Nitrate Poisoning

With new grass paddocks about to be grazed it is important to remind you of the risks associated with nitrate poisoning. Nitrate poisoning is quite common in new grass paddocks especially short rotation Italian ryegrasses, and it can occur in crops such as sorghum or oats.

What is nitrate?

Nitrate is made by micro-organisms in the soil from the breakdown of organic matter or N from fertilisers. It is one of the two forms of N that plants can take up from the soil for use to make protein. Nitrate is most likely to accumulate in soil that has been cultivated following the harvest of crops, as oxygen is introduced into the soil stimulating soil bacteria and additional organic matter is present due to the breakdown of the crop residues. If nitrogen fertiliser is also added at this time then nitrate levels will build up to high levels in the soil as the new pasture is not mature enough to utilise the excess that is being produced. The new pasture tends to reach maturity and thus maximum uptake of nitrate about the time it is due to be grazed. If excess amounts of nitrate are present in the soil then there is an increased risk of excess levels of nitrate being present in the pasture. Plants convert nitrate to proteins via photosynthesis, so weather conditions such as rainy or overcast days prevent photosynthesis and lead to the accumulation of nitrate to toxic levels in the plant leaves.    

What happens and what are the signs?

Microbes in the rumen usually convert nitrate to ammonia, however when the amount of nitrate goes over 1% the rumen microbes are unable to convert all the nitrate in the grass which results in an accumulation of a compound called nitrite in the rumen. Nitrite is absorbed into the blood stream where it combines with the red blood cells preventing oxygen from binding with them. By the time 20% of the red blood cells have become affected the animal begins to show signs such as breathing faster and looking drunk. They may also salivate and froth at the mouth, and as more red blood cells become affected the cows start to gasp for breath.

Severely affected animals will often go down, and by the time 80% of red blood cells are affected they will die. The animal is essentially suffocating. 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 if you suspect nitrate poisoning?

Call the vet immediately and outline how many animals are affected. Extra help may be required if the outbreak is severe.

The first thing to do is move animals that are able to walk out of the paddock. If possible feed them some maize silage, PKE or hay. Treatment of affected animals is possible with a product called methylene blue (MB) injected into the vein. This “knocks” the nitrite off the red blood cells allowing them to carry oxygen again. In severe poisonings, more nitrite may be absorbed from the gut following treatment causing relapses, and requiring further treatment.

How to avoid nitrate poisoning?

If growing annual ryegrass in paddocks that have been cropped it is likely that less N fertiliser will be required as there will be moderate to high levels of nitrate already present in the soil. Reducing the amount of N applied to new grass and/or timing of fertiliser applications to when plants are more mature and thus will utilise the N better is advisable. 

Always test for nitrate levels in at risk crops, especially grass crops before first grazing, or in cloudy    conditions. If there is a risk of high nitrate levels then ensure cows are fed before grazing the crop. Only graze the crop for 1-2 hours, preferably in the afternoon when the crop has had maximum exposure to sunlight to help decrease the nitrate levels. If the crop is very high in nitrate, cows can get poisoned after the first grazing, however poisoning will most likely occur on day 3 and 4 of grazing, as bacteria that form nitrite from nitrate are beginning to multiply in the cow’s gut.