Nitrate Poisoning
In recent year there have been a number of isolated outbreaks of nitrate poisoning in cattle. This year is no exception. Although nitrate poisoning is not common compared with diseases such as salmonella or bloat it can be lethal with significant mortality rates.The outbreaks are usually associated with grazing of cattle of any age group, on capeweed dominant paddocks. These paddocks are more commonly newly sown pastures with young capeweed or regrowth of a brassica crop. They may or may not have been sprayed to control broadleaf weeds.
Typically the disease occurs when hungry cows are placed in such paddocks in overcast and cool conditions. Nitrate levels in plants appear to be greatest in these conditions, especially following a bout of warm weather. Plants also take up nitrates following long periods of dry weather. This is due to concentration in the roots. Once rains arrive, these are released into the plant leaf material in levels toxic to stock.
Nitrates will convert to nitrites in the rumen if conditions are suited. The nitrites are the active substance causing the disease. Cows will become distressed, and will have laboured breathing. This is accentuated if they are moved. As the symptoms progress, they sit and can quickly die through lack of oxygen. Treatment involves injection of methylene blue into the blood stream. Unless cows are treated quickly losses can be high.
Although there have been cases where to half the animals within a group have died, this is rare. Susceptible paddocks can be grazed with caution, making sure that cattle have had access to hay before or that they are at least not moved into such areas after long periods with out feed.
Long term avoidance of this disease can be through rigorous control of broadleafs early in the season and spraying of crop paddocks before resowing in autumn.
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