Forages that are grazed or put up for hay as frost begins to appear may contain potentially toxic compounds that can cause problems for livestock producers, especially in areas that have been under drought stress or early fall frost.

The most common toxicity problems are associated with either nitrate, which is converted to nitrite in the body after consumption, or prussic acid (cyanide) poisoning, both of which impair the oxygen-carrying capacity of the blood.

Both of these potential problems can occur in forages during drought or when normal growing conditions have been interrupted or altered such as by frost, hail or rapid growth after a rain.

Prussic Acid

Prussic acid poisoning is mainly confined to sudangrass and forage sorghum, which both contain high levels of compounds that can release cyanide under certain situations. Sorghums and sorghum-sudangrass hybrids both contain more prussic acid than sudangrass. There are selected varieties, such as Piper and Trudan sudangrass, that tend to be lower in prussic acid.

If plants are damaged by freezing or trampling, or new growth is stimulated, cyanide concentration and release is increased, resulting in an increased chance of poisoning. New leaves and shoots contain from two to 25 times more prussic acid than stems, so new growth following a frost, dry conditions or when the crop has been grazed down should be considered suspect.

Before you regraze these pastures with cattle, new growth should be at least 18 to 20 inches for sudangrass and 24 to 30 inches for sorghum-sudangrass hybrids. As temperatures drop to levels where no regrowth takes place, you can wait five to six days for the forage to dry before turning cattle back in to graze.

The prussic acid content of sudangrass hay decreases by as much as 75 percent while curing, so well-cured hay is rarely hazardous for livestock.

Nitrate Poisoning

Nitrate poisoning is not limited to the sorghums and sudangrasses. Excess nitrate levels can occur in many plants because it is normally taken up by plant roots. Problems are generally found when growing conditions are disrupted, such as with low light intensity, drought, frost, hail, herbicide and nitrogen fertilizer applications, and disease.

Frost or hail damage can greatly reduce the leaf area of plant. This limits the ability of the plant to convert nitrate to protein, so the plant will accumulate nitrate. Temperatures of 70 degrees to help push nitrate into protein formation, but as temperatures cool down to 50 degrees protein formation is reduced and nitrates may begin to accumulate.

Some plants are more likely to accumulate nitrates, including pigweed, lambsquarter, kochia, wild sunflower, Russian thistle, oats, barley, millet, corn, sorghum and sudangrass. Grasses such as brome grass and legumes such as alfalfa are not normally high in nitrates.

Younger plants usually contain higher nitrate levels, but dry conditions and high soil levels of nitrate can lead to potential problems in mature plants. Plant leaves and grain do not contain appreciable levels of nitrate. The highest concentration of nitrate is in the lower stalk.

Unlike prussic acid, nitrates do not dissipate when forages are put up for hay. Harvesting at a more mature stage can reduce the concentration of nitrates, and ensiling forages may reduce nitrate concentrations by 40 to 60 percent during the fermentation process.

Caution When Fall Grazing Alfalfa

Many livestock producers graze alfalfa aftermath in the fall months and should be cautioned ofbloat, especially following a killing frost. As the plant cells freeze, the cell wall is ruptured,causing severe damage to the plant, literally killing the aboveground live material. This damage enhances the production of soluble leaf proteins that, in theory, are the principal foam?causing agents in legumes. Bloat is characterized as a condition that results in the formation of a frothy stable foam in the rumen, a retention of gas, and an inhibition of the eructation mechanism.

The recommendation for grazing frost killed alfalfa is to wait 5 to 7 days after the killing frost (less than 28 degrees Fahrenheit) before grazing. This will allow the live tissue to fully break down, minimizing the soluble leaf proteins, and making a much safer feed base for ruminant livestock.

Minimize Fall Grazing Risks

While grazing risks or hazards are likely low, some managerial precautions are suggested to prevent problems. If there is any question that there may be nitrates or prussic acid present in the forage stand, then having some forage samples analyzed for these potential poisons can help determine if accumulated levels are to an extent that may be of concern. Cattle should not be turned out hungry allow animals to fill up on hay before turn out. Since some of the lush fall grazed forages are low in fiber, cattle should be allowed access to pasture or place straw or hay in the field to allow cattle an opportunity for more optimum fiber intake. Additionally, for the first several days, cattle should be monitored for bloat or other digestive upset.

For questions or more information please call the NDSU Extension Pierce County office at 776-6234 ext. 5.

Schmidt is the NDSU Extension agent for Pierce County.