Nitrate Poisoning

With more use of crops for dairy cows and higher nitrogen application to grass, most people are familiar with the term "nitrate poisoning". However, as veterinarians, we still see cases every year and they are always a very costly incident.

Well known culprits for nitrate poisoning are sorghum crops, turnip leaves, oats and new grass paddocks, especially short rotation Italian ryegrasses. Climatic conditions which predispose to high plant nitrate levels are: drought (nitrate builds up in and around roots of plant) followed by rain (sudden uptake of nutrients and growth spurt of plant) and also: cloudy weather with otherwise good growing conditions.

What is nitrate? Nitrate is the building block for protein materials in the plant. It is taken up from the soil, then converted into protein by the action of photosynthesis (means by which plant converts sun's energy into plant material). Some plants store more nitrate than others. In general, the highest concentration of nitrate is found in the roots and in the stalks and stems, rather than the leaves. Plants with high nitrate levels tend to be grown in well fertilised soils, and have often had nitrogenous fertilizers applied. Plants containing more than 1% potassium nitrate are toxic to grazing animals.

Which animals are affected? Sheep, cattle, deer, goats all get nitrate poisoning.  Cattle more susceptible, and sheep most resistant.

What happens and what are the signs? When plant levels are below 1%, nitrate is converted in the rumen to ammonia and hydroxylamine. When the amount of nitrate goes over this level, the reduction of nitrate to ammonia is limited, and there is an accumulation of nitrite, which is nitrate (N03-) less one oxygen atom (NO2-). The nitrite is then absorbed into the blood stream where it combines with haemoglobin.

Haemoglobin is a protein molecule in the red blood cells, and carries oxygen. Oxygen "jumps" on and off haemoglobin quite happily, but nitrite forms methaemoglobin, which does not carry oxygen, and does not go back to haemoglobin. By the time 20% of the haemoglobin has been converted to methaemoglobin, the body is beginning to suffer, and you see cattle start to breathe faster and become unbalanced (they look drunk in the early stages). They will also salivate and froth at the mouth, and then start to gasp for breath. Affected cows will then go down, and by the time 80% of the haemoglobin is methaemoglobin, they die. The staggering is due to lack of oxygen to the brain. The gasping for breath is basically a reflex to try and get more oxygen into the system. The animal is essentially suffocating.

How long does this take? a cow can consume a toxic amount of nitrate in one hour, and will start to show signs very soon after. If cows are grazing a toxic paddock, then there will very quickly be more than one animal affected and it is a real emergency.

What do you do? Call the vet and outline how many animals are affected. The vet will get extra help if the outbreak is severe. In the meantime, remove animals that are able to walk out of the paddock. If possible, feed them a high energy feed – eg. maize silage, PKE, hay. When the vet arrives, they will start treating cows that are down, and move on to those less affected. The treatment is to inject methylene blue (MB) into vein. This converts the methaemoglobin into oxyhaemoglobin. In blood MB is converted into leucomethylene blue which reduces the methaemoglobin to oxyhaemoglobin. If more nitrate is absorbed from the gut, the animal may relapse and need more treatment.

How to avoid nitrate poisoning? Test for nitrate levels in at risk crops, especially after a drought, or in cloudy conditions. If crop is above 1% nitrate content, then give other food first, especially high energy food, so that rumen bacteria can work at full speed. Graze the crop for 1-2 hours only. Graze in afternoon when the crop has had maximum exposure to sunlight, which decreases the nitrate levels. Poisoning most likely to occur on day 3 and 4 of grazing, as bacteria forming nitrite from nitrate are beginning to multiply in gut. However if crop is very high in nitrate, can get poisoning after first grazing.

Ali Cullum
Intelact Dairy Consultant
Animal Health Centre Morrinsville