A fallow year

14 Apr 2013

There is an increasing acceptance that we’re going to see a lot of land in true fallow this summer. This is because in many situations there is little prospect that a profitable crop can now be sown. Consequently, we’ve been receiving queries on the management of a true fallow.

From my point of view, the introduction of glyphosate was a ‘game-changer’ in terms of fallow management. There is now no longer a need to keep moving the soil to stop weeds establishing. In the past, fallows were often adopted in order to control perennial weeds. Repeated cultivations were timed to try to exhaust their perreniating organs, such as the rhizomes of common couch. This is now not necessary and so an awful lot of diesel and earthworms can be saved and more emphasis can be placed on using fallows to improve soil structure.

It is very doubtful that repeated cultivations during the late spring and summer will contribute much to reducing the grass weed, including black-grass, seed burden in the soil. Perhaps the initial cultivation, if it is carried out in April, may stimulate some black-grass seed to germinate and so there needs to be some knowledge as to where the seed bank may lie.

If it is suspected that much of the seed is in the upper layers of the soil, the choices are to have a shallow initial cultivation to try to stimulate germination or, in order to take advantage of more time being available, to plough them down to a depth from which they cannot emerge. However, all this may be theoretical chat as the majority of the fields in question have already been cultivated and the black-grass weed has been dispersed throughout the surface layers. 

It may be more important to prepare a stale seedbed well before the next crop is sown in the autumn. This may prove worthwhile provided that there is some moisture to support germination. 

So the main objectives of a fallow are to improve soil structure and to provide a break from cropping. In my opinion, a cover crop has to be sown to improve soil structure. Not only will it pump out water and hopefully subject the soil to wetting and drying cycles but the vegetable matter produced, particularly the roots, will result in a ‘living soil’. Using a cover crop to dry the soil may be essential if sub-soiling is required in order to repair soil damage. In addition, provided that there is plant cover in August a cover crop will prevent wheat bulb flies laying eggs.

So an investment in a cover crop is worthwhile. This view is supported by a project that The Arable Group, now NIAB TAG, carried out for HGCA a few years ago (HGCA Project Report No 414). In this project, the second wheat in a ‘fallow wheat: fallow wheat’ rotation yielded 1 t/ha less than the second wheat in a ‘mustard; wheat; mustard; wheat’ rotation.

Mustard is often the first choice for a fallow. However, this may not be a wise choice on farms that have a high proportion of oilseed rape in the rotation. This is because mustard is botanically similar to oilseed rape and may share the same root diseases that have been identified as being a likely contributory cause of reduced yields associated with tight rotations of the oilseed rape crop. The problem is that we do not know if mustard will increase the level of these root diseases in the soil, particularly as it will be in the ground for such a short time.

The alternatives to mustard may be a grassy crop such as rye, which does not host take-all, or even a short season rye-grass. A lot will depend on seed supply and the farm rotation. However, whatever the choice, it is important that the seed from the cover crop is not allowed to shed and become a weed in its own right. Also, make sure that any black-grass or other pernicious weeds are prevented from shedding viable seeds.  

Let us hope that this year is unusual and that in the future we are not forced by the weather to have fallows.  However, it is worth writing down your experiences with fallow crops this year as you never know what the future will bring.