HOW DOES NITRATE ACCUMULATE IN CROPS?
Rapidly growing plants absorb nitrogen in the form of nitrate (NO3-). Usually, nitrate is rapidly converted into other forms of nitrogen in the roots. Nitrate accumulation within the plant depends on two factors: the rate of uptake from the soil by the plant and the rate at which the plant converts it to other forms.
Under some growing conditions, the nitrate uptake is faster than the conversion process. Nitrate accumulates in the plant and nitrate poisoning can occur when the plant is eaten by stock.
There are a wide range of agricultural crops and weeds that can accumulate high nitrate levels these include perennial and annual grasses, brassicas, red root, oats, wheat, barley, maize, and forage sorghum.
WHAT GROWING CONDITIONS CAN LEAD TO HIGH NITRATE LEVELS?
There are a number of management and environmental factors that can result in high nitrate levels. These include:
- Drought stress. Nitrate builds up in the soil during dry periods. Uptake of nitrate is often high when it rains and the plant begins to grow rapidly.
- Overcast and cloudy weather. During overcast weather, photosynthesis can be slow and this affects the plant’s ability to turn nitrate into less toxic forms.
- Frosted, diseased or stressed crops. The rate of nitrate breakdown is slower in stressed plants.
- Young or rapidly growing plants. Often uptake of nitrate is faster than the rate of breakdown in young and/or rapidly growing crops or pastures. This can be further accentuated by the use of nitrogen fertilizers although nitrate poisoning can still occur in crops that have not had an additional application of nitrogen.
It is impossible to accurately predict the nitrate level of a crop. A nitrate test must be conducted if you suspect that your crop may have high nitrate levels.
HOW DO I TEST FOR NITRATES?
Nitrate can be tested quickly and cheaply (around $30 per sample) at most commercial laboratories. The following sampling protocol is recommended.
- Collect forage or grass samples in the early morning. Levels tend to decrease during favorable (sunny and warm) daytime conditions. An early morning sample will give you the maximum nitrate reading.
- Collect a representative sample of the material that the animal will be eating (e.g. for a crop that will be grazed, take the entire plant from grazing height upwards). The stem tends to have higher nitrate levels than the leaf, therefore, the stem should be included in the sample if the animals will be eating it.
- Collect the sample in a paper bag. Samples in plastic bags can sweat and this results in a loss of nitrates.
- Submit the sample as soon after collection as possible and preferably in the early morning. In most cases, a result will be available the same afternoon.
Retest if required. Forage nitrate levels can change rapidly and moderate-high nitrate level crops should be re-tested if environmental conditions change. Contact your veterinarian for advice.
Various methods of reporting feed nitrates are used by laboratories. The following table should aid in interpreting laboratory results.
Table 1: Nitrate levels and feeding recommendations
|Nitrate Ion (%)||Nitrate Nitrogen (ppm)||Comments|
|0.0 – 0.44||<1,000||Safe to feed under all conditions|
|0.44 – 0.66||1,000 – 1,500||Safe to feed to non-pregnant animals. Limit use for pregnant animals to 50% of the total diet on a DM basis|
|0.66 – 0.88||1,500 – 2,000||Safely fed if limited to 50% of the total DM intake|
|0.88 – 1.54||2,000 – 3,500||Feeds should be limited to 35-40% of the total DM intake. Do not feed to pregnant animals|
|1.54 – 1.76||3,500 – 4,000||Feeds should be limited to 25% of the total DM intake. Do not feed to pregnant animals|
|0ver 1.76||>4,000||Feeds containing these levels are potentially toxic. DO NOT FEED|
Reference: Pioneer Forage Manual, A Nutritional Guide. Pioneer Hi-Bred International Inc., 1990
WHAT HAPPENS IF MY STOCK CONSUME HIGH NITRATE FORAGES?
When an animal consumes nitrate containing plant material, the nitrate is usually converted to nitrite. Rumen microbes transform nitrite into ammonia and this is used to build proteins. However, if the nitrate level is very high, the rate of nitrite breakdown can be slower than the rate at which it is formed. Nitrite is absorbed across the wall of the rumen into the bloodstream. Once in the blood, nitrite takes the place of oxygen in haemoglobin which is the molecule that transports oxygen around the body. As a result, the animal becomes starved of oxygen.
Symptoms of nitrate poisoning include a grey/brown discoloration of the mucous membranes, a rapid pulse, breathlessness, and frequent urination.
Subclinical nitrate poisoning can lower animal productivity.
Severe poisoning can result in abortion in pregnant animals. Death can occur rapidly if large amounts of nitrate are consumed.
If you suspect nitrate poisoning, move the animals immediately from the suspected high nitrate crop or pasture and contact your local veterinarian immediately.
PREVENTING NITRATE POISONING
- Apply nitrogen fertilizers judiciously. Do not apply excessive amounts of nitrogen to crops or pastures. Several small applications will be safer than one large application.
- Test plant nitrate levels. At risk crops (e.g. greenfeed cereal crops (including forage sorghum), young or stressed greenfeed maize crops, frosted or drought-stressed maize silage crops, brassicas, young or rapidly growing grasses) should be tested prior to feeding, ensiling or haymaking. Your local veterinarian should be able to arrange a nitrate test if you suspect that the nitrate levels may be high.
- Delay crop harvest. Warm, sunny days will help to reduce nitrate levels in most crops as long as moisture is not limiting. Waiting for a few days after a frost or drought-breaking rains is recommended. Always nitrate test before feeding or harvesting.
- Do not feed high nitrate crops while they are damp. Damp feed appears to be more toxic. Wait for dew or rain to dry before feeding the crop.
- Feed low nitrate feeds first. Always feed high nitrate crops directly after animals have been fed on a low nitrate forage (e.g. mature pasture, grass silage, maize silage or Lucerne). This will ensure that the rumen is full before the animals begin feeding on the high nitrate crop which reduces the risk of gorging. Feeding a high carbohydrate feed (e.g. maize silage or grain) in conjunction with a high nitrate feed will help the conversion of nitrate to ammonia within the rumen.
- Dilute high nitrate feeds. Where nitrate levels are high, feeding a small amount over a longer period of time will ensure that the overall dietary nitrate level is reduced. Aim to keep dietary nitrates within the acceptable levels (Table 1). Several small feeds per day of a high nitrate feed will be less risky than the same amount fed once a day. Watch water nitrate levels when feeding high nitrate diets.
- Do not feed high nitrate crops to young, pregnant or unhealthy animals. Young animals (less than 6 months), pregnant animals and animals that are in poor health will be more affected by high nitrate levels.
- Consider making silage. Where it is necessary to harvest a high nitrate crop consider making silage rather than hay. Nitrate levels do not decrease during haymaking however they usually reduce during the ensiling process. Actual decreases in crop nitrates during ensiling have been variable. Always test for nitrates before feeding silage made from a high nitrate crop.