Maximising the benefits from cover crops through species selection and crop management (Maxi-Cover crop)

In a recent survey of UK farmers, the most cited reasons for not growing cover crops were: (i) they did not fit with the current rotation (ii) expense and (iii) difficulty of measuring their benefit to crop production. The Maxi Cover Crop project aimed to address some of these issues by characterising the performance of a range of cover crop species, both individually and in mixes of increasing complexity, under field conditions, and by performing a cost/benefit analysis on the systems used. The project has measured, assessed and quantified the impacts of the cover crops on soil properties and the performance of the subsequent two crops in the rotation. It has also provided new data on the depth of rooting and root density from different cover crop species and mixes, and developed a template for growers to perform a cost/benefit analysis of using cover crops in their rotations. A key feature of the work has been the use of tramline trials on commercial farms to complement the work undertaken on the more traditional field experimental plots. These sites provided more data from field scale comparisons carried out in commercial farming systems on a wider range of soil types.

The results have confirmed that early establishment (August rather than September) is important to maximise the benefits of cover crops, particularly to ensure good crop cover and nutrient recovery. Above-ground, radish, buckwheat and a mix comprising radish, buckwheat and phacelia were quickest to establish. However, below ground, it was the rye cover crop that produced the greatest amount of roots, both early in the season and at destruction (average root length density or RLD, of 3.5 cm root/cm³ soil to 60cm depth, compared to a RLD of 1.2 cm/cm³ from the volunteers/weeds on the control treatment; note root assessments excluded the tap root); rye also had the widest root diameter (at c. 0.23 mm). Phacelia roots were slower to develop, but by destruction had a high RLD (3.4 cm root/cm³ soil), particularly in the topsoil, and also produced the narrowest roots (c. 0.19 mm). It also had a high specific root length, or SRL, (length of root in metres per unit of root biomass in grams, at 268 m/g), suggesting it explored more of the soil for a given root biomass compared to the other cover crop treatments (e.g. vetch at 188 m/g). A high SRL is considered to be important for soil structural improvement, although there was little evidence of changes in soil properties following the different cover crop treatments at the large plot experimental sites, and no relationship observed between cover crop rooting and spring crop rooting. However, there was some evidence of soil improvement (lower penetration resistance, lower bulk density and improved visual structural score) following a single year of growing a cover crop mixture at two of the tramline trial sites on medium textured soils; earthworm numbers were also increased where a five species mix had been grown (the other two tramline trial sites were on heavy textured soils).

On average, the different cover crops took up between 30 and 50kg N/ha, although up to 90kg N/ha was recovered by the vetch and clover cover crops following early establishment at one of the sites (medium-textured soil type). Highest N recovery was associated with either species that were able to fix additional N (i.e. clover and vetch) or established good above- or below-ground biomass, early in the season (radish, phacelia and rye). The N uptake by the cover crop treatments was not detected in the spring barley crop or soil at harvest; the fate of cover crop N remains a key research question.

It was clear from both the large plot and tramline trials that cover cropping on heavy textured soils can result in increased topsoil moisture, probably due to the crop cover preventing surface evaporation. In these circumstances and depending on the weather, late destruction (late March/early April) and incorporation of a high cover crop biomass (less than one week prior to drilling the cash crop) can result in a poor seedbed for subsequent cash crop establishment, leading to lower crop yields.

There was also clear evidence of a negative impact of growing a cereal cover crop (oats and particularly rye) on the subsequent performance of the spring barley crop, in terms of rate of crop establishment, rooting to depth and ultimately grain yield, providing robust evidence that cereal cover crops (as a single species) should not be grown ahead of a spring cereal cash crop. The reason for this is uncertain, but N immobilisation or pest and pathogen carry-over (‘green bridge’) have been cited as possible causes. Whether this can be negated by using the cereal cover crop in a mix or how much of the mix can be cereal is unclear. However, cash crop yields in the replicated plot experiments were not reduced following cover crop mixes comprising between 55 and 83% spring oat. None of the trialled mixes included rye. These results have implications for the CAP EFA greening rules (BPS 2020), which require cover crop mixes to include a cereal and non-cereal. However, although the results suggest cereal cover crops should be part of a mix rather than grown as a straight (when followed by a spring cereal cash crop), there was no clear evidence that the performance of the other cover crop species (notably radish and phacelia) was improved by inclusion within a mix. Indeed, the highest cumulative margin was achieved by growing oil radish as a monoculture, rather than within a mix.

There was a trend for a higher spring barley grain phosphorus (P) concentration and grain P offtake where buckwheat had been grown compared to the control treatment (volunteer/weeds). This additional evidence corroborates that reported in the literature. However, the mechanism for increasing the solubility of soil P is uncertain, as rooting by the buckwheat and total above ground biomass production was low compared to the other species evaluated.

Cumulative (two-year) margins were calculated for all of the seven study sites (20 comparisons), with most (95%) showing a reduction in margin compared to no cover crops (ranging from + £64/ha following oil radish on a clay loam to - £476/ha following a two species mix on a clay soil; average of - £150/ha), due to the absence of a sufficient yield benefit to compensate for the additional seed and establishment costs. There was also no evidence that soil type influenced the economic performance of cover crops. However, the greatest reductions in margins were where cover crops resulted in a significant yield reduction as a result of poorer establishment conditions, which tended to be on the heavier soil types.

As well as providing robust scientific evidence of the physiology and performance of a range of cover crop species, Maxi Cover Crop has provided useful insights into the practicality of using cover crops across a range of soil and climatic conditions. The involvement of commercial farmers in trialling some of the mixtures and methodologies of crop establishment has been particularly valuable, with some farmers changing practices as a result of their involvement in the work.