An introduction to cover cropping

Tim Martyn

`

Freshwater eutrophic

@ only 35 g P per litre

OR

35 millionths of a gram

160m3 ha 10m3 ha 433m3 ha 381m3 ha

SS 213kg ha 9kg ha 719kg ha 1551kg ha PP 859g ha 37g ha 3029g ha 5762g ha

Catch and Cover Crops

• An extra crop between two of the main crops in a rotation

• Soil protection (min- or no-till drills)

• Fertility building

• Supplementary stock feed

• Replacement of main crops that have failed

• Livestock for grazing or folding

• Sow early for improved establishment/effect

Pros

• Building soil organic matter (SOM) – green manure, particularly on sandy soils

• Reducing suspended solids in runoff – retaining soil

• Nitrate scavenging – early est. best

• Improving soil structure

• Aeration/Drainage

• SOM & soil structure increases water retention

• Weed suppression

Cons

• Reliability of establishment – early establishment best

• Appropriate soil conditions for establishment – not usually suited to heavy or dry land

• Depletion of soil water in spring

• Clean land to minimise weed control problems

• Fertilise - Uptake of soil plant available nutrient

• Possible delays in seedbed preparation

Carbon:Nitrogen Ratio

• Bacterial biomass has a C:N ratio of 6:1 • Well decomposed humus between 10 and 12:1 • Other organic remains have much more C • Breakdown of material having wide C:N ratio, such as

cereal straw (140:1), is slow unless N is readily available

• Soil bacteria uses ‘plant available N’ (mineral or inorganic N) in the decomposition process

• N used (immobilised) becomes organic N, mostly unavailable to the plant. However, it can be made ‘plant available’ at a later date by a process called mineralisation.

Pests and Diseases

• Must not harbour pests or diseases of any main crop in the rotation, e.g. rape is highly susceptible to clubroot

• Do not use brassica cover crops in rotation with beet

• Rye, IRG or Westerwold preferred in rotations including cauliflower, swedes or OSR

• Green bridge if grasses present – aphids and cereal fungal diseases

• Slugs

Source: Nickersons

Cover Crop Seed Costs

Crop £/kg Seed rate, kg/ha Seed cost, £/ha

Forage rape (treated) £5.75 10 £58

Mustard £3.00 20 £60

Phacelia £7.00 10 £70

Westerwolds £2.00 35 £70

Red clover £6.60 15 £99

Forage rye £0.60 180 £108

Buckwheat £3.00 75 £225

Summary

• Remove compaction – avoid soil destabilisation

• Early establishment following harvest when soil temp. higher, prior to winter, to maximise root and canopy growth

• Good seed to soil contact imperative together with minimal soil movement

• Avoid smearing if slot seeding – waterlogged slot

• Care required with drill selection

Cover and Catch Crops

Crop Options

• IRG or rape on a scarified corn stubble following harvest, ploughed in, grazed or sprayed off then strip-tillage

• Rye/IRG/Westerwolds RG establish best when sown in early autumn

• White or Ethiopian mustard between harvest and drilling

• Oilseed radish

• Legumes and Phacelia

• Mixtures/undersowing – consider following crops

Further options

• Buckwheat - Fast establishing producing a thick bushy plant which sheds seed in the Autumn. Allelopathic effects?

• Phacelia – an extremely fast growing crop. It’s dense growth habit smothers out most weed competition. Care must be taken to destroy the crop as Phacelia easily reseeds itself.

• Hybrid Forage Brassicas – a Forage Rape x Kale cross that is quicker growing than Kale and more winter hardy than Forage Rape. Forage Brassicas are ideal for late sowings.

• Yellow Sweet Clover - a biennial plant which can last for several years due to it’s ability to regenerate itself through shedding seed. Yellow sweet Clover has a sweet smell and produces large amounts of nectar making it highly attractive to insects which in turn attract birds. Best sown with an annual companion crop to produce a cover in first year.

• Texsel Greens – an easy to establish, quick growing crop which is frost hardy and ideal for suppressing any weeds

Nitrogen Fixing and Accumulating

Yield t dm / ha Legume % N in plant kg / ha N fixed kg / ha White clover/grass 6.04 67 117 248 Red clover/grass 9.46 79 112 343 Lucerne/grass 7.53 74 115 320 Beans 3.80 100% 130 275 Vetches 7.00 100% 150 160

Nitrogen Scavenging & Accumulating

Nitrogen Scavenging

• Work by IGER found growth over-winter of rye to be generally better than Westerwolds or IRG

• Consequentially higher herbage N recovery

• Rye has greater frost tolerance (leaf proteins)

• Rye can be established later in autumn but early drilling gives better establishment (regional)

• Establishes better in free-draining (aerobic) soils

• Faster earlier est. means higher LAI

Source: Nickersons

Nematode Reduction

• Nematode reduction – oilseed radish and mustards

• Harvest date and the time available to sow the cover crop

• Weather – most cover crops are best sown in July and August and will need moisture to germinate

• Soil type (determine the sowing rate)

• Ability to mow and incorporate the cover crop in late autumn

Source: Nickersons

Biofumigation

• Plant breakdown releases chemical substances known as Isothiocyanates (ITC): Ethiopian and white mustards, oilseed radish

• Prussic acid production : Sudan grass

• Used to control, Fusarium, Pythium, Rhizoctonia fungal diseases and soil borne insects.

• Likely to reduce beneficial mycorrhizal soil fungi

• Works well on light textured, low organic matter soils.

Benefits of Biofumigation Crops

• Natural Control of soil borne pests but also beneficial ones too

• Traps and re-cycles nitrogen via the plant

• Improves soil structure by increasing organic matter

• Improved soil water retention

• Reduces soil erosion

Yield of grain (t/ha, 15% moisture) of

wheat grown with and without N, either conventionally or in a clover understorey

Treatment 1995 1996 1997 Bi-cropped, no input 1.59 1.41 1.56 Bi-cropped, low input 3.11 2.4 2.57 Conventional, moderate input 8.63 8.19 6.0 Conventional, high input 10.09 9.76 7.34

Source: Burke et al

Bi-cropping - Pros

• Permanent understorey of a companion crop, i.e. white clover.

• Crop drilled using a Hunter stripseeder (strip-till drill)

• White clover may help to interrupt spread of splash borne fungal disease spores

• Non-inversion of soil promotes beneficial invertebrates and mycorrhizal fungi

• Uniform green canopy may reduce aphid alighting (

Bi-cropping - Cons

• Competition for moisture, sunlight and nutrient by companion crop

• Restricts choice of herbicide a.i.

• Can harbour pests (slugs, etc) but also provides cover for beneficial predators (ground beetles) – balance required

Summary

• Soil protection – conservation tillage

• Nitrogen fixer or scavenger

• Pest reduction

• Consider following crop when selecting CC

• Bi-cropping

• Environmental benefits beyond reducing DWPA – wildlife