South Florida Beef - Forage Program

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"Article of the Month"
August 2000

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Nitrate Poisoning in Grazing Cattle
Questions and Answers

John Arthington, PhD
Range Cattle Research and Education Center
University of Florida – IFAS

What is nitrate poisoning?

Nitrate poisoning in grazing cattle occurs following the consumption of plants that contain excessive amounts of nitrate. Nitrate is naturally converted to nitrite and then to ammonia by microbes in the rumen. Acute or chronic nitrite poisoning occurs when the amount of nitrate consumed exceeds the rumen's ability to convert into ammonia. This excess nitrite is then absorbed into the bloodstream where it complexes with hemoglobin. Bound with nitrite, hemoglobin in unable to bind and transport oxygen causing a buildup of methemoglobin in the blood. Methemoglobin has a chocolate brown color, which is a telltale sign of acute nitrate poisoning in cattle. By blocking the transport of oxygen to peripheral tissue, acute nitrate poisoning results in suffocation.

How do plants accumulate excessive nitrate?

Plants take up nitrate in normal growth and developmental processes. During photosynthesis the plant converts nitrates into amino acids and proteins. Nitrate accumulation will occur most readily in the structural components of the plant closest to the ground. Plant leaves are usually low in nitrate. Nitrate problems are most common in young growing plants, which are rapidly taking up nitrate to fuel growth.

Excessive plant nitrate accumulation may occur in at least two ways:

1) Cultivated crop forages (typically annuals) which are grown on heavily fertilized and cultivated soil tend to be the most common accumulators of nitrate. Problems with excess nitrate accumulation may be most common in soils heavily fertilized with poultry or livestock manure. Typically, commercial fertilizers applied at recommended rates are not contributors to excessive nitrate accumulation.

2) Plant stress and environmental factors which decrease the photosynthetic ability of the plant will also contribute to nitrate accumulation:


a. Hail or wind damage that may decrease plant leaf cover will decrease photosynthesis and increase nitrate accumulation.

b. Drought stressed plants will rapidly accumulate nitrate after a rainfall. In these situations, the harvesting of suspect pastures should be delayed by several days.

c. Multiple cloudy days following a rain, especially in newly fertilized forages, will contribute to nitrate accumulation.

What are symptoms of nitrate poisoning?

Two types of nitrate poisoning may occur, chronic and acute.

1. Chronic nitrate poisoning is probably the most troublesome of the two, because detection is difficult. In chronic poisoning situations nitrate accumulation may decrease weight gain and reproductive performance affecting both the number and weight of calves at weaning. Abortions due to a lack of oxygen flow to the fetus may also occur. Fortunately, in grazing environments chronic poisoning is rare, as the cattle are able to adapt to excessive nitrate consumption over a period of time, or gain access to other forages lower in nitrate.
2. Acute nitrate poisoning is the more common of the two. Cattle consuming excessive amounts of nitrate will often exhibit difficulty in breathing, rapid mouth breathing, bluish skin color around eyes and mouth, and muscle tremors. Although not readily visible, blood with a chocolate-brown color is a classic symptom of acute nitrate poisoning. Animals experiencing acute nitrate poisoning may die within 4 hours.

What can I do if I notice acute nitrate poisoning?

Cattle suffering from nitrate toxicity may be treated by an intravenous injection of methylene blue (1 to 2 % solution). Response time is critical, as acutely affected animals may die within a few hours. It is important to note that methylene blue is not an approved drug for use in food animals. Please consult your veterinarian for more information.

What levels of nitrate are suspect in causing poisoning?

If you suspect your forage may be high in nitrate it is a good idea to have it tested. Many commercial laboratories offer nitrate testing in forage crops. It is important to note that laboratories vary in their method of reporting nitrate concentrations. Results may be reported as nitrate (NO3), potassium nitrate (KNO3), or nitrate nitrogen (NO3-N). The values provided below are commonly accepted ranges for determining the safety of forage nitrate concentrations (Table 1) and conversions for comparing reporting results (Table 2). There is a great deal of variability in the range of susceptibility between cattle and these ranges are provided only as general guidelines.

Table 1. Ranges in safety of nitrate in forages
%NO3	%KNO3	%NO3-N	Recommendations
0 to .50	0 to 1.0	0 to .15	Generally considered safe
.50 to 1.5	1.0 to 1.5	.15 to .45	Caution - should dilute intake
> 1.5	>1.5	>.45	Danger - Do not feed
*Values are reported on a dry matter basis

Table 2. Formulas for converting methods of reporting
Potassium nitrate	Nitrate x 1.6
Potassium nitrate	Nitrate nitrogen x 7.0
Nitrate	Potassium nitrate x 0.6
Nitrate	Nitrate nitrogen x 4.4
Nitrate nitrogen	Potassium nitrate x 0.14
Nitrate nitrogen	Nitrate x 0.23
Parts per million (ppm)	% x 10,000
%	Parts per million (ppm) / 10,000
From: Kvasnicka, B. and L.J. Krysl. Cattle Producer's Library CL 620. Nitrate Poisoning in Livestock. University of Nevada.

How may I prevent nitrate toxicity?

1.	If you suspect a forage to be high in nitrate, have it tested first before harvesting or allowing animals to graze.
2.	When grazing suspect pastures, allow initial access only to a few animals. Monitor carefully to determine if the pasture is safe for the entire herd.
3.	Do not harvest fertilized forages immediately after a rainfall, which has followed a period of drought.
4.	Do not harvest fertilized forages immediately after several days of heavy cloud cover.
5.	It is wise to test all forages known to be nitrate accumulators prior to harvest. These include, but are not limited to; hay made from small grains, sorghum, and sudan.
6.	High nitrate forages, which must be harvested, should be cut higher from the ground by raising the cutting bar. This will leave behind the lower stem, which tends to be the highest accumulator of nitrate.
7.	Hale and drought damaged forages which are high in nitrate can be salvaged into a quality usable feedstuff if they are ensiled.
8.	If you have harvested forage that is high in nitrate (in the caution range) and you need to use it due to lack of feed, try diluting with other low nitrate forages. For instance, you may place one bale of each forage out at the same time. Remember, always watch closely for signs of toxicity and never feed these high-nitrate forages to hungry cattle.

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