Nitrate toxicity in cattle can happen quickly and can result in multiple deaths in the herd. There are many factors that lead to death including the level of nitrates in feed consumed and how hungry cattle were at the time feed was offered. The amount of nitrate in the soil can increase significantly during drought conditions, so when a rain event occurs, grass grows rapidly and the uptake of nitrate by plants can be high, especially in the first seven days after rain.
Additional soil and environmental conditions that can contribute to the accumulation of high nitrate levels in plants include:
- Pasture where Nitrogen fertiliser has been applied,
- Low levels of sulphur and molybdenum in the soil,
- Areas where stock have congregated and urinated/defaecated (e.g. yards),
- Cloudy or cold weather,
- Herbicide application (especially hormone-type phenoxy herbicides),
- Wilting.
The nitrate levels are highest in young plants and decrease as the plant matures. The highest concentration of nitrates occurs in the parts of the plant closest to the soil. Therefore, there is less nitrate in leaves than stalk or stem and the flower and seed usually contain little to no nitrate. Anything that stunts plant growth increases nitrate accumulation in the lower part of the plant (Thompson, 2020).
Certain plant species are also known to accumulate higher levels of nitrate than others and have been associated with nitrate/nitrite poisoning. These are listed in the following table:
| Crops/Pasture | Weeds |
| Oats | Capeweed |
| Rye Grass | Mintweed |
| Maize/Corn | Variegated Thistle |
| Sorghum | Pigweed |
| Lucerne | Crown Beard |
| Rape | Cat’s Head (Caltrop) |
| Turnips | Redroot |
| Kikuyu | Marshmallow |
| Sugar Beet Tops | Fat Hen |
| Sudan Grass | Blackberry |
| Wheat/Barley | |
| Soybean | |
| Pearl Millet |
Nitrate/Nitrite Poisoning
Nitrate is not always toxic to livestock, but at elevated levels, poisoning can occur. In the ruminant, nitrates are converted to nitrite by the digestion process and nitrite is then converted to ammonia. This ammonia is converted by rumen microbes into protein which is then used by the animal for growth.
Ruminants can usually tolerate fairly high levels of nitrate if the intake is spread over the whole feeding day and if enough carbohydrate is provided to fuel rumen microbial activity. When a large amount of nitrate is consumed, it has a caustic effect on the lining of the gut. Signs of nitrate poisoning include abdominal pain, diarrhoea and salivation (Robson, 2020).
Prevention and Management Strategies
To help reduce the intake of large amounts of nitrates the following strategies are recommended:
- Continue to supplementary feed in the first few weeks after the break of drought.
- Avoid grazing livestock on high-nitrate pastures for 7 days after rainfall, on cloudy days, frosts or days when high temperatures cause wilting.
- Avoid grazing stressed plants or when regrowth is sprouting.
- Prevent access to high-risk weeds around yards and sheds.
- Livestock receiving fodder that is rich in carbohydrates tolerate higher levels of nitrate because the energy from carbohydrates such as grain helps rumen microbes to convert nitrite to ammonia.
- Watch stock frequently when grazing potentially risky feed. Animals that are stressed or in poor health or condition will be more susceptible to nitrate/nitrite poisoning.
- Feed hungry stock on dry hay or mature grass before allowing free access to immature cereal crops.
- Prior to feeding, test Nitrate levels of high risk feed such as drought-stressed, small-grain forages or hay and silage. Rapid test kits are available from Feed Central.
- To allow time for bacterial populations in the rumen to adapt to the increased intake of Nitrate, introduce livestock to high risk feed slowly.
- Provide access to good quality water at all times. Ensure that water does not contain high levels of nitrates.
- Never feed mouldy hay.
- Make sure you donít overstock pastures when grazing high-nitrate forages.
Caution is advised when combining other feed additives such as non-protein nitrogen, growth and performance enhancer and ionophores such as Monensin.
The Importance of Minerals:
Trace mineral supplements and a balanced diet may help prevent nutritional or metabolic disorders associated with long-term excess dietary nitrate consumption (Thompson, 2020).
Even though there is adequate energy and protein in green grass after drought breaking rain, clinical or sub-clinical mineral deficiencies can restrict livestock performance.
In addition to broad spectrum minerals, extra Phosphorus, Sulphur and Magnesium are important for the following conditions:
- Phosphorus, is a production mineral. If your country is deficient in phosphorus and there is green feed, this is an ideal time to supplement with Phosphorus to maximise production.
- Extra Sulphur is beneficial for stock grazing forage sorghum, as well as assisting to control of ticks and fly.
- Magnesium is a key mineral to help to prevent grass tetany for stock grazing improved pastures such as ryegrass and other cereals.
Maximise livestock productivity when grazing green feed:
From the onset of grazing green feed, the following steps will assist livestock be more productive:
- Introduce quality supplements early.
- Provide animals with effective fibre.
- Allow rumen microbes time to adjust to feed changes.
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