Keith VanSkike, Twin Creeks Extension, Agronomy & Natural Resources
Freezing temperatures change plant metabolism and composition, and different forage species respond differently to cold stress as the fall progresses. 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. The consumption of large amounts of prussic acid interferes with oxygen utilization, potentially causing animals to die from asphyxiation (respiratory paralysis). Avoiding grazing at night when frost occurs is a good management option, as most toxins are produced within hours from the freeze event. If there is a killing frost, such as the ones observed in most of Kansas this last week, it is advised to avoid livestock grazing these pastures for up to three to seven 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 to wait 10-14 days with no 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. 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 result in nitrate accumulation in parts of the plants that are still growing. Examples of forages that may have high nitrate levels include alfalfa, corn, oat and other small grains, sudangrass, and sorghum sudangrass, Johnsongrass, etc. . If cutting alfalfa for hay, the final cutting should occur right after the first killing freeze, before too many of the leaves have dropped, to reduce losses in nutritive value. Producers should be prepared to enter the fields as soon as soil moisture conditions allow. If grazing alfalfa fields, the best practice is to wait a few days after the freeze before releasing livestock to the field as frost-damaged alfalfa. Protect from rabbits and deer if you have young seedling shrubs/trees make sure you protect them. Use ¼ hail screen or 1/2” chicken wire at least 2 feet high or taller. Animals can scar and eat away outer bark layers. Now great time to treat for bindweed, thistle, brood leaves.