* Nitrogen levels could spike in stunted corn plants

* Farmers test forage for toxin that can sicken animals

* Unharvested corn acres could be most in a decade

By Michael Hirtzer and Meredith Davis

CHICAGO, Aug 15 (Reuters) - The worst U.S. drought in five decades has parched the land and decimated crops. It now threatens to deal a second blow to farmers, who may have to throw out tonnes of toxic feed.

Growers are rushing to check the nitrate levels of that silage, the stalks and leaves that corn farmers often harvest to feed to locally raised cattle or hogs.

Agriculture groups are warning farmers that drought-hit plants may have failed to process nitrogen fertilizer due to stunted growth, making them poisonous to livestock.

Exceptionally early spring planting has caused a crush of early summer requests for the tests. Farmers are also expected to chop down a near-record swathe of their fields for silage to make up for this year's poor yields.

"We've had a lot of walk-in business and normally we are not a walk-in business," said Lola Manning, a 30-year employee of Agri-King, a laboratory that tests for nitrates and other toxins. "At this point it's the busiest I've seen it."

Manning said the facility, approved by the National Forage Testing Association, checked about 400 samples -- roughly double the norm -- in July.

So far, few samples have shown elevated levels of toxins, she said. But late-season rains -- far too tardy to help salvage the corn crop -- could prompt mostly mature plants to draw even more nitrogen out of the soil and into the stalks.

"The tests are coming out OK but as soon as they have rain, the situation will change," Manning said.


SO FAR, SO GOOD

Two months of dry weather and high heat that stunted plants and shriveled ears likely caused the absorption of excessive amounts of nitrogen, experts say. Instead of being distributed safely through the plant, the chemical built up in the lower portions of the stalk at potentially toxic levels.

Kenny Wagler, a dairy farmer in Nashville, Indiana who also farms 2,500 acres (1,000 hectares) of corn and pasture, is testing his corn for the first time since the last major drought in 1988.

"It's almost never a factor," said Wagler, who raises about 1,500 dairy cows and cattle, adding that he is testing this year on recommendation from his farm nutritionist.

Nearly half of what he typically harvests to sell as a cash corn crop will be cut for silage this year because most of the plants had no ears of grain.

In the worst-case scenario, silage with high levels of nitrate can be absorbed into an animal's bloodstream, causing poisoning leading to death.

The absorption causes hemoglobin to be converted to methemoglobin, which is incapable of transporting oxygen and so can be fatal to the animal, according to the U.S. Department of Agriculture. Symptoms of nitrate poisoning include labored breathing, rapid heartbeat, weakness, lack of coordination and blue-gray discolored skin.

Extensive losses of livestock are an unlikely, extreme scenario, beef and dairy experts say.

"Certainly there are instances of dead cattle from nitrate," said Chris Hurt, agriculture economist at Purdue University. "Widespread education has helped reduce the problem."

But nitrate-laced silage would force those farmers to buy extra feed grains in order to sustain their animals.


LOW GRAIN YIELDS, MORE SILAGE?

Silage is usually harvested while plants are still green and contain a high level of moisture. It is then fermented, often in silos. Many dairy farmers raise corn specifically for silage, in part to avoid having to buy feed elsewhere.

The rest of the crop is allowed to mature and is harvested as grain to be sold to elevators for export or feed use, or to ethanol makers.

Farmers are expected to harvest more of their corn crop for silage than usual this season due to poor yields, which are forecast by the USDA to be the lowest in 17 years.

As many as 9 million acres -- or 9 percent of the corn crop -- may not be harvested for grain this year, according to USDA data released last week. That would be the most abandoned acres in a decade. Much of that will be used instead as silage.

At Agri-King in western Illinois, tests cost $8 per sample for nitrate. Farmers are advised to take six stalks, chop them up and put them into a bag for testing.

Nitrate levels under 4,400 parts per million are considered safe while those over 15,000 ppm are considered potentially toxic and should not be fed to livestock, said Randy Shaver, extension dairy nutritionist at the University of Wisconsin.

At between 8,800 and 15,000 ppm, silage should be limited to less than half of the total feed ration and well fortified with minerals, data from that university showed. However, acceptable nitrate levels vary slightly from state to state.

"We've had quite a few tests that have come in at 14,000 parts per million or higher, and that seems to come up after a rain," said Travis Meteer, a beef extension specialist at the University of Illinois, one of several universities to issue bulletins about nitrates in silage in recent weeks.


LIVESTOCK PAIN, CORN'S GAIN

If the silage proves to be toxic, farmers like Wagler could be forced to cull their herds, as many ranchers are doing. Or they could buy additional grains from the cash market to feed their livestock -- incurring extra expenses in a year when some of their income will depend on crop insurance claims.

Extra demand could add fuel to corn prices, which have already rallied more than 60 percent in two months to a record as drought deepened across two-thirds of the country.

"It will mean higher feed costs for livestock producers," said Roger Elmore, a professor of agronomy and a corn specialist at Iowa State University. "In addition to the drought, forage quality and the quantity will be less.

"We'll have less forage out there, so that price will also increase. All of that increases the cost of production for livestock producers," he added. (Editing by Dale Hudson)

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