Too much nitrate can be nasty!
When we get a call-out for suspected nitrate poisoning we drive as fast as we’re legally allowed and literally run from our ute to the affected animal(s) as seconds can make a difference! Nitrate poisoning can be nasty and with the recent drought conditions potentially increasing the risk of it we thought a quick refresher on it wouldn’t be a bad idea…
Nitrate is naturally present within grass/plants and in itself is not toxic to animals. However once eaten it is converted by micro-organisms in the rumen to nitrite then to ammonia and used as building blocks for protein. If nitrite levels in the rumen rise faster than it can be converted to ammonia then it is absorbed into the bloodstream. Within the bloodstream it binds to haemoglobin (the oxygen carrying part of blood) turning it into methaemoglobin which cannot transport oxygen. If methaemoglobin levels get too high an animal will die from lack of oxygen supply – it’s essentially the same as carbon monoxide poisoning.
If caught early treatment is relatively cheap and easy however, like many things, nitrate poisoning is certainly far better prevented in the first place! Feed nitrate levels can be tested either through a laboratory or using an at home nitrate test kit. If levels are high (that being greater than 2.0 grams nitrate per kilogram dry matter) it is ideal to avoid letting stock graze that feed, however the reality is that this is often not possible. In such circumstances follow the basic principles below to help decrease the risk:
Aim to reduce total amount of nitrate affected feed consumed and decrease rate at which feed is eaten. Most problems occur in the first few days of feeding so introduce stock gradually, starting with one to two hours, over seven to ten days. Do not put stock on when they’re hungry - fill them up first, such as with hay. Additionally, ensure you provide a break that is long across the face and not too deep so all animals have good access as this will reduce the likelihood of some individuals over-eating. Lastly, also always ensure stock have access to fresh clean water.
Be aware of particular plant risk factors. These include age of plant - all young rapidly growing forage and fodder crops can accumulate excess nitrate during summer and autumn. Allow the crop to mature but feed before it has flowered. Grazing level of plant is also important as nitrate levels tend to be higher in the lowest third of the stalk. Plant type should also be considered - there may be increased danger with first grazing of newly sown perennial ryegrasses, first couple of grazings of short rotation ryegrasses (eg. Tama, Tabu, Feast and Archie), forage crops (eg. oats, triticale, sorghum), and brassica crops (eg. turnips, chou, rape).
Dull, cloudy and humid weather is a major environmental risk factor as it means reduced photosynthesis which means a decrease in conversion of nitrite to ammonia. Moderate drought stress causes nitrate to accumulate within the plant, while severe drought stress reduces nitrate uptake through roots but, once it rains, nitrate uptake is rapid and plant levels will remain high for several days. Lastly is fertiliser use. High levels of nitrogen fertiliser can predispose plants to nitrate accumulation particularly if plant growth is limited by other factors, such as lack of moisture or low temperatures.
Monitoring stock well and knowing some basic symptoms of nitrate poisoning will ensure early identification of a potential issue. Symptoms usually show within an hour or two of eating excess nitrates and include increased salivation, diarrhoea, weakness/muscle tremors, open mouth breathing and being “down”. Death can occur within one to 24 hours after eating toxic feed.
If you have any concerns about possible nitrate poisoning then remove all animals from the suspect feed quietly (you do NOT want to stress them!) but quickly. Provide them with a carbohydrate rich feed, such as good quality hay, maize silage or palm kernal and seek advice from your vet immediately!