The prevalence of weeds in organic fields can be reduced by following five trial-based practical rules.
If you want to avoid major weed problems in your organic fields, you should apply five basic rules when planning your crop rotation.
According to Associate Professor Bo Melander from the Department of Agroecology at Aarhus University, you need to do the following:
These five basic rules are based on, among others, results from three permanent organic crop rotation experiments that included three different soil types and other geographical conditions. Two of the experiments lasted from 1996 to 2009 while the third – also initiated in 1996 – is still active. According to Bo Melander these long-term experiments are priceless.
- Practical experiments allow us to gather valuable experience in relation to weed prevalence in different kinds of crops. Most people will notice whether a certain pre-crop has led to significant weed problems in a crop, but usually do not remember earlier incidents. In this case, permanent crop rotation trials can be of significant help; they can also clarify the actual effects on the weeds when several cultivation factors are at play, Bo Melander explains.
Five basic rules to reduce weeds
Diversity of crops and sowing time are both well-functioning cultivation methods that are used to avoid serious weed problems. The higher level of diversity, the better chances are of preventing considerable weed propagation. The life cycle of especially annual weed species is destroyed when the crops – whose life cycles the weeds have adapted to – are not grown continuously. Various sowing times in spring and autumn should also be applied as this will help increase diversity.
An even better method is to grow at least 20 percent perennial nitrogen-fixating crops for mowing at regular intervals. These may include grass-clover or alfalfa. The longer these crops stay in the fields, the more of the soil’s weed seed bank will be depleted. Grass-clover and alfalfa that is cut at least three or four times during the growth season will inhibit the growth of both annual and perennial weeds. In addition, they will supply nitrogen to the cropping system and increase the general fertility of the soil.
However, grass-clover will not solve all weed problems. Couch grass can propagate during a growth period of grass-clover. Therefore, crop rotations must provide room for weed management against couch grass. The weed management should always be carried out prior to the growing of row crops and legumes as these crops may result in significant couch grass propagation.
Row crops such as maize, beets and vegetables should be kept clean as they may constitute serious sources of weed propagation. Cleaning of row crops may result in significantly reduced levels of both weed seeds and belowground organs capable of vegetative propagation. In the crop rotation experiments performed at AU Flakkebjerg, the reduced weed levels could be detected for up to four years.
Different crops have different abilities to suppress weeds and these should be utilized. Winter rye, for instance, grows quickly in the spring and reaches a certain height that can suppress most weeds. Oats have considerable leaf cover during most of the growing season and are thus competitive against weeds.
Legumes, such as lupines, faba beans and peas, can increase weed propagation. In early summer these crops have a competitive leaf cover, but at later growth stages they are more open. This allows room for perennial weed growth in particular. Further, legumes also provide nitrogen for the perennials if the growth of perennial weeds is not terminated post-harvest.
The results come from the Core Organic Plus project PRODIVA.
For more information please contact: Associate Professor Bo Melander, Department of Agroecology, e-mail: firstname.lastname@example.org, tel.: +45 8715 8198, mobile: +45 2228 3393
Sustainable Pest Management is one of the research areas in which the Department of Agroecology is particularly strong and from which results are delivered in line with national and global societal challenges and goals.