Addressing Key Cover Questions - Agrii - Connecting Agri-science with farming

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April 27, 2015

Addressing Key Cover Questions

Cover cropping shows considerable promise but major questions remain over its true value for heavy ground with black-grass problems in particular, believes Agrii trials manager, Steve Corbett.

We recorded spring cereal yield benefits of up to 1.8 t/ha from cover cropping in our research last season. But our experience to date on difficult, bad black-grass ground certainly hasn’t all been positive.

Even though our recording showed it trapped 20 kg N/ha and gave a much greener crop of wheat from October-sowing, for instance, a short term cover of brown mustard in place of bare stubble did nothing for yield as a result of much higher black-grass populations.

Equally, in our spring wheat work we’ve been able to match the yield increase we saw from a proprietary winter cover mix simply by switching to a narrow tine on the farm’s Claydon drill to restrict spring black-grass germination. So there’s clearly a lot more to learn.

That’s why we’re trying to tease apart the key components of cover cropping success at our black-grass technology centre. And, I have to say, from an initial sceptic I am becoming quite enthusiastic about the opportunity the technique offers.

Even 10 weeks of short term cover ahead of winter wheat sowing last autum has been noticeably valuable in drying out the site’s heavy clay ground and improving its structure.

However, in this and parallel field-scale trial work ahead of spring wheat, we’re recording clear   differences between the four different cover crops we’re exploring alongside cultivation-only regimes.

From what we’ve seen so far, the key question we need to ask with cover cropping is what are we trying to achieve alongside better winter or spring crop performance ?  Is it nutrient trapping, soil structure improvement or grass weed control we really want ?

If it’s mainly the latter, I have to say black-grass isn’t a bad short term cover!  Unlike other temporary crops, it allows you to spray off successive weed flushes with glyphosate for the best weed seed bank depletion.

Unsurprisingly, while they controlled black-grass well, the stale seedbed regimes forming the main controls in our trials failed to do anything for soil structure, leaving ground min-tilled for the past 11 years showing marked compaction below 15-20 cm and lying very wet right through to spring drilling despite the dry March.

By early November, clear but quite different improvements in soil structure and drainage were recorded under the various cover crops we sowed in mid-August with a single pass Lemken Karat system.

With its strong tap root, oil radish made a good attempt to break through the compacted layer. Decent lateral rooting also improved crumb structure.  Much better crumb improvement and compaction busting came from a combination of black oats and vetches. In fact, this was nearly as good as a decent cultivation.

In contrast, though they worked well for soil structure and drying in the top few centimeters, both phacelia and white mustard were unable to make any impact on compaction below 10-15cm. They are clearly water pumps more than anything else.

Mid-October black-grass levels were highest in oil radish. The black oats and vetch also appeared to compete less well with black-grass than the phacelia and white mustard, but a more open structure allowed very good control with the glyphosate cover destruction.

Because they give far less opportunity than stubbles for reducing the weed seed bank with glyphosate, one of the things really becoming clear in our work is the need to prioritise black-grass control when establishing the succeeding crop – either in late autumn or spring.

Drilling a competitive winter wheat variety directly through the green canopy following spraying-off with glyphosate in late October has proved very successful in this respect, with the Claydon  performing especially well.

Even so, quite a bit of soil movement at drilling with standard tines has meant more black-grass emergence in the seedbed than we would have liked. So we’re looking to use narrow tines on the drill in future to reduce the challenge.

It’s very much work in progress here, alongside our detailed variety competitiveness, cultivation system and rotational studies for better black-grass management. So far, we’ve been very encouraged by the improvements in soil structure that appear possible in a very short time.

Providing the right cover crop can be grown for the task in hand and black-grass kept soundly asleep after it, the technique could be really valuable in improving heavy ground even where there are difficult grass weed populations.

The proof of the pudding will, however, be in securing sufficient immediate yield and input-saving benefit to more than cover the cost of both the extra seed and time costs involved. With financial pressures as acute as they currently are this may not be easy. But then again, every little will clearly help.