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Nitrate Toxicity Again a Concern in 1999

Livestock Update, September 1999

Mark Wahlberg, Extension Livestock Specialist, Virginia Tech

Nitrate is accumulated in plants that undergo such stress that they cannot convert the nitrogen taken up from the soil into plant protein. It can also occur when the plant is provided such an abundance of available nitrogen that its own metabolism is overwhelmed with the supply. This second situation is not common and is seen only when forages are overfertilized due to application errors, equipment malfunctions, or overapplying liquid manures.

The typical stress causing nitrate toxicity is drought. Crops often found with high nitrate levels are corn, sorghum, sudangrass, and millet because these plants are normally fertilized well with nitrogen. However, nitrate toxicity occurs in perennial grasses and in some fast-growing summer weeds.

When the animal consumes excess nitrate, it is converted to nitrite, which is then absorbed across the rumen wall into the bloodstream. Nitrite takes the place of oxygen in the hemoglobin molecule, which is the compound that carries oxygen throughout the body. Therefore, the animal becomes oxygen-starved. Symptoms include rapid breathing, unthriftiness, and poor appetite. Nitrate toxicity can cause abortions and offspring at birth that are weak. The blood of a nitrate poisoned animal is dark brown as opposed to the bright red of a normal animal.

Nitrate accumulates in the lower portion of a plant. It drops to a normal level in plants fairly soon after they have resumed normal growth. Three to five days is often long enough. Ensiling is a good way to reduce nitrate in a feed because the microbial activity occurring during fermentation converts nitrate to safe compounds. Hay-making does not reduce nitrate levels, though.

Under the current conditions in late summer of 1999, the highest risk will come within the first 3 to5 days following a big rain. Unused nitrogen in the soil is quickly taken up by the plants while the plants have not fully returned to normal growth. Thus, the nitrate accumulates. After several days this nitrate is converted to a safe compound. Be extra cautious of pasture at this time. Harvesting of crops likely to have accumulated nitrate for silage or hay should be delayed following such a rainfall.

Feeds thought to be suspect should be sampled and analyzed for nitrate. Many labs, including the Virginia Tech Forage Testing Lab, do this work. Fresh or moist samples should be delivered promptly to the lab to reduce the chance of nitrate loss or conversion. If this cannot be done, samples should be sealed in plastic bags and frozen before shipping.

The table below gives nitrate values for feeds and relative safety as feed for livestock.

Nitrate Ion, % Nitrate Nitrogen, ppm Feeding Recommdendation
0-0.44 <1000 Safe under all conditions
0.44-0.66 1000-1500 Safe for Non Pregnant.
Max 50% of DM for Pregnant Animals
0.66-0.88 1500-2000 Max 50% of DM
0.88-1.54 2000-3500 Max 40% of DM.
Not for Pregnant Animals
1.54-1.76 3500-4000 Max 25% of DM.
Not for Pregnant Animals
Over 1.76 >4000 Do Not Feed

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