Watch out for nitrate poisoning when the autumn rains come.
This is when stock eats the fresh new “autumn flush” pasture that grows after a long dry period. It is usually worst with new grass but can happen on old pasture too. It is also common with crops like rape, kale, green-feed maize, or some of the fast-growing forages like millet and sorghum. It is often worst when days are dull and photosynthesis in the plant is not working efficiently.
During the dry summer period, nitrate accumulates in the soil, and is carried into the plant after rainfall often reaching high levels. The nitrate inside the cow’s rumen is converted to nitrite and this has toxic effects.
Deaths can be very quick. Cattle lie down and look very sleepy. They need urgent veterinary help and the antidote is methylene blue.
If you are concerned, send a sample of the pasture or crop you intend to graze to the vet clinic for testing. On any lush crop – no matter what it is, only turn stock on for about 15 minutes in the first grazing. This gives the animal’s digestive system time to adapt, and if there are high nitrate levels, it will reduce the risk of poisoning.
But the best test is a lab test through your veterinary clinic. Then you can be sure.
Typical examples of nitrate poisoning were 17 cows that died when a herd was given access to a paddock, previously heavily fertilised for a squash crop that had become overgrown with weeds. The weed “red root” is very prone to high nitrate levels after a drought.