Kansas City, Mo. ? Ranchers are being urged to watch what they feed their cattle because some drought-stressed plants that were harvested for animal feed or whose stalks remain in fields contain dangerous levels of nitrates.

University extension officials in Kansas and Missouri said high nitrate levels can cause pregnant cows to miscarry or kill adults. Animals that eat the nitrate-laden plants stumble and appear to be suffocating because nitrate poisoning inhibits the ability of blood to transport oxygen.

Nitrates are a naturally occurring form of nitrogen found in the soil. Normally, little nitrate accumulates in plants because they rapidly convert nitrate to amino acids and proteins. But when conditions are dry, as they were this past summer, the roots will take up nitrate faster than the plant can convert it.