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Every year, nitrate toxicity problems show up somewhere when nitrates accumulate in forages due to drought or other stresses. High levels in a lab test are a warning of potential problems, but nitrate itself doesn’t cause the toxicity.
Rumen metabolism first converts nitrate (NO3) in the consumed forage to the nitrite form (NO2), then to the ammonia form (NH3). This ammonia is subsequently metabolized to protein.
Any condition which limits or slows down this conversion process may cause nitrite (NO2) to build up in the rumen. Nitrite is absorbed into the blood stream and combines with hemoglobin to form “methemoglobin”, which cannot carry oxygen. When methemoglobin is 50% to 65% of the total blood hemoglobin, animals can die from suffocation.
There is more to assessing nitrate toxicity potential than just the number on the lab report. Consumption rate and rate of passage through the rumen can be critical factors. Problems occur if the rumen is filled very quickly with a high nitrate forage and/or if digestion is slowed down. Sort of like slugging a baler or combine. Fewer problems occur with smaller amounts of forage consumed over a longer time.
Here is an example of how consumption rate might affect the risk of toxicity. An 800‐pound cow will consume about 2% of its body weight per day as dry matter (DM), when eating average quality forage. That’s about 16 pounds of dry matter per day (lb DM/day). Assuming grass pasture at 15% dry matter (85% moisture), the cow would consume about 107 total pounds of “wet” forage per day.
Experts say that dairy cows on pasture will graze an average of 8.5 hours per day and average about 60 mouthfuls per minute. That is 30,600 mouthfuls per day. Running the math, that’s 0.056 ounces per mouthful of wet forage which will contain 0.0084 ounces of dry matter.
Servi‐Tech Labs uses a dry matter forage concentration of 1400 ppm NO3‐N (nitrate‐nitrogen) as a threshold to warn of potential toxicity problems. This 1400 ppm NO3‐N would be equivalent to 39.7 milligrams of NO3‐N per ounce of dry matter. So each mouthful of a 1400 ppm NO3‐N forage at 15% dry matter would contain 0.33 milligrams of NO3 ‐N.
At 0.33 mg of NO3‐N per mouthful, it would take 31,515 mouthfuls for the cow to consume 10,400 mg of NO3‐N. At 60 mouthfuls per minute, the cow would graze for just under 9 hours to consume all that nitrate.
Let’s suppose our cow was eating a dry forage (15% moisture, 85% dry matter) with a lab test of 1400 ppm NO3‐N. That same mouthful now has 0.0476 ounces of dry matter containing 1.89 milligrams of NO3‐N. At 60 mouthfuls per minute it would only take 92 minutes for the same cow to consume the same 10,400 milligrams of NO3‐N in this dry forage.
Same cow. Same size mouthful. Same lab test. Same critical dose of NO3‐N. Two different dry matter contents. It took over five times longer for the cow to consume the critical dose with the wet forage as the dry forage. That’s because the cow’s dry matter intake rate was five times faster with the dry forage.
When nitrates are a potential problem, two important feeding strategies are:
Practically speaking, more frequent snacking rather than eating a single heavy meal helps reduce potential nitrate risks. Each mouthful counts.
Differences in forage quality, feeding practices, animal diet, and the environment can all affect whether a specific forage might present a potential problem.
The type and amount of fiber in a forage largely determines its quality. For example, the Relative Feed Value calculation uses both acid detergent fiber (ADF) and neutral detergent fiber (NDF) percentages.
The ADF content (primarily cellulose and lignin) is used to estimate digestible dry matter. Lower ADF contents tell us the forage has a higher nutrient density. The NDF content (cellulose, lignin, and hemicellulose) is used to indicate bulkiness of the forage. This is related to the dry matter intake or the amount of feed an animal will consume as percent of their body weight.
Lower quality forage has higher ADF and NDF content, with correspondingly fewer digestible nutrients and lower digestibility. Lower quality forages will also take longer to pass through the rumen than will high quality forages.
Rumen microorganisms are responsible for converting nitrate to nitrite to ammonia to protein. Ruminant nutrition is all about feeding those microbes properly. A large and active population of rumen microbes are necessary for animal performance. A small or malnourished microbial population will be less effective at converting nitrate to protein in a timely manner.
We know that leaves are more nutritious than stems. We also know that nitrates are more concentrated in the lower portion of the stem than the upper portion. When animals consume leaves, they are consuming a high quality forage that is nutrient dense when compared to the lower stalk. Leaves also have lower NDF so the rate of passage through the rumen is more rapid.
When animals consume the lower stalk, not only are they consuming the fraction with the highest nitrate level, they are consuming the fraction with the lowest quality. Rate of passage is slower than leaves, so the time spent in the rumen increases.
So we have the forage fraction (lower stems) with the lowest nutritional value and highest nitrate content. There are fewer nutrients for the rumen microbes that are trying to detoxify this nitrate‐rich forage that is also passing through the rumen more slowly. This quickly ramps up the potential for a toxicity problem.
The net result is fewer nutrients, less or slower microbial activity, and slower conversion of nitrite molecules to ammonia. This provides more opportunity for nitrite to pass through the rumen wall, enter the blood stream, form methemoglobin, and reduce the ability of the blood to carry oxygen.
The first step is to test the forage for nitrate content. A result of 700 to 1000 ppm NO3‐N (nitrate as nitrogen) should trigger a warning. Two keys to dealing with high nitrate forages are to manage the intake rate and to assure a balanced diet. More frequent meals with smaller amounts can help regulate intake. Depending on animal size, feeding three to five pounds of grain may go a long way to help reduce nitrate toxicity potential.
(Note: For more information, see Crop File 6.05.024, “Managing high nitrate feedstuffs ‐ Toxic dosage approach”, available at www.servitechlabs.com under the “Resources”, “Articles” tab.)
http://www.hpj.com/opinion/nitrate‐by‐the‐mouthfulpart/article_143ed618‐9d9c‐5853‐b63a‐eb93f6c690b9.html
http://www.hpj.com/opinion/nitrate‐by‐the‐mouthfulpart/article_de289e9b‐cacf‐509c‐8ec3‐b6b2f3c98a5a.html