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Livestock sickness or death from "nitrate poisoning" is a concern when feeding grass-type forages, like sorghums, corn, and small grains. Feedstuffs that are properly sampled and analyzed can be rated according to their relative ability to cause nitrate toxicity.
ServiTech uses analysis results expressed as "parts per million nitrate-nitrogen" or "ppm NO3-N" to rate nitrate toxicity potential. Nitrate poisoning occurs most often in cattle and less often in other ruminants.
Nitrate occurs rarely in horses, swine, or other monogastric animals. If toxicity does occur in nonruminants, it is often due to consuming spilled fertilizer or high-nitrate water.
Table 1 gives the nitrate-nitrogen levels and livestock hazard ratings used by the ServiTech laboratory. These ratings assume cattle to be the principal species of concern.
¶ Table 1. Livestock Hazard Ratings |
|
| NO3-N ppm, dry matter basis | General hazard rating |
| < 700 | Very low |
| 700 - 1400 | Low |
| 1400 - 2100 | Medium |
| 2100 - 2800 | High |
| 2800 - 3500 | Very high |
| > 3500 | Extremely high |
Following are general guidelines to use when interpreting laboratory results for nitrate-nitrogen in forages for mature livestock. The hazard ratings should be adjusted when dealing with animals in poor condition, are under stress, have health problems, or are smaller sized. These animals probably cannot tolerate nitrate concentrations that would not affect larger or healthier animals.
Very low: Considered safe to feed for all classes of livestock.
Low: Considered safe to feed for non-pregnant ruminants and horses. Limit use of this forage to about one-half (50%) of the total dry matter in rations for pregnant ruminants.
Medium: Limit use of this forage to about one-fourth (25%) of the total dry matter in rations for pregnant ruminants. Limit to about one-half (50%) of the total dry matter in rations for nonpregnant ruminants. Considered safe for horses.
High: Limit use of this forage to about one-third (35%) of the total dry matter in rations for nonpregnant ruminants. Not advised for use with pregnant ruminants. Considered safe for horses.
Very high: Limit use of this forage to about onefourth (25%) of the total dry matter in rations for non-pregnant ruminants. Do not use for pregnant ruminants. Suggest limiting use of his forage to about three-fourths (75%) of the total dry matter in horse rations.
Extremely high: This forage not advised for use in a free-choice feeding program. Suggest limiting this feedstuff to less than one-fourth (25%) of the total dry matter in rations for non-pregnant ruminants; one-half to two-thirds (50% - 65%) of the total dry matter for horse rations. Not advised for use with pregnant ruminants.
The risk of nitrate toxicity is usually based on the nitrate concentration of the forage, assuming it is the only feed source in the animal diet.
Actual toxicity risk depends on the nitrate concentration of the various forages in the diet, feed intake, contributions from water or other nitrate sources, animal body weight, and condition.
The daily nitrate intake per pound of body weight is a better way to evaluate nitrate risk. Nitrate intake of 14 milligrams per pound of body weight (mg NO3-N/lb BW) or less is considered safe for non-gestating cattle. Dosage of 5 mg NO3-N per pound of body weight or less is considered safe for pregnant cattle.
Toxicity problems can be compounded by high nitrate levels in the drinking water consumed by the livestock. Water testing from 31 to 100 ppm NO3-N (or over) should be used with caution, especially when feeding high nitrate forages.
Results expressed as "ppm NO3-N" cannot be used to directly interpret results from other laboratories that are reported using different units.
Other laboratories may express their laboratory results differently, like "percent nitrate (% NO3)". Although the forage contains the same amount of nitrate, the numbers on the lab report may be quite different.
This is similar to reporting the temperature of boiling water as "212°F" or as "100°C". In both cases, the water contains the same amount of heat energy, but the scale is different and must be interpreted differently.
These reporting unit differences can be confusing when comparing ServiTech results and hazard ratings from other sources. If necessary, other values can be converted using the following formulas.
1.0 % = 10,000 ppm
NO3-N X 4.43 = NO3
NO3-N X 7.22 = KNO3
For example, a result reported as 2500 ppm NO3-N (nitrate as nitrogen) would be equivalent to a result of 11,075 ppm NO3 (nitrate):
2500 x 4.43 = 11,075
The ServiTech Laboratory nitrate analysis results are reported on a “dry matter” basis. Some sources may report results on an “as fed” or “wet” basis. Results on an as fed (AF) basis can be converted to a dry matter (DM) basis by dividing the result by the dry matter percentage as follows:
DM result = %AF / (%DM / 100)
For example, if a laboratory analysis report shows 1000 ppm NO3-N on an as fed basis with a dry matter content of 85% (or 15% moisture), the dry matter concentration would be 1176 ppm ppm NO3-N.
= 1000 ppm / (85% / 100)
= 1000 ppm / 0.85
= 1176 ppm (on dry matter basis)
Table 2 shows the differences that occur when different units are used to report the same nitrate concentration.
¶ Table 2. Conversions for Different Nitrate Hazard Toxicity Reporting Units |
|||
| NO3-N ppm | NO3 ppm | NO3 % | KNO3 % |
| 700 | 3,100 | 0.31 | 0.51 |
| 1400 | 6,200 | 0.62 | 1.01 |
| 2100 | 9,300 | 0.93 | 1.52 |
| 2800 | 12,400 | 1.24 | 2.02 |
| 3500 | 15,500 | 1.55 | 2.53 |
| 4200 | 18,600 | 1.86 | 3.03 |
| 4900 | 21,700 | 2.17 | 3.54 |
| 5600 | 24,800 | 2.48 | 4.04 |
| 6300 | 27,900 | 2.79 | 4.55 |
| 7000 | 31,000 | 3.10 | 5.05 |
Undersander, D., et. al. Nitrate poisoning in cattle, sheep, and goats. Univ. of Wisconsin Coop. Ext. Serv., Madison WI. (accessed 08/30/2010) http://www.uwex.edu/ces/forage/pubs/nitrate.htm
Hibberd, C.A., 1993. Utilization of high nitrate forages by beef cows, dairy cows, and stocker calves. Proc. of Mgmt. of High Nitrate Forages for Beef and Dairy Calves, Enid OK, 04 May 1993. Oklahoma Ag. Expt. Station, Stillwater OK. 12 pg.
Tucker, B. 1984. Nitrate-nitrogen accumulation in forages. Textbook supplement for AGRON 3213. Dept. of Agronomy, Oklahoma State Univ. Stillwater OK..12 pg.