What to look for
- Sudden death in a number of animals
- Animals found alive are depressed, have difficulty breathing and are uncoordinated
- Abortion may occur in pregnant cows weeks after they are exposed to toxic levels of nitrate
- The blood of recently dead animals is a brownish colour
Cause - rapid build up of nitrate in the rumen
Nitrate is a normal component of plants and is usually converted in the rumen to nitrite which is, in turn, changed to ammonia. If nitrate levels in plants are higher than usual and/or the conversion of nitrite in the rumen is too slow, nitrate concentrations in the rumen can build up to toxic levels. Excess nitrate is absorbed into the bloodstream where it binds with haemoglobin and reduces the ability of the blood to carry oxygen.
Nitrate can build up in many plants including pasture species such as rye grass, fodder crops such as millet or brassica, or weeds such as capeweed.
Build up is most likely to occur when:
• the weather is overcast
• plants are having a spurt of growth following a period of stress
• if nitrogenous fertilizers have recently been spread on the pasture
Animals appear to be able to adapt to higher levels of nitrate and this can be assisted by providing some roughage such as hay when animals are grazing lush pastures.
Other diseases with similar signs
Any disease that causes sudden death such as anthrax, bloat, enterotoxaemia (pulpy kidney) or milk fever can be confused with nitrate poisoning.
Confirming the diagnosis
Consider nitrate poisoning when the disease outbreak involves a number of animals that have been on potentially toxic pastures, and the blood of recently dead animals is a brownish colour. Your vet can test blood, urine or fluid from inside the eye for the presence of methaemoglobin. Plants can also be tested for nitrate, but care is needed to interpret the results.
If you suspect nitrate poisoning quietly move the remainder of the mob to a paddock that has less toxic pasture and provide alternative feed such as hay or silage. If sick animals are identified early it is possible for your vet to treat them with methylene blue injections.
• Grazing of lush pastures, especially, if cattle have access first thing in the morning or the conditions are overcast
• Introducing hungry animals to rich pastures without access to roughage, particularly following a period of stress
• Recent application of nitrogenous fertilizers
• Check pasture nitrogen levels before grazing
• Strip graze high risk pastures
• Pre-feed hungry cattle with hay prior to grazing high risk pastures
• Wait for sunny conditions to reduce pasture nitrate levels