Grazing Clover After a Light Freeze

Cyanide Poisoning

• Prussic or hydrocyanic acid formation in pasture grass or forage after fall frost is usually the first concern of most cattle, sheep and horse owners. Fatal cyanide poisoning can happen fast, and ruminants -- cattle and sheep -- are most vulnerable because digestive bacteria release the toxin as a byproduct of digestion. Rapidly transported through the bloodstream, animals quickly suffocate because hydrocyanic acid prevents cells from taking up oxygen. Among popular pasture grasses and forages, johnsongrass, sudangrass hybrids, sorghum-sudangrass hybrids and white clover have the potential to produce prussic acid poisoning after frost.

Preventing Poisoning

• Because young, rapidly growing plants typically produce higher levels of hydrocyanic acid, new stems and leaves produced after frost damage can be particularly dangerous. Extended drought can have the same stress-related effect as frost, with new rain stimulating similar levels of toxic vegetation. For tall pasture -- sudangrass 18 inches or taller, sudangrass 30 inches or taller -- you can return animals to pasture after three or four days. Also, feed hungry livestock generous portions of hay before turning them out to graze. For shorter frost-injured pastures -- including clover pastures -- wait at least 10 days to two weeks before allowing animals to graze, so toxins will dissipate. Remove animals again after the next frost. Frost-damaged plants chopped for hay or silage are usually safer, but analyze sample before feeding.

Livestock Bloat

• Many pastures mix high-fiber forages such as sudangrass with alfalfa, clovers and other protein-rich legumes. Livestock owners don't always realize that fatal pasture bloat is a much more likely hazard after fall frost than prussic acid poisoning. Frost-injured clover, alfalfa and other perennial cool-season forages don't have much capacity to produce toxic hydrocyanic acid, but their high levels of soluble protein can cause "frothy bloat," or a stable mass of foam in the rumen that blocks the release of digestive gases. In cattle and sheep, bloat can be fatal within one hour.

Preventing Bloat

• Young protein-rich pasture has the highest risk of causing bloat. Grazing clover, alfalfa or other legume forage following a killing frost can cause fatal bloat, but the risk declines as the pasture dries. Allow at least one week for clover or alfalfa to dehydrate or "dry down" before allowing livestock back into the pasture to graze. If your pasture contains more than 50-percent grass, bloat danger is greatly reduced. Reduce the risk further by feeding substantial amounts of hay to cattle on bloat-provoking pasture. Introduce cattle with full rumens into a new pasture in the afternoon, after the dew is off the clover.