Beware of nitrate poisoning in drought affected regions

Cases of nitrate poisoning are being reported in the Hawke's Bay region after recent rain.
Wednesday, 8 July 2020

While typically associated with new pastures and forage crops, anecdotal evidence suggest it is also occurring in stock grazing older pastures.

Below is a summary of nitrate poisoning written for B+LNZ by Ginny Dodunski of Total Vets. It was first posted in May, but given the unusual climatic conditions, it is worth reminding farmers to be aware of this often fatal condition.

Why is the risk higher after drought?

Drought conditions allow build-up of nitrate in and around roots of the plant. When followed by a significant rain event we get a sudden uptake of nutrients and rapid plant growth.

The warm and overcast weather that accompanies these good growing conditions limits photosynthesis, and plants are unable to convert nitrates into protein. Forage crops, especially cereals, and autumn sown new pastures are the top candidates for this phenomenon.

How does it happen?

Ingested plant nitrates get changed to nitrite in the rumen. This is rapidly absorbed into the bloodstream where nitrite binds with haemoglobin on the red blood cells and prevents them from doing their job of carrying oxygen to the tissues.

Animals become oxygen starved and without prompt treatment can die. Your vet can race out and administer an antidote I/V, but prevention is far preferable!

Cattle are the most susceptible, especially pregnant cattle. Pregnant ewes are more susceptible than lambs but be aware that in bad years lambs can go down with it too.

What might you see?

Rapid breathing, weakness, tremors and imbalance are the first signs. Animals often look ‘drunk’ in the early stages. As the condition progresses, animals salivate and froth at the mouth, and then start to gasp for breath and go down. But the progression of these signs can be so fast that all you find is dead animals. Animals can ingest enough toxic feed in one hour to start showing signs.

How do I minimise the risk of nitrate poisoning?

  • TEST ANY FEED YOU THINK COULD BE A RISK.
  • I can’t overemphasise this. Don’t ring your vet or agronomist and ask if your crop/grass is risky, we can’t tell over the phone!!!!
  • Most rural vet clinics will have a nitrate dipstick test available that they can run in-clinic. Levels under 50mg/L are safe to feed, 50-100 is safe to feed with some provisos, over 100 is dangerous, but levels run up to 500mg/L – ask for the exact number.
  • Others may send your samples to the laboratory; if this is the case for you, ensure you request a QUANTITATIVE nitrate test; this gives the exact percentage of nitrate in the crop: 2% is dangerous, but 5% is far more dangerous.
  • Be aware that nitrate levels may vary across a paddock, so sample small amounts from many spots. But we don’t want a whole feed bag full, half a bread bag of grass or oats is plenty.
  • For brassica crops it is helpful to test the leaf and stem separately so don’t just submit leaf. Bring in small chopped bits from across the paddock rather than a great big bag of whole plants. Pop samples in refrigerator if you’re not bringing it in immediately.
  • Be aware that high nitrate crops can take many weeks to drop. Keep checking them at 7 days intervals. Crops at the 500mg/L level may take 6 weeks or more to fall into the safe range.

Can I feed off a high nitrate crop?

The answer to this is ‘maybe’. And PLEASE get good advice and have a clear plan that everyone understands if you intend to take this risk. The guidelines above are there for a reason and you take this risk at your own responsibility.

This is where it’s important to know the exact nitrate level. Any access to feed at 500mg/L is probably too much of a risk, whereas very careful access to feed at 100mg/L may be lower risk.

Crops in the 1-2% range or at the 100mg/L level may be able to be fed off in a very limited way. Remember the 1-hour rule above. Stay close and don’t allow more than one hour of access at first (20 to 30 minutes is much safer) small breaks, and good power going through your fence are all good rules.

Put animals onto their break with full tummies (feed out baleage or hay first), and if possible, also a good source of high carbohydrate/energy food, to enable rumen bacteria to work at full speed. Graze in afternoon when the crop has had maximum exposure to sunlight, which may help drop the nitrate levels.

However, hopefully by testing you will be able identify the sources of feed on your farm that you’re able to safely feed now, while you wait for the nitrate levels in others to drop.

Find out more

There is more information about nitrogen toxicity in the following B+LNZ resources:

  1. Extreme dry toolkit (PDF, 2.3MB)
  2. Guide to Feed Planning (PDF, 3.3KB)
  3. Guide to Cattle Farming (PDF, 6.56MB)