Management of grazed brassicas and forages

Generally, no additional concentrates are needed when grazing brassicas, although minerals should always be available. Brassicas are high in calcium which may predispose pre-calving cows to hypocalcaemia if they are not fed low calcium forage, such as straw or hay, at the same time.

Brassicas are moderate to low in phosphorus and magnesium. Dry cow rations may require additional phosphorus and magnesium to meet the animals’ requirements in late pregnancy. They are also low in trace elements, particularly copper, iodine and selenium.

Livestock must have access to hay, straw or grass to promote saliva production and healthy rumen function. Bales placed in the field during summer reduce the need for machinery to go into the field during the winter, minimising soil compaction and run-off from wheelings. An adequate water supply is essential at all times.

Some brassicas, such as kale, are high in anti-nutritional factors that can affect copper and iodine metabolism. AHDB research has shown that providing a multivitamin bolus is important to maintain the mineral status of heifers outwintered on kale and can increase milk fat content in early lactation.

Fodder beet has a high sugar content, and the leaves are particularly high in oxylates. If overconsumed, this can affect calcium metabolism and result in kidney failure. Because of fodder beet’s high yield, the electric fence should be moved slower than other crops. However, this can increase poaching and soil damage in challenging weather conditions and requires careful management.

Compounds within the brassica cause the skin to be sensitive to sunlight and can result in skin damage. This usually occurs when crops are grazed too early when they are still growing and is more common with rape and kale. It will generally affect young animals and animals with low pigment levels, such as those with white heads or faces, and can cause the head and ears to swell, resulting in blisters and scabs.

Treatment: remove stock.

Prevention: ensure crops are grazed when mature.

Nitrates accumulate in the leaves of the crop, usually where fast-growing crops are grown in soil with high nitrate levels after rain has followed a dry spell. Cool and overcast conditions and high N fertiliser use increases the risk.

Effect: abdominal pain, scour, weakness, muscle tremors, drooling, mouth breathing or coma. Can cause death. May look similar to milk fever.

Treatment: remove stock and feed high quality forage. The vet may recommend the use of methylene blue.

Prevention: introduce stock slowly and avoid high use of nitrogen fertiliser.

Consider the risk of bloat, as brassicas can be rapidly degraded in the rumen. It is essential to feed fibre alongside the crop and introduce non-hungry stock gradually.

Brassicas, especially root crops, contain glucosinolates, which block the uptake of iodine from the diet. Brassicas are low in iodine, increasing the risk of iodine deficiency, affecting the thyroid gland and the hormones it produces.

Effect: A swollen thyroid gland in severe cases. More of a problem if pregnant animals are grazing crops, as it can cause stillbirths and increased prenatal mortality. It can affect fertility.

Treatment: Remove stock and treat with an iodine supplement.

Prevention: Bolus animals with iodine and avoid grazing pregnant animals.

Excess levels of amino acid compound S-methyl cysteine sulphoxide (SMCO) in the plants, which causes anaemia and appetite loss. The levels of SMCO are worse when soil phosphate levels are low, and nitrogen and sulphur levels are high. SMCO levels also increase when crops are flowering.

Effect: Weakness, red urine (particularly if the animals are grazing kale), reduced appetite, performance and fertility, goitre.

Treatment: Remove stock.

Prevention: Soil test and involve an agronomist in fertiliser decisions. Avoid sulphur fertiliser unless needed. Ensure adequate copper and selenium levels in the diet.

Think about all diseases that are common when animals are introduced to new or increased levels of feed, such as pulpy kidney, redgut, pasteurellosis and polioencephalomalacia (cerebrocortical necrosis or CCN). Vaccinate wherever possible and when the risk warrants it.