Dr. Calvin Trostle, Extension Agronomy, TAMU Dept. of Soil & Crop Sciences, Lubbock, (806) 746-6101, firstname.lastname@example.org
A short version of this item originally appeared in the “Sorghum Insider,” an online newsletter of the Texas Grain Sorghum Association.
I have received several calls from growers in the past three weeks about prussic acid and nitrate accumulation in forages. These inquiries are routine in the fall starting in mid-October in the Texas High Plains as the first heavy frosts and freezes occur. Then questions progress downstate as low temperatures reach further into Texas.
Prussic acid becomes an issue at the first heavy frost/light freeze on anything in the sorghum family. This includes grain sorghum, sorghum/sudan, forage sorghum, redtop cane, hegari, sudangrass, and yes, even Johnsongrass. This releases plant compounds due to cell rupture that frees prussic acid, or cyanide (HCN). Properly cured (dried) forages should not have an issue with prussic acid. Prussic acid is mostly in the leaves, but larger stalks that can’t dissipate the prussic acid as quickly take more time to dry.
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 (Figs. 1A-B). 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 general advice on prussic acid is to avoid grazing for a minimum of seven days after a freeze event.
Is prussic acid that big a deal? I have heard cattle can handle it.
I have heard this too. You will not find this thinking in any state Extension literature. Here is the gist of this statement, which has perplexed me for 20+ years: When cattle graze a forage that has prussic acid potential—that is, there is not at the moment prussic acid but ‘cyanogenic glucosides’ (dhurri) reside in the plant tissue—the process of mastication (chewing) by ruminants releases some of these compounds to form prussic acid. This mimics the results of frost/freeze and prussic acid forms. At low levels, as toxic as this cyanide (or HCN, prussic acid) is, it is not a major health issue. However, when a heavy frost/freeze occurs large amounts of prussic acid are potentially released at once. These are in turn the levels that endanger animal health.
Figures 1A-B. Basal tiller regrowth in grain sorghum near harvest (A) and after harvest and mowing (B) of stalks. Young regrowth is susceptible to prussic acid development without a frost or freeze. Cattle released into a field with regrowth (A) are drawn to this fresh tender regrowth thus at higher risk.
I may not reflect this scenario accurately, but this was the common statement of Dr. Ted McCollum, former Texas A&M AgriLife Extension beef cattle specialist, Amarillo. I was long perplexed by Dr. McCollum’s statement because it is a fact: animals die from prussic acid poisoning.
Leonard Lauriault, long-time New Mexico State University forage agronomist, Tucumcari, generally agrees with Dr. McCollum’s thinking—to a point. That point is higher levels of prussic acid. See more from Mr. Lauriault below with regard to sampling for prussic acid. For some additional explanation of this angle on prussic acid see the United Sorghum Checkoff Program document at the end.
The bottom line with prussic acid remains: it IS a threat to animal health. There is no clear-cut view on how much prussic acid is safe. In the past some labs and animal scientists suggested prussic acid <200 ppm is probably OK to feed. But due to the inaccurate nature of sampling, transporting, and testing prussic acid the results of a prussic acid test have a significant level of uncertainty. Most labs now report the presence of prussic acid, and let you decide how to manage your crop or hay.
Does hybrid pearl millet develop prussic acid?
In general, no. Some literature suggests there is a very low potential for accumulation. However, in practice this is not the case. Years ago encountered a lab report that claimed prussic acid was high in a millet sample. The lab was dismissive of the well-known fact of this low potential in hybrid pearl millet. “Now we know,” they said. The sample should have been retested. Millet is not a member of the sorghum family and you and people you might sell millet hay to can assume that prussic acid is not an animal health issue.
Nitrate in Sorghums and Other 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) 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 Texas A&M AgriLife document “Nitrate and Prussic Acid in Forages” is at https://agrilifeextension.tamu.edu/library/ranching/nitrates-and-prussic-acid-in-forages/ (Fig. 2). This document discusses both prussic acid and nitrate, what field environmental and weather conditions can lead to a concern, and how to recognize them.
Fig. 2. Texas A&M AgriLife’s primary document for nitrate and prussic acid concerns in forages. This document discusses in further detail the issues in these two potentially toxic accumulations in forages, the factors that cause them, and how to manage them.
Testing for Prussic Acid and Nitrate in Forages
For prussic acid testing, I especially recommend the Texas Veterinary Medical Diagnostic Lab, https://tvmdl.tamu.edu/ This lab’s mandate is animal health. That includes what animals eat. I believe they have the best understanding of how to properly sample, transport, and submit samples for prussic acid poison testing. This is important because sampling, transport, and timing can significantly impact measured values. See “Cyanide and Nitrate in Forage” (Fig. 3) at https://tvmdl.tamu.edu/wp-content/uploads/2018/03/B.014_0519.pdf for sampling and shipping guidelines.
Fig. 3. Texas Veterinary Medical Diagnostic Lab’s primary document for how to properly collect forage samples for prussic acid and nitrate analyses.
Among the four TVMDL locations only the College Station lab performs the prussic acid test. Do not send your samples to Canyon, Center, or Gonzales. They will notify you to recollect a sample to send to College Station.
If it is nitrate only you need you can have that done any regional lab including the Texas A&M AgriLife soil, water, and forage lab in College Station. This test is straightforward and does not have the sampling and transport issues that prussic acid does.
For prussic acid samples you should collect samples mid-afternoon and drop locally and request overnight shipping for morning delivery. Samples to TVMDL will be run early afternoon the next day. Do not ship a sample on Friday. It won’t run until Monday. If you are close to College Station you can drive your sample to the lab.
TVMDL’s instructions in the above guide are specific to each of prussic acid and nitrate:
- Prussic acid–they prefer to test leaves only (from 8 to 15 stalks); see specific instructions if sampling standing forage or baled hay. The container you ship in is important.
- Nitrate—they prefer to test only the base of the stalk (from 8 to 15 stalks)
- Test both prussic and nitrate from the same sample?—TVMDL will use the leaves for prussic acid, the rest of the plant for nitrate (this could give a lower value of nitrate)
Bottom Line—Knowing how to best protect animal health
The above documents provide much more detail on prussic acid and nitrate in forages. Leonard Lauriault at NMSU-Tucumcari notes a grower actually might not need a prussic acid test. Why? “If the conditions are right for elevated and risky prussic acid potential from a frost or freeze, you must assume that prussic acid is present.” You can’t wait for a test. The cattle have to come off any sorghum family forage immediately. And you know you have to wait at least seven days before you could graze again. So maybe a test helps you feel better about your situation.
A case study—Snyder, TX, 2002
Prussic acid vs. nitrate and how these compounds occur can be confusing. About 2002 I was called to look into a situation near Snyder that resulted in 31 dead dairy replacement heifers. The veterinarian stated tests showed there was prussic acid in the rumen (was that from mastication only?). But the sorghum/sudan was very dry when baled according to the farmer. In fact, it remained in the windrow an extra 5 days to dry off the moisture from a shower. This did not make sense—Any prussic acid should have dissipated. I core sampled the bales with a Penn State hay sampler wondering if just possibly I could find traces of prussic acid in the bale. No prussic acid rather the samples were 0.7% and 1.2% nitrate. After more questions, I learned from the farmer there was a heavy infestation of pigweed on the north end of the field. Pigweed is a notorious accumulator of nitrate! One of those bales must have been fed Saturday morning. The heifers were found dead Sunday afternoon.
For additional reading—United Sorghum Checkoff Program Info./Dr. Brent Bean
USCP has a concise document that also summarizes well some complexities of prussic acid and nitrate in forages. Agronomy director Dr. Brent Bean dealt with these issues frequently during his distinguished career as Texas A&M Extension agronomist from Amarillo. See the following:
- Avoiding Prussic Acid (cyanide) and Nitrate Poisoning in Drought Stressed Sorghum in Livestock