– Jason Hartschuh, Extension Field Specialist, Dairy Management and Precision Livestock, Ohio State University Extension
Sorghum-sudangrass hybrids have high to intermediate toxic potential.
This year is presenting many challenges when it comes to forage toxicity and ensuring that we have safe feed for our livestock. Drought conditions have led to an active discussion on forage toxicity and the potential for frost in the future presents an additional concern. These environmental conditions increase the risk of prussic acid poisoning, nitrate poisoning, and increased bloat in multiple methods of feeding forages. Recent rainfall has finally allowed for rapid forage growth but unfortunately in some species, this rapid growth poses a risk to your livestock during future frost events.
What is prussic acid toxicity?
Prussic acid toxicity is the accumulation of prussic acid (i.e. hydrogen cyanide) in forage plant tissue. Prussic acid is the product of a reaction between two naturally occurring plant molecules, cyanogenic glycosides and degrading enzymes. Plant cell walls usually separate the two, but a frost event freezes the water in a plant cell, rupturing the cell wall and allowing the formation of prussic acid.
What variables contribute to prussic acid toxicity?
Forage Species
The forage species that are the main concern when it comes to prussic acid toxicity are our warm-season grasses and the most common and their toxicity potential are:
- Grain sorghum: very high to high toxic potential
- Indiangrass: high toxic potential
- Sorghum-sudangrass hybrids and forage sorghums: high to intermediate toxic potential
- Sudangrass hybrids and varieties: intermediate toxic potential
- Piper sudangrass: low toxic potential
- Pearl millet and foxtail millet: low toxic potential, but be cautious of nitrate poisoning this year
Prussic acid toxicity is also found in plants such as johnsongrass, shattercane, and wild cherry, that are not planted as forage but may accidentally be consumed by livestock. It is good practice to identify these plants around your pasture to manage, be sure to always pick up and discard fallen limbs after a storm to prevent accidental consumption.
Fertility
There is a higher chance of prussic acid toxicity in forages that have had a high rate of nitrogen fertilization or manure history and in soils that have high nitrogen to low phosphorus imbalance. Be sure to make fertilization decisions based on regular soil tests.
Plant maturity
The concentrations of prussic acid are higher in young and rapidly growing stands of forages because they contain more cyanogenic glucosides. With recent rainfall events after a severe drought causing rapid growth, many plants have a lot of new plant growth of concern. After a non-killing frost, forages can produce this new growth from the base of the stand.
Prussic acid poisoning symptoms
Symptoms of prussic acid poisoning can manifest quickly. The prussic acid can quickly enter the bloodstream and inhibit the livestock’s ability to utilize oxygen. Symptoms can include drooling, labored breathing, staggering, and convulsions. If you observe any of these symptoms, contact your local veterinarian.
How can we reduce potential prussic acid poisoning?
Grazing
To reduce prussic acid poisoning in a grazing system, take steps to reduce the grazing of frost-damaged and the new growth of the forage species described above. Common precautions include:
- Remove livestock from pasture on nights when frost is forecasted, prussic acid can form quickly and with a light frost.
- After a killing frost, do not graze until the forage is dry, usually 5-7 days.
- After a non-killing frost, do not graze for two weeks, if the non-killing frost results in new growth do not graze until 10 to 14 days after there is a killing frost. The time after a killing frost will allow the forage to field cure and dissipate the prussic acid.
- If the forage has a high potential for prussic acid poisoning, do not graze new growth.
- To reduce selective grazing of forages with new growth utilize heavy stocking rates and rotational grazing.
Green chop
Green-chopping forages will not significantly reduce the level of prussic acid in forages. Green-chopping has the benefit of making it less likely that livestock can selectively consume frost-damaged tissue, but if the forage is frost-damaged it can still be toxic, so feed with caution.
Silage and Hay
Making hay or silage out of forage with prussic acid concerns can be safe with a few precautions. Forage can be mowed for dry hay any time after a frost due to a decrease in prussic acid content throughout the drying process. If the hay is not properly cured and dried prior to baling, it should be tested before feeding. For silage, it is best to wait 5-7 days after a frost to chop and to wait to feed that silage 8 weeks after ensiling. The ensiling process allows a significant amount of the prussic acid to escape as a gas during the fermentation process. Be cautious of this gas through if you are storing forage in a silo and need to enter it for any reason be sure to make sure fresh air is blown into the silo for your safety.
Other Forage Concerns
Nitrate Accumulation
Prussic acid poisoning is not the only concern for feeding forages after frost. Frost can slow the growth of many actively growing plants, not allowing the conversion of the nitrates absorbed into proteins, leading to a toxic accumulation of nitrates in the plant tissue. This same risk is present in drought conditions where drought in the stressor limits nitrate conversion. It is important to note that making hay does not reduce nitrate concentration, so if you have hay made during the drought or after a frost send a sample to be tested and dilute it with supplemental feed to ensure animal safety. Nitrate toxicity can present symptoms similar to prussic acid poisoning in lethal cases and symptoms like weight loss and reduced production in chronic cases. For a more in-depth look at nitrates see Forage Nitrate Toxicity a Major Concern and Drought Worsens.
Bloat
Frost damage can also affect the grazing of our forage legumes like alfalfa and clovers. To reduce instances of bloat, allow primarily legume pasture to dry for a few days after a killing frost. Other mediation methods include; supplementing a fiber source like dry hay or grazing cornstalks post-harvest, making sure livestock are not excessively hungry going to fresh pasture by feeding them other forage before turning out on pasture each morning, and utilizing a bloat protectant.
Testing Forages for Toxicity
If there is a forage where prussic acid or nitrates are a concern, sending a sample to a lab is the best way to confirm if a forage is safe to feed. We have compiled a list of labs and called them on October 4, 2024, to confirm if they offer either test. There may be other labs that are not listed that offer the same tests.
It is very important to call the lab if you plan to send a sample to get guidelines on how to collect, handle, and ship the sample to ensure that the test performed is standardized and accurate. This is particularly important for samples with prussic acid since it is a gas and if handled incorrectly can be lost from the sample, resulting in a sample that appears safe to feed when it is not.