Varieties that consistently suppress weeds are generally more desirable in organic systems (although this might be outweighed by marketing necessities) as opposed to varieties that tolerate weeds (and which potentially allow weeds to develop and return seeds to the soil seed bank).
Organic varieties: should show quick germination and establishment, rapid early and vigourous growth, and the ability to rapidly cover the soil and shade it (prostrate or tall varieties) to outcompete weeds at an early a stage in the crop cycle as possible. Varieties are well known to differ in architecture and competitive ability and whilst those that outcompete weeds are preferred it should also be bourne in mind that those with erect foliage or that can tolerate some degree of mechanical weeding are also likely to be useful. Some crop types or varieties might produce allelochemicals that prevent weeds from developing or germinating although information on this is generally lacking.
In grassland systems the choice of variety may be dominated by forage value, but if there is opportunity the most vigorous species should be selected, as these will determine the productivity of the whole ley period. The trend in conventional cereal production has been to grow the taller stemmed varieties for their weed suppressing ability. Some farmers have stayed with the shorter stemmed varieties and employed a weed topper/cutter which will remove and, ideally, collect weed seed-heads so long as there is a difference in height between crop and weed. New research in wheat is investigating leaf angle development, height and speed of development on weed suppression to aid farmer variety choice (see below).
Seed size: varieties with a larger seed size have been shown to exhibit greater initial vigour of emergence and growth, which may subsequently provide extra competitive ability. If there is a choice available then the most vigorous species should be selected, as these will be more likely to outcompete weeds and suppress their development.
Clean seed: It is important that the crop seed is free from contamination by weed seeds. Organic farmers are obliged to use organically produced seed and this should be clean. It is important if saving seed that it is taken from weed free crops, and ideally, professionally cleaned. Tolerance levels of contamination should be low although they are not generally well defined as of yet. Tolerance levels of dock in Switzerland are one seed per 100g.
- The Centre for Organic Seed Information at www.cosi.org.uk aims to provide a one stop shop for the organic movement on all things to do with organic seed. Run by the Soil Association and NIAB and supported by DEFRA, COSI provides users with the opportunity to search for organic and non-organic variety performance data, catch up on the latest news and find out more about the way the standards are developing at both a UK and European level. COSI also links to the organic X seeds website that details the availability of all organic seed in the UK. COSI also provides a forum for producers to exchange views, experiences and concerns with other producers and the seed industry.
- WECOF is an EU project developing enhanced organic weed strategies, one strategy of which is control of weeds through improved crop and plant architecture. An overview of trials with winter wheat is available at weed control through crop architecture.pdf .
- Looking at cereal varieties to help reduce weed control inputs , a paper from the WECOF project.