This article originally appeared in the September 2015 issue of Harford County's "Ag Notes" newsletter.
Last month, our featured toxic plants were those in the Prunus genus, which most notably includes cherries, peaches, and plums. The toxic agent in Prunus species is hydrogen cyanide. Hydrogen cyanide – sometimes also called prussic acid – is also the toxic agent in sorghums. Under certain conditions, sorghum plants can also cause nitrate poisoning.
Sorghum is a genus of plants within the grass family. Grain sorghum is grown commercially by some farmers in Maryland and is also sometimes used as fodder for animals. Two other common sorghums are sudangrass and johnsongrass. Sudangrass is most frequently planted as a summer cover crop and sometimes harvested for forage. Johnsongrass is invasive and considered a noxious weed because it is extremely competitive.
While grain sorghums and sudangrass are intentionally planted, johnsongrass may be present as a weed. Johnsongrass is a coarse, tall-growing grass that can reach 7-9 feet tall when mature. It generally grows in clumps and spreads rapidly via rhizomes, although it also propagates through seed. Leaves are rolled into the bud, and the leaf has a prominent white midrib. The ligule is membranous and have a tuft of hairs at the top. There are no auricles. In our area, the seedhead usually appears in late July. The seeds are purple and are loosely arranged to form a purple pyramid.
Prussic Acid Poisoning
Like in plants of the Prunus species, the hydrogen cyanide in sorghums is stored in the form of non-poisonous cyanogenic glycosides. When the leaf tissue is ruptured – such as when chopping for forage or chewing by an animal – these glycosides come in contact with enzymes in other parts of the plant and/or saliva and are broken down into their constituent compounds: hydrogen cyanide and sugar. Hydrogen cyanide (prussic acid) acts as a poison by preventing red blood cells from releasing oxygen, essentially causing an animal to suffocate. The poison acts quickly, and animals are usually found dead.
Cattle, sheep, goats, and other ruminants are more likely to be poisoned because bacteria in the rumen speed up the process of releasing hydrogen cyanide from the cyanogenic glycosides. Furthermore, the acidic stomach or non-ruminant animals inactivates some of the enzymes that facilitate the break down process.
Unlike plants of the Prunus species, sorghums are not always toxic. Animals are able to detoxify prussic acid when it’s consumed at low enough doses, and under ideal growing conditions sorghums contain very little prussic acid. Under some conditions, however, the level of prussic acid present in plants can increase to unsafe levels. Prussic acid is found in higher concentrations when:
Poisoning is rarely a concern with hay, even when the hay was made from plants containing high levels of prussic acid, because the drying process inactivates the poison.
Accidental poisoning can be avoided by proper grazing management. Even if you aren’t intentionally growing sorghums, it’s a good idea to scout your pastures for invasive johnsongrass. Livestock are most commonly poisoned when grazing pasture that has recently been affected by drought, frost, or recent grazing, because the immature regrowth will be higher in prussic acid. Don’t graze animals on immature sorghum (the recommended minimum grazing height varies by species) or when plants are stressed by drought. If plants have been affected by frost, wait at least a week before grazing.
Sorghums can cause also nitrate poisoning in ruminants. All plants contain some amount of nitrate, but some species of plants – including sorghums – can accumulate high levels of nitrate if they have been heavily fertilized and are stressed by drought. Under normal conditions, plants absorb nitrogen from the soil and immediately convert it into plant proteins. During times of drought, however, some plants continue to absorb nitrogen but don’t have enough water to keep growing. Thus, nitrates accumulate.
Nitrates are an important part of an animal’s diet because they are used to build proteins. Problems result when the amount of nitrate consumed overwhelms the rate at which the body can convert it. The bacterial enzymes present in the rumen convert nitrate into nitrite and then convert nitrate into ammonia, which then becomes amino acids and finally proteins. When large quantities of nitrate are consumed, the conversion of nitrate to nitrite exceeds the conversion of nitrite to ammonia, and toxic level of nitrite result. Nitrites alter the body’s hemoglobin such that it can no longer transport oxygen, and the affected animal essentially suffocates.
If an animal has consumed a large amount of nitrate, onset of symptoms is rapid and usually occur within an hour. Clinical signs difficulty breathing, muscular weakness, below normal body temperature, loss of coordination, blue coloration of the mucous membranes, and brown discoloration of the blood. Reproductive issues, including abortions, can occur in animals that chronically consume lower levels of nitrates. Animals that have been poisoned can sometimes recover if treated by a veterinarian quickly. Administration of methylene blue can repair the altered hemoglobin, thus reversing the poisoning process.
Ruminants are much more commonly affected by nitrate poisoning than non-ruminants because the enzymes involved in the conversion of nitrate to nitrite are found in the rumen but not in the stomach of non-ruminants. These enzymes are present in the hindgut of non-ruminants, but by the time feed reaches this area it is not retained in the gut for enough additional time to cause toxicity issues.
Nitrate accumulation is also known to occur in orchardgrass, tall fescue, corn, and some broadleaf weeds. If you’ve heavily fertilized a field and drought ensues, suspect that nitrate accumulation may have occurred. There are several management strategies for dealing with high-nitrate forages:
While it’s generally safe for mature livestock to consume this water, keep in mind that feeding high-nitrate feed in addition to providing water containing nitrates may contribute to nitrate poisoning.