Overwinter cover crops tend to have a mixed reputation amongst Scottish farmers but, with many crops being harvested earlier than usual this year, now is an ideal time to think about trying them out.

Cover crops sown in the autumn can give a range of benefits, including improving soil structure, reducing leaching losses of nutrients, suppressing weeds and reducing soil erosion. Of course, there are also potential downsides such as the cost of seed and extra cultivations, the potential for green manures to become weeds themselves and the chance that they could carry over disease into the next crop.

However, with the careful choice of crops and appropriate management, many of these risks can be reduced. The main criticism from growers seems to be that cover crops do not establish sufficiently in the short period of good conditions following harvest, which is why the early entry possible this year is such a good opportunity to get to grips with them.

So, what may be grown as a cover crop? In short, many of the benefits may be realised just by having something in the ground but for optimum performance it is worth considering your goals for growing a cover crop and what is likely to do well on your farm.

One of the simplest cover crops is a winter cereal sown with minimal inputs and ploughed in spring. While most cereals will perform well in this role perhaps the best choice is rye.

This is widely used in continental Europe as it establishes well off a late sowing, is extremely winter hardy and can be grazed in the spring if needed. It also has a dense root system which makes it ideal for locking up nutrients and binding the soil together.

Another useful feature of rye is that it produces substances that can reduce the growth and survival of other organisms (allelopathy). This may help reduce weed growth but it can also reduce the germination of seeds of a following crop, especially if it has small seeds.

This means that rye should be ploughed in well before sowing. It is also worth remembering that rye does not act as a break crop for take-all, although it is not itself affected.

Other crops that deserve consideration include mustard, chicory, winter radish and black oats. Each has its own pros and cons – beyond the scope of this article – but all have a ‘unique selling point’ that make them a good choice in the correct circumstances.

A further option is growing a mixture of cover crops. Mixtures are typically 20-40% more productive than solo crops and will have a wider range of characteristics. The benefits of these can be seen in the large variety of mixtures now on offer from seed merchants.

Legumes such as clovers, with their ability to fix atmospheric nitrogen into a form accessible to a following crop, might be thought of as attractive option for cover crops, but their use overwinter can be problematic. Many are small-seeded and struggle to establish well in the back end, particularly if conditions are cool and damp. Research at SRUC over the last three years has shown that of all the legumes tested, vetch was the most reliable as an overwinter cover crop at sites in Aberdeenshire and Midlothian. Its large seed size (like a small pea) helps it establish quickly and it is highly winter hardy.

It even survived well through last year’s hard winter and although much of the leaf material was knocked back, it quickly recovered in early spring.

One caution about overwinter legumes though – don’t expect much in the way of nitrogen fixation. The small plants have very limited capacity and the bacteria responsible are not very active at low temperatures.

A further consideration is whether you want to grow your cover crop to be an Ecological Focus Area (EFA) under CAP greening regulations because ‘green cover crops’, as they are called, have a strict set of rules that need to be followed. As these have been changing on a yearly basis it is worth seeking out the latest version.

A key consideration is that the cover crop must be sown before November 1, undisturbed until December 31 and be a mixture of two or more from the following list – alfalfa, barley, red clover, white clover, mustard, oats, phacelia, radish, rye, triticale and vetch. Previous rules preventing it being used to support agricultural production have now gone, so it may be grazed from January, but not taken to harvest.

Ready-to-sow EFA-compliant mixtures are available to buy or you can make your own from the list above. Good rules of thumb are to aim for the partial seed rates of the chosen species to add up to a full seed rate, and to have no more than 40% legumes in the mix.