Chilly Mornings, Chilly Concerns: Managing Frosted Forages

October 11, 2024 7:45 AM
Blog Post

Chilly Mornings, Chilly Concerns: Managing Frosted Forages 

Why is Frost a Concern?

Frost is one of the primary environmental factors that can lead to increased prussic acid content in sorghum species, forage sorghum, sorghum x sudangrass, and sudangrass. When a frost occurs, the stress it places on the plant can cause an accumulation of dhurrin, a compound that breaks down into prussic acid (also known as hydrocyanic acid). If livestock consume forage with elevated levels of prussic acid, it can result in acute toxicity, leading to symptoms such as muscle twitching, staggering, and even death within minutes.

Management Guidelines for Frost Events

To mitigate the risk of prussic acid toxicity following a frost, it’s important to take immediate action:

  1. Remove Cattle Before the Frost: When a frost is forecasted, cattle should be removed from sorghum fields to prevent consumption of potentially toxic forage.
  2. Wait One Week After Frost: Cattle should not graze the field for at least one week following a frost. If another frost occurs within the week, the waiting period should extend an additional seven days, effectively restarting the clock.
  3. Regrowth: Following a non-killing frost, scanning the field for regrowth is invaluable because new tillers can have high levels of dhurrin.  If the field has a considerable amount of regrowth waiting till the new growth is over 18 in tall or is killed by a killing frost is important to avoid high levels of prussic acid.
  4. Safe Grazing After a Killing Frost: After a killing frost, prussic acid levels diminish to safer levels. One week after the killing frost, the forage is generally safe for livestock to graze. Additionally, densely planted sorghum species can help protect the interior from a killing frost. Walking into the field can help determine if the plants were killed.

Feeding Alternatives

For producers concerned about managing sorghum forage after a frost, other methods such as silage and baleage offer safer options. The chopping and fermentation process involved in creating silage can reduce prussic acid content by over 50%, making it a safer choice than grazing or hay. Grazing is particularly risky as animals will selectively graze the leaf material which will accumulate higher levels of dhurrin.  Haying at this point in time would be VERY difficult to dry down, additionally haying, contradictory to previously thought,  does not reduce your risk of prussic acid potential.

Testing for Prussic Acid

If there is any doubt about the safety of sorghum forage following a frost, it is recommended to send samples for lab analysis. Fresh forage samples should be sealed in a high-quality plastic bag and chilled before being sent to a laboratory for testing.

For prussic acid testing, send samples to:

Veterinary Diagnostic Laboratory
Iowa State University
1850 Christensen Drive
Ames, IA 50011

By following these guidelines, sorghum species can continue to be a safe and valuable forage resource for livestock. Proper management after frost events is critical to avoiding prussic acid toxicity and ensuring livestock safety.

Is Frosted Alfalfa Toxic?

Frost injured alfalfa, clovers, and common perennial cool-season forage grasses Do NOT have the potential to form hydrocyanic acid, are NOT considered toxic and can be safely grazed or harvested for hay or silage following a frost.  However, there can be a slightly higher risk of bloat for grazed alfalfa and clovers the first few days after a frost. Producers should follow normal bloat preventing grazing management when grazing alfalfa and clover.

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 or 24 degree F freeze that lasts for at least four to six 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 was 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. 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 over wintering and regrowth the following spring. When these late-recovering plants experience a killing freeze a few days or weeks later, they will be physiologically weaker and more susceptible to winter injury.

 

Category: 
Authors: 

Joshua Michel Field Agronomist in NE Iowa

Joshua Michel is a field agronomist in Northeast Iowa for Iowa State University Extension and Outreach.

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