Nitrate can accumulate in many weed, crop and pasture plants. Forage sorghum, grain sorghum, sudan grass, sudan grass hybrids and pearl millet are well recognised nitrate accumulators.
Plants accumulate nitrate when soil nitrate is high but conditions are not suitable for normal forage growth, which would allow the nitrate to be converted to protein. This can occur during droughts, cloudy weather, cold weather, when plants are wilted, after herbicide application or with combinations of any of these factors.
Nitrate will persist in hay. Nitrate is converted to nitrite in the rumen. The nitrite is absorbed and prevents the blood from carrying oxygen. The blood changes to a brown colour (the colour of methaemoglobin).
Plants containing more than 1.5 per cent potassium nitrate (KNO3) on a dry weight basis are potentially dangerous for hungry stock. The nitrate content on a dry weight basis of sorghum crops in Queensland has been recorded as high as 6.4 per cent. Sorghum hay in Queensland has reached levels of 12 per cent.
Animals can tolerate higher concentrations of nitrate if it is introduced to them gradually, as this allows time for modification of bacterial populations in the rumen. These additional bacteria help the animal break down the nitrates. However, any sudden increase in feed intake or the feeding of supplements containing monensin can lead to poisoning because they reduce the animal's capacity to break down this nitrate.