Page 1011 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Nitrate and Nitrite Toxicity Chapter | 65 943

VetBooks.ir two samples from each bale should be collected. An indi- present and conversion of nitrate to nitrite exceeds the
ability of the rumen flora to convert nitrite to ammonia.
vidual investigation might reveal, e.g., that part of the
Hungry cattle and sheep introduced to stockyards contain-
bales have low nitrate content, while some may contain
toxic amounts. Thus, extreme care should be emphasized ing a dominant or pure growth of button grass
in the interpretation of relative risk, since there is always (Dactyloctenium radulans) suffered acute nitrate nitrite
the potential for localized toxic concentrations to be toxicity in four incidents in inland Queensland between
missed in the testing. 1993 and 2001 (McKenzie et al., 2004b). The nitrate con-
Nitrate content is not reduced by drying and baling as tent of the button grass from within the stockyards ranged
hay. High nitrate hay fed to cattle months after baling can from 2.44% to 7.87% nitrate in dry matter and from out-
cause deaths and abortions. Mortality can be striking, as side the stockyards ranged from less than 0.12% to
in a case in Nebraska in which Amaranthus/Kochia hay 0.24%.
with 2.99% nitrate and sudangrass with 4.88% nitrate
were fed to 390 cattle, resulting in the deaths of 226 and PHARMACOKINETICS/TOXICOKINETICS
42 abortions (Hibbs et al., 1978). High nitrate summer
hay fed during the winter of 1977 78 killed cattle in Action of the rumen flora reduces nitrate to the much
Oklahoma (Haliburton and Edwards, 1978). more toxic nitrite relatively rapidly, which is normally
Risks from high nitrate content forages can be reduced further reduced to ammonia and utilized by the microor-
by some management strategies. As nitrate is water solu- ganisms. Nitrite is absorbed into the blood when the
ble, high nitrate forages that are senescent can have some intake of nitrates and the production of nitrite exceed
of the nitrate leached from the forage by precipitation. the capacity of the rumen flora to further metabolize the
However, if high concentrations were present, there may nitrite. In some cases, preformed nitrite in hay may
still be toxic amounts present after leaching. Corn stalks shorten the period from ingestion to onset of signs. Both
left in a field had an average decrease of only 30% after nitrates and nitrites are absorbed into the blood but the
90 days (Johnson et al., 1992). Properly ensiled forages absorbed nitrite is the proximate cause of methemoglobin
can have a significant amount of nitrate utilized by the formation that results in poisoning. With IV administra-
fermenting microbes, but ensiled, high nitrate forages tion of nitrite, peak methemoglobin formation was
should always be tested prior to feeding to determine observed at 1.5 h after the onset of the infusion (van’t
whether adequate nitrate has been lost or if additional Klooster et al., 1990), which likely is due to peak nitrite
dilution is required to ensure safe feed concentrations. accumulation and not due to a delay in the formation of
Since nitrates are soluble, drained silage fluids could have the methemoglobin.
a significant nitrate content and pose a potential hazard. The nitrate ion is primarily eliminated in the urine of
It has been understood that rumen microorganisms can monogastrics and preruminant calves, but ruminants elim-
adapt to and utilize increasing amounts of nitrate in the inate much smaller amounts (Casteel and Evans, 2004).
diet. The period of maximum acclimation occurs within 6 This is likely due to ruminants readily recycling nitrogen
days (Allison and Reddy, 1984). However, adaptation can through the rumen to enhance overall utilization. The
be lost within a few days. The ability of rumen microor- half-life of nitrate is estimated to be 9 h in adult cattle
ganisms to safely reduce nitrate and further reduce nitrite and more than 24 h in the bovine fetus (Johnson et al.,
can be increased by feeding corn-based supplements to 1992). The half-lives of nitrate and nitrite in the blood of
cattle (Burrows et al., 1987; Nolan et al., 2016). In addi- sheep are 4.2 and 0.5 h, respectively (Schneider and
tion, it has been shown that adaptation to increased nitrate Yeary, 1975). In comparison, the elimination half-life for
may also have some systemic effects, as induction of met- nitrate is reported to be 44.7 h in dogs and 4.8 h in ponies
hemoglobin reductase activity was observed in cattle (Bruning-Fann and Kaneene, 1993), while elimination
(Godwin et al., 2014). half-life for nitrites is reported to be 0.5 h in dogs and
It has been stated that rate of intake for nitrate con- 0.57 h in ponies.
taining forages is a major factor in the potential for toxic
effects (Burrows and Tyrl, 2001). Ingestion of dry forage MECHANISM OF ACTION
containing high nitrate may have greater risk than green
forages, as the rate of intake on a total dry matter basis The nitrite anion causes vasodilation and oxidizes ferrous
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and the rate of microbial exposure to nitrates is much fas- iron (Fe ) in hemoglobin to the ferric iron (Fe ) state
ter (Geurink et al, 1979). To reduce forage trampling, forming methemoglobin, which then cannot accept molec-
farm management may employ limited grazing periods, ular oxygen. The formation of methemoglobin is likely
allowing hungry cattle to consume a large amount of for- rapid with the cumulative development occurring as
age for short periods. This increases risk because of the nitrite is absorbed. As the percentage of methemoglobine-
time dose relationship that exists when excess nitrate is mia rises, oxygen starvation of tissues increases and blood