Investigations into the potential of cover crops are seeking to help farmers identify what value they can bring by pinpointing agronomic and financial returns. Martin Rickatson spoke to NIABTAG’s Ron Stobart.
With spring cropping playing an increasingly important role in UK arable rotations, and heightened awareness of the value of non-mechanical solutions to soil structure and compaction problems, interest is growing in the use of cover cropping.
But making an additional investment in seed, soil movement and spraying-off at a time when a field would normally be costing the business little requires commitment and understanding from farmers of both the immediate and longer term benefits.
That is according to Ron Stobart, of NIABTAG, responsible alongside others from the research group and other bodies for a number of recent studies into the potential and practicalities of cover crop integration into arable rotations.
Their findings suggest cover crop use can produce a financial benefit, but that returns are not all immediate in the season following the crop.
Mr Stobart says: “As part of the Sustainable Intensification Platform programme, for 10 years now we’ve been working at a range of levels to investigate cover crops, spanning everything from small plot testing to big replicated field trials and farm practice work.
“At the most basic level, what we’ve seen is that clearly identifying the reason for growing the crop is essential. Farms with different soils, cropping and rotations will have different needs and expectations, and it’s important before committing to cover cropping to identify your primary objective, whether that be management of erosion, run-off, leaching, compaction or other factors.
“It’s equally important to identify ways of measuring the benefits. In trials, we assess this through tests ranging from simple soil digs to measurement of soil organic matter levels, porosity, friability and resistance levels. Most of those can also be done simply on-farm.
“And it’s essential to look at any possible conflicts – for example, brassicas can be a useful cover crop element, but there could be issues on land already regularly growing oilseed rape.
“Then there’s cover crop destruction, which will depend on factors including climate, soil type, ability of the farm’s drill to handle residue, and even the availability of livestock to allow grazing-off.”
While seed mixture costs range from £20-80/hectare, it’s often possible to trial small areas for little investment to assess their merits, Mr Stobart says.
“That offers the opportunity to compare trial cover crop and autumn-cultivated land side by side to then examine differences in structure, so the farm’s cultivation regime can be assessed, and possibly reduced or partly replaced by strong-rooting cover crops.”
At its sandy loam Morley site in Norfolk, NIABTAG’s New Farming Systems (NFS) research project comprises a series of large-scale, long-term, replicated experiments designed to examine the potential for improving arable system sustainability, stability and output.
It includes a range of pre-spring crop cover crop regimes using brassica and legume mix bases, plus the use of long-term clover bi-crops, where a semi-permanent clover remains in the stubble.
Results indicate benefits of enhanced soil characteristics, positive yield responses and improvements in financial margins over fertiliser input associated with the use of specific cover crop approaches.
“This provides evidence of cover crops’ potential, but the range of cover crop options assessed differ in their management requirements and likely end results,” says Mr Stobart.
“Choice of species should be guided by circumstances and the desired goal, with fertility and soil structure enhancement among most farms’ key aims.”
Work at Morley, where long-term field studies began in autumn 2007, is comparing rotations, nitrogen dose interactions, the interaction of cultivation method and cover crop on crop performance, and how different mixes work, using a fodder oil radish and, for the rotations study, a legume species mixture using white clover, trefoil, lucerne and crimson clover.
Soil porosity is assessed via measurement of infiltration rates, with an increase from 0.78mm/min to 2.19mm/min recorded in one trial where a clover bi-crop system had been used, attributed to development of a more open soil structure created by the clover bi-crop rooting.
“In addition to aiding crop root development, these higher infiltration rates cut run-off, and consequently hence soil erosion, nutrient loss and diffuse pollution risk,” says Mr Stobart.
“Previous research suggests cover cropping, both legume- and non-legume-based, would also be expected to reduce nitrate leaching. When considering all the cover crop approaches, we’ve seen the highest mean yield response over standard practice coming from this clover bi-crop approach, being potentially linked to both the changes in soil characteristics within the system and the nitrogen provision afforded to the crop from legume-based bi-cropping.”
NIABTAG is also working on cover crop development with breakfast cereal maker Kellogg’s, as part of its Origins initiative to bring the food manufacturer closer to its farmer suppliers and improve supply chain efficiency and consumer sustainability demands.
Mr Stobart says: “Farmers in the Origins group are focused on practical ways to improve soil structure and fertility, benefiting farm performance and system resilience and delivering wider nature benefits. Cover crops can contribute here, and there’s strong interest in how they might best be used to support these goals.
“Origins farmers have been involved in a cover crop trial programme. Notable findings include soil characteristic improvements such as structure and earthworm activity, and environmental enhancements including nitrate leaching reductions. The work has also generated improved cover crop selection criteria and best practice guidance.”
During 2014-15, cover crops were established as part of the programme on 17 fields farmed by nine Origins growers across Yorkshire, Lincolnshire, Gloucestershire and Northamptonshire, with usage varying according to farm and ranging from multiple strips of different cover crops in the same field through to split field comparisons and whole field areas.
“On some sites, different management practices – establishment or input regimes – were also applied, while cover crop choices also varied with location, encompassing grasses, brassicas, legumes, borage types and polygonums.
“We assessed crop counts and green area indices, soil and crop nutrient contents, soil structure and biological factors such as earthworm counts. Despite the range of cover crop species and drilling parameters there was a strong relationship between sowing date and autumn growth, relatively little difference in cover crop population, and some suggestion conventional drilling resulted in larger autumn green area indexes (GAIs), while single-pass drilling resulted in reduced spring weed populations.
“On two sites we were also able to evaluate the impact of a low dose of starter fertiliser (20–40kg/ha N) on cover crop trial areas. Mean data suggest it had little impact on cover crop population, resulted in a small increases in autumn GAI and broadly doubled autumn weed emergence, encompassing both broad-leaved and grass-weeds, including black-grass.
“In a further comparison, areas sown with cover crops all retained more N than comparable control areas left unsown. Assuming this N would have otherwise been leached, the mean data across all sites would suggest a reduction in leaching of 38kg/ha, equivalent to 40%.”
Research has demonstrated the yield responses from cover crops are often rotational and not solely in the following spring crop, adds Mr Stobart. Across the range of cover crop types and management systems within the Origins programme, early sowing appears to promote greater cover crop growth, while brassica species and cereals such as oats provided greater biomass scores and higher winter survivals, in contrast to legumes, with other species such as phacelia in between, although this was variable between sites and sowing dates.