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Animal toxicity from "hydrocyanosis" or "prussic acid poisoning" can be a concern when harvesting sorghums as a forage source. Sudans, weedy sorghums, and other plants also have the potential to cause prussic acid toxicity. Crop File 6.05.031 “What is Prussic Acid Toxicity?” discusses the mechanism in more detail.
Samples should not be dried, heated, or frozen before they are submitted to the Laboratory for analysis. Otherwise, HCN may be lost and the sample results will not represent the field conditions.
Place samples in a zipper-lock plastic bag for shipping.
Seal the bag and deliver to the nearest Servi-Tech
Laboratory by the fastest means available. Refrigerate - but do not freeze - samples if they have to be held and cannot be delivered immediately.
Ensiled forages are not likely to contain high HCN levels. Much of the HCN will be lost by heating that occurs during the fermentation process, but this can result in forming highly toxic gases in confined silos and storage structures.
Collecting a random sample is important, because HCN content can be greatly affected by plant size and time of day.
Collect samples in late morning or early afternoon, if feasible. Glycoside levels increase during the morning, peak and level off during midday, then begin to decline in the afternoon and evening.
Collect samples of leaf tissue rather than stems. Leaves may contain two to 25 times more HCN than stems. Livestock are also more likely to eat leaves before they eat stems.
One method to collect random samples is to walk a transect across the field, diagonally to the row direction. Chose a random number, like “17" or “23".
Start walking and count the number of paces or rows, then sample the plant immediately ahead at that numbered pace or row (e.g., 17th row or 23rd pace). Proceed along the transect the same distance, then sample the next plant, etc., etc. until a total of 15 to 20 plants have been sampled.
Sample size is important. Collecting samples from fewer than 15 to 20 plants may not accurately represent field conditions.
Use a core sampler to collect samples from at least 20 bales that have come from a single field. Collect samples from 20% of the bales if the field yielded more than 100 bales.
Core as deeply as the sampling tool allows. The first couple of inches of the bale surface will be weathered by exposure to the elements and may not accurately reflect the condition of the inner portion of the bale.
Servi-Tech laboratory results are expressed as "ppm HCN" ("parts per million hydrogen cyanide") on a dry matter basis to rate the relative toxicity hazard for livestock. Following are the analytical ranges and interpretations used by the ServiTech Laboratory to estimate the potential for livestock toxicity symptoms to occur.
¶ Table 1. Interpreting Prussic Acid Analysis Results |
|
| ppm HCN (dry matter basis) |
Relative rating |
| under 300 | Very low |
| 300 - 600 | Low |
| 600 - 1200 | Medium |
| 1200 - 1800 | High |
| over 1800 | Very high |
Following are general interpretations assuming the forage will be fed to mature, healthy livestock in good condition. Younger animals or those in poor condition could show toxicity symptoms at lower HCN levels. Crop File 6.05.033, “Managing High Prussic Acid (HCN) Forages” provides some feeding management suggestions.
(under 300 ppm HCN):
Forage should not cause prussic acid poisoning. Considered safe for all classes of livestock.
(300 - 600 ppm HCN):
Forage not likely to cause prussic acid poisoning. Considered safe to graze, green chop, bale, or ensile under most conditions.
(600 - 1200 ppm HCN):
Forage may be potentially toxic if used in a grazing or green chop program. Suggest feeding at a restricted rate. Harvesting by baling as sun-cured hay or by ensiling are better choices because they reduce the HCN toxicity hazard. Allowing plants to mature will help lower HCN levels. Retest forage before feeding.
(1200 - 1800 ppm):
Forage can be potentially toxic if used in a grazing or green chop program, so should be fed at a restricted rate. Harvesting by baling as sun-cured hay or by ensiling are better choices because they reduce the HCN toxicity hazard. Allowing plants to mature will help lower HCN levels. Retest forage before feeding.
(over 1800 ppm HCN):
Forage is potentially very toxic if used in a grazing or green chop program. Feed at very restricted rate, if at all. Harvesting by baling as sun-cured hay or by ensiling are better choices because they reduce the HCN toxicity hazard. Allowing plants to mature may help reduce HCN levels. Retest forage before feeding.
Thiex, N.J. Prussic acid poisoning. South Dakota State Univ., Coop. Ext. Serv. Bull. FS-805.
Fjell, D., D Blasi, and G. Towne. 1991. Nitrate and Prussic Acid Toxicity in Forage: Causes, Prevention, and Feeding Management. MF-1018. Agronomy and Animal Science Extension, Kansas State Univ., Manhattan. 4 pg
Stanton, T.L. and J. Whittier. 2006. Prussic acid poisoning. Fact Sheet No. 1.612. Colorado State University Extension, Fort Collins CO. 2 pg