Under certain adverse environmental conditions (drought) many weed and crop plants accumulate nitrate to potentially toxic concentrations. Nitrate-accumulating weeds include pigweed (Amaranthus spp.), lambsquarter (Chenopodium spp.), dock (Rumex spp.) and nightshades (Solanum spp.). Potentially troublesome crop plants include corn, sorghum, oats, barley, beet tops and wheat.
Nitrate is also found in fertilizers and is a common contaminant of water. Thus, exposure to these sources can cause intoxication if exposure is of sufficient
- Toxic principle
- nitrate. Nitrate is reduced in the rumen to nitrite which is the ultimate toxin.
NO3 → NO2
- as a salt, nitrate is toxic for ruminants at 0.5 g/kg (single oral dose). Forages containing > 0.2% nitrate and water containing > 1000 ppm are potentially toxic. Plants can accumulate 3 to 4% nitrate under appropriate conditions. Nitrate is not very toxic for monogastrics since it is not efficiently reduced to nitrite. However, nitrite is toxic for monogastrics. Unlike cyanide, nitrate does not volatilize and therefore dried forages are toxic.
- the iron in hemoglobin is oxidized from ferrous to ferric iron. This results in the formation of methemoglobin. Methemoglobin has significantly reduced oxygen carrying capacity.
- Clinical signs
- dyspnea, sudden death, “muddy” mucous membranes, “brownish” appearance to blood.
- Laboratory diagnosis
- significant methemoglobin, high serum, ocular fluid or other body fluid nitrate concentration (> 20 ppm in serum or body fluids, > 50 ppm in ocular fluid). Measurement of high levels of nitrate in plants or water.
- “brownish” discoloration to blood, muscles
- directed at reducing methemoglobin to hemoglobin ü Remove from source ü A 1% solution of methylene blue is generally given at a dose of 4 to 15 mg/kg at 4 to 6 hour intervals. Methylene blue is reduced to leukomethylene blue, which in turn reduces methemoglobin.
- Test forage prior to feeding. Ensiling high nitrate forage may lower nitrate concentrations to acceptable levels.