Prussic acid is another name for cyanide. There is long-time concern about this animal health issue especially in sorghum family forages where prussic acid potential is much higher. Forthcoming information is changing and correcting our understanding of prussic acid and its animal injury potential. This may alter our advice on sorghums frosted and frozen in the fall.
A Brief Explanation of Prussic Acid (Cyanide, or HCN) in Sorghums
Sorghums in particular have much higher levels of a series of compounds called cyanogenic glycosides. These compounds contain a cyanide group (HCN) as part of a larger molecule. The specific cyanogenic glycosides in sorghum are collectively called ‘dhurrin.’ Dhurrin is higher in younger sorghum plants, and you may see advice not to graze until sorghums are at least 18 or even 24” tall. Dhurrin is much higher in leaves than in stalks. In the past the more common reason not to graze before 18-24” height was due to enabling plants to be better established and reach a more rapid growth stage that could better keep pace with grazing demand.
The cyanide group in dhurrin and other cyanogenic glycosides is not free cyanide. It is best regarded as potential cyanide, or HCN-p. The release of the HCN-p group is a process fostered by specific enzyme(s) generally known as beta glucosidase(s). When sorghum plant tissues are frozen cells are disrupted and the HCN-p in dhurrin comes in contact with beta-glucosidase. Free prussic acid, or cyanide, is produced. Ingestion of large amounts of sorghums at this point could be a threat to animal health.
However, the process of mastication by livestock can mimic the cell rupture of a freeze enabling contact of dhurrin compounds with the enzyme. Furthermore, for livestock, in the rumen there are also beta-glucosidases that contact dhurrin/HCN-p further releasing prussic acid to the free, toxic form. This is an animal health concern, especially for ruminants.
Historical Prussic Acid Advice vs. New Testing Observations
For a long time—decades—the traditional advice for managing livestock on sorghum forages has been:
- Remove cattle from grazing frosted and frozen sorghums to allow the potential or likely increase in prussic acid to dissipate (gas off) from the forage before reintroducing animals to the forage.
- For sorghums that have been cut for forage do not bale until the forage is fully “cured”, that is dried. This, too, allows any prussic acid to dissipate and thus animals can then be safely fed the forage now or in the future.
Former Texas A&M AgriLife beef cattle Extension specialist Dr. Ted McCollum, Amarillo, questioned this advice. The first few times he explained this to me (or tried to!), I was perplexed. What he said did not align with our Extension literature. Texas A&M AgriLife Extension and many other states said the same things about the dangers of prussic acid in cattle centered around the two above points.
Subsequent study of this earlier advice and their sources of information are now questioned. It appears much of the original information was inaccurate if not wrong. I credit Texas A&M AgriLife Extension beef cattle specialist Dr. Jason Banta, Overton, for leading a re-examination of this question in Texas. He is collaborating with like-minded researchers in other states including Kansas State University, to better understand prussic acid, its potential, and its actual presence in sorghum forages. AgriLife Extension agronomists Dr. Vanessa Corriher-Olson, Overton; Dr. Jourdan Bell, Amarillo; and Dr. Reagan Noland, San Angelo have assisted with this work.
Today there is a better understanding of prussic acid potential and the need to differentiate between HCN-p and free HCN to know if and when there is a threat to animal health. Researchers like Dr. Banta are careful to ensure they are properly documenting HCN, its forms, timing of free HCN and why, etc. before revising new guidelines.
Today’s Research Response to Above Bullet Points about Prussic Acid
Those investigating prussic acid potential, particularly in sorghums, note several points:
- Cattle that are already grazing on sorghums have likely built up some tolerance to free prussic acid. Ruminants can detoxify some low to perhaps moderate level of HCN. The question we don’t know is how much increase in free HCN is there when a freeze occurs? The dhurrin level in sorghums appears to remain unchanged after a freeze, but the free HCN can increase. How much is “safe” is unknown and probably risky to attempt quantify at this point.
- Dissipation of free HCN from heavily frosted/frozen sorghums and cut sorghums. Where did this thinking come from? It is in almost all Extension literature. Recent data suggests that free HCN does not dissipate or ‘gas off’ as has long been described.
Correct Test for Prussic Acid in Sorghums and other Forages
The correct test for prussic acid potential and free prussic acid, HCN, is a two-step test. It appears that historically many tests for prussic acid have bene incorrect—and misleading. They only conducted the first step: the measurement of free prussic acid. Yes, this is the immediate threat to animal health. I have noticed a change in 20+ years where labs would provide a specific number of prussic acid concentration in forages in parts per million (ppm) and a scale of what was safe to feed. This sometimes reflected whether animals were healthy or pregnant. These numbers did not agree among states. Today I see labs most likely stating, “presence of prussic acid” and thus issuing caution to forage and livestock operators.
However, Dr. Banta has noted that most labs do not perform the second step in chemical analysis. They only measure current free HCN. It is perhaps if not likely more important to measure prussic acid potential, HCN-p. The freezes, mastication, and ruminant digestion will suggest what level of this potential cyanide is released thus a threat to animal health.
Where to get the correct two-step prussic acid test?
Servi-Tech, Amarillo, TX (https://servitech.com/locations) is the only lab in Texas we currently know that performs the correct two-step test. The Texas Veterinary Medical Diagnostic Lab apparently does not conduct the second step. The Texas A&M AgriLife Soil, Water, and Forage Testing Lab in College Station does not offer the test.
Producers often want a nitrate test in forages at the same time as prussic acid. This is a routine analysis at any lab. This still requires differences in collection. Prussic acid is an issue in the leaves, which should be tested (not the whole plant). Nitrates are usually an issue in the bottom of the stalk. (If nitrates are not an issue in the lower stalk, then they are not an issue anywhere else.)
Additional Fall Prussic Acid Sources of Concern
Prussic acid can also be an issue in the new fresh growth at the base of the plant from a grain sorghum field near and after harvest. This can also occur from the base of sorghum/sudans for forage or hay. Also, the least-known potential issue with grazed sorghum/sudans, Johnsongrass, etc. is in the summer. Under drought conditions when the sorghum/sudan is struggling to grow, rains come, and the fresh growth can be hot.
The Bottom Line with Prussic Acid Remains for Now—Play it Safe
Prussic acid, in either form—free HCN or potential HCN-p that is not yet released from the cyanogenic glycoside dhurrin—IS a threat to animal health. Due to the inaccurate nature of sampling, transporting, and testing prussic acid the results of a prussic acid test have some level of uncertainty. Until further research better quantifies the risk of prussic acid and its potential then possibly revises safe grazing and feeding guidelines, livestock producers should be cautious and limit exposure of cattle especially to sorghum forages when there is a question of animal safety. This would still involve removing cattle from sorghums with a freeze though this may ultimately not be necessary. Proper testing—the two-step chemical analysis like ServiTech, Amarillo, TX uses—is encouraged.
Nitrate in Sorghums and Other Forages
Unlike prussic acid there are no changes in understanding of nitrate, where it accumulates, what levels are potential an animal health concern, and how to manage it. The AgriLife Extension guide below has the proper information for understanding, testing, and managing possible nitrate issues in all forages.
Nitrate accumulates in the bottom of the plant when they are not growing. Plants are still accumulating—but not assimilating—the N into plant proteins or other components. This nitrate concentrates in the bottom (up to 12 inches or so) of the stalk. When you mow hay, the nitrate level is fixed. It does not dissipate. When nitrate is high in a forage (near 1.0% and higher) for healthy animals, it can be blended with low nitrate forage. Or at hay harvest one can raise the cutter bar (if a swather) a few inches. This reduces yields but leaves a significant amount of nitrate in the field. Cattle can develop some tolerance to nitrate in forages over time.
Unlike prussic acid toxicity, nitrate issues are not unique to sorghum family forages. Other crops like corn, several small grains, hybrid pearl millet and several weeds (including pigweed/carelessweed/Palmer ameranth) also have potential nitrate issues. Nitrate poisoning from irrigated sorghum forages is rare. These plants are actively growing and assimilating uptake nitrate into plant structures.
The primary AgriLife document for nitrates (and also prussic acid, but information is changing) at Texas A&M AgriLife is “Nitrate and Prussic Acid in Forages” is at https://agrilifeextension.tamu.edu/library/ranching/nitrates-and-prussic-acid-in-forages/ (Fig. 4). This document discusses both prussic acid and nitrate, what field environmental and weather conditions can lead to a concern, and how to recognize them. All labs can conduct a nitrate test in forage samples.
This weekly agronomic Memo for Texas A&M AgriLife Extension county agents is compiled by Dr. Calvin Trostle, Professor & Extension Agronomist, Lubbock, (806) 777-0247 (mobile), [email protected] TAM-AAMM tips will be collected at http://_(to_be_determined) Permission is granted to AgriLife Extension personnel to use this information as you see fit for Extension education purposes (newsletters, web posting, social media, etc.).