Sorghum is an important forage crop for livestock production in Australia as frequent drought and high summer temperatures reduce forage production from pasture. According to Pioneer Seeds (2020), planting a forage sorghum crop can have a number of advantages for graziers as the fodder crops can grow well in poorer soils; they can withstand dry conditions and remain green at very low moisture levels; the lower leaves do not dry out as the plant matures; and therefore they retain a higher crude protein content and as an animal feed they have the same energy levels as maize or other cereals.
Although there can be significant benefits and weight gains that can be achieved in having livestock graze sorghum species, there are some important things to consider in order to successfully graze this forage.
What is Prussic Acid Poisoning?
A number of plants (particularly sorghums and related species) can accumulate large quantities of prussic acid. Prussic acid poisoning is caused by the release of hydrocyanic acid (HCN) in cattle (and potentially other livestock) grazing sorghum-type forages. These toxic compounds are found in the outer tissue of the plant and the enzymes that enable prussic acid production are located in the leaf tissue.
Any event that ruptures the plant cells (chewing, trampling, chopping, drought, freezing, wilting etc.) and allows the compound and enzyme to combine will produce prussic acid. When a plant containing high levels of prussic acid has been eaten, the toxin can enter the bloodstream and rapidly spread throughout the body. Death by suffocation can be the result as prussic acid inhibits oxygen utilization by the cells in the animal body (McCuistion, 2020). Clinical signs such as muscle tremors; rapid and laboured breathing; salivation; runny eyes; eliminating urine and faeces; the mucous membranes of the mouth and eyes may have a blue colouration; staggering and collapse; can occur within 15-20 minutes to a few hours after consumption of the toxic forage.
Animals are able to detoxify a certain amount of HCN with (Vough, 2020) estimating that a 450 kg cow should be able to detoxify at a rate of about 0.5 g of HCN per hour. This means that cattle and sheep should be able to consume forages containing small amounts of HCN without ill effects or signs of prussic acid poisoning. Although the detoxification of cyanide to the harmless thiocyanate is a rapid process; if animals eat large amounts rapidly they are at the greatest risk of poisoning since the rate of HCN formation exceeds that of detoxification. Levels of 0-25 mg of HCN/100g of dry plant tissue have been considered as safe for grazing, levels of 50-75 mg/100 g as doubtful and concentrations of greater than 100 mg/100 g as highly dangerous (Vough, 2020).
The amount of prussic acid found in plants is also affected by soil fertility as soils high in available nitrogen and low in phosphorus increase the potential of prussic acid (Boyles, 2020). The greatest risk of stock succumbing to poisoning is grazing after a period of drought or a series of frosts. The young dark green growth or regrowth, especially in sorghum species can be highly toxic and dangerous to livestock. As plants mature and plant height increases, the risk of prussic acid poisoning reduces.
Hungry animals are at greater risk as they will normally consume a larger amount of toxic material in a short time. The overload of prussic acid can easily overwhelm an animal’s ability to metabolise prussic acid to the non-toxic thiocyanate.
It’s important for producers to be aware that cyanide-producing potential is present during any stage of the sorghum crop’s life cycle but the risk is of poisoning is higher when growing plants are stressed by: (Queensland Government , 2020)
- hail damage
- insect damage (grasshoppers and caterpillars).
Therefore, stressed regrowth is considered the most dangerous portion of the crop to graze and extreme care must be taken at this stage to avoid poisoning. The lowest risk of cyanide accumulation is when plants are flowering and seeding.
Strategies to Help Avoid Prussic Acid Poisoning
Like always, prevention is better than a cure, and here are a few management practices that can be incorporated if you are planning on running stock on forage sorghum over summer:
- Never allow stock to graze sorghum that is less than 50 cm high and avoid grazing drought-stressed, immature, wilted or frost-damaged plants.
- Get any risky feed sources tested for prussic acid before allowing stock to consume forage. If buying sorghum hay, make sure it was cut during low-risk conditions
- Feed stock hay before turning them out on forage to avoid gorging by hungry stock. It might be helpful to limit grazing to a short period of time in the first day.
- Strip grazing with a back fencing helps by allowing animals to uniformly graze the crop down to 20 cm stubble before being rotated to another area and the back fencing prevents the new growth from being grazed until it is at least 50cm high.
- Supplementation with a professionally formulated sulphur supplement can help increase the animal’s efficiency at converting prussic acid to the non-toxic thiocyanate.
- If you suspect prussic acid poisoning, call your veterinarian promptly.
How Supplementation with MegaMin Extra Sulphur Supplements can Help Stock Grazing Forage Sorghum
One of the key functions of sulphur is being an important detoxifier that helps the body to get rid of prussic acid. To help minimise the risk of losing cattle to prussic acid poisoning a good management practice is to provide stock with enough dietary sulphur to assist the detoxification process.
Forage sorghum is also known to be quite low in sulphur and salt and by providing livestock with enough of these crucial minerals to meet their requirements gives the animal the tools it needs to have a production response. It is a considerable investment in time and money when planting a cereal crop and to get a return on investment it is important that stock graze the crop in its entirety without suffering from metabolic issues.
A trial (1978) into the Response to Salt and Sulphur by Cattle Grazing Sorghum conducted by the NSW DPI and CSIRO found an increase in daily weight gain in the cattle supplemented with sulphur and salt compared to the unsupplemented animals. Providing extra sulphur when livestock are grazing forage sorghum can help the animal to handle the prussic acid and eat more stalk and stem of the plant, therefore increasing intake and allowing the animal to utilise the standing crop which can be reflected in significant weight gains.
Feeding straight sulphur or adding it to a feed ration yourself, comes with an element of risk. Excessive consumption can result in sulphur toxicity and secondary conditions like Polioencephalomalacia (PEM). Symptoms of PEM can include blindness, brain irritation and death. MegaMin Extra Sulphur has been professionally formulated to provide the safe additional sulphur required along with broad spectrum macro and trace minerals that are essential for stock health.
If you are planning on planting a forage sorghum crop for you stock to graze this Summer, reach out to AgSolutions or your preferred local store to find out more about our range of high sulphur products, including;
By Shannon Godwin (BAppSc GDTL)