Steve Wikse, a retired professor of large-animal clinical sciences, College of Veterinary Medicine at Texas A&M University, answers this month's beef cattle health question.

Q -- Is there a delay before placing cattle back on pasture after fertilizing? If so, how long? Is toxicity a concern?

A -- As long as recommended amounts of fertilizer are properly applied, there is no need to delay placing cattle back on pasture after fertilizing. Of course, I'm not about to break the rule of never-say-never. Toxicity problems associated with fertilization can be fatal to cattle under specific circumstances.

To prevent problems, avoid spilling fertilizer during application. Fertilizer should be spread evenly on the pasture. Hungry cattle can ingest toxic amounts of fertilizer if exposed to spilled piles. The risk is greatest under poor-pasture conditions. Spilled poultry litter used for fertilizer presents a special danger. Ingestion of poultry litter that contains chicken carcasses can result in fatal cases of botulism.

Application of high concentrations of nitrogen to pastures of Sorghum spp. (Johnson grass, sorghums) or Brassica spp. (rape, turnips) can lead to nitrate toxicity if plants undergo drought stress.

Ranchers who suspect a pasture has accumulated excessive amounts of nitrate during drought conditions can submit samples to the Texas Veterinary Medical Diagnostic Laboratory to measure nitrate concentration. Pastures with less than 2 percent nitrate on a dry- matter basis are safe for cattle.

Although usually associated with Sorghum spp. forages, nitrate toxicity can occur on any type of pasture. I observed nitrate toxicity due to extreme over-fertilization of a natural pasture. A client of mine finished fertilizing his pastures and had a large amount of ammonium sulfate left over in the fertilizer hopper. He decided to empty the hopper by applying all the fertilizer to a pasture with only a few acres. Months later, I autopsied cattle that died within 24 hours of being placed in the small pasture. Diagnosis: nitrate toxicity.

Nitrate toxicity can result in large losses of cattle. Rumen bacteria convert the excessive nitrate in the forage to nitrite. Nitrite is absorbed into the bloodstream and changes the hemoglobin of red blood cells to methemogloblin. This causes victims to become starved for oxygen because methemoglobin is unable to carry oxygen. A lack of oxygen is the cause of death.

The first symptoms of nitrate poisoning are weakness, staggering and gasping for breath. Several animals may be ill. Severely affected cattle have convulsions and die within minutes to an hour. The blood is brown in autopsy findings. Diagnosis is confirmed by finding elevated concentrations of nitrite in the blood or in ocular fluid.

Two other fatal diseases of cattle have difficult, gasping, open-mouthed breathing as their main symptoms. They are cyanide toxicity and acute bovine pulmonary edema and emphysema (ABPEE). Pasture conditions associated with these two diseases are very different.

Cyanide toxicity, like nitrate toxicity, often occurs when Sorghum spp. is stressed by drought conditions. Pastures suspected of containing excessive concentrations of cyanide can also be tested by the Texas Veterinary Medical Diagnostic Laboratory to determine if they are safe for grazing by cattle. A change in diet from a low plane of nutrition to lush green forage such as abundant Bermuda grass pasture can result in ABPEE. This disease is called fog fever in Britain and in some textbooks. The term originated because in Britain ABPEE sometimes occurs when cattle are allowed to graze lush regrowth of hay fields (called fogage in England). We also commonly encounter outbreaks of ABPEE in cattle moved from poor pastures to lush hay pastures after the final cutting of hay.

As in nitrate toxicity, color of blood in animals with cyanide toxicity is striking. It is bright red. Diagnosis of cyanide toxicity is confirmed by demonstration of high concentrations of cyanide in the rumens or livers of victims.

A diagnosis of ABPEE is based on a combination of grazing history and autopsy findings of froth in the large air passages of the lung, plus enlarged lungs that do not collapse. There is presently no laboratory test to confirm a diagnosis of ABPEE.

As long as recommended amounts of fertilizer are properly applied there is no need to delay placing cattle back on fertilized pastures.

Consult a veterinarian on how to prevent pasture-associated diseases in your cattle.

E-mail questions to beverly.moseley@theeagle.com.