Nitrate poisoning is often reported in cattle supplementary fed hay or silage during droughts. The most common risk factor (other than moderate to high levels of nitrates in the fodder) is that cattle are hungry with a poorly adapted rumen prior to ingesting the fodder. However, an additional risk factor occasionally encountered is a marked variability in nitrate levels from bale to bale.
On 20 July 2020, the owner of a mob of 34 Murray Grey/Angus cows reported that four were found dead that morning. The cows had been fed sorghum hybrid (Sudax) hay since 15 May 2020 without loss.The owner fed a bale every third day but increased to a bale every second day as the cows started calving. The pasture was short and green, with an estimated 50% clover and the remainder phalaris and cocksfoot. The owner fed one bale of sorghum hybrid hay in a self-feeder on the 19 July and found four dead cows on the morning of 20 July 2020.
Aqueous humour samples were collected from three dead cows, one of which was necropsied.
Case 1. A mature Angus cow in fat score 3 and late gestation was necropsied. The peripheral blood was possibly brownish and the liver and other tissues were moderately autolysed. One adult fluke was found in the bile duct. There was a large quantity of sorghum hay in the rumen. No other abnormalities were detected. Aqueous humour was collected.
Case 2. A mature Angus cow in fat score 3 due to calve. Aqueous humour was collected.
Case 3. Mature Murray Grey cow in fat score 3 due to calve. Caught in fence, markedly emphysematous carcass. The collected aqueous humour was blood stained and brownish.
Below are the results of the laboratory tests of the aqueous humour (Aq Hu). The samples were tested at the Regional Laboratory Services, Benalla, Victoria. All samples have elevated nitrate and nitrite levels.
|Aq Hu Ca||1.0-2.12 mmol/L||1.55||1.54||1.99|
|Aq Hu Mg||0.61-1.61 mmol/L||0.94||0.94||1.23|
HAY TEST RESULTS
The owner collected samples from four bales for testing. The samples were submitted to Regional Laboratory Services, Benalla, Victoria. Sample 1 was from the hay that was fed in the hay rack.
(mg NO3/kg dry weight)
(mg NO2/kg dry weight)
(mg HCN/kg dry weight)
The guidelines provided by Regional Laboratory Services state that feed concentrations of greater than 5000 mg of nitrate per kg dry weight are potentially toxic to livestock. The guidelines add that toxicity depends on several additional factors, the most important being that livestock have fully functional rumens before ingesting potentially toxic feedstuffs.
The guidelines add that feed concentrations of greater than 200 mg cyanide per kg dry weight are also potentially toxic, but additional factors contribute, most importantly sulphur status and functional rumen capacity.
Sample 1, from the hay available in the rack at the time the cows died had nitrate levels ten times the potentially toxic level whereas the three other samples had low to moderate levels.
I have encountered a marked variation in nitrate levels from bale to bale on several occasions (although none as extreme as this case). Testing fodder nitrate levels is recommended especially for fodder crops grown on soils high in nitrogen (either natural or applied). However, bale variability indicates that multiple bales should be tested and even then risky fodder fed with caution.
The risk of nitrate poisoning can be reduced by not allowing hungry cattle ad lib access to risky fodder, that time is allowed for rumen adaption, that risky fodder is diluted with other feedstuffs and that livestock are also fed a high energy supplement with the risky fodder (Parkinson et al. 2019).