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This article was published in 1996
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Nitrite Poisoning in Drought Fed Cattle
HC Grant-Frost, DV Mudgee
HISTORY
Property A - Rylstone
Because of the severe 1994/5 drought, a buckwheat crop on the property became severely moisture-stressed and was fed off by a mob of some 50 HF cows in December 1994. What was left of the crop was baled for hay in large round bales and stored on the property.
In late April 1995 the owner began feeding out this hay every 2nd day, and after five such feeds, 10/39 cows were found dead on 10 May. They may have died some 1-2 days before, since the area, being well away from the main property, was not checked every day.
In the absence of D/V Mudgee, D/V Scone investigated the deaths. Because of advanced autolysis no clear diagnosis could be reached, but the owner was told not to feed any more of the suspect hay.
On 30 May 1995, a call was received from the same owner to say that 11/40 HF cows had died overnight. The suspect hay had been fed once more, and these deaths occurred after the 2nd feed.
Aqueous humour and blood were positive to nitrite, using Combur dipsticks, and humour, blood and hay were all positive using the Diphenylamine Blue Reagent. Laboratory tests confirmed the diagnosis.
Property B - Hargraves
On the evening of 8 August 1995, 25/51 Murray Grey cows and calves died within hours of being fed home-grown millet hay. The crop had been sown with nitrogenous fertiliser. The cattle had been fed this hay since May, but only for the last 2 weeks with large round bales. The inner portion of at least some of the bales were described by the owner as damp and hot. The hay had been stored in the open since baling.
Aqueous humour and hay were positive for nitrite using Combur dipsticks and DPB reagent.
DISCUSSION
The obvious question to be answered is:- Why the time lapse between first consumption of the suspect hay and clinical signs? Blood and Radostits provide at least two possibilities:
1. Cattle reduce nitrate in the plant to nitrite in the rumen, and their capacity to do this is enhanced by continued feeding, over a period, of nitrate. This enhanced capacity is due to changes in microbial activity, which obviously takes place over time.
2. High concentrations of nitrite are formed when suspect material is, or becomes, damp and develops some heat.
This factor may be critical in the case of large round or square bales. It is often the case that hot, damp hay material is not reached by the affected animals until the centre of the bale is ingested.
This leads to a supposition that, large bales being relatively new technology, there is a good deal yet to be learnt about the ideal stage of growth of the crop to be baled, drying of the cut crop and subsequent baling. As well, large bales are often seen stored in the open, and therefore subjected to rain, dew and fog - all increasing the moisture content of the material.
The dangers of nitrite toxicity can be minimised by preventing access to suspect material by hungry stock, and by aerating hay overnight before feeding out.
References
1. Blood and Radostits - Veterinary Medicine 7th Edition pp 1279-1282
2. Montomery, J - pers. comm.