The hot and dry conditions we are currently experiencing in West Texas can cause toxic levels of prussic acid to occur in many annual forages. While many plants contain the toxic materials, the greatest culprits are Johnson grass, haygrazers or forage sorghums—sudan hybrids, and various millets.
Cattle may succumb to prussic acid poisoning while grazing these forages if they have become drought-stressed, or in the fall after a light frost.
Livestock should not be allowed to graze these plants in a stressed condition as death can occur. Prussic acid poisoning acts rapidly and can kill animals within minutes. Symptoms may include a brief period of stimulation followed by depression and paralysis.
Signs of colic may be present.
Stupor (loss of sensibility), difficult breathing and frequent convulsions, may result. Death is actually caused by suffocation since oxygen remains in the blood and is not exchanged to the tissues.
Testing for prussic acid in these forages is possible; however, a true accuracy is unpredictable because many plants in the field may have prussic acid and many may not. Just remember a couple of bites of the poisonous plant can result in death. Producers do have an option of swathing forage sorghums that have prussic acid. When swathed, prussic acid leaves the cut forage when allowed proper time to cure before baling. However, re-growth of plant material may still be toxic.
Producers turning into new pastures that have not been grazed should also be cautious of the fact that there could be many colonies of Johnson grass growing. These Johnson grass colonies could contain prussic acid. The best management option is to wait two weeks after good rains to allow the plant to recover from stressed conditions, or 7 to 10 days after a killing freeze before turning cattle into new pastures, haygrazer, or sorghum stubble fields. The 7-10 days will allow the prussic acid to leave the plant just like swathing the hay and letting it cure.
Nitrate poisoning is different in that nitrates accumulate at the base of the plant and move upward. Cattle can tolerate low levels of nitrates in their system, and it is only when high levels of nitrates are fed that it becomes toxic. Nitrates do not leave the plant after swathing like prussic acid does. Nitrates remain in the hay, and that is usually when producers get in trouble, by feeding free-choice, high-nitrate hay in the winter.
Hay can be tested for nitrates, and high nitrate hay can be fed on a limited basis. Drought stress in combination with high fertilizer rates usually creates nitrate problems in haygrazer fields and grain sorghum fields.
Raising the cutting height of the swather can aid producers in reducing nitrate levels. Grazing sorghum fields high in nitrates is not recommended.
For more information, contact the Kent County Extension Office at (806) 237-3751 or the Dickens County Extension Office at (806) 623-5552.
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