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Key pointers to cover crop success

23/07/2021

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Key pointers to cover crop success

Soils have taken a battering over the past two seasons and remedial action will be required in many situations. Cover crops can play a key part in repairing the damage, but must be managed appropriately to achieve the desired outcome.

This is particularly true on heavy land, where the two biggest challenges are optimum establishment of the cover crop and its effective destruction.

Niall Atkinson, Agrovista’s farming systems research and development adviser, says: “By following a few guidelines, growers will greatly increase the chances of growing an effective cover, regardless of soil type, weather, or species.”

Timely establishment

Good timing is critical. Even a few days’ delay in sowing can adversely affect a cover crop’s performance. “Plants need to harvest as much sunlight as possible, as quickly as possible,” says Mr Atkinson. “A short-term cover crop planted ahead of an autumn-sown cereal may only be in the ground for two months, and time is also of the essence for an over-wintered cover as growth slows during the late autumn and winter.”

While timely drilling is key, ideally soon after the combine has left the field, this must not be at the expense of seedbed quality, he adds.

Straw management

The first step to good establishment should be taken before the preceding crop is harvested. The combine’s straw chopper should be set to produce a fine chop that spreads across the full width of the cut. “There will be little time for crop residue to break down before the cover crop is drilled and no stale seedbed cultivations to aid the process,” says Mr Atkinson.

Cultivation strategy

The next step is to assess soil structure and the need for remedial cultivations. If yields in the preceding cash crop were good and pretty even across the field, and steps were taken to minimise traffic and soil damage during harvesting, there is unlikely to be a problem.

Suspect areas warrant closer inspection with a spade. “Recent weather means many soil types could still be wet at depth which, if loosened too deeply, could cause more harm than good, but still incur all the cost.

“Work at Agrovista’s flagship heavy land research site, Lamport AgX, has consistently shown that shallow low-disturbance loosening at 100-125mm together with plant roots has been far more beneficial than deep loosening alone, as well as being more cost effective.”

Sowing

Cover crops can be established in several ways, including direct drilling. Where required, a very shallow cultivation, either as a separate operation or during drilling if appropriate, should be sufficient to mix in the straw and mineralise some nitrogen to help kick-start crop growth.

Cover crop mixtures often consist of seeds of varying sizes that require differing sowing depths. “Modern drills can often deal with this, while older equipment that suits larger seeds will benefit from the addition of a small seeds application kit,” says Mr Atkinson.

“After drilling, the seed-bed can be rolled as necessary – soil moisture conservation is important throughout this process.”

Agrovista is also investigating broadcasting cover crop seed into the preceding crop several weeks before harvest. Work is continuing to pinpoint appropriate timings and species.

Effective destruction

Destruction timing is closely linked to several factors, including land type, cover crop bulk and, most importantly, the carbon:nitrogen ratio.

“Where covers with higher C:N ratio such as cereals and brassicas are grown, especially on heavy land, it is vital to apply glyphosate early, December or January if possible, so plant material has time to break down to release nutrients and benefit soil biology,” says Mr Atkinson.

Early destruction also allows the soil surface to dry, improving drill performance and reducing slotting, whilst minimising the green bridge risk.

Work at Lamport AgX has shown a distinct yield advantage in spring wheat when the preceding black-oat based cover crop was sprayed off in late December/early January, compared with two weeks pre-drilling (see table).

In practice, a second application may be needed in bulky cover crops to ensure a complete kill before drilling.

“Most drills, with some adjustment, will comb through reasonable amounts of cover crop residue provided it remains anchored to the roots,” says Mr Atkinson. “It is best to avoid surface cultivations before drilling.

“Where covers consist of lower C:N ratios, such as vetch, early destruction is less important, especially on lighter land.

“Growers who own specialist drills that work in a standing cover crop can spray just before or just after sowing the following crop.”

Impact of cover crop destruction timing on spring wheat yield (t/ha)

  Early destruction Later destruction No cover crop
2015 10.30 9.12 9.59
2016 8.65 7.75 6.01
2017 8.6 7.31 7.35

Source: Agrovista, Lamport AgX trials