Drought-stressed crops tend to accumulate high nitrate levels in the lower leaves and stalk of the plant (Figure 1). Crops such as forage and grain sorghum, sudangrass, hybrid sorghum-sudan, and pearl millet are notorious nitrate accumulators. Corn and other cereal grains can also accumulate potentially toxic levels of nitrate. Nitrates accumulate in the lower portion of these plants when stresses reduce crop yields to less than expected, based on the supplied nitrogen fertility level. Nitrate toxicity in livestock is caused by its absorption into the bloodstream and binding to hemoglobin, rendering it unable to carry oxygen throughout the body. The result is eventual asphyxiation and death.
Figure 1. Drought-stressed corn plants (top photo) and forage sorghum (bottom photo). Photos by Dorivar Ruiz Diaz and John Holman, K-State Research and Extension.
Animals under physiological stress (sick, hungry, lactating, or pregnant) are more susceptible to nitrate toxicity than healthy animals. Toxicity is related to the total amount of forage consumed and how quickly it is eaten. Generally, forages containing more than 6,000 ppm nitrate should be considered potentially toxic (Table 1). Symptoms of nitrate toxicity may appear within a few hours after eating or not for several days. Signs of toxicity include reduced appetite, weight loss, diarrhea, and runny eyes. However, these are nonspecific symptoms of numerous disorders and are not a reliable diagnosis of nitrate poisoning. Lower nitrate levels can cause abortion without any other noticeable symptoms.
Forage testing
It is wise for producers to test their drought-stricken forage prior to harvest. Levels of nitrates can increase in drought-stressed plants after rain, and delaying harvest may be beneficial. Nitrate testing can be done through several labs, including the K-State Soil Testing Laboratory. Not all labs use the same testing procedure so be sure to use the scale provided by the testing lab. Harvesting the forage 6-to-12 inches above the ground to avoid the highest concentrations of nitrate in the plant is a good practice. Producers should collect a good representative forage sample above this cutting height to get an accurate determination of the nitrate concentration. It's important to consider factors like actual nitrate concentration, storing and feeding methods, and forage availability, as these can significantly impact the safety of your livestock. Ensiling forage will reduce nitrates levels ~50%, but ensiled forage should be tested for nitrate levels prior to feeding. Toxicity is related to the total amount of nitrate in the diet (including water) and how quickly it is eaten. Generally, if forages contain more than 6,000 ppm nitrate, they should be considered potentially toxic (Table 1).
Table 1. Level of forage nitrate (dry matter basis) and the potential effect on animals.
|
ppm Nitrate (NO3) |
Effect on Animals |
|
0-3,000 |
Virtually safe |
|
3,000-6,000 |
Moderately safe in most situations; limit use for stressed animals to 50% of total ration. |
|
6,000-9,000 |
Potentially toxic to cattle depending on the situation; should not be the only source of feed. |
|
9,000 and above |
Dangerous to cattle and often will cause death. |
Management options
Depending on the planned feeding method, a producer may wish to test different parts of the plant. If wrapping the forage into a bale and feeding it directly to livestock, a producer may want to test the lowest part of the stalk to determine the greatest risk of nitrate forage that the animal could ingest. If this value is high, further testing may be needed. If a producer was planning on grinding the bale, a whole?plant sample above what will be left in the field may be a more accurate representation of what will be eaten. If a harvested forage is high in nitrate, grinding and mixing the feed with a low nitrate forage, such as prairie hay, brome, or forage sorghum, will dilute the total nitrates in the animal’s diet and could potentially reduce the risk of poisoning. However, be aware that bunk sorting could occur if the forage mix is feed free choice.
If parts of the field show more stress and potentially differ in nitrate concentration, baled forage from those areas can be segregated. Mark bales and retain their identity in storage. Resample and test for nitrates and forage quality in each field and subgroup separately to determine safe and cost-effective feeding options. While the nitrate concentration does not change after hay harvest, the variability of nitrates across a field and the challenge of collecting a truly representative sample pre-harvest make a thorough post-harvest sample imperative.
High-nitrate forages chopped for silage and properly ensiled are a safer option for livestock feeding. During the ensiling process, potentially 50 percent of the nitrates in the forage will be metabolized by the microbes and can vastly reduce the risk of poisoning. It is still not a bad idea to leave 6 inches of stubble in the field. That is the portion of the stem with the highest concentration of nitrates, and leaving residue will help reduce soil erosion and capture snow over winter.
Grazing high nitrate forages can be a dangerous practice. This option requires very careful management. Grazing pressure should be limited so that animals do not consume the parts of the plant forage testing has shown to be dangerous. Although animals tend to consume the leaves and the top portions of the plant, which contain less nitrates, the risk of consuming a high-nitrate portion of the plant still exists. In addition, the longer the animal is left on a field and the more that animal is forced to eat the remaining forage at the lower portions of the plant, the greater the risk of nitrate poisoning.
For more information, see K-State Research and Extension publication MF3029, “Nitrate Toxicity”, at your local county Extension office or at https://bookstore.ksre.ksu.edu/pubs/MF3029.pdf
Dorivar Ruiz Diaz, Nutrient Management Specialist
ruizdiaz@ksu.edu
Sandy Johnson, Extension Beef Specialist, Northwest Research and Extension Center-Colby
sandyj@ksu.edu
John Holman, Cropping Systems, Southwest Research and Extension Center
jholman@ksu.edu
Tags: grazing forage drought stress nitrate toxicity nitrates