Sorghums, sorghum-Sudan grass hybrids, Sudan grass, and wild cherry leaves and twigs are sources of poisons that are collectively referred to as prussic acid, cyanide or hydrocyanic acid. Most beef producers are well aware of the danger of wild cherry trees and limbs falling in the pasture because the wilted leaves are toxic sources of cyanide. Less well-known are the effects of the sorghum family of forages that can also produce toxic levels of prussic acid.
Some of the plants that may contain prussic acid:
- Grain sorghum
- Sorghum-Sudan grass hybrids
- Sudan grass
Prussic acid occurs normally in these forages in a bound, nonpoisonous state. Certain environmental conditions result in the combination of the chemicals emulsin and dhurrin to produce prussic acid in an unbound state. There are several factors that contribute to prussic acid formation:
1. Plant species - The vegetative portion of all sorghum-variety plants contains prussic acid, but Sudan grass may contain as much as 40 percent less than other sorghums. There are some varieties of sorghum plants that have been developed that are lower in prussic acid content. Observation for the presence of any of these plants, including in fencerows and sacrifice areas, will help prevent poisoning of cattle. Pearl millet does not contain high levels of prussic acid.
Photo courtesy of Helmut Gevert/sxc.hu.
2. Plant parts - Leaf blades contain the highest levels of prussic acid and are generally the grazed portion of the plant. Stems contain less, and grains and seeds contain none. The leaf tillers produced after a drought, frost or mowing are the most dangerous parts of the plant.
3. Plant maturity - Younger, growing plants will have higher concentrations of prussic acid, and plant maturity will dilute the concentration with more stem proportion. However, cattle will normally selectively graze the leaf portion regardless of the plant maturity, so the plant can remain dangerous in a pasture regardless of maturity.
4. Drought - Drought is the primary cause of most prussic acid poisoning. Drought-stricken plants have a higher proportion of leaf material, and in the case of regrowth right after a drought, the new leaf shoots will be high in prussic acid.
5. Frost - Similar to the effects of drought is the effect of freezing. The tops of the plant containing the highest levels of prussic acid that are frosted may appear dead, but the prussic acid content is not diminished until significant wilting occurs. Frosted plants should not be baled or chopped until five to six days after a killing frost. Plants that are frosted and then produce new leaf shoots in warmer weather should be avoided completely.
6. Fertilizer - High levels of nitrogen fertilizer applied to sorghums when phosphorus and potassium levels remain low will increase the prussic acid content of the plants.
Safely feeding sorghum plants:
- Pasture - Most sorghum-based plants can be grazed safely as long as there has been no environmental stress on the plants that creates the unbound prussic acid. Frost and drought are the major stressors that result in unbound prussic acid, particularly when new leaf area is selectively grazed.
- Green chop - Except in the most severe examples of prussic acid buildup such as new tillers produced after a frost, green chop is usually safe because the whole plant is eaten and the stem portion dilutes the acid.
- Silage - Silage is safe nearly all the time because during the fermentation process prussic acid escapes as a gas. It follows to not feed silage that may have high levels of prussic acid until fermentation is complete in at least three weeks.
- Hay - The prussic acid content of dry hay is reduced by about 75 percent and is generally safe to feed. Wrapped bales should undergo reasonable fermentation such as with silage to allow the escape of the prussic acid as a gas.
Symptoms and treatment of animals poisoned with prussic acid are similar to other toxins. Symptoms include staggering, salivation and a high respiratory rate. In many cases, because prussic acid can be highly concentrated in grazed material, the producer will just find dead cattle. Death can occur in less than 20 minutes in some cases. The cumulative effect of consuming lower concentrations of prussic acid - staggering gait, salivation, etc. - is similar to many other health problems. A veterinarian should always be called in under these conditions, and they should be informed about intake of sorghum-based plants if they are available. Treatment with nitrogenous compounds can be effective, but only if administered quickly after the onset of symptoms.
A checklist of actions for the use of sorghum-based plants for cattle:
- Use Sudan grass as a pasture forage instead of hybrids with sorghum because it contains less prussic acid.
- Do not graze sorghum-based plants until they are 18 to 24 inches tall.
- Hybrids should not be grazed until they are 24 inches tall
- Avoid grazing any sorghum-based plants during and right after a drought or frost.
- Do not graze any regrowth after mowing, drought or frost for at least seven to 10 days.
- Intake of leafy portions of the plant higher in prussic acid content will increase when hungry cattle are turned into these pastures.
Dr. John Comerford is associate professor of dairy and animal science at the Pennsylvania