The drought that has kept much of the nation in its grip this summer brings a host of additional downstream worries for growers already struggling with reduced yields.
Cattle are being poisoned by cyanide-laced weeds in Arkansas. Across the Midwest water-soluble fertilizers are concentrating in soils and plants, making them harmful rather than productive. And in Missouri, samples suggest that more than half the corn crop isn't fit for human consumption, thanks to unusually high levels of a carcinogenic toxin.
For farmers coping with the worst drought to hit the United States in decades, it's another chapter in an unfolding disaster that shows no sign of abating. And with climate projections showing more frequent summer droughts in heavy farming areas, these elevated drought-related poisons add to the challenges growers face in a changing climate.
"I've been talking to veterinarians and other folks in Iowa since June, and I'd estimate somewhere around 150 cattle have died from toxic nitrate doses," said Steve Ensley, a toxicologist at Iowa State University College of Veterinary Medicine.
The annual tally is usually less than five, he said.
With nitrate, drought spurs high levels because plants take up nitrogen, applied as a fertilizer, but cannot convert it into useful compounds due to a lack of water.
"If the plant doesn't go through photosynthesis because it doesn't have the water to, the nitrate just stays in the plants," Ensley said.
Nitrate isn't toxic to animals. But microorganisms in the environment convert it to a compound that, at high levels, inhibits oxygen flow in animals, resulting in difficulty breathing, weight loss, lack of appetite, sometimes death.
For humans, nitrate brings two main health issues: blue baby syndrome and digestive tract cancers [pdf].
But the danger to humans is through drinking water. Nitrogen fertilizer not sucked up by plants often washes into streams and lakes. This summer's dryness has sent less nitrogen-laced runoff into Midwest streams, Ensley said.
But the leaching could just be delayed.
"It's probably still in the soil," Ensley said. "When it rains, there's definitely the potential for some heavy runoffs."
Drought stress also causes increased cyanide compounds in weeds that cattle like to eat, as is the case in Arkansas where more than 50 cattle have died this season.
Extended drought can often induce a highly toxic and cancer-causing toxin – aflatoxin, a fungal-byproduct. With the corn harvest underway, aflatoxin worry is pervading the farming community.
"We don't know how bad it'll be yet," said Don Parrish, senior director of regulatory relations at the American Farm Bureau Federation. "But the conditions certainly exist."
Several fungi, or molds, can grow on drought-stressed grain, and several of these produce byproducts are toxic to animals and humans. The most common of these fungi, Aspergillus flavus, consumes the starch inside corn kernels and produces a byproduct named aflatoxin. This fungus also contaminates peanuts, cottonseed, pecans and grain sorghum. The disease causes olive green, moldy growth.
Drought doesn't cause the fungus, but the dryness allows for cracks in the shell protecting the kernels, allowing fungus to get to the grain.
"We've had an increase in aflatoxin all over the state," said Ron Heiniger, professor of crop science at North Carolina State University. "We had the prolonged heat stress, and then recent rainfall.... It's just blowing up."
Ensley said Iowa officials recently started testing for aflatoxin in milk from Iowa dairies; other Midwest states are already feeling the heat.
"There have been (corn) loads in southern Illinois, and northern parts of Missouri with high contamination levels.... A lot [is] getting rejected by grain buyers,” said Allen Wrather, a professor of Plant Sciences at the University of Missouri's College of Agriculture, Food and Natural Resources.
The Missouri Department of Agriculture has found aflatoxin at levels over 20 parts per billion – the legal limit for corn intended for humans or dairy cattle – in 58 percent of samples so far this season. Last season, only 8 percent of the crop was above that mark.
The Illinois Department of Agriculture already had 56 corn samples this season test for aflatoxin over the 20 ppb threshold There were only six samples over that limit in all of 2011.
While carcinogenic to humans, aflatoxin is also toxic to many animals.
"Pigs and cows can handle it a little bit better," Wrather said. But it will kill chicken, cats and dogs if it gets into their food, he added.
The U.S. Department of Agriculture regulates aflatoxin and sets strict contamination limits. Corn destined for humans and dairy cattle has the 20 ppb limit, whereas corn for beef cattle can contain concentrations up to 300 ppb.
But Heiniger said the desperate times are causing desperate measures in his state.
"There's a lack of grain on the market, and, therefore, a lack of vigilance on testing grain by grain buyers," he said. "I'm seeing a lot of grain that would be rejected in a normal year being bought to blend with other grain or be ground up to mitigate the toxic compounds in it."
The true impact of aflatoxin will be known in a few weeks, Parrish said. So far only 39 percent of the U.S. corn crop has been harvested, according to U.S. Department of Agriculture.
Climate scientists warn that these problems won't soon disappear. This year's drought has shown little sign of slacking, and climate modelers expect more of the same: This year's extreme weather may be closer to the future norm, scientists say.
"In the Midwest, as you go ... toward the south and west – places like Missouri, Iowa, Kansas and Nebraska – the future looks warmer and drier," said Jeff Andresen, an assistant professor at Michigan State University who specializes in applied climatology. These, he added, are "not very optimistic predictions for agriculture."
Parrish, of the American Farm Bureau Federation, is optimistic farmers can mitigate drought-stressed crops and excess nitrate runoff.
But for many farmers, the year has been a bust. The nitrate and aflatoxin worries pile onto a growing season already marred with low production. Through September, the Corn Belt was still "slightly" or "abnormally" dry in most areas, according to the USDA. Compared to 2011, corn and soybean yields are down 13 and 14 percent, respectively.
This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.