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. Practically all plants contain detectable amounts of nitrates. Excessive nitrate accumulation occurs when the uptake of nitrate exceeds its utilization in plants for protein synthesis.
Plant species vary in their ability to accumulate nitrate.
Even common barnyard weeds can cause problems. Stalks are highest in nitrate content, followed in order by leaves and grain in decreasing amounts. Hungry livestock are not as likely to selectively graze leaves over stems so allow them to feed before releasing them on a field. Always test fields of concern prior to releasing livestock on them. Any weather condition which reduces plant growth may increase nitrate accumulation.