Plants that contain prussic acid (hydrocyanic acid)
Under certain conditions, these plants contain prussic acid, or hydrocyanic acid), a deadly poison which interferes with the oxygen-carrying ability of the blood. Death in these cases is usually rapid and with few outward symptoms. Members of the Prunus family of plants, especially wild cherries, are dangerous. Peaches, plums, wild cherry, and other stone fruits belong to this group of plants. Wilting of the green leaves caused by frost, storm damage, or by cutting, changes a glucoside found in the leaves to hydrocyanic acid (HCN) and sugar. The sweet, wilted leaves are thus more attractive to animals than normal foliage. Hydrocyanic acid content varies widely, but under some conditions, a few handfuls of leaves may be enough to kill a horse or cow. This type of poisoning should be suspected when sudden death of animals follows windstorms or early sharp frosts. These leaves apparently lose their poison after they have become dry; the limp, green or partially yellowed leaves are the most dangerous.
Sudan grass and sorghums are also cyanogenetic plants. These plants are usually deadly when damaged or frozen. Aftermath sprouts following an early frost are particularly dangerous. Although often listed as dangerous, sudan grass poisoning that occurs when animals trample and later eat plants doesn’t happen often. In dry weather, sudan grass is often pastured to the ground without ill effects. After sudan grass has been repeatedly frozen and the plants are completely dead, it is safe but not very valuable for pasture. Once frozen, sorghum, sorghum sudan hybrids, or their aftermath, should never be pastured. As long as the plants show any green color they may be very poisonous. Both frosted sorghum and sudan grass can be best and most safely utilized by ensiling them for at least two weeks before feeding. Normal ensilage fermentation safely eliminates the poisonous compounds.
Other plants of this group include:
Common milkweed, a perennial that grows 3- or 4-feet high, has a heavy stem and leaves and is frequently found in pastures.
Horsenettle, a perennial plant, 2-feet high, with spiny stems and leaves, and smooth, orange-yellow berries. Fruits are more toxic than the foliage. It’s a common plant in grasslands and fields and is a member of the nightshade family.
Black nightshade, an annual plant, 2-feet high, with many branches. Leaves are variably smooth or hairy. The stems are angled in cross-section and sometimes spiny. Clusters of white flowers, one-fourth inch across, bloom in midsummer and are followed by small, black fruits. Both the foliage and green berries are toxic. The ripe berries are not poisonous. Black nightshade is widely distributed.
Mountain laurels and rhododendrons, evergreen shrubs of the Appalachian Mountains region. Plants grow 5-feet tall and have glossy green leaves. Flowers appear in clusters at the ends of branches. Livestock eat the leaves in early spring when little other foliage is available. Piedmont azaleas are deciduous plants of the Piedmont.
Several varieties of Leucothe, also called Fetterbush or Dog-hobble, are evergreen or deciduous plants found in most regions of North Carolina and other southeastern states. Weakness, nausea, salivation and vomiting are symptoms of poisoning. To prevent poisoning, keep livestock out of areas where these plants are abundant.
Luginbuhl, J-M. 2006. Pastures for Meat Goats. In: Meat Goat Production Handbook, ed. T.A. Gipson, R.C. Merkel, K. Williams, and T. Sahlu, Langston University, ISBN 1-880667-04-5.