We have a bonus blog from Dr Jennifer Banfield-Zanin of Stockbridge Technology Centre (North Yorkshire) this month. It's all about the challenges of establishing a white clover living mulch and balancing the health of the clover against its competitiveness with the cash crop.

Why the interest in living mulch?

There is growing interest in the use of clover living mulches, and the role they can play in delivering multifunctional benefits to arable production and in meeting sustainability goals. Research has shown that they have potential to improve soil fertility, health and structure, to reduce weed pressure, as well as pest and disease incidence, whilst also having positive impacts on biodiversity. In addition to supporting carbon storage and reducing the risk of soil erosion, as a legume clover is also able to fix atmospheric nitrogen into the soil for provision to the crop. This in mind, clover has the potential to reduce expensive nitrogen fertiliser inputs when well-managed. There are clearly many potential benefits to incorporating clovers in arable rotations!

Establishing white clover in sunny (and dry) Yorkshire

At STC, we have been trialling white clover (Trifolium repens) as a living mulch in cereal crops over the course of the DIVERSify project. White clover lends itself well to partnering cereal crops. It has a creeping growth habit, keeping relatively near ground level, and as a consequence it does not interfere with most farm machinery. It is one of the more persistent clovers in the UK, showing good frost tolerance and is able to last between 5-10 years, depending on conditions. In the UK, it can produce reasonable biomass annually (around 7t DM/ha), with good, albeit variable, nitrogen fixing potential (this typically reaches 150kg N/ha annually, but can vary between 50-450kg N/ha) (Rosenfeld & Ryan, 2011).

There are, however, challenges to growing with a white clover living mulch. It is relatively slow and patchy to establish, and this can allow weeds to get a foothold. The clover also needs to be knocked back and managed in advance of crop sowing to mitigate impacts on crop yield. Over the course of our field trial, however, we have found that this can be a double-edged sword – and the weather plays no small part in this! What works one year, might be too much or too little stress for the clover, depending on the climate.

Under our normal North Yorkshire conditions, we have previously found white clover to grow and recover very well on our farm site. That said, we’ve also found that it tends to do so well, that it competes strongly with the cereal crop and can lead to notable yield reductions. The approach we have previously taken at STC to mitigate this, is to mow the clover prior to drilling, and further challenge it by applying a low-rate herbicide. We then use strip tillage to shallow-cultivate and drill into the prepared seed bed, which allows the crop to establish and ‘get away’ before the clover closes in again (Figure 1).

Figure 1. The clover living mulch at STC, following mowing (left). Strip tillage cultivates bands in the living mulch into which we drill crop seed (right), in contrast to direct drilling into a full clover cover or standard conventional practice.

We have had good success with this historically, and the first year of the DIVERSify field trial followed this trend. In 2018, despite a very dry summer, with drought conditions, the clover had recovered well over the usual wet spring, allowing it to grow strongly.

By contrast, in the second year of the trial (2019) we had a very dry spring. Winter conditions had been relatively average prior to management of the clover and drilling. The clover was subjected to the usual management regime; it was mowed, the low-rate herbicide applied, with the crop drilled a few weeks later. After this point, however, the weather dried up, with functionally no rain until much later in the summer. As a consequence, we found that the clover had been overstressed, and failed to recover strongly.

Knock-on effects

In our DIVERSify trials we have three treatments: (1) a standard monoculture control of wheat (usual land prep, direct drilled), (2) a strip-tilled clover living mulch (so, it gets strip-tilled and drilled into the prepared strips, leaving some 50% clover cover at time of drilling), and (3) a direct drill into an undisturbed clover living mulch (100% clover cover at time of drilling). In 2018, we had excellent weed suppression in the clover living mulch treatments. Yield was however reduced. The summer drought made this a challenging year and the presence of clover appeared to challenge the cash crop, although in the plots on which we used strip tillage prior to drilling, as opposed to direct drilling in a full cover of clover, the impact on yield was notably mitigated (in comparison to the monoculture: wheat grain yield was reduced by 75% in the direct drilled full clover cover plots, but in the strip-tilled clover plots the reduction in yield was 64%).

In 2019 however, the prolonged dry weather and poor establishment led to a significant period of time where the clover failed to outcompete weeds, allowing these to get a foothold (Figure 2). In order to manage these, we had to resort to multiple low-rate applications of a relatively clover-safe herbicide. These continued to knock the clover back throughout the season, further weakening it and preventing the clover from achieving good ground cover levels. Cumulatively, although this meant that crop yield was not significantly different in the strip tilled treatments as under monoculture treatments, this also meant that a key benefit of using a clover living mulch, namely weed suppression without input, was not met.

Figure 2. Following a dry spring, clover failed to recover well or rapidly, allowing weeds to gain a foothold and establish.

Alternative management options?

These contrasting results highlight the balance in managing the clover living mulch. In 2018, it performed extremely (you could argue too) well, and competed strongly with the wheat crop, whereas in 2019 the clover never really recovered from the spring management, and weeds got a foothold that carried into the season.

A potential solution to the challenges may be more frequent mowing, particularly in-crop. Certainly, mowing is a key component in the management of white clover living mulches. The ability to continue mowing through the growing season, however, could provide a means of managing both any weeds that establish as well as clover competition throughout the growing season, and should negate any potential need for herbicide inputs. This should also reduce the levels of stress of the clover, improving the overall resilience of the living mulch to natural environmental fluctuations and climatic challenges year on year.

Management must balance maintenance and health of the clover living mulch, and weed suppression, with competition of the clover against the crop, in order to mitigate impact on yield. Overstressing the clover may reduce impact on crop yield, however it can then lead to weakened clover competition against weeds, allowing these to establish a foothold. Not a straightforward challenge, and a crystal ball for the weather wouldn't go amiss, but the potential is there nonetheless!

References

Rosenfeld, A. & Rayns, F. (2011) Sort Out Your Soil. Wilkinson, I. & Lane, S. (eds). Cotswold Seeds & Garden Organic.