Management recommendations common to both prussic acid and nitrates include:
• Test first, don’t gamble. Keep in mind, different labs use different tests that have different scales.
• Feed animals with a known safe feedstuff(s) and have them full before introduction to potentially problematic feeds. Don’t turn in hungry.
• Ensiling will reduce concentrations of either by 40-60% in well-made silage, but silage put up under less-than-optimal conditions could still contain very high levels. If extremely high before ensiling, a 50% reduction may not be enough to result in safe feed.
• Dhurrin concentrates in the upper portion of the plant and with more plant growth (>24”), concentration levels may be diluted if measuring the whole plant.
• Nitrate concentrates in the base of the plant and is least in head and leaves, grazing or cutting high can reduce nitrate levels in the forage.
• Do not harvest drought stressed forage within 7 to 14 days after good rainfall to reduce the levels of accumulated nitrates.
If testing before grazing, samples should reflect what the animals are expected to consume, generally leaves and upper portion of the plant. Sample a minimum of 15 sites across a given field. One method is to sample from each corner and the center by walking diagonal lines and sample plants every 50-100 steps or as appropriate for field size.
We expect levels of nitrates and prussic acid to be variable across a field, so more samples are better than less. A rule of thumb is to sample 10 to 20 % of the bales per field or cutting as a minimum. Be aware of areas of the field that exhibited more plant stress than others. If large enough areas, you may want to sample them separately. Your acreage size and feeding methods likely factor into this decision. Use a forage probe that cuts across all plant parts in a bale rather than a grab sample from individual bales or windrows. The Cottonwood extension offices can help with sampling procedures and equipment.
Prussic acid in sorghum following a freeze event
Frost causes plant cells to rupture and prussic acid gas forms in the process. Because the prussic acid is in a gaseous state, it will gradually dissipate as the frosted/frozen tissues dry. Thus, risks are highest when grazing frosted sorghums and sudangrasses that are still green. New growth of sorghum species following frost can be dangerously high in prussic acid due to its young stage of growth. Prussic acid content decreases dramatically during the hay drying process and during ensiling. It is recommended to wait ten days until after a killing freeze before grazing. Sorghum and sudangrass forage that has undergone silage fermentation is generally safe to feed.
For more complete information on these problems see these publications Nitrate Toxicity, Prussic Acid Poisoning, and Managing the Prussic Acid Hazard in Sorghum. If you have samples with high prussic acid concentrations, and are willing to share information on variety, growth, fertility, and harvest conditions, it will be helpful as we strive to better understand this issue.
Alicia Boor is the Agriculture and Natural Resources agent with K-State Research and Extension – Cottonwood District. Contact her by email at aboor@ksu.eduor call 620-793-1910.
