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Nitrate toxicity is a potential threat to livestock that graze or pasture summer annual crops, especially those that have been damaged or stressed. Potentially toxic nitrate levels may show up in many crops and weeds. Crops that may accumulate nitrate include corn, grain sorghum, sudans, forage sorghum, millets, and small grain forages. Problem weeds include pigweed, kochia, Russian thistle, and others. Nitrates do not typically accumulate in legume crops that might be grazed as emergency forage.
Feed intake cannot be effectively restricted when animals are grazing. Grazing forages that are suspected of high nitrate must be carefully managed. Toxicity symptoms appear rapidly, often within 30 to 90 minutes after livestock consume the forage.
Total nitrate intake is the critical problem, rather than the amount in the single forage. For example, a “dangerous” level in a forage that makes up only half the total dry matter intake could be perfectly safe. Likewise, forages with lower nitrate levels may adversely affect livestock if animals are nutritionally stressed and suddenly eat a large amount of the forage.
If it is necessary to pasture a high nitrate forage, the amount and rate of nitrate intake must be restricted. Just how a producer can achieve this objective will vary from operation to operation.
Avoid heavy stocking rates in high nitrate pastures. Overgrazing may pressure some of the animals to consume the lower stalks, which are high in nitrate, thus increasing potential for toxicity.
Nitrate levels are usually lowest in leaves and highest in the lower stalk (see Table 1, drought damaged corn example). This pattern of nitrate distribution (low in upper leaves, highest in lower stalk) would be similar for sorghums, sudangrasses, small grains, and other grass crops.
¶ Table 1. Nitrate Levels in Drought‐Stressed Corn |
|
| Plant Part | ppm NO3-N |
| Leaves | 64 |
| Ears | 17 |
| Stalk: | |
| Upper ⅓rd | 153 |
| Middle ⅓rd | 803 |
| Lower ⅓rd | 5,524 |
| Whole plant | 978 |
Nitrate levels are also highest in young, immature plants, then decline as the plant matures. Plants exposed to the right conditions may have a high nitrate content at maturity.
This is a problem because livestock tend to graze selectively. They will prefer to eat the shorter, less mature, and more succulent plants first. These plants often have higher nitrate levels than more mature plants. Livestock will typically consume the leaves first, which are lower in nitrate and graze the stalks later.
Nitrates start to concentrate in plants when they are under stress that retards their normal growth or development. High nitrates can be triggered by any condition that interferes with normal growth. These conditions include drought, cool temperatures, cloudy weather that limits light, and others. Spraying with a growth‐regulating herbicide (like 2,4‐D or dicamba) has been known to result in nitrate accumulations.
Nitrate can continue to be a concern even after rain follows a prolonged dry spell. The precipitation dissolves nitrates in the soil. These nitrates are quickly taken up by the plant root system as it absorbs water. This can double or triple the plant nitrate level in a matter of hours (see Table 2).
¶ Table 2. Rainfall Effect on Nitrate Content of Forage Sorghum |
||||
| Plant Part | Before Rain | Time After Rain | ||
| 24 hr. | 48 hr. | 7 days | ||
| ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ ppm NO3‐N ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ | ||||
| Leaves | 385 | n/a | n/a | 1,015 |
| Upper ⅓rd | 645 | 735 | 875 | 1,035 |
| Middle ⅓rd | 680 | n/a | 1,015 | 1,465 |
| Lower ⅓rd | 2,045 | 5,125 | 8,330 | 5,645 |
| (Note: Rain occurred on Sept. 15; “caution” for 700 to 1,400 ppm NO3‐N) | ||||
Introduce livestock gradually to the high nitrate forage, preferably over a period of several days. A twice‐per‐day feeding schedule may be required.
Animals can adapt gradually to elevated nitrate levels in their diet, if the nitrate is introduced gradually. Nitrate levels in growing plants may change rapidly, so the potential for nitrate toxicity can change almost from day‐to‐day.
Feeding 2 to 3 pounds of grain (or 3 to 5 pounds of concentrate) per head per day allows ruminant animals to tolerate more nitrate in the diet, possibly by supplying more energy. Feed a limited amount of low nitrate hay or silage before allowing animals to graze. This partially fills the rumen so limits the total amount of forage consumed while pasturing.
Overall good livestock management is required, because animals under stress are also more susceptible to high nitrates in their ration.
Sample standing forages carefully. The sample must accurately represent the field. Growth stage is a good guide for sampling. Separate samples may be required from different fields, different planting dates, or different hybrids.
One sample should consist of three or four plants from four or five different spots in the field, yielding a total of at least 15 to 20 plants. Plants should be cut off at ground level, but do not include the roots.
Chop the whole plants into 2‐inch to 3‐inch pieces, then mix thoroughly. Remove handfuls of mixed forage to collect at least one quart of the chopped material. Place the sample into a plastic bag and seal. Be sure to get a representative amount of leaves and stems.
Ship samples via the quickest way to prevent sample deterioration during transit. Heating can drive off nitrates, resulting in an artificially low lab result.
It may be necessary to sample more than once. A sample taken only once will not be valid for the whole season, because nitrate levels can change dramatically, even over a few days.
Samples of standing forage must be collected carefully to be representative of the field situation. Annual pastures may require sampling more than once per season because nitrate levels change according to growth conditions.
Rasby, R., et. al. 1988. Nitrates in livestock feeding. NebGuide G74‐170. Univ. of Nebr. Coop. Ext. Serv.
Corah, L., et. al. 1988. "Nitrates ‐ Well worth worrying about this year" in Beef Tips. Kansas State Univ. Coop. Ext. Serv.
Higbee, J.M., et. al. Nitrate hazards to livestock. Mimeo from Univ. of Wisconsin Coop. Ext. Serv.
Stolenow, C. and G. Lardy. 1998. Nitrate poisoning of livestock. Pub. V‐839. North Dakota State Univ. Coop. Ext. Svc., Fargo ND. (accessed 08/30/2010) http://www.ag.ndsu.edu/pubs/ansci/livestoc/v839w.htm
Rankins, D. and D. Ball. 2004. Nitrate poisoning of cattle in Alabama. Pub. ANR‐112. Alabama Coop. Ext., Auburn Univ. http://www.aces.edu/pubs/docs/A/ANR‐0112/
Adams, R., T. McCarty, and L. Hutchinson. 2010. Prevention and control of nitrate toxicity in cattle. DAS 92‐107. Penn. State Univ. Coop, Ext., University Park PA http://www.extension.org/pages/Prevention_and_Control_of_Nitrate_Toxicity_i n_Cattle