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Nitrate poisoning in merino ewes introduced to a canola stubble

Bruce Watt, Central Tablelands Local Land Service, Bathurst, and Patrick Staples and Erika Bunker, State Veterinary Diagnostic Laboratory, EMAI, Menangle

Posted Flock & Herd July 2017


Ruminants are particularly susceptible to nitrate poisoning because rumen microflora convert nitrate to ammonia via highly toxic nitrites (Merck Veterinary Manual, 2016). NSW laboratory submissions from January 1 2007 until 8 December 2015 include 57 cases in which nitrate poisoning was suspected or confirmed. Most (43) were in cattle, while 13 cases occurred in sheep and one in goats. While these case reports are yet to be analysed it is considered that most cases of nitrate poisoning in NSW occur when hungry livestock gain access to forages high in nitrates.


On December 5 2015, the owner of a mob of 400 September shorn 3-5 year old merino ewes found 17 dead the morning after moving them into a canola stubble paddock. The ewes were mustered 36 hours previously for classing and to wean the lambs. The ewes walked to and from the yards without difficulty. The canola stubble had few obvious weeds apart from dry annual ryegrass (Lolium rigidum), barley grass (Hordeum leporinum). Abundant dry variegated thistle (Silybum marianum) was found on rocky outcrops in the paddock. The surviving ewes that had been immediately moved to a lucerne paddock showed no signs of illness.

nitrate poisoning ewes one ewe found dead
Figure 1. One of 17 ewes found dead in canola stubble.


Four ewes were necropsied. The only consistent finding on necropsy of the mildly autolysed carcases was wet, red lungs and frothy white foam in the trachea. Some had haemorrhages in the epicardium and the rumens contained dry, fibrous material.

nitrate poisoning ewes frothy tracheal contents
Figure 2. Frothy tracheal contents


Aqueous humor samples revealed normal levels of calcium, magnesium and glucose but markedly elevated levels of nitrate and nitrite. There is no history of anthrax on this property or district and smears taken for anthrax were also negative.

SAMPLE Normal range 1 2 3 4
Glucose <4.4 mmol/L 2.0 0.5 0.1 0.3
Ca 1.0-2.4 mmol/L 1.65 1.42 1.73 1.07
Mg 0.48-1.5 mmol/L 1.01 1.62 1.06 0.94
D-lact 0-0.5 mmol/L 0.0 0.0 0.0 0.0
Ammonia 0-200 umol/L 497 1490 586 691
NĂ­trate <10 mg/L 318 286 200 365
Nitrite <1 mgl/L 9.3 9.7 7.3 12.0
Table 1. Aqueous humor test results

Samples of the canola stem and chaff were collected from the paddock at the time of the investigation. Samples of dry canola leaf and dry variegated thistle leaf were collected by the owner on the 24 December. All samples had elevated nitrate levels and levels in the variegated thistle leaf were particularly high.

SAMPLE Dry weight % Nitrate (mg NO3/kg dry weight) Nitrite (mg NO2/kg dry weight)
Canola leaf 93 5750 <35
Thistle leaf 88 11200 <35
Canola stem/chaff 93 6670 <35
Table 2. Nitrate and nitrite levels of canola residue and dry variegated thistle leaf


Because the deaths occurred suddenly, in a new paddock following 36 hours off feed, hypocalcaemia and nitrate poisoning were considered the two most likely diagnoses in this case. Anthrax was considered to be unlikely both because there is no history of anthrax in this area and because the pattern of deaths, a large overnight mortality with no further illness or death, was not regarded as consistent with anthrax. Nonetheless it was considered important to exclude anthrax.

Ruminal acidosis was considered to be a possibility although the ewes had been introduced into canola stubble, not cereal stubble (and therefore spilled grain). Enterotoxaemia and ammonia/protein toxicity were considered possible but unlikely.

Many of these possibilities were ruled out based on testing of aqueous humor, especially in the light of the diagnostic levels of nitrate and nitrite.

Variegated thistle is regarded as a nitrate accumulator and is well known as a risk of nitrate poisoning. In this case both the canola residue and the dry thistle leaves were high in nitrates and both remain candidates as the source of toxin, although there was no evidence that the sheep had grazed the thistle.


  1. Merck Veterinary Manual, Overview of nitrate and nitrate poisoning www.merckvetmanual.com accessed 20 July 2016


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