Some farmers are beginning to graze drought stressed corn stalks and are encountering nitrate toxicity problems. Remember that nitrate accumulates in the stalk and is more concentrated in the lower stalk than the leaves and upper stalk. If forage and grain yield was reduced by more than 50 percent (even in spots of the field), stalks should be tested for nitrate before grazing. Some corn stalks have been found with toxic levels of nitrate.
If a test comes back with toxic levels are nitrate, what are the options?
1) Nitrate is soluble and some will wash out of the leaves and stalk with heavy rains. Consider that rains greater than 1 inch will likely reduce nitrate to some extent. Forage should again be tested after the rain event to determine nitrate levels remaining. Thus waiting to graze corn stalks may reduce the problem.
2) Corn stalks with toxic levels should be diluted with other feeds to reduce the nitrate intake:
Feeding a supplement, such as corn, can reduce the nitrate concentration and provide energy so that nitrite can be metabolized to some extent.
Limit feed the corn stalks, for example, allowing grazing only 2 to 4 hours per day (time depending on nitrate level), so that animals consume sufficient hay, other crop residue, or silage to dilute the nitrate.
Knowing the nitrate level of the feedstuff is crucial before feeding to cattle. We tend to think of animal deaths from nitrate (nitrite actually) and many will think if no animals died they got by.
However, subclinical nitrite poisoning can also cause considerable damage to animals. Prolonged exposure to nitrate/nitrite at subclinical levels may cause degeneration of the vascular tissues of the brain and other organs such as lungs, heart, liver, kidneys and testes. The affected animals may appear to be in normal health, but will abort their young, fail to settle to service, suffer a decline in lactation or lack ability to adjust to cold temperatures.
Thyroid function is also impaired by nitrate causing a decrease in iodine uptake by the thyroid gland. Fortunately, iodized salt contains enough iodine to counteract levels of nitrate that would reduce uptake but not show any other signs of poisoning. Be sure to have iodized salt blocks available to cattle grazing corn stalks this year.
Corn stalks have long been grazed for beef cattle and dairy heifers and dry cows. It is a good feed, however this year special attention must be paid to nitrate levels of the corn stalks so that animals do not suffer from acute (death) or subacute toxicity.
~Dan Undersander, UW-Madison Agronomy