Nitrate poisoning in livestock

Sheep and cattle are usually able to tolerate and metabolise nitrates in the diet, however poisoning can occur when stock suddenly encounter high levels in the diet without an adaptation period. 

Nitrates in plants and weeds

Common plant and weed species that accumulate nitrates include:

• capeweed
• pigweed
• caltrop
• variegated thistle.

Nitrate/nitrite poisoning is common during drought or after subsequent rain. This is due to soil nitrate increasing greatly as a result of reduced leaching and lower plant uptake, in combination with increasing availability due to organic matter decomposition.

Factors which cause nitrate to accumulate in the plant include:

• drought
• cloudy or cold weather
• herbicide application
• wilting

The amount of nitrate in plant tissues also depends on:

• plant species
• stage of maturity
• part of the plant
• fertiliser application
• soil nitrogen availability

Nitrate concentrations are usually higher in young plants and decrease as plants mature. Most of the plant nitrate is also located in the bottom third of the stalk, hence the leaves contain less nitrate and the flowers or grain contain little to no nitrate.  

When to watch for nitrate poisoning

If you have recently applied nitrogen fertiliser, the risk of nitrate poisoning is higher. The nitrogen in the fertiliser needs rainfall to become available therefor do not introduce stock into a paddock that has had nitrogen applied until 2 weeks after the first effective rainfall event following application.

Producers also need to be mindful of recent herbicide applications. It is always critical to comply with any herbicides grazing withholding period which is stated on the label. Another consideration is that many common broadleaf herbicides will increase the palatability of some weeds. If there are weeds like the ones listed above, they become more palatable and therefore the risk of nitrate poisoning is much higher.

Clinical signs of nitrate poisoning

Symptoms appear within a day of stock grazing affected plants, though sudden death is also possible, so gradual transition onto these feeds is required.

Signs of nitrate poisoning are:

• diarrhoea
• vomiting
• salivation
• abdominal pain.

How to prevent nitrate poisoning

Prevention is pretty straightforward - avoid putting hungry stock straight out onto lush green feed. Fill them up on hay first and shift them on later in the day rather than in the morning, as nitrate levels in the plant are highest in the morning and lowest in the afternoon.

Read more about introducing stock to lush feed.