Cover crops are becoming the new ‘in’ thing for arable farmers, and more are trying them out, often without really knowing what they are doing. Seed merchants are getting in on the act, and there’s an increasing amount of conflicting advice out there. Farmers want a plan they can follow, and here is one for any practical farmer who wants to succeed with them.
Writes Mike Donovan in this article from @Practical Farm Ideas issue 24-4
Steve Townsend has been into soil management and cover cropping since leaving the Eco-tilling team in Monsanto in 1997 to start Soil First Farming which advises farmers on soil issues using a hands-on approach. Steve is also into liquid crop nutrients and has developed a number of formulations which help farmers achieve improvements in their crops. During his time with Monsanto he became an expert in glyphosate. As farmers across the globe moved to minimum and zero tillage methods, Monsanto found their Roundup sales doubling every four years.
Working with soil became more than a job and Steve devoured all that he could on the topic, in particular the books and records kept by pre-chemicals farmers. Soil, and it’s degradation, has an immensely long history, which goes back to the Romans and Egyptians – “did you know that the main grain port of the pharaohs, used in 1400BC, is now 30 miles inland? Silt, the best part of the soil, makes up that delta, as it has been piled up over the millennia.” The erosion and degradation continues today, with rivers in the UK and elsewhere carrying millions of tonnes of productive soil downstream
and into the seas each year.
Practical ‘can-do’ talk at the Real conference
This article is the result of Steve providing a scintillating 12 minute talk ‘Managing Cover Crops’ in the 2016 Oxford REAL Farm Conference. He has kindly shared his visuals with us. At the conference there was little time for pre-amble, but at our subsequent meeting there was time to set the scene. Soil enthusiasts are scattered across the globe. While the same broad message is common among all who understand how to protect, build and use soil advantageously, the detail from each is importantly different.
To understand soils there’s a need to understand these variations, and since starting his business in 1998 Steve has travelled extensively, learning the techniques of soil conscious farmers in many countries. It’s experience he brings back home. So when we talk about docks in organic grassland, he suggests one looks at potash levels, in particular the phosphate P : K potash ratio. If there’s too much K, as there often is in parts of the South West and South Wales, the docks get out of control.
Soil carbon is the main factor for production in soil. Nutrients are less available when carbon declines. The decline in yield is inevitable. It can be held up by chemical intervention but this always needs to be increased in order to maintain yield. Carbon content is created from rotting organic matter, especially crop residues.
“Chopped straw is one of the best sources of soil carbon, and is far too valuable to be carted off to power stations. Left for worms, bacteria and fungi to break down, straw, together with the moisture we get, adds vital carbon to any soil.”
The spider’s web of soil
The structure and workings of any soil is as complex as a spider’s web. The different components interlock to make it work, and the whole is fragile.
Ploughing and cultivating damage the web, but are sometimes necessary in order to provide a structure for it to grow. Ploughing in Roman times was no more than a two or three inch scratch on the surface – something which enabled a seed to be covered over. Move forward a few centuries and the early mouldboard plough came along, and was a brilliant way to control weeds. These first ploughs worked to a depth of a few inches only. Horsepower was limited. With tractor power ploughs became wider, and therefore worked deeper, as the depth of the furrow is always around half the width. Digger bodies pulled up soil which had been sequestering carbon for hundreds of years. Exposed to sunlight and air the carbon deteriorated through oxidation, and when the power harrow came along the process was accelerated. Oxidation causes a major loss in soil carbon. With less carbon the soil is harder to work, so where once a light pass with the power harrow gave an onion bed, farmers were finding they had to do two passes to get an effective seedbed for corn.
Low cost winter feed which builds soil condition as well
Cover cropping seeds
Steve is emphatic on this – start simple. So you plan to grow a cover between harvest and a spring sown crop. That makes sense, as essentially farming is the art of capturing and using the sun’s energy, through photosynthesis. Bare soil, be it cultivated or stubble, captures no solar energy, but growing plants do. They grow roots, harvest nutrients from air and soil, and provide a habitat for microorganisms (as well as birds and insects).
So he suggests starting with mustard, or leaving volunteer oilseed rape to grow. Success builds confidence, and you want to reduce the risk of failure. You don’t want to spend too much money, so start with inexpensive seed like mustard, so you can budget no more than £20/ha/ year on the first three cover cropping seasons. The mustard, linseed, oats or barley produce good root mass.
In years 3 – 6 go for high biomass plants, such as sunflowers, beans in addition to the mustard and so on. The budget could be £30/ha, but you are beginning to think there’s some sense in the system. After year six you can add plants with higher bio-diversity, but also higher seed prices. Vetches and clovers will add N but work better in soils which are inherently fertile.
There’s a big difference between species and variety of cover crop seeds. Go into a field of phacelia (which is not supposed to be frost hardy) during the winter and you’ll find some plants have been killed by the frost and others are doing fine. The seed was bought as ‘phacelia’, the species, so the bag contained a number of different varieties. We don’t buy cereals as ‘winter wheat’ but as ‘Claire’, so we need to know what variety we are buying.
Slug damage in turnips which were overseeded into wheat. The surviving turnips did well, but some parts were obliterated.
When to get the seed drill out
There’s a simple answer – as soon as possible. You need to organise the cover crop drilling to be done while the combine harvester is in full swing. The soil will still have moisture in it, enough to get your mustard off to a good start. Steve says “don’t think of overseeding to start with, as the risks are far greater than drilling. Remember, you want a crop.” As soil fertility improves, so broadcasting becomes a more reliable way to put these seeds on.
Spinning them out with a fertiliser spreader has problems. Mustard goes on at 10kg/ac and stubble turnips 3-7kg. Seed doesn’t travel the same way as fertiliser, and will find any narrow gap to escape being broadcast at all. Mixing with fertiliser makes the spreading rate easier, but the seed separates in the hopper. E_B from Norfolk explained on TheFarmingForum how he spins turnip seed on 2 – 3 weeks before harvest using a spreader that goes no further than 16m. His tramlines are 24m apart. As soon as the bales are off he fills in the gaps with the spinner. If it’s dry he drills the gaps with his Kverneland Evo tine drill, just scratching the surface.
Broadcast seeds grow very much better in fertile top soil which has been worked over by worms and other critters. Your first season top soil could be badly degraded. When those plants are small, look after them like any other crop. If there slug challenge, get some pellets out, and if they look poor, perk them with 15-30 kg/ha of N. Your aim is to have a good crop just the same as cash crop.The investment will pay off, because the next year it will grow better.
Cover crop destruction
Autumn sown covers that are to be terminated in the spring ideally should be sprayed with glyphosate the day of drilling. Moisture translocation stops 24 hours after spraying so it’s important rain doesn’t fall between spraying and drilling as the cover crop could become the perfect mulch stopping the soil from drying out.
Maybe there’s a lot of grass weeds, and black grass. In which case spraying needs to happen early, by the end of January, to effect good control of these weeds.
A clean crop which is free of blackgrass might be grazed off, but not too hard, so the soil remains covered. Organic farmers and those who disapprove of glyphosate can consider the Rodale crimper roller which flattens and bruises the stems of most plants sufficiently to kill them off, though results have been few and mixed with this technique.
Sheep on oversown stubble turnips
How about grass and soil condition?
Once again, Steve says “look at the soil” and try to copy what happened in the world’s natural grasslands. Bison herds were constantly moving, eating the tops of grasses and moving on. Yet we like to graze the sward to an inch or two. We’re told to never let grass go to head – yet when cattle break into a hay field they dive at the seed heads. Grazing tightly causes swards to become open, need reseeding, as this grazing weakens the preferred species allowing less productive grasses to creep in.
He’s seen mob grazing in the USA and understands the theory and results behind it. Grass and herbage build up biomass in an ‘S’ growth curve, which means a lot happens, or could happen, just after the conventional farmer grazes the sward off. Get the grazing controlled so the that spurt of growth, which includes the creation of the seedhead happens, and the quantity of material produced is far greater than when grazed conventionally. The sward must then be allowed to recover – the bison have moved on!
Close-up of the grazed field showing a worm ridden where they collect straw prior to dragging it down using holes they have made.
Back fencing is the most productive action in a strip grazing system. It takes little time to shift the fence. The back fence stops the stock from returning and snipping off the new shoots, which causes the plant roots to weaken. Those new shoots are fed by goodness from the roots. When the grass is short, the roots are also short.
As it gets longer and more mature, so energy is put into the rooting system so they grow bigger, allowing the grass to recover quicker from the energy in the bigger root mass. The best grasses are quickly damaged in swards which have no rest from grazing as the roots never have the time to recover. Weeds take over.
Grazed lightly, the sheep are moved on before they damage to soil. Waste makes essential biomass.
Steve refers to Prof William Albrecht who spent many years studying the best soils and who wrote many illuminating works on soil balancing. Andre Voison is another luminary, particularly in grass, rotational grazing and grass production. Soil is exciting material which farmers have almost been encouraged to neglect for too long.