{"id":2208,"date":"2021-11-08T20:55:54","date_gmt":"2021-11-08T20:55:54","guid":{"rendered":"https:\/\/agrilife.org\/texasrowcrops\/?p=2208"},"modified":"2021-11-08T20:55:54","modified_gmt":"2021-11-08T20:55:54","slug":"prussic-acid-and-nitrate-in-forages-especially-sorghums","status":"publish","type":"post","link":"https:\/\/agrilife.org\/texasrowcrops\/2021\/11\/08\/prussic-acid-and-nitrate-in-forages-especially-sorghums\/","title":{"rendered":"Prussic Acid and Nitrate in Forages, Especially Sorghums"},"content":{"rendered":"

Dr. Calvin Trostle, Extension Agronomy, TAMU Dept. of Soil & Crop Sciences, Lubbock, (806) 746-6101, ctrostle@ag.tamu.edu<\/a><\/p>\n

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A short version of this item originally appeared in the \u201cSorghum Insider,\u201d an online newsletter of the Texas Grain Sorghum Association.<\/em><\/p>\n

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I have received several calls from growers in the past three weeks about prussic acid and nitrate accumulation in forages.\u00a0 These inquiries are routine in the fall starting in mid-October in the Texas High Plains as the first heavy frosts and freezes occur.\u00a0 Then questions progress downstate as low temperatures reach further into Texas.<\/p>\n

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Prussic acid becomes an issue at the first heavy frost\/light freeze on anything in the sorghum family.\u00a0 This includes grain sorghum, sorghum\/sudan, forage sorghum, redtop cane, hegari, sudangrass, and yes, even Johnsongrass.\u00a0 This releases plant compounds due to cell rupture that frees prussic acid, or cyanide (HCN).\u00a0 Properly cured (dried) forages should not have an issue with prussic acid.\u00a0 Prussic acid is mostly in the leaves, but larger stalks that can\u2019t dissipate the prussic acid as quickly take more time to dry.<\/p>\n

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Prussic acid can also be an issue<\/u> in the new fresh growth at the base of the plant from a grain sorghum field near and after harvest (Figs. 1A-B).\u00a0 This can also occur from the base of sorghum\/sudans for forage or hay.\u00a0 Also, the least-known potential issue<\/u> with grazed sorghum\/sudans, Johnsongrass, etc. is in the summer.\u00a0 Under drought conditions when the sorghum\/sudan is struggling to grow, rains come, and the fresh growth can be hot.<\/p>\n

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The general advice on prussic acid is to avoid grazing for a minimum of seven days after a freeze event.<\/p>\n

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Is prussic acid that big a deal?\u00a0 I have heard cattle can handle it.<\/em><\/p>\n

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I have heard this too.\u00a0 You will not find this thinking in any state Extension literature.\u00a0 Here is the gist of this statement, which has perplexed me for 20+ years:\u00a0 When cattle graze a forage that has prussic acid potential\u2014that is, there is not at the moment prussic acid but \u2018cyanogenic glucosides\u2019 (dhurri) reside in the plant tissue\u2014the process of mastication (chewing) by ruminants releases some of these compounds to form prussic acid.\u00a0 This mimics the results of frost\/freeze and prussic acid forms.\u00a0 At low levels, as toxic as this cyanide (or HCN, prussic acid) is, it is not a major health issue.\u00a0 However, when a heavy frost\/freeze occurs large amounts<\/u> of prussic acid are potentially released at once.\u00a0 These are in turn the levels that endanger animal health.<\/p>\n

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Figures 1A-B<\/strong>.\u00a0 Basal tiller regrowth in grain sorghum near harvest (A) and after harvest and mowing (B) of stalks.\u00a0 Young regrowth is susceptible to prussic acid development without a frost or freeze.\u00a0 Cattle released into a field with regrowth (A) are drawn to this fresh tender regrowth thus at higher risk.<\/p>\n

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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.\u00a0 I was long perplexed by Dr. McCollum\u2019s statement because it is a fact:\u00a0 animals die from prussic acid poisoning.<\/p>\n

Leonard Lauriault, long-time New Mexico State University forage agronomist, Tucumcari, generally agrees with Dr. McCollum\u2019s thinking\u2014to a point.\u00a0 That point is higher levels of prussic acid.\u00a0 See more from Mr. Lauriault below with regard to sampling for prussic acid.\u00a0 For some additional explanation of this angle on prussic acid see the United Sorghum Checkoff Program document at the end.<\/p>\n

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The bottom line with prussic acid remains<\/u>:\u00a0 it IS a threat to animal health.\u00a0 There is no clear-cut view on how much prussic acid is safe.\u00a0 In the past some labs and animal scientists suggested prussic acid <200 ppm is probably OK to feed.\u00a0 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.\u00a0 Most labs now report the presence<\/em> of prussic acid, and let you decide how to manage your crop or hay.<\/p>\n

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Does hybrid pearl millet develop prussic acid?<\/em><\/p>\n

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In general, no.\u00a0 Some literature suggests there is a very low potential for accumulation.\u00a0 However, in practice this is not the case.\u00a0 Years ago encountered a lab report that claimed prussic acid was high in a millet sample.\u00a0 The lab was dismissive of the well-known fact of this low potential in hybrid pearl millet.\u00a0 \u201cNow we know,\u201d they said.\u00a0 The sample should have been retested.\u00a0 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.<\/p>\n

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Nitrate in Sorghums and Other Forages<\/em><\/strong><\/p>\n

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Nitrate accumulates in the bottom of the plant when they are not growing. \u00a0Plants are still accumulating\u2014but not assimilating\u2014the N into plant proteins or other components. \u00a0This nitrate concentrates in the bottom (up to 12 inches or so) the stalk. \u00a0When you mow hay, the nitrate level is fixed. \u00a0It does not dissipate. \u00a0When nitrate is high in a forage (near 1.0% and higher) for healthy animals, it can be blended with low nitrate forage.\u00a0 Or at hay harvest one can raise the cutter bar (if a swather) a few inches. \u00a0This reduces yields but leaves a significant amount of nitrate in the field. \u00a0Cattle can develop some tolerance to nitrate in forages over time.<\/p>\n

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Unlike prussic acid toxicity, nitrate issues are not unique to sorghum family forages<\/u>.\u00a0 Other crops like corn, several small grains, hybrid pearl millet and several weeds (including pigweed\/carelessweed\/Palmer ameranth) also have potential nitrate issues.\u00a0 Nitrate poisoning from irrigated sorghum forages is rare. \u00a0These plants are actively growing and assimilating uptake nitrate into plant structures.<\/p>\n

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The primary Texas A&M AgriLife document \u201cNitrate and Prussic Acid in Forages\u201d is at https:\/\/agrilifeextension.tamu.edu\/library\/ranching\/nitrates-and-prussic-acid-in-forages\/<\/a>\u00a0 (Fig. 2).\u00a0 This document discusses both prussic acid and nitrate, what field environmental and weather conditions can lead to a concern, and how to recognize them.<\/p>\n

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Fig. 2<\/strong>.\u00a0 Texas A&M AgriLife\u2019s primary document for nitrate and prussic acid concerns in forages.\u00a0 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.<\/p>\n

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Testing for Prussic Acid and Nitrate in Forages<\/em><\/strong><\/p>\n

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For prussic acid testing, I especially recommend the Texas Veterinary Medical Diagnostic Lab, https:\/\/tvmdl.tamu.edu\/<\/a> \u00a0This lab\u2019s mandate is animal health.\u00a0 That includes what animals eat. \u00a0I believe they have the best understanding of how to properly sample, transport, and submit samples for prussic acid poison testing. \u00a0This is important because sampling, transport, and timing can significantly impact measured values.\u00a0 See \u201cCyanide and Nitrate in Forage\u201d (Fig. 3) at https:\/\/tvmdl.tamu.edu\/wp-content\/uploads\/2018\/03\/B.014_0519.pdf<\/a> for sampling and shipping guidelines.<\/p>\n

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Fig. 3<\/strong>.\u00a0 Texas Veterinary Medical Diagnostic Lab\u2019s primary document for how to properly collect forage samples for prussic acid and nitrate analyses.<\/p>\n

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Among the four TVMDL locations only the College Station lab performs the prussic acid test<\/u>.\u00a0 Do not send your samples to Canyon, Center, or Gonzales.\u00a0 They will notify you to recollect a sample to send to College Station.<\/p>\n

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If it is nitrate only<\/u> you need you can have that done any regional lab including the Texas A&M AgriLife soil, water, and forage lab in College Station.\u00a0 This test is straightforward and does not have the sampling and transport issues that prussic acid does.<\/p>\n

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For prussic acid samples you should collect samples mid-afternoon and drop locally and request overnight shipping for morning delivery.\u00a0 Samples to TVMDL will be run early afternoon the next day. \u00a0Do not ship a sample on Friday.\u00a0 It won\u2019t run until Monday. \u00a0If you are close to College Station you can drive your sample to the lab.<\/p>\n

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TVMDL\u2019s instructions in the above guide are specific to each of prussic acid and nitrate:<\/p>\n

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