Loading...Today, weed control is so successful we can efficiently eliminate almost all weeds that want to spread in the field. But this approach might be creating more challenges than it solves.
As we eliminate all weeds, including those that pose no real threat to crop yields, we unintentionally make way for problem weeds like black grass and Canada thistle to dominate. Could allowing a more diverse range of less competitive weeds help control these problem species?
Weed Management Or Weed Diversity
Dr. Christoph von Redwitz, a researcher at the Julius Kühn Institute in Brunswick, thinks so. “The goal in recent decades has always been a black field,” explains the agronomist. “But that’s not necessary. A certain amount of weed infestation is tolerable.” Studies are now showing that fields with diverse weed communities are particularly desirable as they result in a lower yield loss with the same degree of weed cover than one-sided weed infestation.
The aim is not to ban crop protection completely or to allow problem weeds to grow freely. The key is balance and to develop diverse, site-specific weed communities on individual areas without the presence of highly competitive problem weeds.
Weeds reduce the yield, or do they?
Of course, wild weeds can reduce yields or make harvesting more difficult, but new findings are challenging this notion. The PlantCom project, a collaboration between Dresden University of Applied Sciences (HTW) and the Hofgut Eichigt organic farm, examined how diverse weed communities impact the crop yields.
On a trial field at the farm, researchers sowed between one and twelve weed species among spring wheat and other cereals. The surprising result highlighted that greater weed diversity increased raw and grain yield in wheat.
The number of species was less important here than a balanced ratio of weed flora. The diverse weeds supressed individual competitive species very efficiently and also reduced the total biomass of the weeds. “It is therefore more important in practice to achieve a high species diversity in the weed flora than to exclude individual species that are known to be negative,” says Prof. Dr. Arne Cierjacks, one of the project leaders.
It’s always a question of getting this balance right.
Dr. Marion Rasp
Dr. Marion Rasp, project manager and deputy managing director of the Bavarian Kultur LandStiftung who works in the field of arable weed conservation.
“It’s always a question of getting this balance right: reducing problem weeds and promoting rare, less competitive species.” The Mousetail (Myosurus minimus) or large Venus’s Looking Glass (Legousia speculum veneris), for example, are so small and non-competitive that they generally do not have a yield-threatening effect even in larger numbers. There are good reasons why they are such a rare sight in the field.
More Species, More Diverse Interactions
The Saxony trial results highlight that the impact of a diverse weed flora on wheat cannot be universally applied to all weed communities, crop types, locations, or environmental conditions. The success of diverse weed management depends on a range of factors, and simply increasing the number of species is no guarantee of positive outcomes.
The study found that wheat, barley, and oats generally tolerated weed diversity without significant yield losses. However, corn was a different story; it proved highly sensitive to any type of weed, with a single sowing reducing yield by as much as three-quarters. Interestingly, wheat grown amid a diverse flora was better nourished, demonstrating that weed diversity can have beneficial effects on certain crops.
Nevertheless, field mustard (Rhamphospermum arvense) negatively impacted wheat nutrition because it consumes large amounts of sulphur and nitrogen to produce mustard oil glycosides, compounds that can inhibit wheat growth.
Is weed infestation therefore desirable?
Scientists are only just beginning to understand which weed species thrive well together under specific environmental conditions, and how many species are needed to achieve desired outcomes.
“It is really so complex that it is often impossible to quantify it in detail,” says Dr von Redwitz. So far, there are no definitive answers as to whether and when diverse weed flora benefits the farm. “No study has gone so far as to say that ten species reduce yield loss by more than 5%,” he adds.
However, from an ecological perspective, increasing weed diversity could be beneficial. A species-rich weed community can help maintain a stable and productive agricultural ecosystem. Diverse weed roots and root exudates improve soil health by supporting natural pest antagonists and enhancing insect pollination.
The PlantCom project, for example, found that even small plots far from field edges increased insect numbers. Furthermore, a diverse weed flora affects nutrient mobilization and distribution, creating a complex web of competition and mutual support among plants across species lines.
No Worries About The Seed Bank
The challenge lies in promoting a variety of arable companion species without increasing the total biomass of weeds, which could harm yields.
Trials at HTW Dresden found success with at least twelve additional companion species without human intervention. “The species efficiently kept each other in check,” explains Prof. Dr. Arne Cierjacks.
The goal in recent decades has always been a black field.
Dr. Christoph von Redwitz
It therefore also seems unlikely that the sown species will continue to spread in subsequent years or develop dominant populations that are difficult to control. “The danger posed by the seed bank is generally overestimated,” says Dr von Redwitz. “Even on areas with a full seed bank that have not been treated for years, the yield can be increased to 100% within one season – thanks to efficient weed control.”
Precision Farming Offers Opportunities
Dr von Redwitz believes smart farming technologies offers good opportunities for managing diverse weed communities more easily in the future.
Image recognition must be trained to differentiate between crops and weeds at species level. Understanding the parameters that determine the success or failure of a diverse weed community would also need to be known. Initial findings are beginning to shed light on how to better assess weed-crop interactions and these insights could eventually guide precision farming systems in deciding which plants to control and which to leave alone.
In the ‘Better Weeds’ project, weed maps are also being created using GIS-based image data with the help of artificial intelligence (AI). Combined with farm-specific information and the characteristics of different weed species, these maps help develop site-specific weed control plans that balance competitive effects with ecological benefits.
While this approach is still in its early stages, and more needs to be understood about plant community interactions, there is a clear path forward. “We do know how to do it better,” he says. “Today’s technology already offers practical solutions like leaving spraying windows and not harrowing areas with rare species.”
When can a weed flora start to be considered diverse?
This must be assessed differently, depending on the location. For example, the pheasant’s eye (Adonis vernalis) family is particularly species-rich and can develop on shallow skeletal and calcareous soils. “Nowadays, a field with more than 20 species is already relatively diverse by comparison,” says Dr von Redwitz. These are still frequently occurring species.