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Northwest Iowa Dairy Outlooks

A local discussion of current science and issues concerning dairying in northwest iowa

With near-record high temperatures this past week, it is hard to remember that the average “first freeze date” for the Tarrant County area is November 22. For cattlemen that date is important because after a freeze there is the potential for prussic acid poisoning of animals grazing forages in the sorghum family such as johnsongrass, sudangrass, forage sorghums and grain sorghum. Some other plants like chokecherries, wild cherries and mountain mahogany also have compounds that breakdown and result in liberated cyanide, which we commonly call prussic acid.

Dr. Ted McCollum notes that the cyanide is part of a larger molecule called a cyanogenic glycoside and as long as the cyanogenic glycoside remains intact there is only the potential for toxicity. Dr. McCollum explains the science that takes place in order for the potential to be realized, noting that something has to trigger the enzymatic action to liberate the cyanide molecule from the glycoside. The beta-glucosidase enzymes that liberate cyanide from the parent glycoside are found in the plant tissue. In the intact plant tissue, the cyanogenic glycosides are found in vacuoles while the enzymes are found in the cytosol. In order for the cyanide to be released the plant tissue must be damaged so that the glycosides and the enzymes come together. The enzymes are also produced by ruminal microbes.  Cutting, crimping, mastication, trampling, hail damage, and frost/freeze disrupt cellular structure and allow the glycosides and enzymes to mix and liberate cyanide from the parent glycoside.  Introduction into the ruminal environment presents the glycosides to the microbial enzymes and releases cyanide. 

For cattlemen, the bottom line is when prussic acid is ingested by cattle it is quickly absorbed into the blood stream, and blocks the animal’s cells from utilizing oxygen.  Thus the animal dies from asphyxiation at the cellular level.  The first symptoms of prussic acid poisoning are accelerated and deep respiration. The nose and mouth may become filled with foam, and in some cases, involuntary urination may occur. These symptoms are followed by depression, inability to stand, severe difficulty in breathing, and finally death. Animals affected by prussic acid poisoning exhibit a characteristic bright red blood just prior to and during death.

Because prussic acid is one of the fastest acting poisons known, prompt diagnosis and treatment are required. Extremely low levels of prussic acid are toxic: only 2 mg per pound of body weight per hour will kill an animal.

For frost/freeze conditions there seems to be a two-phase zone where prussic acid poisoning is a potential problem. First, a freeze on plants, particularly if there is a lot of young and new growth (prussic acid does not affect older growth very much), disrupts plant cells and can bring about prussic acid immediately as enzymes catalyze reactions producing prussic acid. According to Dr. McCollum, this early phase is usually only a problem with a freeze rather than a frost, but it is better to err on the side of caution if the low temperature is not known. Many producers prefer to pull their livestock off of sorghum and sorghum/sudans even after a frost to be safe. Dr. McCollum considers a minimum of three days for not grazing freeze-injured sorghum and sorghum/sudan forage.

The second and more serious phase for prussic acid potential is the occurrence of rapid growth of young forage and new tillers due to warm temperatures after a frost or light freeze. These small shoots are very dangerous because of their potentially high prussic acid content and appeal to livestock. Producers are advised to wait a minimum of seven days or more after new growth begins before grazing the forage again to avert potential poisoning.

Because cyanide begins to dissipate as soon as the plant begins to die, it is critical that producers hand-carry or ship overnight all samples to be tested for prussic acid. A good sample for prussic acid testing consists of leaves from 10 to 12 plants. Refrigerate but do not freeze the samples in transit to the lab.

For more information read the publication Nitrate and Prussic Acid in Forages on the Tarrant County Urban Agriculture website at:








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