Feeding Frosted Forages

By Mark Sulc 

I am beginning to get questions about toxicities that can develop after forages are frosted. There is potential for some forage toxicities and other problems that can develop after a frost. Prussic acid poisoning and high nitrates are the main concern with a few specific annual forages and several weed species, but there is also an increased risk of bloat when grazing legumes after a frost.

Nitrate accumulation in frosted 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 is not hazardous to grazing animals, but greenchop or hay cut right after a freeze can be more dangerous. When in doubt, send in a sample to a forage testing lab and request a nitrate test before grazing or feeding a forage after a frost.

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. Some labs provide prussic acid testing of forages (see a partial list at the end of this article). 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. Call the lab to confirm what guidelines they suggest and that they are still doing the test.

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 (a native prairie grass) 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 after frost are listed below in decreasing order of toxicity risk:

• Grain sorghum = high to very high toxic potential
• Indiangrass = high toxic potential
• Sorghum-sudangrass hybrids (“sudax”) 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:

• Johnsongrass
• Shattercane
• Chokecherry
• Black cherry
• Elderberry

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 common after the first autumn frost. New growth from frosted plants is palatable but can be dangerously high in prussic acid.

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. Rain after drought plus young stages of plant maturity can combine to cause toxic levels of prussic acid in forages.

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 has more risk. After frost damage, cyanide levels will likely be higher in fresh forage as compared with silage or hay that is wilted in the field. 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.

Grazing Precautions Against Nitrate & Prussic Acid Poisoning

The following guidelines will help you avoid danger to your livestock this fall when feeding species with nitrates or prussic acid poisoning potential:

• 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.
• Do not 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.
• 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. I have seen where frosts after drought ending rains seems to maintain the high nitrate levels in forages (high nitrates can be maintained all winter in the forage0, so don’t assume frosts will reduce the nitrate levels. Keep testing the forage and only graze or feed it when and IF the nitrate levels drop to safe levels!
• 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 and wearing respirators designed to filter out nitrous oxides.

Greenchop

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 do not 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 any time 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.

Species That Can Cause Bloat After Frost

Forage legumes such as alfalfa and clovers have an increased risk of bloat when grazed one or two days after a hard frost. The bloat risk is highest when grazing pure legume stands and least when grazing stands having mostly grass.

The safest management is to wait a few days after a killing frost before grazing pure legume stands – wait until the forage begins to dry from the frost damage. It is also a good idea to make sure animals have some dry hay before being introduced to lush fall pastures that contain significant amounts of legumes. You can also swath your legume-rich pasture ahead of grazing and let animals graze dry hay in the swath.  Bloat protectants like poloxalene can be fed as blocks or mixed with grain. While this an expensive supplement, it does work well when animals eat a uniform amount each day.

Frost and Equine Toxicity Problems

(source: Bruce Anderson, University of Nebraska)

Minnesota specialists report that fall pasture, especially frost damaged pasture, can have high concentrations of nonstructural carbohydrates, like sugars.  This can lead to various health problems for horses, such as founder and colic.  They recommend pulling horses off of pasture for about one week following the first killing frost.

High concentrations of nonstructural carbohydrates are most likely in leafy regrowth of cool-season grasses such as brome, timothy, and bluegrass but native warm-season grasses also may occasionally have similar risks.

Another unexpected risk can come from dead maple leaves that fall or are blown into horse pastures.  Red blood cells can be damaged in horses that eat 1.5 to 3 pounds of dried maple leaves per one thousand pounds of bodyweight.  This problem apparently does not occur with fresh green leaves or with any other animal type.  Fortunately, the toxicity does not appear to remain in the leaves the following spring.