A complete guide to feeding brassicas

Brassicas offer a source of high-quality out-of-season feed that can be grazed in situ to reduce overall wintering costs on livestock farms.

Growers can get the best out of them by following a few simple rules on allocation and feeding.

Below we offer advice on:

Poor performance on brassicas is often the result of underestimating the total quantity of feed on offer – or overestimating crop utilisation.

To get it right, start by calculating how much energy each animal requires, then take sample cuts of the crop to gauge availability per hectare, advises Rhidian Jones, of RJ Livestock Systems.

“You should always have an idea of the feed requirement in megajoules of energy of your group of animals, and how this translates into hectares or square metres of crop per day, based on the yield you have measured,’’ he says.

Finishing animals eat 2.5-3% of their bodyweight in dry matter (DM) a day and waste about 0.5% of their bodyweight.

See also: 7-step guide to planting brassica crops

For accurate feed planning it is essential to measure the DM yield of the crop.

This can be done by using a 1m square frame, a seed bag, a pair of garden shears and a set of scales.

• Take several random samples from each field by placing the frame in the forage crop and using the shears to cut all plants within the frame at about 10cm from the ground.
• Individually bag the material from each square metre and weigh it.
• Multiply this weight by 10,000 to calculate fresh yield/ha.
• Taking kale as an example, if a square metre yields 5kg of fresh weight, multiplying this by 10,000 gives a fresh weight/ha of 50,000kg.

Strip grazing is preferable to mob stocking and should lead to better utilisation of the crop.

Aim for long, narrow strips to allow all animals access to the crop at the same time.

Incorrect feed allocation is most commonly caused by an incorrect break area, so knowing the crop face length and crop yield is vital to setting the size of break.

To calculate this, you need to know the number of animals and the kg DM/animal/day to be offered, as well as the DM/head of baled silage offered, says Mr Jones.

• Take the figure for the DM yield of the crop and divide by 10,000. This will give you the DM yield/sq m.
• The kg DM allocated to each animal is worked out by dividing the sq m required for each animal by the DM yield/sq m.
• The number of animals in the group multiplied by the sq m per animal gives you the area needed for the break.
• Dividing the break area by the length of the break will give you the width of the break. For example, a break of 710sq m divided by  a 200m face length gives a 3.6m break width.

Graze each section of the field behind an electric fence before moving stock on to another section daily, at a similar time of the day; move twice daily in wet weather.

Set up two fences in front of animals so that a large proportion of the crop isn’t trampled if they break out; this also makes it quicker and safer to move them to a fresh break.

Introducing the crop

Livestock must gradually adapt to brassicas over at least a week to allow rumen microbes to adjust to the new diet, reducing the risk of digestive upsets.

Allow stock access to the crop for one to two hours a day initially, building up to unrestricted access after seven to 10 days, Mr Jones recommends.

Provide a wide access run-back area to increase utilisation and animal welfare, and also to reduce mud contamination of hides and fleeces.

“Start grazing from the top of a sloping field, rather than the bottom, to reduce run-off,’’ says Mr Jones. “Avoid channelling stock through gateways to adjoining fields to minimise poaching.’’

Brassicas should form no more than 70% of the diet, with 30% coming from a fibre source such as silage, straw or hay.

It is often easier to place bales in the field before the crop is sown or before it is long enough to be damaged.

However, sometimes this does not work out: for example straw might not be available until the brassica crop is well established.

Bales can be placed in the fields later, but allow for some damage to the crop.

Forage can be fed in a ring feeder or, in the case of straw, spread out along the feed face.

If bales of straw/hay are placed in the field beforehand it is a good idea to at least give them a single wrap in plastic for weatherproofing.

No additional concentrates are generally needed when grazing brassicas, although minerals should always be available.

Brassicas are high in calcium which may pre-dispose 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, so dry cow rations may require additional phosphorus and magnesium to meet the animals’ requirements in late pregnancy.

These crops are also low in trace elements, particularly copper, iodine and selenium, so supplementation is needed.