The fertility-building period, or ley, will influence the weed population. If it is well managed it can act as a weed suppressing phase. It is important to choose the right species and ensure they establish quickly, especially for grassland systems where the ley may last for several years. Establishment of leys can be easier in the autumn period than in the spring because sowing in spring coincides with the main spring flush of weeds. The seedbed needs to be well prepared, and good contact made between seed mix and, ideally, moist soil to achieve good establishment. The choice of fertility building crop is also important. Rotations with grass leys have been shown to be beneficial in reducing weed seed numbers compared with rotations that do not include a grass phase. Grassland systems, which have temporary leys rather than permanent pasture, will provide the opportunity to control perennial weeds during the cultivations between ploughing and reseeding.
Grassland or clover/grass leys are an important part of the organic farming system in the UK. On livestock farms grassland forms the basis of the production process, in arable systems the ley is used primarily for maintaining or restoring soil fertility. The grass may be managed as a short, medium or long-term crop and this may determine the composition of the desirable sward species and the nature of the associated weeds. The seed mixture for a ley may include a relatively simple mixture of grasses and legumes or may be more complex and contain a range