Grazing sorghum and millet

13 Feb 2019

Callen Thompson, Senior Land Services officer – Mixed Farming 

Over the last few weeks I have had many questions regarding the safety of grazing sorghum and millet. In particular, crops that are short and/or moisture stressed.

Due to the ongoing dry weather, there is some grain sorghum crops around the district which may not make harvest and producers are looking to either graze or make hay. Nitrate and prussic acid poisoning are a significant risk in both situations and if you are planning to use these crops as feed, great care should be taken.

Nitrate and Nitrite poisoning

Nitrate poisoning can occur when grazing both sorghum and millet, as well as a range of other plants such cathead. It is caused by livestock consuming plants that contain high concentrations of nitrate.

Nitrate will often accumulate in the plant in times of stress, including drought, cloudy or cold weather or after an application of herbicide (often after spraying with 2,4-D). Nitrate concentrations are usually higher in young plants or regrowth of older plants. Most of the plant nitrates are contained in the bottom third of the stalk, so leaves are usually a lower level and flowers and grain should contain none.

Hay production does not reduce the level of nitrate within the plant and as such nitrate poisoning can still be seen when feeding hay including canola, millet or sorghum hay. The fermentation that occurs during silage production decreases the level of nitrates within the forage but it is still a good idea to do a test prior to feeding it out.

Signs of nitrate poisoning usually appear 6-24 hours after the plant has been consumed. Stock will exhibit rapid, noisy and difficulty breathing, blue/chocolate coloured mucous membrane and dark or chocolate coloured blood, salivation, bloat, tremors and staggering.

If you have a paddock that you are concerned about, get it tested. The risk of poisoning can be reduced by:

• When introducing stock, make sure they are full so they don’t gorge themselves
• Do not overgraze the paddock so that stock only eat leaves rather than the stem
• Cattle are more susceptible than sheep. If sheep are happily grazing, it doesn’t mean you can put cattle in the paddock
• Feed for short periods of time, the rumen microbes can adapt to higher nitrate levels if introduced slowly. Make sure that stock are getting fed from other sources when not grazing the forage sorghum/millet
• Observe stock frequently when grazing
• Do not put stock on young, short plants or regrown plants until it is at least 1m high in high risk situations (high soil nitrogen/following dry conditions).

Prussic acid poisoning

Prussic acid poisoning can occur in sorghum and sorghum-sudan hybrids. It will not occur in millets. The risk of poisoning is high following dry conditions, when there has been rain and the plant rapidly regrows. It can also occur when grazing short regrowth after harvest or cutting for hay.  Stress events such as frost or chemical application can also increase the risk of poisoning.

Poisoned stock are usually found dead with no clinical signs observed. This is because the signs of poisoning usually occur 15-20 minutes after the toxin is eaten. Stock will die within 1-2 hours, in some cases death occurs only 2-3 minutes after showing symptoms. Symptoms include breathing difficulties, stumbling and staggering, moaning and muscle tremors. The symptoms are similar to nitrate poisoning, but prussic acid poisoning leads to bright red mucous membranes rather than blue or chocolate coloured membranes caused by nitrate poisoning.

As with Nitrate poisoning, if you have a paddock that you are concerned about, get it tested. The risk of poisoning can be reduced by:

• Do not graze drought stressed, wilted or frost damaged plants
• Do not graze sorghum that is less than 50cm
• When introducing stock, make sure they are full so they don’t gorge themselves
• Cattle are more susceptible than sheep
• Test hay and silage. Prussic acid levels in silage can reduce with time, but they will not decrease in hay
• Supplement with sulphur
• Knowing the risk level of your sorghum. Some varieties have lower levels than others. Grain varieties are often high risk. Speak to your local agronomist or seed supplier to find out more.

Key Points:

• If sorghum is stressed, get it tested for nitrates and prussic acid. Test for nitrate only in millet
• Prior to introducing stock, fill them up with hay
• Introduce stock slowly, then monitor them closely
• Do not graze short sorghum or millet, especially regrowth.

DPI has some great primefacts on nitrate poisoning and prussic acid poisoning

For more information, or to get samples tested, contact your Local Land Services Ag advisory team member or District Vet.