Potential Weed and Sorghum issues for livestock

There have been some sheep deaths from grazing summer weeds.

With the late rains & bare ground there has been some plantings of forage sorghum.

Below is info from Victoria on potential issues FYI.

Hopefully there won’t be any issues.

Sorghum

Sorghum is a popular forage crop used in livestock production systems capable of producing substantial quantities of feed during the warmer months of the year. Sorghum is a feed source that can be grazed, ensiled or made in to hay.

Negative characteristic of sorghum from an animal health perspective is its capabilities to cause cyanide and nitrate poisoning. Sorghum can contain lethal levels of prussic acid better known as cyanide. As well as cyanide, sorghum can have elevated levels of nitrates. Both nitrates and cyanide can cause significant animal health problems including death. Careful management can greatly lower any potential animal health risk associated with feeding sorghum to livestock.

Minimising the Risk of Cyanide Poisoning

• Selective breeding by seed companies against negative traits like high levels of prussic acid has resulted in sorghum varieties that are relatively safe. Grain sorghums and sweet sorghums generally have higher prussic acid levels than other varieties. Your seed supplier should be able to provide you with recommendations as to the suitability of specific varieties for grazing.
• New growth and /or regrowth plant material will typically contain high levels of prussic acid. When stock consume the new growth or regrowth material the prussic acid is digested, releasing cyanide which is absorbed by the animal. The amount of cyanide absorbed can be sufficient to kill an animal. For this reason it is recommended not to graze sorghum when it is less than 45 cm in height for short varieties and 65cm for tall varieties. Although this decreases the risk of poisoning it does not guarantee safe grazing.
• Avoid grazing hungry stock on sorghum. When introducing stock onto sorghum it is recommended that they receive plenty of roughage before going into the paddock. This will decrease the amount of material they can consume in the first grazing period and decrease the risk of ill effects.
• Maintain a close eye on stock for the first 48 hours when introducing stock on to sorghum. It is ideal to watch the animals continuously for the first 2 hours. Affected stock often die within minutes of eating a lethal dose of sorghum.
• Provide stock with a sulphur lick (10% sulphur in a salt lick). Sulphur plays an important role in detoxification of cyanide as it is released in the digestive process. Some research has shown that sulphur added to the diet may also result in increased production when animals are grazing sorghum exclusively.
• If buying sorghum silage, have it feed tested for cyanide and nitrate levels before feeding to stock. Sorghum that is dangerous to graze will also be dangerous to livestock after it has been cut for hay or silage.

Minimising the Risk of Nitrate Poisoning

• Nitrates accumulate in many different plants with sorghum being one plant well recognised for this negative trait. There are many conditions that result in nitrate accumulation. Generally, plants accumulate nitrates when soil nitrate is high but conditions are not suitable for normal forage growth. Moisture stress, cloudy weather, and herbicide application are factors that can increase accumulation of nitrates.
• To minimise the risk associated with nitrate poisoning it is important that hungry stock are not introduced on to sorghum. Take special care if the crop has been moisture stressed or the crop has a high uptake of nitrogen influenced by fertiliser application.

Treatments for Cyanide and Nitrate Poisoning

Monitor stock closely and if any stock show signs of distress contact your local Animal Health staff or your local veterinarian for specific advice and assistance. There are products that can assist with the recovery of animals suffering nitrate and cyanide poisoning, however professional advice should obtained to get an accurate diagnosis and treatment plan.

Summary of Key Points

• Select a safe variety of sorghum for grazing.
• Avoid grazing plants under 45 cm for short and 65 for tall varieties.
• Avoid grazing stressed plants or fresh growth shortly after the plant has been stressed.
• Avoid putting hungry stock onto sorghum. Make sure stock have plenty of roughage prior to grazing sorghum.
• Provide stock with a source of sulphur. (Salt block with 10% sulphur).
• Monitor stock closely during the first 2 hrs and then keep a close eye on stock for next 48 hrs.
• Always test sorghum silage for nitrate and cyanide levels.

References

DEDJTR &F Note Queensland. Summer Forage Crops – Sorghum, 24 December 2003.

DEDJTR &F Note Queensland. Cyanide (prussic acid) and nitrates in sorghum crops Risk management, 10 June 2005.

Monty S. Kerley. Potential for nitrate or cyanide toxicity in cattle grazing sorghum-sudan during drought conditions, University of Missouri Extension. 15 June 2006.

Aust. Journal of Experimental Agriculture and Animal Husbandry: volume 19 December 1979. J.L. Wheeler and D.A. Hedges. Deficiencies of sodium and sulphur in sorghum forage for animal production in eastern Australia.

Agnote, No:C19. Forage Sorghum. A.G. Cameron, Principal Pastures Agronomist, Darwin. Department of Primary Industry, Fisheries and Mines, Northern Territory Government, 2006.