Nitrate poisoning can result in a number of deaths and abortions and therefore can be a huge blow to the system. Autumn is the main risk period and you need to be extra cautious during this period.
Many common pasture plants including ryegrasses, cereals (oats and maize) and brassica crops (kale, rape) naturally accumulate nitrates. Nitrate is the major plant form of nitrogen. Certain conditions favour nitrate accumulation, for example; warm, wet and overcast conditions common in autumn. This is especially true on high fertility soil or soils recently fertilised with nitrogen.
Low sunlight reduces the plants usual process of converting the nitrates into “safe” ammonia. Although any insult such as insect damage, frost and freezing can also reduce the nitrate conversion process and result in high levels of nitrate in the plant. Plant stunting and wilting associated with drought is another source of plant stress.
Other risk factors include over application of inorganic nitrogenous fertilisers and excess application of farm effluent. It takes 4-6 days for fertiliser added to ryegrass to dissolve and be taken up by the crop. Nitrogen levels peak 10-14 days post application in optimum growing conditions. Any stunting of growth will extend this. Brassica crops can take up to 6 weeks to return to “safe” values.
A single intake of nitrate over a short period is more toxic than chronic accumulation over several days/weeks. Allowing hungry cattle access to sources of nitrates will increase the likelihood of toxicity.
Toxic Effect of Nitrate
If nitrate levels are excessive then the rumen’s bacteria are overloaded and cannot complete the conversion of nitrate to ammonia and an intermediate form, nitrite, builds up. The accumulation of nitrite is then absorbed and bonds to the blood molecule haemoglobin, converting it to methaemoglobin. Methaemoglobin is incapable of binding oxygen.
Often the result is sudden death (less than 5 hours after ingestion) or abortion. Some cows may show clinical signs such as increased salivation, abdominal pain, diarrhoea, muscle tremors, weakness, staggering, open mouth breathing and blue mucous membranes and the classic brown discolouration of blood.
Diagnosis is usually on history and sudden death of cattle. To confirm, urine and blood can be used to test the concentration of nitrate.
If nitrate poisoning is suspected then cattle should be removed from the suspected source and fed a high quality diet such as hay. The antidote for nitrate poisoning is methylene blue, this is given intravenously with the least severely affected animals treated first. This treatment is off label and has long withholding periods, 4 days for milk and a 180 days for meat.