Brassicas (plants of the cabbage family) are commonly used as a source of additional feed for beef cattle and youngstock. They provide valuable additional energy and are also often much higher in protein than standard forages. Brassicas grown to feed cattle include swedes, kale, rape, leaf turnips and standard turnips. However, if fed incorrectly significant problems can be associated with the feeding of brassicas.
Five different toxicities are associated with feeding brassicas:
1) Nitrate poisoning
Brassica stalks have a large capacity to concentrate nitrates, so nitrate toxicity is a significant problem, particularly if high nitrogen fertilisers are used
Many species of brassica, including rape, contain photodynamic agents which may lead to skin damage as the result of photosensitisation
All brassicas contain a variety of glucosinolates which block the production of thyroid hormone and can cause goitre. Glucosinolates have also been associated with poor fertility. The roots of the crop tend to contain more glucosinolates than the tops or leaves.
4) S-methylcysteine sulphoxide (SMCO)
This amino acid accumulates in the plant during the growing season. This compound is associated with goitre and with anaemia
5) Sulphur alone
The previous 2 toxicities are due to sulphur-containing compounds. Additionally, on their own the high sulphur levels in brassicas can reduce the absorption of copper and selenium causing deficiency of these minerals. High sulphur diets have also been associated with the nervous disease cerebro-cortical necrosis (CCN).
As there is a wide range of potential problems there is also a wide range of possible clinical signs. If you’re feeding brassicas and you get poor productivity or unusual diseases get your vet involved. The two most important are summarised below:
- Abdominal pain
- Muscle tremors
- Drooling of saliva
- Blue discolouration of the mouth
- Mouth breathing
- Reluctance to move
- Haemoglobinuria (red urine) particularly if the animals are grazing kale
- Reduced appetite / reduced production
- Reduced fertility
- Goitre (swelling of the thyroid gland)
The clinical signs along with the grazing history are generally sufficient to provide a diagnosis of brassica poisoning. There are no routine tests for brassica poisoning.
Post mortem can be used to confirm nitrate toxicity, SMCO anaemia, and goitre.
Stop feeding the brassicas, feed a relatively high carbohydrate diet with quality forage . Veterinary treatment with methylene blue can be very effective at reversing the changes in the blood, particularly in the early stages.
No specific treatment except for removal of the brassica.
Brassicas should ideally be fed as only as one part of the diet. They are effectively more like a concentrate than a forage feed, so if possible feed additional forage alongside the brassicas.
Animals should be introduced to these crops gradually and preferably with full stomachs. Strip grazing of brassicas, particularly at the start of feeding, is a good way of controlling intake (and improving the efficiency of use of the crop).
Sulphur concentrations in general (and SMCO concentrations in particular) can be reduced by minimising the use of sulphur fertilisers on fields used for brassicas should be minimised to reduce.
Supplementation of trace elements, particularly iodine, can be beneficial in preventing the clinical signs of deficiency. Ask your vet or nutritionist for advice before supplementation.