Precautions for Feeding Frosted and Drought-Stressed Forages

 

Livestock owners feeding forage need to keep in mind the potential for some forage toxicities and other problems that can develop this fall. High nitrates and prussic acid poisoning are the main potential concerns. 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.

 

 

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 been dry of late. Corn, oat and other small grains, sudangrass, and sorghum sudangrass, and many weed species including johnsongrass can accumulate toxic levels of nitrates. Even alfalfa can accumulate toxic nitrate levels under severe drought stress.

 

Before feeding or grazing drought stressed forage, send in a forage sample to be tested for nitrates. Most labs now offer nitrate tests, so it is likely that you can get a forage nitrate test by your favorite lab. Several labs are listed at the end of this article that do nitrate testing. This list is for your convenience and no labs are intentionally omitted. Check your chosen lab’s website or call them and follow their specific instructions about how to collect and handle the sample. The cost is well worth it against the risk of losing animals.

 

See the following references for more details:

 

Nitrates in Cattle Sheep and Goats (University of Wisconsin Extension) https://fyi.extension.wisc.edu/forage/nitrate-poisoning-in-cattle-sheep-and-goats/

 

Nitrates and Prussic Acid in Forages (Texas Cooperative Extension) http://forages.tamu.edu/PDF/Nitrate.pdf

 

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, test the forage for nitrates before grazing or feeding it.

 

 

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

 

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:

Species not usually planted for agronomic use can also develop toxic levels of prussic acid, including the following:

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.

 

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. This is because cyanide is a gas and dissipates as the forage is wilted and dried for making silage or dry hay.

 

 

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.

 

 

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

 

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.

 

 

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.

 

 

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.

 

 

 

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.