By Mark Sulc
Livestock owners feeding forage need to keep in mind the potential for some forage toxicities and other problems that can develop late this season. Nitrate and prussic acid poisoning associated with drought stress or frost are the main potential concerns to be aware of. These are primarily an issue with annual forages and several weed species, but nitrates can be an issue even in drought stressed perennial forages. There is also an increased risk of bloat when grazing legumes after a frost. Each of these risks is discussed in this article along with precautions to avoid them.
Drought stressed forages can accumulate toxic nitrate levels. This can occur in many different forage species, including both annuals and perennials. Several areas in Ohio have become very dry late this summer into fall. Corn, oat and other small grains, sudangrass, and sorghum sudangrass, and many weed species including johnson grass can accumulate toxic levels of nitrates. Even alfalfa can accumulate toxic nitrate levels under severe drought stress.
Before feeding or grazing severely drought stressed forage, the forage should be analyzed for nitrates. Most labs now offer nitrate tests, so it is likely that you can get forage plants tested for nitrates by your favorite lab. A number of labs are listed at the end of this article that have nitrate testing available. This list is for your convenience and no labs are intentionally omitted. Check your chosen lab’s web site and follow their specific instructions about how to take and handle the sample.Tthe cost is well worth it against the risk of losing animals.
See the following references for more details:
Nitrate accumulation in frost forages. Freezing damage slows down metabolism in all plants, and this might result in nitrate accumulation in plants that are still growing, especially grasses like oats and other small grains, millet, and sudangrass. This build-up usually isn’t hazardous to grazing animals, but greenchop or hay cut right after a freeze can be more dangerous. When in doubt, send a forage sample to a forage testing lab for nitrate testing before grazing or feeding it.
Prussic Acid Toxicity
Several forage and weed species contain compounds called cyanogenic glucosides that are converted quickly to prussic acid (i.e. hydrogen cyanide) in freeze-damaged plant tissues, or under drought conditions. Several labs provide prussic acid testing of forages. Sampling and shipping guidelines should be carefully followed because prussic acid is a gas and can dissipate during shipping leading to a false sense of security when no prussic acid is found in the sample.
Drought stress can affect prussic acid poisoning risk. Drought-stunted plants can contain or produce prussic acid and can possess toxic levels at maturity. Prussic acid poisoning can be associated with new regrowth following a drought-ending rain, which is likely the case in some parts of Ohio now. Rain after drought plus young stages of plant maturity (see below) could combine to cause toxic levels of prussic acid in forage this year.
Plant age affects toxicity. Young, rapidly growing plants of species that contain cyanogenic glucosides will have the highest levels of prussic acid. Pure stands of indiangrass can have lethal levels of cyanide if they are grazed when the plants are less than 8 inches tall.
Species with prussic acid poisoning potential. Forage species that can contain prussic acid are listed below in decreasing order of risk of toxicity:
Grain sorghum = high to very high toxic potential
Indiangrass = high toxic potential
Sorghum-sudangrass hybrids and forage sorghums = intermediate to high potential
Sudangrass hybrids = intermediate potential
Sudangrass varieties = low to intermediate in cyanide poisoning potential
Piper sudangrass = low prussic acid poisoning potential
Pearl millet and foxtail millet = rarely cause toxicity
Species not usually planted for agronomic use can also develop toxic levels of prussic acid, including the following:
- Black cherry
It is always a good idea to check areas where wild cherry trees grow after a storm and pick up and discard any fallen limbs to prevent animals from grazing on the leaves and twigs.
Frost affects toxicity. Cyanogenic glucosides are converted quickly to prussic acid (i.e. hydrogen cyanide) in freeze-damaged plant tissues. Prussic acid poisoning potential is most commonly associated the first autumn frost. New growth from frosted plants is palatable but can be dangerously high in prussic acid.
Fertility can affect poisoning risk. Plants growing under high nitrogen levels or in soils deficient in phosphorus or potassium will be more likely to have high prussic acid poisoning potential.
Fresh forage is more risky. After frost damage, cyanide levels will likely be higher in fresh forage as compared with silage or hay. This is because cyanide is a gas and dissipates as the forage is wilted and dried for making silage or dry hay.
Prussic Acid Toxicity Symptoms
Animals can die within minutes if they consume forage with high concentrations of prussic acid. Prussic acid interferes with oxygen transfer in the blood stream of the animal, causing it to die of asphyxiation. Before death, symptoms include excess salivation, difficult breathing, staggering, convulsions, and collapse.
Ruminants are more susceptible to prussic acid poisoning than horses or swine because cud chewing and rumen bacteria help release the cyanide from plant tissue.
According to a Texas Cooperative Extension Factsheet, “Animals consuming forages with nigh nitrate levels cannot complete the conversion of nitrate to protein, and toxic nitrite levels accumulate. Nitrite is adsorbed directly into the bloodstream through the rumen wall, where it combines with hemoglobin to form methhemoglobin. Hemoglobin carries oxygen in the blood, but methhemoglobin does not. The formation of methhemoglobin can cause an animal to die from asphyxiation, or lack of oxygen. The animal’s blood turns brown instead of the normal bright red. Monogastrics (i.e., hors-es, mules, swine, etc.) are less sensitive to nitrate toxicitythan ruminants. An animal’s conditioning affects its ability to assimilate or tolerate nitrates, so consult your veterinarian before feeding forage that contains nitrates.”
The following guidelines will help you avoid danger to your livestock this fall when feeding species with nitrates or prussic acid poisoning potential:
- Under drought conditions, allow animals to graze only the upper one-third to one-half of the plant or the leaves of coarse-stemmed forages if the nitrate levels in these plant parts is safe. Monitor animals closely and remove them quickly when the upper portion of plants is grazed off.
- Generally, forage nitrate levels drop significantly 3 to 5 days after sufficient rainfall, but it is always safer to send in a sample for testing before grazing or feeding forage soon after drought stress periods.
- Making hay does not reduce nitrate levels in the forage, but the hay can be tested and diluted sufficiently with other feeds to make it safe for animals.
- Ensiling forage converts nitrates to volatile nitrous oxides, or “silo gases”. These gases are highly toxic to humans. Safety practices include removing tarps from a portion of the silo a day or two before removing the silage from the bunker.
- Do not graze on nights when frost is likely. High levels of toxic prussic acid are produced within hours after a frost, even if it was a light frost.
- Do not graze after a killing frost until plants are dry, which usually takes 5 to 7 days.
- After a non-killing frost, do not allow animals to graze for two weeks because the plants usually contain high concentrations of prussic acid.
- New growth may appear at the base of the plant after a non-killing frost. If this occurs, wait for a killing freeze, then wait another 10 to 14 days before grazing the new growth.
- Don’t allow hungry or stressed animals to graze young growth of species with prussic acid potential. To reduce the risk, feed ground cereal grains to animals before turning them out to graze.
- Use heavy stocking rates (4-6 head of cattle/acre) and rotational grazing to reduce the risk of animals selectively grazing leaves that can contain high levels of prussic acid.
- Never graze immature growth or short regrowth following a harvest or grazing (at any time of the year). Graze or greenchop sudangrass only after it is 15 to 18 inches tall. Sorghum-sudangrass should be 24 to 30 inches tall before grazing.
- Do not graze wilted plants or plants with young tillers.
Green-chopping will not reduce the level of nitrates and is not likely to greatly reduce the level of prussic acid present. However, green-chopping frost-damaged plants will lower the risk compared with grazing directly, because animals are less likely to selectively graze damaged tissue. Stems in the forage dilute the high prussic acid content that can occur in leaves. However, the forage can still be toxic, so feed greenchop with great caution after a frost. If feeding greenchopped forage of species containing cyanogenic glucosides, feed it within a few hours of greenchopping, and don’t leave greenchopped forage in wagons or feedbunks overnight.
Hay and Silage
Prussic acid content in the plant decreases dramatically during the hay drying process and the forage should be safe once baled as dry hay. The forage can be mowed anytime after a frost if you are making hay. It is rare for dry hay to contain toxic levels of prussic acid. However, if the hay was not properly cured and dried before baling, it should be tested for prussic acid content before feeding to livestock.
Forage with prussic acid potential that is stored as silage is generally safe to feed. To be extra cautious, wait 5 to 7 days after a frost before chopping for silage. If the plants appear to be drying down quickly after a killing frost, it is safe to ensile sooner.
Delay feeding silage for 8 weeks after ensiling. If the forage likely contained high levels of cyanide at the time of chopping, hazardous levels of cyanide might remain and the silage should be analyzed before feeding.
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