Livestock farmers should be careful when allowing their animals to graze any regrowth in pastures that are recovering from the exceptional drought much of New Zealand has experienced this year – because farm animals are especially vulnerable to nitrate poisoning at this time.
During a drought nitrate levels can increase greatly in the soil because there is no leaching, a reduced uptake by pasture plants and the soil organic matter decomposes during the drought, releasing more nitrogen (N).
Nitrate uptake by plants will be high once the drought breaks, especially during the first week after rainfall has resumed.
Stock losses from nitrate and nitrite poisoning can be disastrous if hungry animals are allowed free access to such pastures.
Cattle, sheep, deer and even goats are all prone nitrate poisoning, cattle being most susceptible and sheep the least sensitive.
After a drought has broken any cattle grazing young autumn-sown ryegrass can also be very prone to this disorder, and sometimes greenfeed cereal crops are also toxic when sown.
When cattle having been put onto toxic pasture in a morning they can show signs of nitrate poisoning within four hours.
The nitrate needs to be converted into more toxic nitrite in the animal's rumen before toxicity symptoms are seen by the farmer.
Nitrate is not always toxic to animals but when ruminants eat feed containing high nitrate levels the nitrate is converted to nitrite by rumen microbes.
Poisoned livestock appear to be drunk, staggering and their condition deteriorates rapidly, leading to death. Most animals are found dead.
Farmers can help to prevent deaths due to nitrate poisoning by:
- Checking their livestock regularly for signs of poisoning during this after-drought period.
- Testing new pasture and suspect crops before grazing them after the drought. If farmers don't have their own nitrate testing kit they can take a pasture sample to a veterinary clinic and should be able to receive a report the same day.
- Nitrate levels build up in pastures during the night and sunlight lowers the levels, so a farmer should introduce cattle to new grazing either in late morning or early afternoon.
- Feeding some hay or silage will ensure that cattle are not hungry before putting them onto a new grazing block.
- Reducing the time period cattle are grazed on suspect pasture. Allow cattle less than two hours grazing on a suspect pasture or fresh and lush crop or newly sown pasture.