Background
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 waters. Cover crops have been included in the ‘Sustainable Farming Incentive’ actions for soils (‘CSAM 2: Multi-species winter cover’) which is one of the environmental schemes 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 effect on the subsequent spring cash crop 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.
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 (White et al., 2016). A survey of UK farmers on cover crop use highlighted the uncertainty of their effects on crop available nutrient supply (Storr et al., 2019). Furthermore over 80% of the farmers were destroying cover crops with herbicide. Given ongoing uncertainty over the future of glyphosate use in the UK, and the movement to ‘regenerative’ farming practices associated with an 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 destruction method and timing of destruction on nitrogen cycling to more accurately quantify the timing and amount of nitrogen released in order to help guide nutrient management planning.
This work built on the research undertaken in the Portsmouth Water and Affinity Water funded Nitrogen release from cover crops (NiCCs) project (Report on how cover crops can refine nitrogen use published | ADAS) by focussing on the effect that cover crop destruction method and destruction timing have on nitrogen release i.e., how much and when, as well as the impact on the weed burden. The information will be used to underpin advice on the appropriate management of cover crops in terms of the crop available nitrogen supply and weed management.
Aims and methodology:
The aim of this study was to quantify the effect of two contrasting cover crop destruction techniques (chemical vs mechanical destruction), and the impact of destruction timing (early vs late) on nitrate leaching, soil nitrogen supply (and hence crop nitrogen fertiliser requirements) and the performance of the following spring oat cash crop. In addition, the impact of a ‘reduced’ fertiliser nitrogen application rate on the performance of the spring oat crop was assessed. The study also evaluated the effect of cover cropping on yields and gross margins to demonstrate the benefits and trade-offs of cover crops for the farm business. The work was undertaken over one cropping season (2023-2024), on a commercial farm in Hertfordshire. Two covers were tested:
- No cover: weedy stubble
- Cover crop: spring linseed (38%), turnip (6%), millet (36%), buckwheat (14%) & Phacelia (6%) @ 13 kg/ha
Ground cover was destroyed in the spring at two timings (mid-Jan or mid-Feb) using two methods: mechanically by either rolling on a frost in January or mowing in February and chemically using glyphosate at both timings.
Key findings:
- Drilling a cover crop was effective at capturing nitrogen that would potentially otherwise have been lost over winter via nitrate leaching, with over 35 kg/ha captured in the above ground biomass. Nitrate concentrations in the drainage water were also initially (i.e. immediately prior to cover crop destruction) lower where a cover crop had been grown.
- High rainfall following destruction led to an increase in nitrate concentrations in the drainage waters where a cover crop had been grown, but not following destruction of a weedy stubble, indicating that some of the cover crop N may have mineralised and been lost via leaching. This suggested that early destruction of a cover crop increases the risk of leaching, not only due to the absence of crop cover/N uptake, but due to the breakdown of cover crop residues. However, over the whole measurement period (Jan – Mar), the total N leached from the cover crop treatments was 50% lower than on the weedy stubble.
- For some cover crop species mixes, below ground biomass can be quite large, but these have a higher C:N ratio suggesting slower breakdown rates. For example, the turnip in the mix studied had greater biomass below ground than above, but the C:N ratio was 26 (roots) compared to 10 (shoots).
- Topsoil mineral N following cover crop destruction showed small, but consistent treatment differences which aligned with the NiCCs-1 findings i.e. that greater/more rapid nitrogen release occurs following chemical destruction, with mechanical destruction methods delaying or reducing nitrogen release. The greatest topsoil mineral N levels were measured following chemical destruction of the cover crop at the later timing.
- Differences measured in spring soil and biomass nitrogen suggested a reduction in manufactured nitrogen fertiliser of 30 kg/ha was possible where the cover crop had been grown. A reduction of 60 kg/ha below the recommended rate (145 kg/ha) was also evaluated across all treatments.
- Reducing N fertiliser by 30 kg/ha had no negative impact on spring oat yields where the cover crop had been grown, but a 60 kg/ha reduction did. Over all treatments (averaged across all N rates), the presence of a cover crop increased yields by 0.1 t/ha (compared to the weedy stubble) and mechanical destruction reduced yields by 0.2 t/ha (compared to glyphosate). These differences were not statistically significant but align with the findings of the NiCCs-1 project.
- The highest spring oat yields were recorded following a cover crop which had been destroyed chemically at the later timing. This treatment had the highest spring topsoil SMN levels and suggest that on this soil type and for this season, the gap between cover crop destruction and spring crop establishment should be minimised to make most efficient use of the nitrogen within the cover crop.
- The cost of growing a cover crop (£100/ha) was completely offset by an increase in crop yield and reduction in N fertiliser costs. SFI provided additional income, increasing margins by 25% compared to where no cover crop had been grown. Mechanical destruction and early destruction timings reduced this margin.
- Although crop yields were reduced (by c. 0.6 t/ha) following a 60% (60 kg/ha) reduction in N fertiliser, this was again offset by the SFI payment on the cover crop treatment, so was not loss-making. Reductions in N fertiliser use not only lead to cost savings but also reduce a farm’s carbon footprint.
Overall, these results support earlier findings from the NiCCs-1 project of the benefits of cover crops in capturing nitrogen that would otherwise have been lost over winter and supplying it to the following crop, enabling a reduction in N fertiliser use without any impact on crop yields. However, early destruction may result in an increase in nitrate leaching losses in a wet spring. The results suggest that in order to benefit from cover crop nitrogen release (and avoid the risk of increased leaching following destruction) the gap between destruction and drilling a cash crop should be kept to a minimum, although this may be dependent on the soil-type and following cash crop. Moreover, as with NiCCs-1, mechanical destruction delayed/reduced N release and had a negative impact on crop yields.
