Written by Ed Trotter
Cover crops – Working with the options
Cover crops come in all shapes and sizes and they are all to be loved, but really everyone is after the biggest, boldest cover crop around. It is now the pub talk of regenerative farmers, how big they managed to grow their cover this year and the envy of all neighbours as they drive past a knee-high jungle of biomass and species.
Inevitably, a lot of it, is like many things in farming, is down to the weather. The expensive ‘Ferrari’ mixes as I call them, can do no good without some moisture or heat at the right time. This has always been the frustration with sowing cover crops, but now at least we have good funding to have a go and see what they can do. With warmer autumns now the norm, cover crops can double or triple in size from mid-September to end of October, which can make up for the slow start we can often see.
Getting the Best from Cover Crops
What I have been discussing with clients in recent years, is how to get the best from cover crops and consider them being front and centre of the farming system. When you read books such as David Montgomery’s ‘Hidden half of nature’, one of the biggest take aways is how important covering the soil is, living roots and returning biomass to the soil for the biology. This can transform a soil, especially when it has been poorly treated for so long. But as everyone knows, there are cover crops and then there are COVER CROPS. If anyone has ever stood in a field where the biomass of green comes over your knees (and I am 6ft 3!) and it is thick to walk through, this is a completely different situation to a ‘cover crop’ drilled mid-September into straw and gets to a meagre 2” tall. These are called the same thing but anyone who has experience with covers will know they are far from it.
A farmer claiming they have been cover cropping for years with no benefit, should realise they are vastly different and know the time it can take to realise the benefits, but also how to manage these situations. The smaller of the two counterparts will offer some small benefits and I would never discourage from farmers growing these if that is all they can achieve in that year (not every year do we get good opportunities). But, this is the distinction between cover crops which can really help develop a soil and one which is a nice thing to do, but lacks impact. It is certainly my view the more you focus on cover crops the more you will get out of them and the outcomes can be incredibly good for any soil.
Establishment of Cover Crops
A trend over past 5 years has been to grow more winter barley in order to establish oilseed rape earlier and evade the pesky flea beetle. This still seems hit and miss on performance of the crop but it begs the question if you are dropping oilseed rape, should you drop winter barley? On farms where we can sensibly grow winter barley, I am in favour of keeping winter barley in the rotation for the early entry for big cover crops, this is on top of the practical benefits the crop brings. The practical issue of getting oilseed rape AND cover crops in early on top of every other harvest job in august has disappeared for many who have dropped osr from the rotation.
Cover Crop Mixes and Rotations
I am a fan of exploring the options of mixes which could contain multiple species as well as oilseed rape, so the mix goes in as a OSR/cover crop mix and whichever wins the battle gets to keep its place. This is nothing new with buckwheat as a companion but this obviously lacks persistency and biomass, but we will rule out brassicas unless using expensive Clearfield seed/herbicides. Growing spring barley before winter barley allows less contamination from winter wheat (where not deep cultivating), but also manages the grassweed issue somewhat than multiple winter crops one after the other. It also then allows a good early cover crop to precede a spring break rather than trying to grow a cover crop after spring barley late on. I am also a fan of growing second wheats where sensible, particularly with an early variety like Extase, for the same principle. An early harvested second wheat can also offer early entry for a cover crop and chopping the straw here will be easier than with the winter barley.
Boosting Cover Crop Growth
Making up for lost time in a wet or excessively dry season may be needed to achieve a ‘big’ cover crop. I would argue that a cover crop going in early to some moisture has a good chance to get away and mop up nutrients, a later sown one however may need a helping hand. If the option of a liquid applicator is there then additions of nitrogen and possibly phosphorus could be useful in building biomass to enable the root system to develop and do its job. 10-30kg of N concentrated to where it needs to be can make a significant impact on growth. But also you can add your carbon sources and biological products or brews. This is actually in my opinion one of the best opportunities to place biology, into a warm soil and (hopefully) some moisture heading into autumn for it to develop and thrive for the whole duration of the cover crop and pass over to the spring crop. Solid fertiliser applicators are a little more restricted but some urea down the spout can give a cheap and cheerful boost to push growth on. As we are using more modest amounts of N in the spring and growing more spring oat or malting barley crops inevitably this means we do not have the same high amounts of N floating around in the soil than we would have done before on tight OSR and milling wheat rotations. Seed treatments are also being looked at now, including biological ones which can speed up and aid germination and early growth.
Destruction of Cover Crops
It is a well known fact that cover crops can cause hangover effects onto the following spring crops, particularly spring cereals like barley and oats. In my opinion the best way to deal with ‘big’ cover crops is to graze them early i.e. early December. This is partly for regrowth so that we can have some green leaf which we can spray in spring, rather than spraying a grazed nub of radish which pushes a leaf out a few weeks after your pre-drill roundup has been applied in march. This can also happen with volunteers like barley or even the oats in the mixture, which can cause headaches in your crop later on trying to spray them out. Early grazing and in good conditions when the soil is not too wet will allow soil to rest for several months and a timely glyphosate pre drill later onto green leaf.
Timing of Cover Crop Termination
If sheep are not available or they are likely to poach the soil then spraying roundup from potentially mid December or early January depending on size of cover or availability of conditions. It is a misconception that a living cover crop will ‘suck out’ moisture in enough quantity to dry out the soil better in late winter/early spring, plants do not transpire in high amounts until I would estimate mid-late April. In late February the cover crop needs to be fully dead and starting to disintegrate for a spring barley to be established in march. The cover crop dying back allows the sun and wind to access the surface of the soil and extract the moisture, if you leave a tarpaulin sheet on a soil the moisture cannot escape, that is the same principle as a thick cover. As everyone knows with a spring crop like barley, the earlier the sowing the better the crop can withstand adverse conditions later like drought. This is also amplified in direct drilled spring crops which can lack rooting opportunity so earliness is key.
Carbon to Nitrogen Ratio Considerations
It is also worth noting the carbon to nitrogen ratio of any cover crop you are establishing, the higher the figure the slower and more difficult it is to break down (i.e. more lignin). So a high C:N ratio will need destroying earlier, especially preceding spring barley for example. Cover crops before crops like beans are less of a worry from a carbon point of view because they seem to establish okay themselves from depth away from the high carbon surface and manufacture their own N later on.
Analyzing Cover Crop Benefits
What we can also do with cover crops is take a cutting of a square metre and send off for analysis, this can give a good indication of N,P,K,S,Mg plus traces contained in the cover which has been extracted and to be returned over time. This will however take years to be converted and processed to soluble nutrients for crops to utilise and most likely not the immediate crop afterwards, except some N which might be available from legumes in the mix. Once multiple ‘good’ cover crops have been established in a field then the benefits start to compound each other, with even more soluble nutrition and the biology and overall soil ecosystem to boot.
