By Bruce Anderson, UNL Forage Specialist
Cover crops are reportedly good for many purposes. They can reduce erosion, fix nitrogen, add organic matter, breakup hardpans, feed soil microbes, control weeds, and improve water infiltration. They might remove excess moisture, keep soils cool, or reduce evaporation. Hopefully, they maintain or improve our stewardship of the soil resource and result in improved cropping system profits.
Another potential benefit of certain cover crops is the capture of soil nitrates, retaining the excess nitrogen for use by future crops instead of losing the nitrates to leaching, runoff, or volatilization. When these nitrates are retained primarily belowground in the roots, livestock can safely graze or eat hay or silage from the cover crops. However, many cover crops can accumulate nitrates in aboveground tissue, which poses a risk to livestock consuming these crops.
Many popular cover crops that also are used as forages can accumulate nitrates, including cereals like rye and oats, summer forages like sorghums and millets, and brassicas like turnips and radishes. Many weeds like pigweed, lambsquarter, sunflowers, and kochia are known nitrate accumulators. Legumes like peas, beans, clovers, and vetch usually do not accumulate nitrates but under stress such as drought, hail, or a sudden hard freeze the chance that they will have high nitrates, at least temporarily, increases.
So – can these cover crops be used safely as forage? In most situations, yes. While many plants used as forage/cover crops can cause nitrate toxicity, most conditions in which they are grown do not lead to excessive nitrate levels. And even when nitrate levels are elevated, the forage often can be used safely if certain precautions are followed.
The key to safe use is a two-step process. First determine if the growing environment might produce high-nitrate forage. If so, then sample and test the crop for nitrates, using the test results to guide safe feeding of the forage to livestock.
Weather conditions that stress plant growth – drought, hail, frost or low temperatures – often contribute to nitrate accumulation. Heavily manured soils, mineral imbalances, or excessive nitrogen fertilization also increase nitrate risk. When cover crops that are known to accumulate nitrates have been growing under any of these conditions, sample and test for nitrates before using them for forage.
Hail can create some highly variable conditions that make forage use decisions challenging, especially in fields initially planted to corn. The original crop may or may not have been destroyed. Fields may or may not have been replanted with a forage crop. And soil nitrogen may or may not be high enough to cause extra nitrate risks.
Special sampling methods. The methods used to sample forages for nitrates often need to differ from those used when testing forage quality. Sampling for forage quality seeks to represent the average of the entire lot of forage. With nitrates, though, it often is important to know the worst case scenario, or what is the highest concentration of nitrates that might be consumed by the animals.
For example, some fields have sites that are more droughty than other areas. Hay harvested from these sites might be expected to contain more nitrates due to drought stress. Samples could be collected just from this hay and tested. Other samples might be collected from the remainder of the hay. If the stressed hay contains potentially risky nitrate levels, it can be separated from the rest of the hay and fed in ways that reduce the risk, such as combining it with a much lower nitrate containing feed to reduce the overall concentration of nitrates in the diet. Numerous other methods to safely feed high-nitrate forages can be found on-line at (http://extensionpublications.unl.edu/assets/pdf/g1779.pdf).
Collecting useful samples from fields that will be grazed can be challenging because of selective grazing by the animals. This is especially true when several species or a cocktail mix has been planted. Separate samples could be collected to represent the expected diet as well as the most risky diet. For example, in a corn field previously damaged severely by hail there may be well-eared stalks, barren stalks, regrowth tillers from damaged stalks, various weeds, and whatever might have been sown as a cover/forage crop after the hail. This diverse mixture might provide excellent nutrition as livestock select their diet but if, in that process of selection, the animals pick out the one type of plant material with an excessive nitrate concentration, losses could occur.
To determine if such a risk exists, it might be necessary to collect separate samples from many different plant species, plant parts, or areas of the field. This can be temporarily labor intensive and somewhat expensive but the loss of just one animal usually is much more costly.
Each of these samples will be very subjective, however, and biased according to the sampler’s ability to select the appropriate plants and plant parts. When interpreting the results of the laboratory analysis for nitrate content in these samples and making decisions about the safety of grazing, use extra caution to protect from sampling errors.
Bottom line. Cover crops usually can be very useful and safely fed as forages. However, individual situations can develop risks of nitrate poisoning. Recognize the risks and take proper precautions to avoid livestock losses while using valuable forage from cover crops.