It’s a bumper year for summer forage crops such as turnips, forage rape, millet, and sorghum. But these tasty feeds are not without their risks. Below are some diseases we can see when animals are grazing crop. Usually only a couple of animals are affected, but occasionally a large number of animals fall sick. Interestingly, many of these diseases actually result from how the rumen microbes metabolise the various compounds naturally present in these crops.
Nitrate/nitrite: sudden death!
- Nitrate/nitrite toxicity can occur in brassicas, sorghum and cereal crops. As shown in the diagram below, when plants have high levels of nitrate, rumen microbes convert this to nitrite. The nitrite crosses into the red blood cells and oxidises hemoglobin. The blood cells are then unable to carry oxygen (the blood will appear brown). Animals die if enough of the blood is affected.
- As well as sudden death, cattle can present with gasping, tremor, staggering, and dribbling urine.
- Cyanide is not just for spies, it can be a bitter pill for farmers too as a cause of sudden death in cattle. Sorghum contains a substance that rumen microbes can convert to cyanide. The cyanide crosses into the bloodstream and prevents red blood cells from releasing oxygen to the tissues (the blood remains cherry red).
- Animals will stagger, tremor, gasp and die.
Bloat (Brassicas, clover)
- When cows graze plants that high in soluble protein and carbohydrates and low in fibre, the rumen bacteria grow rapidly. The gas they produce can incorporate into a foam. Unlike free gas, the foam is unable to be burped out, and the rumen expands in size instead.
- Bloated cows will grunt and drool as they struggle to breathe.
Polioencephalomalacia (a.k.a. PEM, Vitamin B1 deficiency)
- Brassicas and chicory can be high in sulphur. Rumen microbes convert this to hydrogen sulphide gas. When cows burp, they inhale the hydrogen sulphide which then impairs the normal function of the neurons of the brain.
- Animals present with neurological signs such as blindness, circling, knuckling, champing, star-gazing, staggering, and recumbency.
Haemoglobinuria (a.k.a. “red water”)
- Mature, flowering brassicas can contain higher levels of S-methyl-cysteine sulphoxide (or SMCO for short). The rumen microbes convert this to dimethyl sulphide which oxidises the hemoglobin in red blood cells.
- The spleen destroys these damaged red blood cells. The cell remnants pass out in the urine, turning it red.
Jejunal Haemorrhagic Syndrome (a.k.a. JHS or “bloody guts”)
- We see this occasionally in animals grazing brassicas. Overflow of carbohydrates from the rumen to the intestines promotes growth of Clostridia bacteria. The clostridia produce a toxin that damages intestinal tissues.
- Animals will present with abdominal pain, dehydration and pass dark tarry faeces or a blood clot. Sometimes the blood clot is so severe that it can cause a complete intestinal obstruction.
- Photosensitisation in cows grazing brassicas is a little complicated. Plant compounds (glucosinolates) are converted to products that damage the liver. This means that the liver doesn’t clear phylloerythrin (the breakdown product of chlorophyll). Instead, phylloerythrin deposits in the skin. It reacts with the sun’s UV rays resulting in inflammation and pain.
- Animals will be agitated, restless, with swollen white or hairless skin – you can see photos of this on one of our previous posts.
How to avoid forage crop poisoning
Below are some key points to reduce the risk of disease:
- Don’t allow hungry/unaccustomed stock to have allow free access to crop
- Take care with young/fast growing crops as these have increased risk of nitrate/nitrite and cyanide toxicity
- Reduce the amount of toxic species that animals graze, such as mixing with other plant species or replacing with a less toxic crop
- Be aware of frost, insect damage, fertiliser and overcast weather conditions as these can increase the risk of nitrate/nitrite toxicity. Frost, insect damage, heat, hail, and wilting increase the risk of cyanide poisoning
If you have any questions about the clinical signs or treatments of crop diseases, give the clinic a call to talk to one of our vets.