The first frost of the year can set unwary cattle producers up for some harsh health setbacks in the herd, including prussic acid poisoning, nitrate poisoning and bloat.
Nebraska Extension educator Ben Beckman reminds growers to wait five to seven days after a frost before grazing to minimize risks to cattle. If haying or cutting, consider the danger of nitrates, and wait five days, raising the cutting height 6 to 8 inches. If the frost was non-killing, new shoots or regrowth can be especially high in prussic acid and nitrates, so pull any animals present in the pasture and don't allow them back until the plant has been fully killed or new growth is at least 18 inches high.
PRUSSIC ACID AND GRAZING
For sorghum, sudangrass, sorghum-sudangrass hybrids, and milo, a frost can mean the formation of prussic acid. This is a form of cyanide, which, when ingested, can lead to death. Ruminants are especially susceptible.
Keeping livestock out of these areas for five to seven days after a frost limits the risk, but each time there is a frost, the timer resets until the entire plant is dead. New shoots and regrowth have the highest concentrations of prussic acid.
Best use of these forages after a frost may be haying or cutting for feed. As the forage dries, prussic acid volatilizes, and 50% or more of what was initially there is lost. Fermentation on ensiled sorghums reduces prussic levels too. Send silage or hay samples to a lab for analysis if there are concerns that levels may still be too high.
Grasses are especially susceptible to nitrate, especially oats, sudangrass and millets. The nitrates accumulate in plants after a freeze, and they don't dissipate like prussic acid -- meaning haying or green chopping can create more of a danger to cattle.
Nitrates concentrate in the lower portion of plant stems, so wait to hay or chop five days after the frost, and keep cutting height at 6 to 8 inches. Ensiling can lower levels of nitrates in plants harvested for silage.
If animals are grazing an area likely to have high levels of nitrates, reduce stocking rates and increase their ability to selectively graze. Pull cattle off the forages entirely once the upper two-thirds of the plants are eaten.
BLOAT COMMON AFTER A FREEZE
High-quality forages including alfalfa, clover (ladino and white), and fresh small-grain shoots may lead to issues with bloat after frost. This is because of damage in the plants that ruptures cell walls and makes protein and minerals more readily available. Naturally higher protein species like legumes have a greater chance of causing bloat.
Introduce grazing animals to suspect forages with a full stomach and limit grazing them, supplementing with hay or other feeds/forages. Bloat reducers can be useful if consumed uniformly and regularly. To help with this, mix supplements into a daily fed ration, or use a molasses or a salt-based block.
For more on the effects of a freeze on forages, go here: https://beef.unl.edu/….
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