Summer annual crops that experience sustained periods of dry weather can contain high levels of nitrates, which can have detrimental effects on feeding. Crops with high nitrates can also be dangerous for people as well. Producers should watch out for hazardous silage gases in these crops.
Shortly after ensiling green plant material, oxygen is used up, and the nitrates in the plant may be released as nitric oxide (NO). This gas escapes from the silage and combines with oxygen in the air to form toxic nitrogen dioxide, a lethal orange-brown gas that smells similar to laundry bleach.
Proper fertilization (with both macro and micro minerals), combined with good cultural methods — such as proper weed, insect and disease control — reduce the chances of nitrogen dioxide gas production when you ensile the crop. Apply adequate nitrogen, but don't overdo it. As a guide, corn needs 1.2 pounds of nitrogen per bushel yield, while sudangrass used for silage should receive no more than 75 pounds of nitrogen for each harvest. Since this includes both nitrogen in the soil and applied, follow the recommendations on soil analysis reports and be sure to factor in nitrogen from any slurry applications.
Another way to avoid dangerous nitrate levels is to use disease and insect resistant varieties and/or spray to help control insect and disease damage to leaves and roots. In addition, keeping fields relatively free of weeds can help prevent deadly gases. Weeds can make silage dangerous even though there is no nitrate in the silage crop itself.
After a period without moisture, plants can take up nitrate rapidly following rainfall. Therefore, harvest the crop before large fall rains, or wait at least five days after a rain event. Plants damaged by hail or frost should be harvested immediately before they take up nitrates.
Nitrates accumulate in the bottom portion of the plant so raising the cutter bar to leave 12 inches or more of stalk in the field can be effective in reducing nitrate levels in the resulting silage. This may lead to a yield reduction in 15 percent, but the material harvested will have higher quality, as well as the lower nitrate risk.
Bacterial fermentation during ensiling converts 40 to 50 percent of nitrates, so the risk of poisoning is considerably reduced. Using research-proven quality forage inoculants can help ensure silage has safe nitrate levels — in addition to helping retain dry matter and maximizing nutrient preservation and stability at feedout. The lactic acid bacteria (LAB) Pediococcus pentosaceus 12455, provides an efficient, fast fermentation fueled by sugars generated by high activity enzymes, while the high dose rate Lactobacillus buchneri 40788 is uniquely reviewed by the FDA for improved aerobic (feedout) stability.
If your forage crop is at risk for elevated nitrate levels, send a sample to a commercial lab for analysis.