The nitrate ion itself is relatively non toxic, it is when the nitrate is broken down in the digestive system that the trouble occurs. Nitrate becomes nitrite which becomes ammonia, which becomes a protein. It is the nitrite phase in the stomach(s) where some of the nitrite can enter the bloodstream where it changes into a chemical that reduces the bloods ability to carry oxygen. Oxygen starvation of the tissues occurs, along with a different process in which Prussic acid causes death of the tissues. Nitrate poisoning happens when animals eat grass, hay, or drink water that contain large amounts of the chemical. This can come from plants, or from fertilizers used for farming.
Nitrates occur in most plants and in water, but sometimes the levels get so high as to become toxic. Grazing animals are usually poisoned by forage and hay. Nitrate consumption of as little as 0.05% of the animal’s body weight may be lethal. Problems occur most often during cool temperatures, cloudy days or drought; also where heavy use of nitrate fertilizers occur. Nitrate formation increases when soil temperatures are 80-90°F.
Low light (cloudy days) and night time causes nitrates to build up in plants, it is dispersed when sunlight hits the plants. The accumulation of nitrates in plants is a natural process. The plant gathers nitrogen through its root system and then stores it as nitrate in the stems. This nitrogen is later converted to protein in the leaves. Horses can tolerate up to 0.50% nitrate in their total dry matter diet. A rule of thumb is to select hay for horses that contains no more than 0.65% nitrate ion on a dry matter basis. Poisoning happens quickly, animals may be found dead before symptoms are noticed.
Here is what to look for:
- Animal stays out of herd
- May collapse, fall in their tracks
- Unsteady gait
- Shallow and rapid breathing
- Rapid pulse
- Frequent urination
- Frothing at the mouth
- Death accompanied by muscular reflex movements
- White of eye, tongue, lips may have a blue-brown discoloration
- Blood is chocolate brown in color
- Pregnant animals may abort
Other issues include animals ingesting plants that are "borderline" in toxicity causing abortion, reduced milk flow, lower weight gains, vitamin A deficiency.
Plants to Watch Out For:
- Feeding rations high in carbohydrates will reduce and some times prevent losses from nitrate poisoning.
- Control weeds that accumulate nitrates. Freshly sprayed plants may become more palatable, so defer grazing of sprayed areas.
- During periods of cool or cloudy weather. avoid grazing a suspect area if possible. During periods of sunlight allow’ animals to eat large quantities of dry forage and then graze the area.
- Test the nitrate content of forage when in doubt.
- Distinguish nitrate poisoning from prussic acid poisoning or grass tetany, so the appropriate treatment may be administered.
Oat hay moistened with water can convert nitrates to nitrites in a short time.