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The potential hazard for nitrate toxicity is based on the dry matter nitrate concentration found during lab analysis. The hazard ratings assume the forage represented by the sample is the sole feedstuff available to the animal.
Toxicity potential that is based only on the lab analysis does not consider everything that affects how an animal will actually react. A good evaluation of nitrate toxicity potential should include the following:
“The dose makes the poison” is an old saying that applies to nitrate toxicity. All forages contain some level of nitrate. Nitrate is safely converted to nitrite and then ammonia in the rumen at a certain rate. If the amount of nitrate consumed with the forage exceeds the safe rate of conversion, some nitrite escapes into the blood stream, combining with hemoglobin to form methemoglobin. Nitrate toxicity symptoms become obvious if the escaped nitrite results in a methemoglobin concentration exceeding 10% of the total blood hemoglobin.
Research shows that nitrate toxicity problems are rare if an animal consumes less than 4.5 milligrams of nitratenitrogen per pound of body weight (NO3-N mg/lb-BW) per day. The critical dose increases to 13.0 milligrams if the animal is not pregnant. These amounts keep the methemoglobin concentration at about 3% of the total hemoglobin.
Table 1 shows the expected toxicity symptoms at different dosages. However, different animals vary in just how they respond to elevated nitrates. Some animals can simply tolerate more nitrate in the ration than others, so no approach is absolutely foolproof.
¶ Table 1. Nitrate Dose and Expected Animal Response |
|
| Milligrams of nitrate-nitrogen (NO3-N) per pound of body weight |
Expected animal response |
| 0 – 4.5 | Safe for pregnant cattle |
| 4.5 – 13.0 | Potential early-term abortions Reduced breeding performance |
| 0 – 13.0 | Safe for non-pregnant cattle. |
| 13.0 – 26.0 | Mid- to late-term abortions Weak newborn calves Decreased growth Reduced milk yields |
| > 26.0 | Abortions Toxicity symptoms and death |
The toxic dose approach considers body weight, ration composition, and other factors - not just the nitrate level reported by the laboratory. Treating forage nitrate as a daily dose allows some flexibility for managing high nitrate forages in a feeding program.
The forage analysis results and animal weight are used to calculate the amount of a specific feedstuff that can be tolerated in an animal diet to stay below the critical nitrate dosage. This process has four steps:
Multiply the nitrate-nitrogen result from the feed test by 0.4536 to calculate the milligrams of nitrate per pound of forage dry matter. Repeat if there is more than one forage source.
Multiply the milligrams per pound (from step #1) by the pounds of forage to be fed or consumed daily on a dry matter basis to find the total daily nitrate consumption for each forage.
Divide the total daily consumption by the animal body weight in pounds to find the daily nitrate dose.
Divide the critical level by the daily dose to calculate the maximum fraction of the forage to include in the ration dry matter.
Multiply the daily dry matter intake rate (as pounds per day) by the maximum fraction from the previous step to calculate the maximum amount of dry matter forage to use.
Divide the maximum amount of dry matter by the forage dry matter percentage to calculate the amount of “as fed” forage to include in the ration.
Assume a hay (15% moisture) is to be fed free-choice to 500 lb calves (5 cwt) that consume 2% of their body weight (10 lb of dry matter) per day. The feed test result was 2000 ppm NO3-N on a dry matter basis.
The daily dose for these calves would be 18.1 mg of NO3-N per pound. In this case, the forage should not be the single source of feed because it exceeds the 13.0 mg critical level for non-pregnant animals.
The forage in question should only make up about 70% of the total daily ration dry matter or about 7 pounds per day. The remaining 30% of the ration dry matter should consist of very low nitrate feedstuffs, like grain or concentrate. In the example, the hay had 15% moisture or 85% dry matter, so the feeding rate must be adjusted from a dry matter basis to an as fed basis.
Limiting the as fed hay consumption to 8 pounds per head per day would keep the average daily nitrate consumption at or slightly below the critical level.
Daily consumption depends on the animal size and the forage quality. Dry matter consumption rates are calculated as a percentage of animal body weight. For example, a young, 400-lb animal will consume an average of about 2.0% to 2.5% of their body weight as dry matter or about 8 to 10 pounds of dry matter per day.
The intake rate increases as forage quality increases. Lower quality forages have higher fiber contents, so are bulkier and fill the rumen faster. Table 2 lists the typical dry matter intake rates for various forage qualities.
¶ Table 2. Forage Quality Effect on Dry Matter Intake |
||
| Forage quality | Dry matter intake, % of body weight |
Examples |
| Low quality | 1.5% | dormant grass, straw, stubble, mature stalks |
| Average quality | 2.0% – 2.5% | native grass; mid- to late summer, improved warm-season pasture |
| High quality | 2.5% – 3.0% | alfalfa, small grain pasture; spring cool-season improved pasture |
Managing intake rate is not easy, but is critical to manage high nitrate feedstuffs. Animals cannot sense the level of feed nitrate, so the forage palatability and digestibility usually govern dry matter intake.
The toxic dose is calculated for the entire day’s consumption, not the feed consumed in a single “meal”. A single meal is one feeding that may take a few minutes to two hours to complete. Feeding management involves slowing the rate of intake as much as feasible. “Snacking” on small meals is preferred to eating one large meal.
Hungry animals eat more feed. Eating more forage or eating it faster may exceed the ability of the rumen microbes to detoxify the ingested nitrate.
Follow-up feedings of a high nitrate forage should be delayed for at least two to three hours after the animal has completed the previous feeding. Don’t delay feeding to the point where the animal becomes excessively hungry and prone to eat rapidly - exceeding the rumen microbes ability to detoxify nitrate.
Harvesting forage does allow a producer to manage the intake rate, but livestock can consume a processed forage faster than if it is grazed. Regardless of harvest method, high nitrate forages must be limit-fed to reduce toxicity potential.
The rumen microbes adapt to increased nitrate levels in feedstuffs by increasing the enzyme, nitrate reductase. This enzyme is used to convert nitrate to nitrite. The microbes can then convert the nitrite to ammonia or other nontoxic compounds.
This adaptation process begins within about four hours after the animal starts consuming a high nitrate feedstuff. Full adaptation may take three to six days. Adaptation may not be rapid enough to avoid toxicity, especially if nitrate intake is too high. The microbes simply cannot detoxify the nitrate quickly enough.
Microbes will de-adapt when the ration nitrate level declines. De-adaptation may take as little as four days, so the animals are not protected if daily nitrate intake increases later.
Nitrate in drinking water should be included as part of the total dietary nitrate consumption. Convert the water nitrate concentration (as mg/L or ppm) to amount per gallon (as mg).
• mg NO3-N/gallon = mg/L NO3-N x 3.785
Multiply the nitrate content of the water by the gallons consumed each day.
Hibberd, Rehberger, Swartzlander,& Parrot. 1993. Utilization of High Nitrate Forages by Beef Cows, Dairy Cows and Stocker Calves. Proc. of “Management of High Nitrate Forages for Beef and Dairy Cattle”. Oklahoma State Univ., Coop. Extension Svc. Enid OK. 04 May 1993. 12 pg.
Adams, R.S., T.R. McCarty and L.J. Hutchinson. 2010. Prevention and Control of Nitrate Toxicity in Cattle. Publ. DAS 92-107. Penn State. Univ. Ext.