Plant Factors in Nitrate Accumulation

Nitrate Poisoning Farmers and ranchers depend on the successful combination of livestock and crops. Forage crops, in particular, are important to the producer, but they should be monitored due to plant toxicants that can be a problem. One toxicosis of concern is Nitrate (N03 ) toxicity. Nitrate toxicity of cattle was noted as early as 1895 with corn-stalk poisoning. However, at that time nitrate was not recognized as the principle toxicant. In the late 1930s, after an outbreak of oat-hay poisoning in the high plains region, an indictment of nitrate was finally made. The term “Nitrate Toxicity” should actually be “Nitrite Toxicity.” When nitrate is ingested by a ruminant animal, it undergoes a chemical reduction action to nitrite. This reduction is accomplished by rumen microorganisms. Nitrite is readily absorbed into the bloodstream where it oxidizes the ferrous iron of the red blood pigment hemoglobin to ferric iron producing a modified red blood pigment called methemoglobin. Methemoglobin is incapable of transporting oxygen to various body tissues so the animal exhibits a characteristic chocolate brown blood color prior to and during death which is caused by asphyxiation. Ruminant animals affected in this manner by high nitrate feeds are said to be suffering from methemoglobinemia. Simple stomached animals such as swine and poultry do not have the microorganisms which can make this rapid conversion and are not as susceptible to nitrate toxicity. The environmental scenarios that enhance accumulation of nitrates are similar to those of prussic acid, see OSU Fact Sheet PSS-2904, “Prussic Acid Poisoning in Livestock.” However, the location of the toxicant within a plant is different. Therefore, it is necessary to be familiar with plant factors, livestock factors, and management practices to safely utilize forages that have the potential of this toxicosis.