Grazing brassicas

Grazing brassicas – what are the risks & how can we manage them?

Dr Emilee Johnstone, District Veterinarian

June 2020

Many producers across the region have taken advantage of good summer and autumn rain to sow dual purpose or grazing only Brassica crops which are now providing an abundance of good quality feed with the potential to produce excellent livestock weight gains. However, along with this flush of feed, has come an influx of calls from concerned from producers about the animal health risks associated with grazing Brassica spp.

Just about every crop or pasture we grow can cause livestock health problems – think lucerne, oats, phalaris, and clover.

Fortunately, livestock health problems from grazing brassicas are relatively rare and can largely be avoided by agronomic and grazing management – and this has certainly been the experience of District Vets in our region this autumn. However some problems have been encountered and these need to be managed – including nitrate toxicity, blindness, photosensitisation and bloat.

So, what can you do to help get the most out of your stock grazing brassicas this autumn/winter?

  • Never give hungry animals the chance to gorge themselves on a lush crop. Introduce stock gradually so that their rumen microflora have the chance to adjust to the high quality diet. A few hours per day initially (10am-2pm) for seven days is ideal.
  • While high in crude protein and energy, Brassica crops, like all grazing crops and lush pastures, are low in fibre content. Straw or hay should be available ad lib in the paddock at all times to assist with digestion.
  • Dull, cloudy weather can reduce photosynthesis in the plant, which in turn can allow the nitrate to accumulate. Consider removing stock from the crop during and for a week after these periods if possible, or alternatively increase the amount and quality of hay being fed out.
  • Clostridial vaccination (e.g. 5-in-1) of animals is recommended as high quality feeds can trigger pulpy kidney deaths.
  • Crops recently top-dressed with nitrogenous fertiliser such as urea are considered to be high risk. Avoid grazing any crop within 4 weeks of a nitrogen application.
  • And last, but certainly not least, keep an eye on them – grazing any Brassica or cereal crop is not a set & forget system. Watch them and if you notice any issues contact your veterinarian immediately for advice.

A final comment – research has shown that both sheep and cattle experience a ‘lag phase’ of 10-14 days in weight gains after introduction onto a Brassica crop. The recommendation is to graze a Brassica crop for at least 4-5 weeks to achieve maximum benefits and make up for that initial lag phase.

Looking beyond the rain gauge… iodine deficiency

Heavy rain this autumn could put livestock, particularly sheep and goats, at risk of iodine deficiency and have fatal consequences for lambs and kids. A swelling in the neck of affected lambs and kids is the most obvious clinical sign. This swelling is an enlargement of the thyroid glands caused by iodine deficiency and is known as goitre. Lambs or kids with iodine deficiency will often be small and/or weak rendering them highly susceptible to starvation and exposure.

Iodine deficiency is thought to be associated with a reduction in soil ingestion (an important source of dietary iodine) in years with good autumn rain and lush pasture growth in autumn and winter. Brassica species may also contain high levels of goitrogens which block the uptake of iodine by the thyroid gland – animals grazing these pastures can have 2-4 times the normal requirement for dietary iodine.

While an outbreak of goitre can be challenging to manage, iodine deficiency is easy to prevent by drenching ewes pre-lambing with potassium iodide. This method of supplementation is cheap and accurate, and can even be combined with some worm drenches.

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