It has been fantastic to hear about parts of Australia that have been severely drought-stricken finally receiving enough rain to produce some grass growth. Despite this being great news, there needs to be some caution around letting stock gorge this rapidly growing grass as there is a potential of nitrate poisoning.
What Happens In The Plant
Nitrogen from the soil is absorbed by the plant in the form of nitrates, which are converted into proteins and other nitrogen-containing substances. During drought conditions the amount of nitrate in the soil can increase significantly due to reduced uptake by plants, a lack of leaching and decomposition of organic matter. When a rain event occurs that allows grass to grow rapidly after dry conditions, the uptake of nitrate by plants can be high, especially in the first seven days after rain. To avoid stock losses due to nitrate/nitrite poisoning it is recommended that hungry animals are not given free access to these plants.
Besides drought there are other soil and environmental conditions that can facilitate the accumulation of high nitrate levels in plants and these include:
- Nitrogen containing fertilisers have been used,
- Low levels of sulphur and molybdenum in the soil,
- Areas where stock have congregated (e.g. yards) and urinated/defaecated,
- Cloudy or cold weather,
- Herbicide application (especially hormone-type phenoxy herbicides),
The parts of the plant that are closest to the soil contain the highest concentrations of nitrates. Therefore, leaves contain less nitrate than stalks or stems and the flower and seed usually contains little to no nitrate. The nitrate levels are highest in young plants and decrease as the plant matures. Anything that stunts plant growth increases nitrate accumulation in the lower part of the plant (Thompson, 20200
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:
|Turnip Tops||Cat’s Head (Caltrop)|
|Sugar Beet Tops||Marshmallow|
|Sudan Grass||Fat Hen|
Nitrate is not always toxic to animals, but at elevated levels poisoning can occur. In the ruminant nitrates normally found in forages are converted to nitrite by the digestion process and nitrite is then converted to ammonia. The bacteria in the rumen convert the ammonia to protein that is used by the animal. Ruminants can usually tolerate fairly high levels of nitrate if the intake is spread over the whole feeding day and if enough carbohydrate is also 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).
Nitrite is at least 10 times more toxic than nitrate and nitrite can also be absorbed direct from wet or mouldy hay. If plants that contain high levels of nitrate are rapidly ingested in large quantities the nitrite will accumulate in the rumen and the excess nitrite formed enters the bloodstream where it converts blood haemoglobin to methaemoglobin. This greatly decreases the oxygen-carrying capacity of the blood and results in the tissues being starved of oxygen. If enough methaemoglobin (which cannot carry oxygen) is produced, the animal will die (Stoltenow, 2020). Signs of nitrite poisoning include difficulty breathing (gasping and rapid breaths), increased pulse rate, salivation, excessive urination, abortion, dark, chocolate coloured blood, convulsions and death (Hungerford, 1990).
It has been reported that animals grazing forage containing more than 0.7 g nitrate-N/kg of dry matter may be toxic (McDonald, 2011)
Prevention and Management Strategies
To help reduce the intake of large amounts of nitrates the following strategies are suggested:
- Avoid grazing livestock on high-nitrate pastures or crops for 7 days after periods of rainfall, cloudy days, frosts or days when there are high temperatures that cause wilting.
- Avoid grazing stressed plants or when regrowth is sprouting.
- Prevent access to high-risk weeds around yards/sheds.
- Continue to supplementary feed in the first few weeks after the break of drought.
- Livestock receiving fodder that is rich in carbohydrates tolerate high nitrate and nitrite levels better because the energy from carbohydrates such as grain helps rumen microbes convert nitrite to ammonia.
- Watch stock frequently when put on 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.
- Test feeds and forages for nitrate levels especially from drought-stress small-grain forages.
- Allow cattle time to adapt to increased nitrate intake, this allows time for bacterial populations in the rumen to change.
- 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 don’t overstock pastures when grazing high-nitrate forages.
- Trace mineral supplements and a balanced diet may help prevent nutritional or metabolic disorders associated with long-term excess dietary nitrate consumption (Thompson, 2020). Caution is advised when combining other feed additives such as non-protein nitrogen, growth and performance enhancer and ionophores such as monensin.
After drought breaking rain has been received and livestock have been safely transitioned onto sufficient green pasture don’t forget that there is still a need to balance existing mineral deficiencies and supplementation should focus on the most limiting nutrient. During times when there is adequate energy and protein being supplied in green pasture, mineral supplementation will become a priority via the delivery of a low dose broad-spectrum mineral supplement. If you know your country is deficient in phosphorus then times of green feed is the best time to concentrate on phosphorus. If you are going to have stock grazing forage sorghum or ticks and fly are a constant issue, then supplementing with extra sulphur may be of benefit. Magnesium is also a key mineral for areas where stock are grazing improved pastures, ryegrass and other cereals to help prevent grass tetany.
The early introduction of quality supplements such as MegaMin Mineral Blend, MegaMin Extra Phos 8, MegaMin Extra Sulphur , MegaMin SulPhos, and MegaMin Extra Magnesium, providing animals with effective fibre and allowing rumen microbes time to adjust to feed changes can help animals to become much healthier and more productive from the onset of grazing green feed.