As fall progresses, freezing temperatures become more likely across the Plains and Midwest. Knowing how they can impact our forage crops is crucial for safe use.
ty Freezing temperatures cause metabolic and cellular changes to plants we need to be aware of, with prussic acid formation and nitrate poisoning being the biggest concerns.
First, let’s look at prussic acid. Sorghum, sudangrass, sorghum-sudangrass hybrids, and milo following a frost have broken cell membranes that allow the formation of a cyanide compound called prussic acid. This is naturally occurring in sorghum species and comes from dhurrin, a compound that can break down and release hydrogen cyanide, or HCN. When ingested and broken down by the digestive system, the cyanide released can quickly cause lethal results when consumed in high amounts. Monogastric species like pigs and horses can get prussic acid poisoning in extreme cases, but ruminants are more susceptible.
Levels of prussic acid vary depending on plant species. Lowest risk is sudangrass, with sorghum sudan hybrids of medium risk. High risk are forage sorghums with grain sorghum or milo carrying the highest levels and risk.
Keeping livestock out of fields with these species for seven days after a frost can limit the risk associated with prussic acid. Each time a new part of the plant is frozen, this seven-day timer is reset until the entire plant has been killed. This can make grazing difficult in the fall when freezing events occur regularly but are not enough to fully kill the plant.
New shoots and especially regrowth on previously frost-damaged plants have the highest concentrations of prussic acid. If you notice new shoots after a frost, animals should not be allowed to graze until the regrowth is 15-18 inches tall or seven days after a frost completely kills the plant. This can be especially tricky when warm temperatures follow a frost.
In the past, we have often said that haying or cutting a crop with prussic acid is not a concern, because HCN can volatilize during the drying process. While drying can reduce prussic acid risk, newer work has shown that drying sorghum for hay is not a guaranteed fix. Dhurrin may remain in dried sorghum hay and still release HCN during digestion. Hay made from drought-stressed, immature, or frost-damaged sorghum should be tested before feeding. Similarly, the fermentation process for ensiled sorghums can reduce prussic acid levels, but if you are concerned about high levels of prussic acid in a silage or hay feed, samples can be sent to a lab for analysis.
Another issue to keep an eye on is nitrate poisoning. Grasses are especially susceptible to nitrate poisoning risks. Slower metabolism following a stress like freezing allows nitrates to accumulate within the plant, specifically oats, sudangrass, and millets. Because nitrates do not dissipate, haying or green chopping immediately following a freeze is not recommended and can be potentially dangerous.
Nitrates commonly concentrate in the lower portions of plant stems. Waiting five days before haying or chopping and keeping a cutting height of 6 to 8 inches will help mitigate risk. Like prussic acid, the ensiling process will lower nitrate levels of plants harvested for silage. If grazing, reducing the stocking rate and increasing the animals' ability to selectively graze can lower nitrate risks. Pull animals off once the upper 2/3 of the plant has been consumed to avoid forcing animals to eat the lower portions of plants where nitrate risk is highest. This is not a good strategy for mixtures with sorghum species due to the prussic acid concerns discussed above.
Feeds that may contain high levels of nitrate aren’t necessarily unusable as long as proper action is taken to minimize risk. Send samples to a lab for analysis on those forages that are suspected to contain high nitrate levels. With these results, rations can be developed that limit the amount of high nitrate forage being fed, minimizing risk.
Ben Beckman is a beef systems Extension Educator serving northeast Nebraska. He is based out of the Cedar County Extension office in Hartington. You can reach him by phone: (402) 254-6821 or email: ben.beckman@unl.edu.
