This article was written by Jim Krantz, former SDSU Extension Cow/Calf Field Specialist.
Nitrate and prussic acid poisoning both cause a type of asphyxiation in livestock and the clinical signs that distinguish them are very similar. Both result from the consumption of forages that have been stressed by conditions such as drought or freezing, particularly sorghums, sudangrasses, sorghum-sudangrass mixtures and small grains such as oats.
In both conditions, livestock exhibit quickened respiration rates, difficult breathing, muscle tremors, and visible staggering, all of which result in the collapse of the animal. Death is typically sudden in either poisoning. There are, however, a couple of distinguishing characteristics that define the correct diagnosis in each condition:
- With nitrate poisoning, the tongue and eyes turn a definite blue.
- Ultimately, the one distinguishing characteristic of nitrate and prussic acid poisoning is the color of the blood of the affected animals: With nitrate poisoning, the color of the blood is a dark, chocolate brown while in the case of prussic acid poisoning, the blood will be a bright cherry red color.
Soil fertility may also play a role as a cause of these conditions.
Prussic acid content in plants tends to be elevated by soils high in nitrogen and low in phosphorous. Nitrate levels in the soil appear to be highly correlated to resulting high levels of nitrates in plants. Without this availability, high levels of nitrates in plants is virtually an impossibility. However, high soil fertility levels in themselves do not cause nitrate poisoning. There must be a disruption in the normal growth of the plant which results in the onset of nitrate poisoning. In South Dakota, that disruption is typically drought, but could be freezing temperatures.
The danger of nitrate poisoning from any forage can be diminished by providing low-nitrate feeds as an alternative to a complete diet of potentially high-nitrate forages. Since the lower portions of the
plants contain the highest levels of nitrates, it is recommended that livestock not be forced to consume that portion of the plant when the leaf portions have been consumed.
Quick nitrate test kits are available through your Extension Service that will reveal the presence or absence of nitrates in forages. For more information on Forage Nitrate Poisoning, please see Forage Nitrate Poisoning.