage source.
The Verdict on Equine Consumption
Most equine nutritionists and veterinarians advise against intentionally feeding Johnson grass to horses. The narrow margin between palatability and toxicity, combined with the lack of high-quality nutrition, makes it a poor choice for fodder. While some horses may avoid it due to its bitter taste, others may consume it when hungry, leading to dangerous situations.
Safe alternatives remain standard grass hays like Timothy or Orchard grass, or legumes such as Alfalfa, which offer predictable nutritional profiles without the inherent risks of cyanide and nitrate poisoning. Prioritizing these safer options ensures the long-term health and safety of the equine herd.
