Frosted Forages and Fall Risks: Managing Prussic Acid and Bloat in Grazing Systems

It may have felt like summer was hanging on through October, but this morning’s frost on rooftops and the need for a heavier jacket was a clear reminder, fall is here. And with fall comes frost, which can bring serious concerns for some forage species.

Why Is Frost a Concern for Sorghum?

Frost is one of the key environmental triggers that can increase prussic acid levels in sorghum species, including forage sorghum, sorghum x sudangrass, and sudangrass. When frost stresses these plants, they accumulate dhurrin, a compound that breaks down into prussic acid (hydrogen cyanide). If livestock consume forage with elevated prussic acid, it can lead to acute toxicity; muscle twitching, staggering, and even death within minutes.

Frost Management Guidelines for Sorghum Forages

To reduce the risk of prussic acid poisoning, here are some practical steps:

1. Remove Cattle Before Frost Hits - If frost is in the forecast, pull cattle from sorghum fields to prevent exposure to toxic forage.
2. Wait One Week After Frost - Don’t graze for at least 7 days after a frost. If another frost occurs during that week, restart the clock.
3. Watch for Regrowth - After a light frost, new tillers may emerge and contain high dhurrin levels. Wait until regrowth is over 18 inches tall or until a killing frost halts growth.
4. Grazing After a Killing Frost - One week after a hard freeze (28°F for ~4 hours), prussic acid levels typically drop to safe levels. Dense stands may protect interior plants; walk the field to assess damage.

Feeding Alternatives

If you're unsure about grazing safety, consider silage or baleage. The fermentation process can reduce prussic acid by over 50%. Haying is not recommended, it’s hard to dry down this time of year, and contrary to past beliefs, haying doesn’t reduce prussic acid risk. Grazing is especially risky because animals tend to select leaf material, which contains higher dhurrin levels.

Testing for Prussic Acid

When in doubt, test. Send fresh forage samples to:

ISU Veterinary Diagnostic Laboratory

Iowa State University

1850 Christensen Drive

Ames, IA 50011

Ward Labs offers a relative prussic acid potential test that producers can use on-site. While it doesn’t give a precise concentration, it provides a quick indication of whether prussic acid levels are likely to be a concern. This can be a helpful tool for making immediate grazing decisions.

More information on prussic acid management: CROPR 3199

What About Frosted Alfalfa?

Good news - alfalfa, clovers, and cool-season grasses do not produce prussic acid and are safe to graze or harvest after frost. However, alfalfa and clovers may pose a slightly higher bloat risk in the first few days post-frost. Use standard bloat prevention strategies when grazing.

Is Frosted Alfalfa Toxic?

While frost-damaged alfalfa is not toxic, the risk of bloat increases immediately following a frost or freeze. Even if cattle are adapted to grazing alfalfa, they should be removed from frosted fields.

Watch for wilting, typically 5–7 days after the freeze, as this signals a reduced (but not eliminated) bloat risk. If warm temperatures follow the freeze, bloat risk may rise again, especially since alfalfa can continue growing late into the season.

Once >50% of plants have gone dormant after a killing frost, bloat risk drops significantly. For safer winter grazing, consider:

• Providing bloat blocks or supplements.
• Avoiding grazing when dew is present.
• Ensuring cattle are full before turnout.

Should I Harvest Alfalfa after a Frost?

There is not a simple answer to this question. In general, it will depend whether the frost was a killing frost or not.  A killing frost is not the first light frost of the season; rather, it is a 23- 24 ⁰F freeze that lasts for at least four hours. If the producer does not need the forage, it is best for the alfalfa to remain uncut and standing through the winter. This will help reduce the chances of potential winterkill.

If it were a hard, killing freeze and the producer needs the forage, harvest as soon as possible after the freeze to salvage as much of the nutritive value as possible, as quality will rapidly decline after a killing frost. The longer the delay, the greater the weathering damage, leaching of nutrients, and leaf loss from the standing frosted plants. To improve plant crown insulation over the winter, consider leaving a 5 to 6 inch stubble at this late-season harvest.  

If the frost was a light, non-killing freeze, the tops (terminal buds) of the alfalfa plants may be visibly damaged and growth from the terminal buds will be hindered but will not likely stop the plants' growth for the season. The damaged tops will deteriorate in nutritive quality but the plant will still be attempting to regrow from crown buds and will be using stored sugars. The best management for the plant is to allow it to continue to grow using whatever green leaf area it still has until a hard, killing freeze occurs. Then if the producer needs the forage, it can be cut and harvested for hay or silage, or grazed.

Lastly, alfalfa plants cut immediately after a partial freeze (non-killing frost) and which experience further normal growing temperatures will start new regrowth from crown buds, using accumulated root proteins and carbohydrates that would otherwise be used for overwintering and regrowth the following spring. When these late-recovering plants experience a killing freeze a few weeks later, they can be physiologically weaker and more susceptible to winter injury.