LOVELOCK - The cold weather this past week has raised concerns and questions of feeding certain forage species to livestock. These are very legitimate concerns with the sorghum species. Prussic acid poisoning can occur when feeding frost-damaged sudangrass, sorghum-sudangrass hybrids, forage sorghum, or grain sorghum. These species contain varying concentrations of cyanogenic glucosides, which are converted to prussic acid, also known as hydrogen cyanide (HCN). 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.
russic 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.
Minimizing risk of prussic acid poisoning when grazing:
1. Graze or green-chop only when sorghum grasses are greater than 18 inches tall. Never graze or green-chop sorghum grasses less than 18 inches tall, at any time of the year.
2. Do not graze on nights when frost is likely to occur. High levels of the toxic compounds are produced within hours after a frost occurs.
3. Do not graze after a killing frost until the plants turn brown and are dry. Wait 5 to 7 days to allow the released cyanide to dissipate.
4. If plants begin to grow after being frost damaged (non-killing frost), do not them until the re-growth is 18 inches tall or the entire plant is killed and turns brown by a later killing frost. To be on the safe side, never graze for two weeks after a non-killing frost.
5. Do not graze wilted plants or plants with young tillers.
6. Split applications of nitrogen decrease the risk of prussic acid toxicity, and proper levels of phosphorus and potassium in the soil will also help.
7. Don't allow hungry or stressed animals to graze young sorghum grass growth.
8. Do not graze plants during or shortly after a drought when growth has been reduced.
The fall is also a time when animals grazing on pasture may experience bloat problems. Bloat occurrences can be reduced through several pasture management methods:
1. Plant pastures so that no more than 50 percent of the forage mixture is alfalfa or clover.
2. Fill cattle on dry roughage or grass pastures before turning to legume pastures.
3. Provide grass pasture, hay, crop residue (cornstalks, milo stubble) or grain along with the legume or wheat pasture to reduce pasture intake.
4. Graze in a rotation using different grass and legume pastures.
5. Fertilize and graze to stimulate grasses in the grass-legume pastures. Strip graze or rotational graze grass-legume pastures to force cattle to eat most of the plant material rather than just the succulent top growth.
6. Once cattle are turned to pasture, don't remove them at the first signs of bloat. Mild bloat occurs frequently and repeatedly on alfalfa pasture. Cattle with greatly distended rumens should be removed immediately and treated.
Anytime alfalfa is grazed, and especially during the fall, make sure soils are dry and firm. Grazing alfalfa when soils are soft and wet is a sure recipe for severe damage to crowns and subsequent stand loss over the winter. Frosted and frozen alfalfa does not contain toxic compounds. It is likely, however, that a slightly higher potential for bloat exists for one or two days after a frost or freeze. The safest management is to wait a few days before grazing after a killing freeze, when the forage begins to dry.
For more information on this subject contact the Pershing County Extension Office at (775) 273-2923 or email me at fosters@unce.unr.edu.
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