Forager: Grazing management of brassicas

Thursday 06.09.2018 , News

Livestock should be introduced slowly to brassica crops. Initially, one to two hours access should be given and built up to full-time access over seven to ten days. To avoid digestive disorders, animals should not be introduced to fresh brassicas when hungry, as they could gorge themselves.

Roughage in the form of grass, hay or straw should be freely available to animals being transitioned onto brassica diets. When fully adjusted brassica should make up no more than 70% of the diet, with 30% coming from a good fibre source such as silage, hay or straw.

It is important to be aware that brassicas have a poor mineral profile and are found to be deficient for selenium, iodine, copper and cobalt, so it is essential that animals are supplemented with such, generally in the form of a bolus.

The level of crop utilisation of brassica crops is dependent on many factors such as soil type, weather, brassica type and the level of animal performance required. Utilisation levels of up to 80% can be achieved from brassicas which have a high leaf to stem ratio. Generally, for Redstart, under optimum conditions, a target of 80 and 75% (or less) utilisation is sufficient for maintenance and weight gain, respectively.

To maximise crop utilisation and feed budgeting the correct crop allocation must be given, also relevant where crops are to be sold on a DM yield basis. To do this, the DM yield of the crop must be calculated.

To do this you will need:

  • 0.5m x 0.5m (0.25m2) quadrat
  • A seed bag
  • A garden shears
  • Scales

Using the quadrat a number of representative samples should be taken to get an accurate estimation of yield. All material within the quadrat should be harvested to a residual height of 10cm and the harvested material should be placed in the seed bag and weighed on the scales. To calculate total DM yield per hectare you must multiply the fresh weight per m2 by 40,000, and then multiply by the expected DM percentage. Brassicas have very low DM, typically around 12%.

Total DM yield (kg/ha) = Fresh weight (kg) x 40,000 x DM %

Example for a crop of redstart

Total DM yield (kg/ha) = 1 (kg) x 40,000 × 0.12 = 4,800 kg DM/ha

Typically out-wintered animals have a daily DM intake requirement of 3% of liveweight. For example, a 300 kg animal will require 9 kg DM per day, however, only 70% of this can be made up of brassicas, leaving the brassica requirement at 6.3 kg DM per day, with the outstanding 2.7 kg DM being made up of silage, hay or straw.

A crop of late sown Redstart with an average yield of 4,800 kg DM/ha will last a group of 30,300 kg weanlings for 25 days, receiving an approximate area of 0.04 m2 per day.

Some key management techniques include:

  • Strip grazing, using long and narrow strips to maximise utilisation and minimise trampling
  • Use a double fence for safety and for labour efficiency when allocating strips
  • Back fencing can be done to minimise poaching on grazed areas
  • Ensure animals have access to roughage and fresh water
  • Move stock during the day, to allow frozen crops to defrost before offering fresh allocations (If the crop remains frozen alternative forage will have to be offered and animals not given access to a fresh break)