Managing forages to minimize prussic acid poisoning

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Prussic acid poisoning is a potential concern for cattle producers who feed forage and grain sorghums, Johnsongrass, shattercane, sudangrass, and sorghum-sudangrass crosses. These species all have the potential to have high concentrations of prussic acid under certain conditions. Frost damage and late-season regrowth present conditions that can lead to concerns, so producers should be prepared to take steps to avoid potential prussic acid poisoning problems.

Species factors

Most sorghums and sudangrasses contain a prussic acid precursor (dhurrin) in their epidermal cells. Dhurrin in itself is not toxic. The mesophyll cells located below the epidermis contain an enzyme that under certain conditions converts dhurrin to prussic acid (HCN). It is the prussic acid that is toxic to livestock. Grain sorghum generally has higher concentrations of dhurrin than forage sorghums or sudangrass.

Under normal growing conditions, the dhurrin concentration is low enough that animals can detoxify it before it causes toxicity. Dhurrin concentrations are highest in young plants, new regrowth, and following rapid regrowth after a period of stunted growth, such as rapid growth of drought-stressed plants following a rain, or regrowth following a frost or freeze. Under these conditions dhurrin concentrations can be high enough to poison livestock.

Appropriate management of these forages combined with sample analysis can minimize poisoning risks and allow safe use of these forages. Delaying grazing until minimum plant heights are achieved or until injured or stressed plants have had adequate time to recover or by proper ensiling or conditioning and drying hay can reduce prussic acid concentrations.

Hybrid pearl millet and foxtail millet are generally considered to not have high prussic acid concentrations. Indiangrass, flax, chokecherry, black cherry, elderberry, and some varieties of white clover and birdsfoot trefoil also can cause prussic acid poisoning.

Conditions conducive to high levels of prussic acid

High levels of prussic acid can occur in forage and grain sorghums, Johnsongrass, shattercane, sudangrass, and sorghum-sudangrass crosses under the following conditions:

• Stress. Drought-stunted plants accumulate cyanide and can possess toxic levels at maturity. Generally, any stress condition that retards normal plant growth can increase prussic acid content.
• Leaf damage. Freezing ruptures the plant cells and releases cyanide into the leaf tissue. After a killing frost, wait at least five days or until the frozen leaf tissue has completely dried out before grazing to allow the released HCN gas to dissipate. HCN is also released when plant leaves are physically damaged by trampling, cutting, crushing, wilting, or chewing.
• Regrowth. Prussic acid poisoning is most commonly associated with regrowth following a drought-ending rain or the first autumn frost. New growth from frosted or drought-stressed plants is palatable but can be dangerously high in cyanide.
• Soil fertility. Plants growing in soils that are high in nitrogen and low in phosphorus and potassium tend to have high cyanide concentrations. Split applications of nitrogen decrease the risk of prussic acid toxicity.
• Harvest method. Prussic acid concentrations are greater in fresh forage than in silage or hay because HCN is volatile and dissipates as the forage dries or ensiles. However, if the forage had extremely high cyanide content before cutting, or if the hay was not properly conditioned and cured, hazardous concentrations of prussic acid could remain. Hay or silage that likely contained high cyanide concentrations at harvest should be analyzed before it is fed.
• Growth stage. Cyanide normally is more concentrated in the growing point and young leaves than in older leaves or stems. Young, rapidly growing plants are likely to contain higher levels of prussic acid than older, mature plants. New sorghum growth, especially “suckers” or tillers, following drought or frost are dangerously high in cyanide. Pure stands of Indiangrass grazed when the plants are less than 8 inches tall can possess lethal concentrations of cyanide.

Symptoms of prussic acid poisoning

Symptoms of poisoning include increased pulse rate and respiration, excessive salivation, staggering, spasms, labored breathing, bloating, muscular tremors, foaming from the mouth and a blue coloration of lining of the mouth, a bright red color of the blood,  falling, and severe convulsions. Death from respiratory paralysis follows shortly.

Ways to reduce prussic acid poisoning

To prevent prussic acid poisoning, following options are available:

• Avoid grazing or green chopping young sorghum or sudangrass plants or new regrowth.
• Do not allow hungry cattle to graze where prussic acid may be a problem.
• Feed hay or grain before releasing cattle to pasture to slow intake and reduce the poisoning risk.
• Do not allow animals to graze potentially troublesome plants after a light frost or after rain has ended a summer drought.
• Hay or ensile plants high in cyanide to reduce toxin levels.
• Have representative samples of any suspect forage analyzed before feeding.

Most losses occur when hungry or stressed animals graze young sorghum growth. Ruminants are particularly susceptible to prussic acid poisoning because cud chewing and rumen bacteria both contribute to releasing cyanide.

Feeding grain or hay before releasing animals to pasture slows intake and dilutes the amount of cyanide consumed. Animals do not adapt or become immune to cyanide, but they can detoxify low concentrations of HCN. Under normal growth conditions they can consume forage sorghums or sudangrass containing low levels of prussic acid with no ill effects, while elevated levels that cannot be detoxified fast enough cause poisoning.

Interpreting prussic acid test results

Table 1 shows prussic acid levels in forages considered to be safe to toxic and safety recommendations for animals.

Table 1. Level of prussic acid (dry matter basis) in forage and potential effects on animals
Prussic acid (ppm)	Effect on animals
0-500	Generally safe; should not cause toxicity
500-1,000	Potentially toxic; should not be used as only source of feed
1,000 and above	Dangerous to cattle and will usually cause death

Source: Prussic Acid Poisoning, K-State publication MF-3040, http://www.ksre.ksu.edu/bookstore/pubs/MF3040.pdf

Prussic acid poisoning is related to the amount of forage consumed, the rate of consumption, and the animal’s physiological condition, but HCN levels in forages exceeding 200 parts per million on a wet weight (as is) basis are dangerous. On a dry weight basis, forages with more than 500 ppm HCN should be considered potentially toxic.

Sampling Handling Methods

Sampling handling method may influence prussic acid content of forage samples.  Forage samples may be refrigerated or frozen in sealed plastic storage bags for up to 7 days prior to analysis of prussic acid content but should not be left in unsealed containers in an unprotected environment for an extended periods (Table 2).

Table 2. Effect of sample handling method on chemical composition of forage sorghum
	Treatment
Item	Fresh	Refrigerated	Frozen	Open plastic bag in pickup	Sealed plastic bag in pickup	LSD (0.05)
Dry matter %	27.6a	27.7a	27.6a	71.5b	28.2a	13.9
Prussic Acid (ppm)	597a	551a	536a	109b	419c	90
Crude protein, %DM	17.6a	17.3a	17.5a	18.8b	17.6a	0.6
ADF. %DM	30.7a	30.9a	30.9a	32.7b	31.8c	0.8

Treatments: Fresh = delivered to lab on same day; Refrigerated = sample placed in refrigerator for 7 days; Frozen = sample placed in freezer for 7 days; Open plastic bag in pickup = sample placed in pickup for 7 days in unsealed plastic bag; Sealed plastic bag in pickup = sample placed in pickup for 7 days in sealed plastic bag.

Same letters within a row indicate values with no significant difference (P ≤ 0.05).

Source: Effect of Sampling Handling Method on Prussic Acid (Hydrocyanic Acid) Content of Forage Sorghum. Report of Progress 1086. http://www.ksre.ksu.edu/bookstore/pubs/SRP1086.pdf

For more information, see:

Prussic Acid Poisoning, Forage Facts MF-3040, K. Roozeboom, D. Mengel, D. Blasi, and J. Holman. http://www.ksre.ksu.edu/bookstore/pubs/MF3040.pdf

 Effect of Sampling Handling Method on Prussic Acid (Hydrocyanic Acid) Content of Forage Sorghum. J. Waggoner and J. Holman. Report of Progress 1086 Kansas Agric. Exp. Station pp. 30-31. http://www.ksre.ksu.edu/bookstore/pubs/SRP1086.pdf

Doo-Hong Min, Southwest Area Crops and Soils Specialist
dmin@ksu.edu

John Holman, Cropping Systems Agronomist, Southwest Research-Extension Center
jholman@ksu.edu

Justin Waggoner, Beef System Specialist, Southwest Research-Extension Center
 jwaggon@k-state.edu

Kraig Roozeboom, Cropping Systems Agronomist
Kraig@ksu.edu

Dave Mengel, Soil Fertility Specialist
Dmengel@ksu.edu

Dale Blasi, Extension Animal Science and Industries
dblasi@ksu.edu

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