Nitrate poisoning is a problem that all horse and livestock owners in eastern North Carolina should be aware of. Forages can be tested for nitrate content at no cost and it is recommended that ALL forages be tested before being fed to horses and livestock. You can’t determine the nitrate content of forage just by looking at it.
How do high nitrates occur?

Under normal growing conditions, plants take up nitrogen from the soil, which is then stored as nitrates in the plant. The plant will later convert this nitrate to protein. Any type of stress to the plant, including drought or prolonged cloudy weather, can interrupt this process. The plant may continue to store the nitrates, rather than convert them to protein, and the nitrates will accumulate within the plant.

Are forages that are “sprayed on” (i.e., fertilized with animal waste) more likely to have high nitrates?

No. Any forage can accumulate nitrates under the right conditions. The type of fertilizer used has no effect on whether the hay will be high in nitrates. Excessive fertilization or the improper timing of fertilization does affect whether the forage will be high in nitrates. Environmental conditions that may stress the plants, such as drought, can also play a role.

How Do Nitrates Affect Livestock & Horses?

Nitrates interfere with an animal’s ability to carry oxygen in the blood. High nitrates in forages can cause reduced feed consumption and growth rates, lowered milk production, and abortions. If nitrates reach dangerously high levels, it can cause death.

What are the Signs of Nitrate Poisoning?

Nitrate-poisoned animals show symptoms of suffocation, including labored breathing, lack of coordination, and blue mucous membranes. The most reliable symptom of nitrate toxicity is a chocolate brown coloration of the blood. Other signs include: diarrhea, frequent urination, and frothing at the mouth.

What Nitrate Levels are Safe for Cattle, Sheep, and Goats?

If these animals are gradually introduced to nitrates, they can adapt to tolerate higher levels of
nitrates in the diet. The following table shows the levels of nitrates that can be tolerated in the diet and how those forages should be used.
Management considerations for use in feeding forages with various levels of nitrate:

Level in Forage
(Dry Basis)
Nitrate Ion %

Feeding Precautions
Unadapted Animals

Adapted Animals

0.00 – 0.25
0.26 – 0.50

0.51 – 1.00

1.01 – 1.50

1.51 – 2.00

2.01 – 2.50

2.51 and up

Safe: Generally considered safe.
Slight Risk: Should not make up
more than 50% of total intake for
pregnant ruminants.

Moderate Risk: Do not feed to pregnant
ruminants. Limit to less than 50% total
intake for all others.

High Risk: Exercise extreme caution when
feeding. Limit to 33% of the ration.

Severe Risk: Do not feed to any animals free choice. If using in a mixed ration, limit to 25% of the ration.

Extreme Risk: Do not feed at all.

Extreme Risk: Do not feed at all.

Safe
Safe

Slight Risk

Moderate Risk

High Risk

Severe Risk

Extreme Risk

What Levels of Nitrates are Recommended for Horses?

Forage that tests no higher than 0.50% nitrate is safe for horses. Generally, it’s best not to choose hay for horses that tests higher than 0.65% nitrate. Higher nitrate hays will need to be “diluted” so that the total nitrate in the diet is at a safe level. The Table below shows methods of diluting high nitrate hay in the horse’s diet.

Methods of adjusting the nitrate content of rations when feeding forage high in nitrates

Nitrate% Forage
Maximum % of total ration Grain
Minimum % of total ration
0.5
0.75
1.0
1.25
1.5

100
(0.5 ÷ 0.75) = 67
(0.5 ÷ 1.0) = 50
(0.5 ÷ 1.25) = 40
Don’t feed: It would require too much grain
0
(100 – 67) = 33
(100 – 50) = 50
(100 – 40) = 60
Adapted from L.D. Lewis (1995).