Field choice and establishing brassica and forage crops for grazing

Soil type and field choice are important aspects of managing crop performance and environmental impact. Read about field selection, sowing, weeds, pests and fertility.

Careful field selection is crucial to minimise the risk of soil poaching and run-off. However, when grazing in the summer, there is generally less risk of such environmental damage.

When selecting a field:

• Choose fields where grass production is falling
• The soil needs to be free draining and must dry quickly after rain
• Ideal sites offer shelter for livestock but have sufficient airflow to allow fields to dry out
• Think about how to fence the field, how animals will access water and where to position a run-back (a loafing or grazing area at one end of the field where stock may find shelter) and bales
• Avoid steeply sloping fields or those close to watercourses

Fields with sandy soils, good soil drainage and gentle slopes are preferable to poorly drained, heavy clay soils or steep slopes where considerable run-off can lead to soil erosion.

• Fields should not be near watercourses. Buffer strips (such as uncultivated or undrilled land) should be left at the bottom of slopes or near areas where run-off is a risk
• Avoid vehicles travelling in the field during winter by putting bales out in the summer. Organise feed fences to provide maximum frontage and a narrow strip of accessible fodder
• Avoid overstocking

Soil type has the greatest influence on leaching losses of nitrogen and the level of damage caused by compaction.

There are many soil testing companies available, which can assess soil type and health.

Key points:

• Perform a soil test at least eight weeks before sowing
• pH should be at least 5.6 (optimum 5.8–6.2)
• Apply manures, fertiliser and lime according to soil test results
• Aim to control weeds before sowing

Soil structure and soil health are also important, consider field drainage if fields are prone to ponding.

Early sowing can lead to higher yields, but crops become less digestible as they mature.

Plan ahead for your target grazing period and work back from this date to prevent sowing too early, which will reduce utilisation. 

Seed can generally be broadcast or drilled into a cultivated seedbed.

Where conditions are suitable, direct drill into an existing sward that has been sprayed off with glyphosate.

Direct drilling improves the soil moisture and produces a firmer surface for grazing. This is less prone to poaching compared to fields that are ploughed and then drilled for outwintering purposes.

It is best to:

• Drill when soil temperature is 10°C and rising
• Direct drill into clean, open ground
• Sow seeds to a maximum depth of 10 mm to ensure uniform germination
• Roll after sowing

Treated seed can help protect seedlings against attack from insects and disease.

Common pests include flea beetle, diamondback moth and slugs.

Monitor crops closely throughout growth. Control is usually most cost-effective if the potential problem is spotted early.

Seek advice from a qualified agronomist at the earliest opportunity and take prompt action if a problem is identified.

To minimise disease risks, don't grow brassicas continuously in the same area.

A gap of four to five years (in drier areas) or seven to eight years (in wetter areas) between crops is advised.

If brassicas are grown for two consecutive years, a longer gap is required before the next brassica crop can be grown.

Control weeds in the previous crop or apply a pre-emergence herbicide after drilling.

Table 1. Nitrogen, phosphate and potash for forage swedes and turnips (kg/ha ) (65 t/ha roots lifted)

Table 2. Nitrogen, phosphate and potash for fodder beet (kg/ha ) (85 t/ha roots lifted)

Table 3. Nitrogen, phosphate and potash for forage rape, swedes and stubble turnips (grazed) (kg/ha )

Table 4. Nitrogen, phosphate and potash for kale (grazed) (kg/ha )

Data taken from Nutrient Management Guide (RB209) Section 3.

Recommendations are based on typical yields.

Adjustments can be made for higher or lower yields by estimating crop offtake using Table 3.2 in Nutrient Management Guide (RB209) Section 3 Grass and forage crops.

The recommendations for grazed crops are assuming that all the manures are returned.

For hybrid brassicas, use the recommendations for forage rape, swedes or stubble turnips.

^a Forage rape and stubble turnips, apply no more than 75 kg nitrogen/ha at Index 0 or 1. Further reductions may be made if the soil is moist and has been cultivated.

Note: Tables contain the total nutrient required – remember to deduct nutrients applied as organic materials (Section 2: Organic materials).

Phosphate and potash need only be applied to the seedbed at Index 0 or 1.

In crops where roots are removed (forage swedes, turnips and fodder beet) and the tops are also carted off, potash applications may need to be increased by up to 150 kg/ha.

Where sugar beet or potatoes do not feature in the rotation, magnesium fertiliser is only justified at Soil Index 0, where 50–100 kg MgO/ha should be applied every three or four years.

The exception is fodder beet, which should be treated as sugar beet with a Mg recommendation at Index 1 (Nutrient Management Guide (RB209) Section 4: Arable crops).

For fodder beet, sodium is recommended on all soils except fen silts and peats.

Apply 400 kg/ha of agricultural salt (200 kg Na₂O/ha) well before drilling.

If sodium is recommended but not applied, increase potash by 100 kg K₂O/ha

A boron application may be needed. Soil and plant analysis are useful guides to assess the need for boron.

Soils at risk of deficiency require 25 kg SO₃/ha (Nutrient Management Guide (RB209) Section 3: Grass and Forage Crops: Table 3.28).

Table 5. Example fertiliser application rates and timings

Note: 100 kg per ha equals 80 units per acre. Source: Nutrient Management Guide (RB209) and Germinal GB Ltd.

Brassicas are prone to sulphur deficiency which is shown by the yellowing of the leaves; if suspected, a tissue analysis is the best guide.

25 kg SO₃/ha (8–24 units/acre) is recommended on soils at risk of deficiency.

Seek advice from a FACTS-qualified adviser and use Nutrient Management Guide (RB209) as a guide.