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. Excessive applications or improper timing of commercial fertilizer or animal wastes can lead to excessive nitrate levels in grass pastures or the hay produced from it. High nitrate levels can also be caused by prolonged cloudy weather, droughts, or other plant stresses.
Nitrates interfere with an animal's ability to carry oxygen in the blood.
Excessive nitrates in forages can cause reduced feed consumption and growth rates, lowered milk production and abortions. In severe instances, death will occur. The most reliable symptom of nitrate toxicity is a chocolate brown coloration of the blood. Generally considered safe for all animals. These levels drastically decrease if an animal is pregnant.