During drought years the potential exists for the corn plant to have high levels of nitrates. This is largely due to high soil nitrogen levels that are readily available, but the plant is unable to utilize it because of moisture shortages. As a result the nitrates accumulate in the plant and can occur at toxic levels. Excessive levels in corn when harvested as green chop or made into corn silage and then fed can causehigh blood levels of methemoglobin to occur.
Methemoglobin cannot carry oxygen to animal tissues and can result in a number of symptoms, and in severe cases, death.
Field livestock specialists in Illinois and Iowa have been doing "Field Screening Tests" consisting of a mixture of diphenylamine and concentrated sulfuric acid. This mixture is placed on the split stem of the corn plant and with a resulting color change can assist in identifying fields with nitrate problems. Limited tests done in southeast Iowa by Byron Leu, livestock field specialist, have shown about 50 percent of the plants to have nitrates present. This test does not indicate the level of nitrates, but rather whether it is present or not. In his field testing procedure, several of the plants had nitrates occurring up to 3 feet of stalk height, thus moving the cutting height up will not always solve the problem. Of greatest concern is the direct feeding of green chop corn that has nitrates present. Normally, if the corn plant is chopped and then ensiled, nitrate levels will drop by 50 percent or more.
Further information can be found on the drought web page  from Iowa State University Extension.
This article originally appeared on page 169 of the IC-494(22) -- August 22, 2005 issue.