Roger Bryan, Agronomist, Crop Management Partners, Kent
At a time when the economics of arable farming mean most farms are trying to drive cost out of the business, spending £40-100/ha on seed, fertiliser, slug pellets, labour and machinery to establish a cover crop is not very attractive. You have to buy into the longer term benefits of better soil structure, improved soil biology, reduced erosion risk and nutrient retention. Plus catch or cover crops can also be used to meet your Greening requirements.
Finding the time to send a man and machine off to drill cover crops is not always the highest priority when trying to finish harvest, drill rape and prepare stale seedbeds in catchy weather.
So what are the main benefits?
A 25-40% reduction in nutrient leaching loss has been measured from having a cover crop in place compared to a cultivated soil. With a shift of cross compliance emphasis to the prevention of soil loss by maintaining soil cover (GAEC 4), a cover of 30% can also cut erosion losses by 80%. More importantly, cover crops maintain soil structure through their root growth, preventing soil slumping from winter rainfall.
Organic matter soil levels are falling in arable soils, a good cover crop can return as much as 5t/ha of its dry matter. Not as good as organic manure applications but better than chopping straw.
Do cover crops control black grass?
Weed competition from cover cropping, particularly black grass, has been much publicised. Glyphosate treatment of multiple black grass flushes on an uncropped stubble seems to have limited success in reducing black grass numbers where populations are high. Multiple glyphosate applications also presents a possible long term risk of glyphosate resistance.
Having a cover crop in place will reduce black grass germination purely through crop competition itself. The longer black grass germination can be prevented near the soil surface, the more the black grass seed in the topsoil will decline through decay and predation. In pure swards of oats, Black or otherwise, and pure swards of fodder radish, there is some allelopathic control of black grass but treat this is as bonus control. The effect is greatest where the primary cultivation is no greater than 20-50mm i.e. true no-till direct drill. Unfortunately whilst black grass numbers decline, brome numbers increase under these systems. A mixed rotation, delayed drilling and spring cropping are far more effective methods of cultural black grass control for conventional drilling systems.
How to establish cover crops?
Establishing a cover crop, like any crop, can be a tricky business particularly in a catchy autumn like this one. Broadcasting seed is cheap but not always reliable. Drilling cover crops is a more reliable albeit more expensive method of establishment. Be prepared to treat slugs and apply nitrogen to get crops off to the best possible start. As a rule of thumb, 6 weeks is needed to grab as much carbon as possible to produce as much top and root growth as possible before the sun gets low in the sky and soil temperatures start to drop away.
Some species are easier to establish than others. Oats, linseed, buckwheat, beans & vetch are fairly bomb-proof. Phacelia, rye, radish, sunflower, peas & clover are trickier. Having a mix of species in your cover usually results in something growing. A mix also brings something to the party from each species in terms of rooting depth, nutrient scavenging, beneficial wildlife and weed competition. There is also a school of thought that a mix reduces the risk of soil-borne disease such as fusarium in pulse spp, clubroot and verticillium wilt in brassica spp and nematodes in oats but we don’t have the answer to that yet.
If you are planning a pulse crop in the Spring, best leave out the pulse component in a cover crop. Likewise leave out brassicas, oats and linseed if those crops are planned for the Spring. With verticillium wilt becoming more prevalent in oilseed rape, I feel that brassica spp in cover crops are not a good idea until we have a clearer understanding of the risks. The decline in the rape area and longer rotations may make this risk less critical.
Another concern I have is that brassica cover crops such as mustards can run to flower and seed quickly in warm autumns so be prepared to spray them off early. Also, as oilseed rape growers know, not all brassica seed will germinate in the year of planting so don’t be surprised to see these species appear in crops further down the line.
When to kill-off the cover crop is another tricky decision?
For most, the on-farm cultivation equipment and drill type will be the main driver for when to spray off. Those with true no-till drills such as the JD750 can spray-off the day before and drill into what is effectively a green cover. As a gross generality, tined cultivators and drills are more likely to hook up on wet material. Disc cultivators are more likely to chop up trash but can smear wet soil, disc drills leave slots in unmoved ground. You really need a degree of flexibility and a lot of patience depending on the soil conditions. No one piece of kit will suit all conditions in all seasons. Not what you want to hear when trying to keep machinery costs down.
Some species are difficult to kill off with straight glyphosate so expect to spend a bit more when you spray off and don’t skimp on rates.
A good hard frost can do most of the work for you. However, our continental cousins have harder winters than us so be careful regarding claims of what cover crop species will be killed off in a frost.
Grazing off with sheep is another option and generates useful income by fattening lambs or through a headage payment from a neighbour. Sheep are a useful way to re-cycle the top growth of the cover into organic manures with readily available nutrients. However, their feet can do a lot of damage to topsoil structure in wet soil conditions so the grazing has to be at a low stocking density.
Do cover crops keep topsoils drier? I have seen cover crops retain topsoil moisture in a wet spring making cultivation/drilling difficult and completely dry seedbeds out in a dry spring causing poor establishment so again there is no easy answer. Overwintered ploughed land will cultivate and drill earlier but at what cost to soil structure and soil biology? With cover cropping you have to play the long game with respect to soil structure and soil biology.
How does it affect soil biology?
Having a green cover in fields all winter does encourage all sorts of vertebrate & invertebrate wildlife, soil bacteria & fungi which is great for soil life. Unfortunately that also includes slugs and snails. In time it is suggested that predators will sort out those soil pests but I am still waiting on farms where cover cropping has been in place for a number of years.
When are soil nutrients released back from the cover crop?
This is work in progress by researchers. Different crop species give it up at different times. I think we know that a mustard crop is particularly poor in giving back the nutrients it has mopped up. The thick woody stems of a well-developed mustard crop take an age to break down. Another reason not to let mustards get too big. Legumes are relatively slow to give up nitrogen once killed off. Chopping the cover crop into smaller pieces will speed up the process but that is another operation, another set of wheelings and another cost.
To maximise the build-up of soil nutrients and organic matter in the top-soil over the years, you really need to be on a no-till direct drill system or at the very least a strip-till direct drill system. Burying all that organic matter with deeper cultivations or ploughing will still have some benefit but not as great.
The future for cover cropping?
Sadly, I suspect a lot of the cover crops this autumn have been planted on the back of fad & fashion combined with over-zealous seed salesmen. The wet August & September have resulted in cover crops slow to establish and disappear under huge slug/snail pressure. This has taken its toll on some of the cover crop species.
In time I see cover cropping predominantly as a vital part of no-till/Strip till direct drilling where capturing carbon to build organic matter, stabilising topsoil structure and keeping nutrients in the top 20-50mm is integral to the system. How many farms will adopt that system in an attempt to drive costs down remains to be seen? The success of no-till systems to culturally control grass weeds with a shrinking chemical armoury will have a large bearing on the uptake and longevity of no-till. But that is an entirely different subject!