Background & rationale
The informed management of cover crops is key to the delivery of important UK Government policies including the 25-year Environment Plan and improvements in ground and surface water quality. Cover crops have been included in ‘Action 3’ (add organic matter) and ‘Action 4’ (winter cover) of the ‘Arable and Horticulture Soils Standard’ in the ‘Sustainable Farming Incentive’, one of the environmental schemes being introduced under the Agricultural Transition Plan post-Brexit. Whilst the benefits of cover crops for erosion control and reduced overwinter nitrate leaching losses are well established, the legacy effect for subsequent crops in the rotation is unclear. Of particular relevance is the timing of nitrogen release from cover crop residues and how this influences the nitrogen fertiliser requirement of the following cash crop and subsequent nitrate leaching losses.
The use of cover crops in conventional farming systems has rapidly increased (Storr et al., 2019 ), but guidance for farmers is often very general or inconsistent and not necessarily based on UK experiences (AHDB Research Review No. 90). A recent survey of UK farmers on the use highlighted the uncertainty of the effects of cover crops on crop available nutrient supply (Storr et al., 2019). Furthermore over 80% of the farmers were destroying cover crops with herbicide. Given the uncertainty over the future of glyphosate use in the UK post 2025, and the movement to ‘regenerative’ farming with the associated increase in minimum/zero tillage, more information is required to help farmers develop alternative destruction methods that are economically viable and do not compromise the establishment and management of the following cash crops. Further information is also required to understand the impact of cover crop species and destruction method on nitrogen cycling to more accurately quantify the timing and amount of nitrogen released.
This work has built on the research undertaken as part of the AHDB funded Maxi-Cover crop project  by focussing on the effect that cover crop species mix and destruction method have on nitrogen release i.e. how much and when. The information will be used to underpin advice on the appropriate management of cover crops in terms of the crop available nitrogen supply and long-term impact on nitrate leaching. Measurements have been carried out on two replicated field plot experiments (located in Hertfordshire and West Sussex) comparing 2 cover crop mixes, with a no-cover control treatment, destroyed by two different methods (glyphosate vs. mechanical), with subsequent crops established using minimum or no tillage (6 treatments replicated 3 times = 18 plots).
1. Mix 1: Phacelia (20%) and oil radish (80%); with the oil radish (‘triangle’) being a certified club root resistant variety.
2. Mix 2: Non-brassica mix: Japanese Oats (45%), Buckwheat (45%); Phacelia (10%)
The work is funded by Affinity Water and Portsmouth Water, with the seed mixes kindly supplied by RAGT seeds. This is a collaborative project, working very closely with our host farmers, who will be conducting all farm operations with their own kit/machinery, ensuring the results are representative of commercial practice.
Crop destruction techniques:
At both the Hertfordshire & West Sussex sites, Mix 1 and Mix 2 reduced over winter NO3 leaching losses by c.90% & 70% respectively compared to the control which lost 9 and 23 kg/ha N by leaching, respectively (Figs 1 & 2.)
Spring soil nitrogen supply (SMN + cover crop N) was up to 30 kg/ha greater where a cover crop (Mix 2) had been grown (Fig 3.)
At both sites, topsoil mineral N post-destruction was lower following mechanical compared to chemical (glyphosate) destruction (Fig 4)
A higher incidence of weeds was recorded in the spring barley crop at the West Sussex site, where mechanical destruction had been used on the stubble (wheat volunteers) and mix 1 (oil radish regrowth) treatments (Fig 5)
Spring barley yields were increased by up to 1 t/ha at the West Sussex site following the radish/phacelia cover crop, but only where it was destroyed using glyphosate. Mechanical destruction resulted in a 0.7–1 t/ha yield decrease (Fig. 6). Similar trends were observed at the Hertfordshire site, but spring oat yields were more variable due to high density of blackgrass
The fate of nitrogen retained in the cover crops is the focus of ongoing work, with topsoil mineral N being measured to track it’s release to the growing spring crop, following different destruction techniques; chop & shallow incorporate, glyphosate, rolling on a frost.
- The potential legacy effect of these treatments is currently being determined
- Results will be used to underpin advice for growers on the appropriate management of cover crops, to maximise N recovery and minimise nitrate leaching
 Bhogal, A., White, C. & Morris N. (2020) Maximising the benefits from cover crops through species selection and crop management. AHDB Report PR620 https://ahdb.org.uk/maximising-the-benefits-from-cover-crops-through-species-selection-and-crop-management-maxi-cover-crop