There is no question NO3 contamination of drinking water can result in acute death and/or abortion in ruminant livestock. Cattle are usually reported to be more susceptible than sheep, with monogastrics such as horses and swine being relatively resistant. Surprisingly, we were able to find only one report of NO3 poisoning (from feedstuffs) in wild ruminants383, but, given the physiological similarities with domestic animals, it is reasonable to assume deer, antelope, and elk are also susceptible.
The chronic toxicity of very low doses of NO3 is controversial. Despite repeated attempts (and failures) to reproduce vitamin A deficiency, hypothyroidism, or other chronic forms of NO3 toxicity, experimentally it does not seem that dietary concentrations significantly less than those required for acute intoxication cause measurable ill-effects in domestic ruminants. While there is no question NO3 can produce abortions in ruminants, the dose required appears to be very near that required for acute toxicity. The most scientifically rigorous examination of chronic NO3 toxicity to date332 concluded that water concentrations less than 400 mg/L (the concentration tested) should not pose any hazard to a well-managed herd.
The lowest toxic dose of NO3 in cattle in the experimental studies we reviewed is somewhat less than 200 mg NO3-/kg BW, although there were several experiments that failed to produce any effect at considerably higher (as much as 800 mg/kg BW) doses. Clinical (i.e. anecdotal) reports, in particular those of Yeruham et al.384 and Slenning et al.361, push the minimum toxic dose down to near 100 mg NO3/kg BW. There are some uncertainties associated with these two reports. Yeruham did not specify the amount of toxic whey consumed (we assumed 20% BW when figuring a dose as it occurred in a hot climate), and there was a two-fold variation in analytical results between samples. Slenning et al. suggested other factors, notably overfeeding an ionophore, might have potentiated the toxicity of NO3. The next lowest concentration reported to be acutely toxic was 1% NO3 in Chenopodium hay, which would provide approximately 250 mg NO3/kg BW in cattle under the assumptions outlined in the Introduction. Nitrate in water is additive with NO3 in feedstuffs, with a given dose in water being somewhat more potent than in feed because it is consumed more rapidly.
Assuming negligible forage NO3 concentrations, a water NO3 concentration of 500 mg NO3-/L (measured as NO3- ion) would provide 100 mg/kg BW, which would provide a two-to-three fold margin below the 200-250 mg/kg BW dose above. If forage concentrations are higher (not a rare occurrence in the Great Plains) the permissible water concentration should be adjusted downward.
The NO2- ion is commonly described as approximately 2.5-fold more toxic than the NO3- ion in ruminants (10-fold more toxic in monogastrics), which implies a safe threshold of about 200 mg/L. We were, however, unable to find sufficient experimental studies or well-documented field investigations upon which to base any conclusion about maximum safe concentrations. This is probably due to the fact NO3- is the more stable form of the two in surface waters and feedstuffs, and NO2- is only rarely present in negligible concentrations. Garner describes the minimum lethal dose of NO2 in swine (the most sensitive species) as 40 mg/kg BW358, which translates to 200 mg NO2-/L in drinking water under very conservative assumptions. Intravenous administration (the most potent route of exposure for most toxicants) of 12 mg NO2-/kg BW to cattle and 17.6 mg NO2-/kg BW to sheep did not produce any reported toxic effects.
Obviously, this is an area that needs further research, but we believe, based upon the existing knowledge base, 100 mg NO2-/L (as the nitrite ion) should not cause poisoning in livestock.