Getting an accurate prussic acid measurement
Is there an SOP for proper handling forage samples?
By Jason M. Warner
Forage sorghum varieties are commonly grown each year throughout the United States as they are typically regarded as a source of excellent yielding, high quality forage. One inherent challenge with forage sorghums is the production of prussic acid. Prussic acid may also be referred to as hydrocyanic acid or hydrogen cyanide and may be toxic to beef cattle depending on the level in the diet. Sorghums, sudangrasses, sorghum-sudangrass hybrids and other related species contain dhurrin, a prussic acid precursor, within their epidermal cells which produces prussic acid and the concentrations of which are generally highest in young, rapidly growing plants, stressed plants, or in regrowth following a frost or freeze.
To guide management decisions, forage samples can be analyzed for prussic acid content. However, accurately measuring prussic acid in forage samples is challenging because the compound is volatile and thus will dissipate from plant cells as the tissue dries. This raises questions as to proper handling of forage samples taken for prussic acid analysis as the methodology followed may greatly impact analytical results and only limited information is available to guide handling procedures.
To better understand how the process with which forage samples are handled prior to analysis influences prussic acid levels within samples, a field-level research trial was conducted in western Kansas by faculty at Kansas State University (Waggoner and Holman, 2013; SRP1086 Roundup 2013) [1].
For this trial, a single representative sample of forage sorghum was collected from a field in southwest Kansas prior to being harvested in early October. The sample was collected from standing forage sorghum within approximately 24 hours of a killing freeze, cut to a particle length of ≤ 3”, adequately mixed, and sub-divided into individual plastic bags. Individual bags were subsequently assigned to one of five handling method treatments including:
Fresh sample delivered to the lab in a sealed plastic bag the same day as harvest (FRESH)
Stored in refrigeration for seven days in a sealed plastic bag prior to analysis (REFR)
Stored frozen for seven days in a sealed plastic bag prior to analysis (FRZR
Placed on a pickup dash in an open (unsealed) plastic bag for seven days prior to analysis (PUOPEN)
Placed on a pickup dash in a sealed plastic bag for seven days prior to analysis (PUSEAL).
The different treatments were designed to simulate various ways that samples may be handled in typical production settings. Following the predetermined period of storage length, all samples were submitted to a commercial analytical laboratory for analysis of dry matter, prussic acid, crude protein, and acid detergent fiber concentration.
All nutrient components analyzed were impacted by the method in which samples were handled prior to analysis (Table 1). Prussic acid was not different among FRESH, REFR, and FRZR samples averaging approximately 560 ppm. However, prussic acid content was decreased in samples that were stored in a sealed bag in a pickup for seven days (419 ppm) and decreased even further (109 ppm) when stored similarly in an unsealed bag. As the sample was harvested freshly from the field, dry matter was not different among freshly submitted, refrigerated and frozen samples as well as samples stored in a pickup that were sealed. As expected, samples that were stored in an unsealed bag in a pickup had greater dry matter levels due to evaporation of moisture from the sample. Likewise, relatively minor changes were observed for crude protein and acid detergent fiber levels in samples that were not submitted for fresh analysis and not stored at reduced temperature. The marked decrease in prussic acid levels in samples improperly stored following collection suggests how volatile prussic acid is and illustrates the importance of sample handling prior to analysis. In the case of prussic acid, levels may be underestimated if samples are not handled correctly and data from this project also suggest that freezing for up to 7 days before analysis does not influence prussic acid levels provided samples
[1} Waggoner, J. W. and J. D. Holman, Effects of Sample Handling Method on Prussic Acid (Hydrocyanic Acid) Content of Forage Sorghum, Roundup 2013, Kansas State University, April 2013. Contribution no. 13-247-S from the Kansas Agricultural Experiment Station.
Warner is an Extension cow-calf specialist in the Department of Animal Sciences and Industry at Kansas State University.
