Have you had a dry summer?

During a drought, nitrogen levels naturally rise in the soil.

When autumn rains arrive, it helps young pasture plants to grow very quickly. They can grow so fast, nitrates (a mix of nitrogen and oxygen) build up in their leaves as the plant can't convert them to protein fast enough.

High nitrate levels can be toxic to stock, and it can happen fast. Cows can eat enough for a toxic dose in just one hour.

Nitrates are absorbed very quickly into the bloodstream, which affects the blood's ability to carry oxygen. Animals begin breathing faster, trying to get more oxygen, causing them to be unsteady on their feet, stagger, then fall. They may froth at the mouth or produce excess saliva, mucous membranes (eg, the inside of the eyelid) may turn blue/chocolate brown, they'll start gasping for air, then die.