Practices such as double cropping and cover cropping can boost yields, improve soil quality and provide a longer grazing season for cattle, but high nitrate levels in forages can lead to serious illness, and in some cases, death.
Warm season crops that accumulate nitrogen include corn, sorghums, millet, sudangrass, sorghum sudangrass and sunflower. Cool season nitrate accumulators include plants in the brassica family such as turnips, radishes, collards, rape and kale. Small cereal grains that accumulate nitrogen include oats, cereal rye and wheat.
Plant maturity is one of the main influencers of nitrate levels in plant tissue. Nitrates are higher in young plants and levels decrease as the plant matures. “As the growing season is extended, nitrate concentrations tend to decrease,” said Mary Drewnoski, Beef Systems Specialist at University of Nebraska-Lincoln. “Cover crops planted after wheat tend to have lower concentrations of nitrates in the fall than do cover crops planted after corn silage.” Drewnoski added that nitrates can be a potential issue in plant species that don’t typically accumulate nitrogen if those plants are immature.
Nitrogen availability also influences the level of nitrates in forage. The risk of nitrate toxicity increases when nitrogen availability to plants is high. “Fields with excessive nitrogen remaining from a previous crop are at great risk,” said Drewnoski. “Seed corn fields or hail-damaged corn fields are of concern. Also, cover crops that have had high rates of additional nitrogen applied to stimulate forage growth can result in high nitrates.”
Growing conditions also affect nitrate levels in forage. Any growing condition that slows plant growth has the potential to affect the level of nitrates in plant tissue. Although drought is the most common stressor that results in higher nitrate levels, stunted growth due to herbicide use and plant disease can also contribute to elevated nitrate levels.
High nitrate levels in plants lead to lack of oxygen in the ruminant’s body tissue. Nitrate is reduced to nitrite in the rumen, and nitrite combines with hemoglobin to form methemoglobin, which cannot transport oxygen to tissue. When methemoglobin levels reach 50 percent or higher of hemoglobin content, the animal exhibits acute clinical signs of toxicity including weakness, incoordination, convulsions, accelerated breathing and/or heart rate, and discoloration of mucous membranes. It’s important to note that clinical signs of prussic acid (cyanide) poisoning, which occurs most often when animals are grazing forage sorghums, are similar to those of nitrate toxicity. Blood tests can differentiate between the two.
Since excess nitrates in forages can have serious consequences for livestock, Drewnoski suggests forage testing. “Walk in a random zig-zag pattern across the field,” said Drewnoski, explaining the process. “Hand-pluck to a height at which you intend to graze from multiple locations in the field (about 20), then place those samples directly on ice.”
Drewnoski explained rapid cooling of samples slows down plant enzymes that convert nitrates to nitrite, and also helps slow down bacterial degradation. Samples should be delivered directly to the lab, or if there will be a delay prior to shipping, samples should be frozen for 24 hours prior to shipping. Samples should be shipped early in the week to ensure prompt delivery and handling in the lab.
Once forage test results are returned, look at the levels and determine the risk of consumption by un-adapted animals based on the level of nitrates. “Anything less than 1,100 ppm nitrate nitrogen is considered safe,” said Drewnoski. “Anything from 1,100 to 2,100 ppm is a moderate risk for nitrate toxicity; forages from 2,100 to 3,400 ppm are high risk, and anything greater than 3,400 ppm is a severe risk.”
Drewnoski points out the fact that labs report nitrate in several ways; the two most common of which are nitrate nitrogen and nitrate ion. “If your analysis is nitrate ion, to convert to nitrate nitrogen, simply multiply by 4.43,” she explained. “If it’s in a percent and you need ppm, multiply by 10,000.”
Cattle can be safely grazed on forages that test in the moderate to moderately high nitrate range (1,000 to 3,000 ppm nitrate nitrogen) if certain management practices are carefully observed. Even when nitrate levels are known, it’s a good practice to make sure cattle are filled with hay prior to turning them out on pasture. Feeding hay slows down the initial intake of fresh forage crops and allows the rumen to adjust. “The key thing is the rate of nitrate intake,” said Drewnoski. “The conversion of nitrate to nitrite in the rumen occurs much more rapidly than the conversion of nitrite to ammonia. Nitrite is the toxic compound that can enter the bloodstream, bind hemoglobin and make it incapable of transporting oxygen; thus the animal can suffocate.” With a slow intake of nitrate, the flow of nitrite to ammonia can keep up with the flow of nitrate to nitrite.
The second key management practice is to graze lower risk cattle on high-risk forages. Pregnant cows are at greatest risk — the fetus is particularly susceptible to low oxygen levels that occur with high nitrate forages, and abortion is often the end result. “Using open cows is the best option, followed by growing calves,” said Drewnoski.
Gradual adaptation of cattle to grazing potentially high nitrate forages is another important practice. Animals can safely adapt to higher nitrate levels because the bacteria that can utilize nitrite and convert it to ammonia can increase in numbers as nitrite availability increases in the rumen. Drewnoski says the key factor is to maintain a slow but steady increase in nitrites so bacterial numbers can keep up and avoid a large increase in nitrites in the rumen that can enter the bloodstream.
Drewnoski suggests not putting hungry cattle on high nitrate pastures; instead, adapt cattle by grazing the lowest nitrate field first then work up to the highest nitrate fields. “Allowing the cattle to selectively graze plant parts that are lower in nitrate concentrations will decrease nitrate intake,” she said. “Leaves are lower in nitrates, and cattle will naturally select leaves when grazing. Forcing cattle to eat the stemmy portion of the plant, especially the lower stem, will increase nitrate intake.”
In cases of severe drought stress, which has not been the case for most areas of the northeast this summer, it’s often best to simply not graze cattle on crops that are suffering drought stress until plants have received adequate moisture. Forage testing for nitrate levels is especially important to ensure safe grazing in such conditions.
Another management practice is grain supplementation, which is particularly useful when available forages are relatively low quality and high in nitrates. “We can supply extra energy to the rumen microbes to convert that nitrate to bacterial protein and minimize nitrate concentrations,” said Drewnoski. “However, this may not be a useful strategy with high-quality forages such as brassicas because the ruminally available energy is already quite elevated.”
To help learn how much nitrate levels are affected by annual rainfall, keep accurate notes on weather conditions throughout the year and compare those notes to forage test results. Several years’ worth of good records should help predict potential nitrate issues, but it’s still important to use forage tests for accurate results and healthy animals.