How Much N Does Your Maize Crop Really Need?
Over the past few weeks, I’ve had several conversations about nitrogen (N) management in maize crops for the coming planting season.
Many of the common species sown, such as permanent and annual ryegrass, pose the risk of nitrate poisoning in cattle. Unfortunately, over the years I have come across many good farmers who have lost cattle to nitrate poisoning, and witnessed the stress and pressure it causes. Many animals can be affected in a very short period which means that prevention is key, rather than hoping it won’t happen to you.
Nitrate poisoning occurs when ruminant animals consume forages with high nitrate levels. High concentrations of nitrate in the diet can lead to a rumen accumulation of nitrite, which is very toxic.
Nitrite is absorbed into the blood and combines with haemoglobin preventing it from carrying oxygen in the bloodstream. If a ruminant consumes enough of a high nitrate plant, hypoxia (low levels of oxygen in the blood) can develop within an hour, and death can rapidly follow. So, what risk factors increase nitrate concentration in the plant1?
- Nitrogen application - excess nitrogen fertiliser application can result in nitrate accumulation.
- Growth stage - generally nitrate concentration is higher in new growth and decreases with maturity.
- Plant stress - hail, frost or plant disease can damage leaves and reduce photosynthetic activity.
- Sunlight - shaded plants lack sufficient energy to convert nitrate to amino acids. Extended periods of cold (below 12°C) cloudy weather increase nitrate concentration in the plant.
- Drought - nitrate levels build up in the soil and when it rains, plants can take up excessive amounts of nitrate.
Grazing a risky paddock
When the conditions pose a risk for nitrate toxicity, it is important to manage grazing animals closely to minimise their risk. Essentially farmers should concentrate more on prevention than treatment, as once an animal begins to show nitrate toxicity symptoms there is likely to be only a short period of time to save the animal. Nitrate poisoning in cattle can be minimised by proper grazing management practices that include:
1. Test don’t guess or hope! Nitrate test kits are cheap and an effective way to understand the risk.
2. Avoid grazing with hungry cattle. Feed low risk feeds such as maize silage, hay or grass silage prior to grazing a high nitrate crop.
3. Avoid grazing to a low residual, as nitrate concentrations are higher in the base of the plant.
4. When risk factors are high, avoid grazing risky paddocks until conditions improve. Ideally graze on sunny afternoons when the temperature is above 15 °C and avoid grazing on frosty or cloudy days.
5. Manage smaller, more frequent grazing of high nitrate feed.
6. Observe stock frequently during grazing potentially risky feed. Set your alarm and check on them an hour after you put them on the break paddock. Look for signs of nitrate toxicity, which include staggering animals (they look like they are drunk), rapid breathing and animals lying down.
7. If you do suspect nitrate poisoning quietly remove livestock to a safe area and contact your veterinarian as soon as possible. Stressing animals which are suffering from nitrate poisoning can make the lack of oxygen worse.
I have witnessed cases where farmers have lost animals even when they have applied one or two of the good management practices above. When grazing risky paddocks, I would recommend that farmers apply all the management practices above to reduce the risk. If you are unsure, contact your animal health advisor.
Wade Bell is Genetic Technologies farm systems manager. Contact him at This email address is being protected from spambots. You need JavaScript enabled to view it.
