On Sept. 18, Jim, an organic dairy farmer, called me to discuss possible corn silage toxicity issues that worried him. He milks about 70 cows in Genesee County (NY), and his farm is “regular” organic (not grass-fed).

I’ve been advising him on his crop program for several years. He grows corn for silage and whole ear picking as well as soybeans. He had just finished chopping enough corn to fill a door’s worth in an 18-foot silo. The fresh-chopped silage was on top of about a third of a silo of first-cut haylage, and both crops had been treated with a probiotic silage inoculant approved by his organic certifier.

Jim had heard several nearby dairy farmers express concerns about possible nitrate toxicity in this year’s corn silage that would be fed to their cattle. Most dairy farmers are aware that droughty conditions can seriously disturb the physiology of growing corn. When this happens, a lot of the nitrate from fertilizer – and to a lesser extent from manure – never gets metabolized into plant protein. I agreed to research the nitrate toxicity issues for him.

In so doing, I learned that the current growing season has been quite variable in many parts of the country. As of late September, crops being grown for livestock feed have been experiencing moderate to severe drought stress in several regions. In this column I’ll address what challenges growers can expect with drought-stressed corn silage and what to watch out for.

Crops can be drought-stressed to varying degrees, and the severity of that stress can increase dramatically during hot, dry weather in a short period of time. In my area, WKTV in Utica (NY)’s meteorologist pointed out that all of July, August and the first three-quarters of September is about 50% behind on normal precipitation.

According to Hubbard Feeds, a major risk to be aware of in drought-stressed corn silages is nitrate toxicity. Any forage harvested under drought conditions has the potential to contain elevated or toxic levels of nitrates. Nitrates are converted to nitrites in the rumen – a chemical change from -NO3 to -NO2. These nitrites bind with hemoglobin to form methemoglobin, which cannot carry oxygen. This occurrence deprives the ruminant’s body of that life-essential gas. Fields that were heavily fertilized with nitrogen, or experienced a heavy application of manure prior to planting, are most likely to produce forages with high levels of nitrates in drought situations.

Here are suggestions as to how to minimize the risk of nitrate toxicity in your livestock, for various scenarios. First, drought-stressed forages that have recently been exposed to rain are more likely to have high nitrate levels. As such, waiting to harvest 10 to 14 days after a significant rainfall greatly reduces the chance of high nitrate levels in silages.

Second, ensiling high-nitrate forages prior to feeding can reduce the nitrate levels by converting more than half of them to ammonia, which can be utilized by the rumen. Thank the fermentation byproduct organic acids – like acetic and lactic – for helping cattle and sheep dodge this particular bullet. Also thank rumen microbes for enabling all this biochemistry.

Third, nitrates accumulate heavily in the lower third of the plant. Practicing high chopping – leaving more of the high-nitrate portion of the plant in the field will lower the overall nitrate levels but will also reduce the overall yield. Fortunately, this part of the corn plant tends to be higher in indigestible hemicellulose and lignins (which are also very low in protein).

The fourth point is that forages suspected of being high in nitrates should be tested. Evaluate the test results and utilize these forages based on those test results.

The possible nitrate levels in different parts of drought-stressed fresh corn can vary all over the board. Research tallied by D.R. Hicks and P.R. Peterson at the University of Minnesota show the following average nitrate nitrogen (NO3-N) values in ppm for different parts of the corn plant: 17 for ears; 64 for leaves; 153 for the upper third of the stalk; 803 for the middle third of the stalk; 978 for the whole plant; and 5,524 for the lower third of the stalk.

Other dairy scientists (V.A. Ishler, R.S. Adams and A.J. Heinrichs at the University of Pennsylvania, in “Drought-Related Issues in Dairy Cattle Nutrition”) stress that a NO3-N level in the 1 – 1,000 ppm range is safe to feed under most conditions. In a range of 1,000 – 1,700 ppm, they recommend that dairymen gradually introduce livestock to ration, feed some concentrate, test all feeds and water, dilute to 0.4% -NO3 or 900 ppm NO3-N in the total ration dry matter and restrict single meal size.

When the NO3-N is in the 1,700 – 2,300 ppm range, they are worried about possible acute toxicity. Thus, they recommend feeding such in a balanced ration with concentrates included – and to dilute to 0.4% -NO3 or 900 ppm NO3-N in the total ration dry matter. Then restrict single meal size. If one forage contains more than 0.44% NO3 (or 1,000 ppm NO3-N), test all forages, water and, possibly, concentrates. Include the nitrate intake from water as part of the dietary intake.

According to the above-noted Minnesota researchers, soil organic matter counters high nitrate threats to cattle by fueling microbial processes that can reduce and immobilize nitrogen, preventing it from accumulating to toxic levels in forage. This natural buffering action works through several pathways within the soil’s nitrogen cycle.

Back to our Genesee County organic dairyman: Jim’s soil organic matter generally falls in the 4% – 7% range. Organic standards limiting corn to no more than two consecutive years helps make that happen. The strong suggestion to grow a cover crop between those two years provides additional insurance against nitrate toxicity.