Freezing temperatures change plant metabolism and composition, and different forage species respond differently to cold stress as the fall progresses. Still, damaging frosts significantly reduce forage quality in most forage species. Depending on plant species, these changes in metabolism resulting from freezing temperatures can create possible feeding-related animal disorders, and therefore, there may be a need to alter grazing management.
Prussic acid poisoning
Plants that contain cyanogenic glucosides, such as warm-season annual grasses in the sorghum family, produce larger amounts of cyanide (prussic acid) when damaged by frost (Figure 1). Greater potential for harmful cyanide levels occur in soils with nitrogen content and low in phosphorus or potassium.
Consuming large amounts of prussic acid interferes with oxygen utilization, potentially causing animals to die from asphyxiation (respiratory paralysis). Symptoms such as cherry-red colored blood, staggering, difficulty breathing, spasms, foaming at the mouth, excess salivation, falling, and severe convulsions appear rapidly after forage consumption, sometimes leading to animal death within minutes.
It is extremely important to use caution when grazing these species during the fall. Most toxins are produced within hours of a freeze event, so grazing animals should be removed from the field before a frost or freeze occurs. If there is a killing frost, it is advised to avoid livestock grazing these pastures for up to three days after the frost – as the toxin usually dissipates within 72 hours or until plant tissue is dried out. Fresh forage is riskier as cyanide levels will be higher as compared to dry tissue, silage, or hay. After non-killing frosts, we advise you to wait 10-14 days without additional frost action before grazing.
Prussic acid content decreases significantly when the forage is cut for hay/used for silage, as large amounts are lost as gas during fermentation. Still, it is recommended to delay feeding silage for six to eight weeks following ensiling. Forage cut for hay that contained high concentrations of cyanide before harvest may still have dangerous levels even after drying. Testing is cheap compared to dead cows. Producers can consider mixing nonthreatening forages into the diet to dilute any potentially damaging residual cyanide.
Forages with differing potentials for prussic acid production:
- High: grain sorghum, forage sorghum, and sorghum-sudangrass hybrids
- Intermediate: sudangrass
- Low: piper sudangrass, pearl millet, and foxtail millet
Other species that have the potential to contain toxic levels of prussic acid after frost include Johnsongrass, chokecherry, black cherry, and elderberry..
Nitrate toxicity
The summer of 2023 was extremely dry in many areas of Kansas. Drought-stressed annual and perennial forages can accumulate toxic nitrate levels. This can be worsened after a frost, as freezing damage slows down metabolism and can accumulate nitrate in parts of the plants that are still growing. Examples of forages that may have high nitrate levels include alfalfa, corn, oats, and other small grains, millet, sudangrass, sorghum sudangrass, Johnsongrass, etc. Before feeding or grazing drought-stressed forage, send a forage sample to a commercial lab to be tested for nitrates. Follow your lab’s specific instructions about how to collect and handle the sample. The cost of sampling and analysis is well below that of losing animals.
Alicia Boor is an Agriculture and Natural Resources agent in the Cottonwood District (which includes Barton and Ellis counties) for K-State Research and Extension. You can contact her by e-mail at [email protected] or calling 620-793-1910. K-State Research and Extension is an equal opportunity employer and provider. Kansas State University, County Extension Councils, Extension Districts, and U.S. Department of Agriculture Cooperating.