Crops such as oat hay, sorghum, corn, sudangrass, Johnsongrass, beets, and weeds such as carelessweed, kochia, pigweed, Russian thistle, and nightshade, are examples of plants that accumulate nitrate. There are many more. Plants containing more than 1.5 percent nitrate (as KNO3) dry weight may be lethal to livestock. Sublethal effects may occur in livestock from eating feed containing between 0.5 and 1.5 percent nitrate. Nitrate poisoning can also occur in animals that eat nitrate fertilizers, machine oil, and some natural well and pond waters.
Plants differ in their ability to accumulate nitrate. The type of soil, availability and the form of nitrogen present in the soil, various environment factors and chemical or physical plant damage influence the amount of nitrate in many of these plants. For example drought conditions, frost, or treatment of nitrate-accumulating plants with 2,4-D may cause plants to accumulate excessive amounts of nitrate. Nitrate accumulates primarily in the vegetative tissue of plants while the seed remains safe.
Where and When Nitrate Poisoning Occurs
Nitrate poisoning occurs throughout the United States, primarily in animals that have eaten nitrate-accumulating plants. Harvested and stored forages continue to be toxic.
How It Affects Livestock
Nitrates are converted to nitrite in the gastrointestinal tract. Nitrite causes the production of methemoglobin, a type of hemoglobin that cannot carry oxygen. Thus, the effects of nitrate poisoning result largely from oxygen starvation or, in effect, suffocation.
The amount of plant material required to poison an animal depends on the amount of nitrate in the plant and, to a lesser degree, on the rate at which the plant is eaten. Many factors affect toxicity, but in general about 0.05 percent of an animal's weight of nitrate is near a minimum lethal dose. Poisoning occurs primarily in ruminants.
Signs and Lesions of Poisoning
- Blue coloration of membranes of mouth, eyes, and other mucous membranes (cyanosis)
- Shortness of breath
- Staggering gait
- Chocolate brown blood
- Muddy, cyanotic mucus membranes
- Congestion of rumen and abomasum
- Watering eyes
- Unthrifty appearance
- Reduced milk flow
- Reduced weight gain
- Abortion and infertility
How to Reduce Losses
Crops that accumulate nitrate and grow under conditions favoring nitrate accumulation should be checked for nitrate content. High nitrate forage should be diluted with low nitrate forage to decrease nitrate intake, or if there are indications of nitrate toxicity, the feeding of nitrate-accumulating forage should be discontinued. Signs of vitamin A deficiency have been associated with the feeding of low levels of nitrate suggesting vitamin supplements may be useful.