A few years ago, I received a call in late spring to a farm where a herd was falling over, several had died and many were having trouble breathing. When I got some information from the client he informed me he had held his cows back and had released them all at once into a paddock of new grass.
What was seen is typical of nitrate poisoning, which is a relatively common problem in grazing livestock in our temperate climate, and often follows when livestock graze young, rapidly growing pastures and crops that haven't converted all their nitrates into protein.
The results of nitrate poisoning can be very dramatic, with animals often giving no warning and collapsing suddenly. Accumulation of nitrites in the rumen leads to these being absorbed into the blood stream. The nitrites bind with haemoglobin and thus limits the oxygen carrying capacity of the red blood cells. Animals become oxygen starved and without prompt treatment can die.
Which animals are commonly affected?
Sheep, cattle, deer and goats all get nitrate poisoning. Cattle are the most susceptible, and sheep the most resistant.
What symptoms can I expect to see?
- Rapid breathing, weakness, tremors and imbalance are the first signs (animals often 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 if untreated may 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.
- When administering IV treatments (usually used is Methylene Blue) it should be evident that the blood has turned a ‘chocolate brown' colour. This is due to the presence of high levels of methaemoglobin in the blood (a consequence of nitrites binding to the haemoglobin).
How long does toxicity take to occur?
A cow can consume a toxic amount of nitrate in one hour, and can start to show signs very soon after. If cows are grazing a toxic paddock, then there can very quickly be a number of animals affected and it becomes real emergency. It can take up to four hours for signs to become evident so animals should be monitored closely during this one to four hour period when grazing suspect crops.
What climatic conditions predispose plants to accumulating high levels of Nitrate?
Basically, conditions which cause rapid growth spurts will predispose plants to accumulating high levels of nitrate. It is commonly seen under the following two scenarios:
- Drought conditions allowing build-up of nitrate
in and around roots of the plants followed by a significant rain event (sudden uptake of
nutrients and growth spurt of plant).
- Warm/humid overcast weather enabling good growing conditions but limited photosynthesis opportunity for the plants to convert nitrates into protein.
How do I minimise the risk of nitrate poisoning?
The best advice is to test any suspect crops prior to feeding. Samples need to be at least a full bread bag size. Totally Vets will have samples analysed on the same day if samples are brought in early in the day. 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. If results is above 2% don't feed till retested.