Nitrate Poisoning

Nitrate poisoning occurs when cattle graze high-nitrate plants or accidentally ingest fertiliser. In the rumen, nitrate converts to nitrite, enters the bloodstream and causes methemoglobin to form instead of normal oxygen-carrying haemoglobin. Sheep are more resistant than cattle, deer and goats.

Signs appear within an hour and include salivation, frothing, diarrhoea, abdominal pain, laboured breathing, staggering, tremors, blue/grey gums, and coma or death. Multiple animals are usually affected simultaneously.

Treatment is intravenous methylene blue in saline — the sooner it’s given, the better. While waiting for your vet, move unaffected animals away from the suspect feed immediately. Blood from affected animals is typically chocolate brown, and can be tested along with eye fluid from deceased animals. Note that immobile sheep are vulnerable to bird scavenging, which may influence treatment decisions.

Nitrates accumulate when photosynthesis is limited — during frost, fog, or cold snaps — and plants can become toxic rapidly when rain breaks a drought. Commonly affected feeds include annual ryegrasses, oats, kale, rape, brassicas and some new pastures.

Suspect feed can be tested at your local Vet Centre or with an on-farm kit — results take around 40 minutes from just two handfuls of feed. Retest toxic feeds weekly until safe. Risk can be reduced by grazing suspect pastures in the afternoon during winter and filling stock up on hay before shifting breaks.