Feeding Frosted Forages

Grain sorghum ready for harvest

By Glen Arnold, Putnam County

Northwest Ohio has yet to receive a killing frost this fall end to the growing season. Any frost (light or killing) can make some forages dangerous to livestock because hydrogen cyanide (HCN) is released when leaves are damaged by frost. This is also called Prussic Acid poisoning.

Prussic acid poisoning can occur when feeding sudangrass, sorghum-sudangrass hybrids, forage sorghum, or grain sorghum. These species contain varying concentrations of cyanogenic glucosides, which are converted to prussic acid. As ruminants consume forage containing high levels of cyanide-producing compounds, prussic acid is released in the rumen, absorbed into the bloodstream where it binds hemoglobin, and interferes with oxygen transfer. The animal soon dies of asphyxiation. Prussic acid acts rapidly, frequently killing animals in minutes. Symptoms include excess salivation, difficult breathing, staggering, convulsions, and collapse. Ruminants are more susceptible than horses or swine because cud chewing and rumen bacteria help release the cyanide.

Generally, any stress condition that retards plant growth may increase prussic acid levels in plants. Hydrogen cyanide is released when leaves are damaged by frost, drought, bruising, cutting, trampling, crushing, or wilting. Plants growing under high nitrogen levels or in soils deficient in soil phosphorus or potassium tend to have high levels of cyanogenic glucosides. Species and varieties differ in prussic acid poisoning potential. Sudangrass varieties are low to intermediate in cyanide potential, and sorghum-sudangrass hybrids and forage sorghums are intermediate to high. Piper sudangrass has low prussic acid poisoning potential, and pearl millet is virtually free of cyanogenic glucosides.

The management practices described below can reduce the risk of prussic acid poisoning from forage sorghum, sudangrass, and sorghum-sudangrass hybrids:

1) Graze or greenchop only when the grass is greater than 18 inches tall.

2) Do not graze wilted plants or plants with young tillers.

3) Do not graze plants during or shortly after a drought when growth has been reduced.

4) Do not graze on nights when frost is likely. High levels of the toxic compounds are produced within hours after a frost occurs.

5) Do not graze after a killing frost until the plants are dry. Wait 5 to 7 days to allow the released cyanide to dissipate.

6) Do not graze for two weeks after a non-killing frost.

7) Delay feeding of silage for 6 to 8 weeks after ensiling. Fresh forage is generally higher in cyanide than silage or hay because cyanide is volatile and dissipates as the forage dries. However, hay or silage that likely contained high cyanide levels at harvest should be analyzed for HCN content before feeding.

8)Splitapplications of nitrogen decrease the risk of prussic acid toxicity, and proper levels of phosphorus and potassium in the soil will also help.

9) Don’t allow hungry or stressed animals to graze young sorghum grass growth.

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