By Scout Nelson
Drought creates challenges for ranchers by reducing forage supplies and lowering pasture quality. It can also increase the risk of harmful substances, including nitrates and prussic acid, building up in certain plants.
Prussic acid, also called hydrocyanic acid, can cause cyanide poisoning when livestock eat forage containing high levels. In severe cases, poisoning can lead to death. Understanding the risks and using safe grazing and feeding practices helps ranchers protect their animals.
Kim Ricardo, SDSU Extension Forage Field Specialist, provided guidance on managing prussic acid risks in forage following drought. Her recommendations focus on identifying risky plants, understanding how weather affects forage safety, and taking precautions before livestock graze or consume harvested feed.
Identify Forages That Carry Prussic Acid Risks
Prussic acid poisoning is mainly linked to plants in the sorghum family. These include forage sorghum, sorghum-sudangrass, sudangrass hybrids, and johnsongrass. These plants contain natural compounds that can release prussic acid when plant tissues are damaged.
Prussic acid levels are often higher in leaves and young plant growth. This makes immature regrowth a concern, especially after drought, cutting, grazing, or frost.
However, not every field or plant has the same level of risk. Weather conditions, plant growth, soil nutrients, and recent management practices can affect prussic acid levels. When there is uncertainty about forage safety, testing a sample through a suitable forage testing service is a reliable way to help prevent livestock losses.
Watch for Drought Stress and New Growth
Drought stressed plants may become more dangerous when moisture returns and new growth begins. Ranchers may see green leaves and assume that pastures are ready for grazing. However, fresh growth does not always mean the forage is safe.
Plants that have been cut for hay or grazed heavily may also produce young regrowth with higher prussic acid levels. Frost can increase the risk, while soils with high nitrogen and low phosphorus or potassium levels may also contribute to greater accumulation.
Ranchers should consider recent weather, plant height, the amount of new growth, and forage type before allowing livestock into a field. Appearance alone cannot confirm that forage is safe.
Follow Safe Grazing Practices
Careful grazing management can lower the risk of poisoning. Ranchers should follow these precautions:
Do not turn hungry cattle into risky pastures. Provide safe feed before introducing animals to potentially hazardous forage. This helps reduce the chance of cattle eating large amounts quickly.
Check plant height. As a general rule, wait until sorghum, sudangrass, and related forages reach at least 18 to 24 inches before grazing.
Avoid short regrowth. Plants growing after grazing, cutting, or other stress may carry higher risks.
Allow time after a killing frost. Wait at least 10 days to two weeks before grazing sorghum-family forages after a killing frost. This gives the plants time to dry and allows prussic acid levels to decline.
Use extra caution after a light frost. Risk may be lower when plants meet the recommended height and have limited new growth, but forage safety still requires careful judgment.
These practices help reduce exposure, but they do not guarantee that forage is safe.
Manage Hay and Silage Carefully
Harvested forage also needs attention when plants experience drought or other stress. Prussic acid behaves differently from nitrates because its levels can decline as forage dries and cures.
Hay generally becomes less risky as it dries and cures. Chopping forage for silage can also reduce prussic acid levels compared with fresh forage. Producers should allow silage to ferment for five to eight weeks before feeding it to livestock.
However, harvesting, drying, or storing forage does not automatically remove every risk. Ranchers should test suspect hay and silage before feeding them to animals.
Test Forage and Protect Livestock
Drought makes feed planning more difficult, particularly when weather conditions change quickly. Ranchers can reduce preventable losses by testing questionable forage before grazing or feeding it.
Working with Extension specialists and livestock nutritionists can help producers develop safe grazing plans, manage feed supplies, and make informed decisions about forage use. Taking precautions before livestock enter a pasture or receive harvested feed can help protect animal health and support successful ranch operations.
Photo Credit: gettyimages-r-j-seymour
Categories: South Dakota, Crops, Hay & Forage, Livestock