With many cover crops growing well this autumn thanks to good September moisture, management over winter will be important to ensure soils are in the best state for spring crops.
High biomass cover crops are an excellent tool for protecting soil over winter and reducing leaching, particularly in a wet autumn, where nitrate losses average 50kg N/hectare – double that of a dry season, according to ADAS.
But they can also lead to problems going into spring, particularly for those on heavier soils.
Speaking during the ADAS ‘Cover crops research into practice’ webinar, Dr Anne Bhogal, senior soil scientist at the company, said: “Timing is very important when it comes to cover crop destruction, particularly for those on clay soils and in a wet season. This means leaving a longer window between destruction and spring crop establishment.”
Farmer tramline trials in the AHDB Maxi Cover Crop project lead by ADAS, looking at two-way mix and three-way cover mixes, found soils were significantly wetter where a cover crop was grown.
Dr Bhogal said: “This could then have an impact on the following crop. It has been suggested that a gap of a least four to six weeks [between destruction and drilling] is best, although we still do not fully know if this is the case.”
Glyphosate is the most common destruction method, but there are questions surrounding the chemical’s future availability.
Results from an Innovative Farmers study which looked at flailing, crimping and rolling methods were positive in that none showed a negative impact on the yield of the following crop.
Dr Bhogal said: “Whether that is flailing, crimping, rolling, discing or grazing, we need to understand the impact of these destruction methods on the quality of the subsequent seedbed, the amount and timing of nitrogen release, weed populations, pest and pathogen carryover and this dependence on weather conditions.”
David Miller has been growing cover crops for 10 years on his farm in Hampshire and turned to no-till six years ago. Soil types range from light loam over chalk, heavy clay cap and lots of flints.
Discussing cover crop destruction at the Cereals Live Q&A webinar, he said: “We have a lot of different ways, whether it is destroying the cover crop completely, or whether we drill on the green. Our Cross Slot drill is capable of going through any sort of cover crop left standing. We have had sheep across that have taken it back to bare soil, but I do not think that is the right way to go. We find if we have a reasonably proud cover crop, we try to roll on a frost early in the morning and it will take out an awful lot.”
All fields destined for spring cropping are cover cropped with an eight to 10 species mix, costing between £25-30 per hectare which is recouped through Countryside Stewardship.
Mr Miller said he still uses glyphosate, but he is trying to move away from it for cover crop destruction.
“We are trying to use a specific graminicide to take out volunteers and black-grass and other grass-weeds and really trying to get the cover crop to be more susceptible to crimping or rolling. It is a work in progress.”
Grazing cover crops down will help with destruction, while giving the ground a healthy dose of manure and providing forage for the sheep sector. However, Hutchinsons agronomist Conor Campbell warns careful management is needed to retain the benefits.
“In the past, I have had fields that have been grazed too much, nearly down to bare soil and we had major issues with poaching. You need to be mindful that you do not want to start overgrazing it or run the risk of poaching the land, because you are then negating the benefits of having that cover in.
“You need to work with someone who will not graze it too much and will leave enough cover and protection of the soil that you are still getting the benefit of the cover.”
For Essex arable farmer Simon Cowell, waiting for his heavy clay soils to dry out in spring after cover crops was affecting the following crop. He now allows stubble fields to green up with volunteers and weeds overwinter instead.
He said: “I did cover crops for quite a few years, but I had to wait so late for it to dry out that I ended up with poor crops. I now chop my straw and rake the stubble and let it green up with volunteers and weeds over winter.”
This keeps the soil covered, but allows for better drilling conditions come spring, he said.
“On light land, cover crops are brilliant but it was not really reliable on my ground. We do not get many frosts here, so things stay green and lush all through winter. Cover crops are great for boosting soil biology but I feel I have done that by other means.”
However, careful management of green cover is still important, he said.
“I do not like to leave that green right up until drilling so in the next week or two [in early December] I will spray it off with one litre/hectare of glyphosate which will not kill everything but will knock it back so I can then go again [with glyphosate] about a week before I drill. We are trying not to use much glyphosate so just a really low dose early on, and a bit more just before drilling.”
This also knocks out any emerging black-grass plants which are likely to cause problems later on, said Mr Cowell.
“It is really important to not leave black-grass surviving all winter. Patches of black-grass turn the soil anaerobic and create the conditions it likes and grows best in. Black-grass changes the soil profile and biology to suit itself, so when you put the crop in the only thing that will grow is black-grass. Getting it sprayed off now will stop that.”
During the Maxi Cover Crop study, yield responses to cover crops in the following crop were variable.
Results from Kent saw a marginal 0.1 and 0.4 tonnes per hectare uplift, in the following barley crop. However, in Yorkshire on heavier soils, a yield decrease of 0.4t/ha and 0.2t/ha in spring beans was seen, Dr Bhogal said.
“It is hit and miss and it needs to be a long-term investment,” said Dr Bhogal.
“In sugar beet and potatoes, the benefits are more obvious. However, the hidden benefits such as nitrogen uptake, erosion control and soil structural benefits can outweigh this as a long-term investment.”
In Scotland, cover cropping is not widely practised due to the short weather window after harvest to get the seed in the ground, as well as the subsequent low temperatures and high rainfall which can limit their success.
The cost and work involved in establishment has also long been a deterrent. However, five farmers in the east of Scotland are now looking at sowing cover crops into standing crops pre-harvest to see if the benefits can outweigh the challenges as part of Farming for a Better Climate’s Soil Regeneration Group, facilitated by SAC Consulting.
They have all experimented with broadcasting seed into standing crops prior to harvest, which gives the cover crops several weeks to germinate and establish under the protection of the cash crop in a warm moist environment, perfect for growth. It has the dual benefit of maintaining a living root, which positively impacts the soil’s biodiversity.
Zach Reilly, of SAC Consulting, said: “We do not have the luxury of early harvests to establish the crops, so this is a promising route forward. Early establishment improves the survival rate of the seed, meaning you can sow less than you would later in the year and the group has been clever with using existing machinery, such as broadcasting with a fertiliser spreader or a boom-mounted slug applicator, both of which keep costs down.
“There are still challenges, such as the cost of cover crop seed, and slugs which can damage crops in a wet year, but it is good to see the group exploring options.”