Forage challenges as the weather turns cooler to keep livestock safe

Kyle Verhoff, OSU Extension Educator, Agricultural and Natural Resources, Defiance County and Jason Hartschuh, OSU Extension Field Specialist Dairy Management and Precision Livestock

Frost can impact the toxicity of our forages.

As the year begins to wrap up and temperatures drop, there are countless things to consider including how the coming frosts impact the toxicity of our forages. This past week many portions of the state began to flirt with possible overnight frosts which raises concerns of prussic acid poisoning, nitrate poisoning, and increased bloat as a result of feeding certain fall forages.

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 prussic acid poisoning.

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. 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?

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.

If you are looking to feed silage that was made from forages with a potential for prussic acid toxicity, be sure to allow six to eight weeks for proper ensiling. This can be a safer method for feeding certain forages because 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.

If you are unsure the forage you are looking to feed has high prussic acid toxicity you can send a sample to a lab for analysis, but be sure to follow sampling and shipping instructions properly because the hydrogen cyanide tested is a gas and a test with a false negative and can mislead you into feeding an unsafe forage.

Other fall 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 risk has increased recently due to the dry weather we have received recently that has slowed plant growth.  Nitrate toxicity can present symptoms similar to prussic acid poisoning in lethal cases and symptoms like weight loss and reduced production in chronic cases.

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.