Nitrates a concern for drought-fed cattle

Dec 30, 2011

Texas Agrilife Extension Agent Rick Hirsch said that the 29 head of cattle found dead last week in Henderson County most likely died of nitrate poisoning from tainted hay.

Tissue samples from the cattle and core samples from the suspected hay were sent to laboratories for analysis.

“Although uncommon in normal years, these poisonings occur when cattle eat forages stressed from severe environmental conditions such as drought,” according to Hirsch. “High enough nitrate can drop a cow in an hour,” he said.

Texas’ severe drought this year leaves the conditions ripe for a number of added stresses, including the potential for nitrate poisoning.

According to Jack Whittier, Colorado State University, nitrate accumulation usually is triggered by some environmental stress where plant growth is restricted but absorption of nitrate from soil continues. “The most common stress of summer annuals is drought. Lack of moisture, together with excessive soil nitrogen for existing growing conditions, is a frequent cause of toxic levels of nitrate in sorghums. Other stress factors that favor buildup are reduced sunlight from cloudiness or shading, frost, certain herbicides including 2,4-D, acid soils, low growing temperatures, and deficiencies of essential nutrients like phosphorus, sulfur and molybdenum,” Whittier writes.

Hirsch warned area producers and livestock owners to make sure and test hay, particularly since many were forced to purchase hay from unknown sources this year because of the drought.

He said the most susceptible forages include: Johnson grass, milo stubble, forage sorghum and sudan grass.

While typically a concern in the fall, the amount of nitrate accumulated in a plat depends on two factors: the rate of uptake by the plant from the soil, and the rate the plant reduces it. If uptake exceeds the rate of reduction, large amounts of nitrate can accumulate. If the rate of reduction equals the rate of uptake, there is no accumulation.

Nitrate accumulation usually results from plant stress, such as drought, and is accentuated by excessive soil nitrogen. Most nitrate accumulates in plant stems rather than leaves, and concentration tends to be highest in immature forage. A characteristic symptom of nitrate toxicity in the animal is a chocolate-brown color to the blood.

Good management practices are the key to avoiding poisoning, according to Whittier.

“Nitrate is the primary nutrient form of nitrogen in most soils and is a normal constituent of plants. Normally, nitrate is assimilated so rapidly following absorption from soil that its concentration in plant tissues is low. Occasionally, excessive levels in plants occur. In Colorado, the most notorious accumulators of nitrate are the sorghums,” according to Whittier.

Another concern with nitrate poisoning includes water. Shallow wells contaminated with surface water, water containing animal wastes, and surface runoff from heavy rain after fertilization with ammonium nitrate can also poison cattle.

“Water containing more than 200 ppm NO3 is potentially toxic, especially when feed also contains an excessive level,” Whittier said.

“Although the term ‘nitrate toxicity’ is commonly used, the toxic principle is actually nitrite. Nitrate is converted to nitrite in the rumen. Nitrite is absorbed from the rumen and converts blood hemoglobin to methemoglobin. Methemoglobin cannot transport oxygen to body tissues, so animals die from oxygen insufficiency,” according to Whittier’s June 2011 report on nitrate poisoning.

Symptoms of nitrate poisoning include a grayish to brownish discoloration of nonpigmeneted skin and mucous membranes of the mouth, nose, eyes and vulva.

The animal will eventually begin to stagger, have a rapid pulse, labored breathing and frequent urination, followed by death. The symptoms come on rabidly, within one-half hour to four hours after ingestion, according to Whittier.

“Treatment of nitrate poisoning with 4 percent methylene blue at the rate of 100 cubic centimeters per 1,000 pounds live weight (intravenously) is effective if administered soon after symptoms appear,” Whittier said.

The best way to avoid poisoning is to routinely test forage, especially if the pasture, hay or silage was grown during less than ideal conditions.

According to Whittier, forages high in nitrate can be fed safely with proper management. “The best alternative is to dilute dangerous forage with feeds low in nitrate, preferably concentrates. Unfortunately for many producers, proper dilution makes it necessary to grind and mix. Gradual acclimation to questionable feed is a good practice to minimize risk. Animals should be healthy, on a good nutrition plane, and filled with low nitrate feed before they are allowed access to nitrate-containing forage,” he wrote.

“Hay, straw or fodder suspected of being high in nitrate should not be fed when damp. Damp feed seems to be more toxic,” he added.

Nitrate poisoning can be a serious problem for livestock producers if not considered in their management plan. Drought, excessive soil nitrogen, shade, frost, certain herbicides, acid soils, low growing temperatures and nutrient deficiencies can contribute to high nitrate levels in plants. Do not overlook the nitrate content of water when a nitrate problem arises. Avoid poisoning by routinely testing any forage suspected of containing excessive nitrate. High nitrate forages can be used by diluting it with other feedstuffs and supplementing it with energy. — Traci Eatherton, WLJ Editor