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Agricultural Brassica Crops - General Information

Alan Muirhead

Forage crops can provide a cheaper option for winter feeding.  Outwintering sheep or cattle on a field of kale or stubble turnips can relieve pressure on housing, cut labour costs, cut bedding costs and cut feeding costs.  Brassica feed value is 15%-18% protein and 70% energy the equivalent to top quality silage.  

There are savings in using forage crops compared to silage or barley -

  • Forage Crops - 4p-5p/kg dry matter (DM)
  • Top quality silage - 12p/kg DM
  • Barley - 15p/kg DM

Forage crops are also a good option for a break in grass rotations.  But remember to check the suitability of your soil, your weather and your topography as forage crops do not suit every farm across the UK.

Choose The Right Crop For You

The first decision is  - when does it need to be ready for grazing?  This can dictate which crop to sow as there is a big difference in the growing periods between the likes of kale and  forage rape. There are now 3 groups of forage crops -

  1. Winter hardy main crops - kale & swede, 10 months growing period to reach maximum yield, usually used for outwintering sheep and cattle.
  2. Fast growing - rape & stubble turnips, 6 month growing period but can be grazed after 8 weeks, usually used to bolster feed when grass growth reduces in late summer.
  3. Hybrid crop - kale/rape hybrid, winter hardy & fast growing, can be grazed from 10 weeks onwards, such rapid growth allows more than 1 grazing period over the winter.

There are other factors you have to take into account before deciding what crop is best for you, such as:

  • Usable yield
  • Winter hardy
  • Disease resistance
  • Growing height
  • Regrowth ability
  • Leaf:Stem ratio
  • Digestibility of leaves/stems/bulbs
  • Feeding.
 Sowing & Feeding Times For Forage Crops
Fodder Beet Sowing mid Mar - May
Feeding Oct -  following May
Forage Rape Sowing Mar - Aug
Feeding Jun - Dec
Grazing Turnips Sowing Feb - Aug
Feeding May - Dec
Hybrid Rape/Kale Sowing Mar - Aug
Feeding May - Dec
Kale Sowing Apr - mid Aug
Feeding Jun - following Apr
Stubble Turnips Sowing Mar - Aug
Feeding Jun - Dec
Swedes Sowing Feb - Apr
Feeding Oct - following Apr
Turnips Sowing Apr - Sept
Feeding Sept - following Apr

 

Choose The Right Field

There are important factors to take into account when choosing which field to use:

  • Free draining
  • Not too steep a gradient
  • No threat of run-off or agrochemicals to near by watercourses
  • Fields sheltered by trees/hedges (protect livestock during harsh weather)
  • Careful management

Careful management is the set up of block grazing.  This is important in reducing pressure on the soil and maximising the the grazing of your forage crop.  Block grazing reduces pressure on the soil as new areas are regularly opened up, so livestock are not trampling over the same area over long periods.  Block grazing also prevents trampling of your crop, so all of your crop is more palatable and easy to graze.

Preparation Of The Field

It is important to test the pH of the soil and the nutrient levels, especially phosphate (P). Availability of P is reduced in acidic soils (pH 5.8 or lower), so if your soil is acidic fast acting lime can relieve this problem.  

You can use a no plough approach and just direct drill the seed in, this helps to maintain a firm surface.  If you have a problem with a persistent previous crop or grass patches then it is recommended that you glyphosate and plough the field before sowing.  

Leaving an area uncultivated as a run back for livestock is also a good management option.  This also provides areas where silage or straw bales for roughage can be placed without having to drive through wet fields during the winter, thus reducing the pressure on the soil.  

Sowing

The seed is drilled at a depth of 1cm.  Sow a high seed rate to produce a dense crop. You want to achieve a high leaf:stalk ratio as the leaves have a higher nutritional value compared to the stalks.

  • Kale, Rape & Turnip sown at 6kg/ha or broadcast @ 7.5kg/ha.
  • Suede precision drilled at 1.2kg/ha (it is a different type of seed).

Fertilizer Applications

A standard fertilizer such as 20:10:10 can be used.  The majority of the fertilizer is applied to the seed bed for early summer sown crops.  Although, forage crops do respond well to P so it might be worth while using a fertilizer with a N:P:K ratio of: 

P - 60-80kg/ha

K - 60-80kg/ha

N - 80-90kg/ha

N rates depend on sowing date.  If summer sowing, N should be increased as growth rates are higher.  Kale responds better to N when the leaves are growing, so split applications:

  1. Apply to seed bed when sowing
  2. 6-8 weeks after sowing when leaves are growing.

Remember, as temperature decreases crop yields decrease and so N requirements decrease.  

Disease, Pests & Weeds

Clubroot is the main disease you want to avoid in brassica crops.  Good crop rotation can prevent this disease - always leave at least 5 years between brassica crops.  If clubroot does appear it can be devastating to crop yields.

Slugs are the main pest and can cause considerable damage.  Drilling slug pellets along with the seed can control slug populations.

Fat Hen can be problematic in forage crops, but spraying is not always cost effective. Treating the field with glyphosate before sowing will reduce weeds becoming a problem.

Feed Intakes

The aim is to provide livestock with enough fresh crop for grazing and minimise pressure on the soil.  To ensure the feed face will match the growth requirements of the livestock you have to measure the feed available.  At stages along the feed face cut and weight a square meter off the crop.  

  • Kale 12-15% DM, sample weighing 6-8kg provides 0.75kg DM/m2. Equivalant to 10t DM/ha.  A 500kg cow needs 10kg DM/day, so would eat through 13m2 of kale. So 1ha of kale can feed 100 cows for 1 week.  For sheep it is roughly 1ha of kale to feed 500 ewes a week.  
  • Stubble Turnips or Rape 5-6t DM/ha (around half of what kale provides). 2ha feeds 100 cows a week.
  • Hybrid varieties are between the two so roughly 8t DM/ha.