When we think of field margins, in the first place we think of flowering margins that attract beneficials (insects that that eat pests, pollinators). In contradiction to the name, a field margin doesn’t necessarily have to be at the border of the field. If it is more useful in your situation to install a flower strip in the middle of your field (because this is e.g. a less productive area), please feel free! Just like for hedgerows also for field margins an informed choice need to be made concerning the species to avoid the attraction of harmful insects.
A second important function of the field margins is the use as buffer strip. This is a strip between the field and the watercourse which is not fertilized and the use of pesticides is not allowed. Aim of the strip is to avoid the leach of nutrients or pesticides to the watercourse. This strip is sown, so also here an informed choice of the used species, which have a positive influence on the rest of the field, must be made.