Page 1009 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

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VetBooks.ir Chapter 65

Nitrate- and Nitrite-Accumulating Plants

Jeffery O. Hall

INTRODUCTION Monogastric animals are less susceptible to the toxic
effects of nitrate ingestion. Horses and rabbits can convert
Nitrate poisoning is a commonly encountered disease state
some nitrate to nitrite in the large bowel (Bruning-Fann
in herbivores. Ruminants are the most susceptible to acute
and Kaneene, 1993), but are much less sensitive than
nitrate poisoning. Microflora in the rumen reduce nitrates
ruminants, with nitrate toxicosis in horses being rarely
to nitrites and then ammonia for microbial growth. Excess
reported (Hintz and Thompson, 1998; Oruc et al., 2010).
intake of nitrates may overwhelm further reduction capac-
Other monogastric species are susceptible to the systemic
ity, allowing toxic amounts of nitrite to accumulate and be toxic effects of nitrites, when ingested as nitrites from
absorbed into the blood, where it causes the formation of
nonplant sources (Worth et al., 1997).
methemoglobin. Thus, even though the clinical syndrome
is referred to as nitrate poisoning, it is actually poisoning
by the nitrite ion. Goats browse leafy portions of plants BACKGROUND
and may not ingest toxic amounts of nitrates as readily as
sheep and cattle since higher concentrations of nitrate When evaluating the potential for nitrate poisoning, for-
occur in stalks and stems of accumulating forages. Sheep age and water can be reported in several ways. For clarity,
are not as commonly poisoned as cattle, which may be due this chapter will use nitrate ion concentrations. Other
to the fact that they eat slower than cattle. Ruminants have means of expressing nitrates can be conversion to the
the potential to graze or be fed a variety of grasses and nitrate ion via the following calculations:
weeds that can accumulate large amounts of nitrate that
Potassium nitrate X 0.61 5 Nitrate ion
may prove toxic if eaten in a sufficient dose. Excessive
Sodium nitrate X 0.72 5 Nitrate ion
nitrate may be present in grazed forages and weeds, hay, or
Nitrate-Nitrogen X 4.4 5 Nitrate ion
fresh cut forage fed to animals as green-chop. Excessive
fertilization, drought, herbicides, hail damage, or reduced Plants take up nitrogen from the soil primarily in the
sunlight can enhance plant accumulation of nitrates via form of the nitrate ion. Excessive accumulation of nitrates
increased uptake or diminished metabolism and utilization can be associated with excessive uptake or hindered utili-
for plant protein synthesis. Forages and weeds growing in zation of the nitrates for protein and other nitrogen con-
soil rich in manure waste or in holding pens are a potential taining chemicals. Numerous plant species have the
source of poisoning. It should be noted that ensiling may potential to accumulate toxic amounts of nitrates, but cer-
reduce forage nitrate by 30% or more over 30 60 days, tain ones are more commonly found to have high nitrate
but silage can still contain toxic amounts of nitrates if the concentrations (Table 65.1). Nitrate accumulation in the
original materials were very high. Nitrate fertilizers are stems and leaves of plants may be associated with exces-
commonly used around livestock and accidental ingestion sive concentrations of nitrogen (nitrates or ammonia) in
or feed contamination occasionally occurs. For example, the soil. Young plants are more likely to have high
water hauled in tanks previously used to transport nitrate- nitrates than are more mature plants. For example, nitrate
based fertilizer can be a source of exposure. When evaluat- concentrations decline considerably in all parts of sudan-
ing the potential for nitrate/nitrite poisoning, one must grasses (Sorghum spp.) following maturation or heading
include both forages and water sources, as nitrates and (Whitehead and Moxon, 1952; Mizukami et al., 1997).
nitrites are soluble and may contaminate available water Plants growing in highly fertilized soil, where large
sources. amounts of livestock manure/urine are applied as

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