Given Europe’s desire to cultivate crops in more sustainable ways less dependent on chemical inputs, the problem of weed control in arable crops remains an important stumbling block. To address this, France has made considerable investments in trials and research, with the knowledge gathered providing possibilities for reducing herbicide use while guarding agronomic and economic performance. Here we report on a major meeting held to discuss progress and provide links to a raft of further information.
France has set itself a target of reducing pesticide use by 50% over the course of a decade, which will require sustained efforts to reduce herbicide use since herbicides account for 40% of the nation’s total pesticide use (read more about the new National Action Plan here). Two organisations are helping to lead these efforts: a group of scientific interest (GIS GC HP2E) aiming to develop innovative cropping systems for field crops with high economic and environmental performances, and a multidisciplinary technological network for weed management (RMT FLORAD). The two groups organised a national conference (Sustainable weed management in arable cropping systems , proceedings available to download below) in December, 2015, bringing together researchers, agricultural extension workers, public authorities and farmers, where a set of cropping trials and scientific results were presented and participants invited to provide feedback on their experiences.
The summary report of the meeting (Weed control in new production contexts - Technical solutions and feedback on experience for sustainable management , available to download below) covers concrete experiences of sowing techniques, innovative cropping systems and cover crops, a look at which combinations of practices can optimise weed control, an examination of how these approaches can be used by farmers and what help should be provided to farmers, and what direction future research and development should take.
Concrete experiences
The summary notes that given a background of minimal soil tillage and increasing use of cover crops, a decrease in herbicide dependence could potentially modify the seed bank. This means solutions in all areas of weed management, from sowing techniques to changes in the cropping system, are required to modify weed development. It describes some of the results obtained in different contexts, which can serve as useful guides for those involved in field crop production, on a case-by-case basis, when trying to modify their cropping strategies.
These include some clear-cut trials conducted by Terres Inovia on sowing techniques and intercropping in oilseed rape. These trials showed that direct drilling was highly effective in a particular situation (weed invasion, lack of effective herbicides, weed seed bank consisting largely of species displaying little or no dormancy), reducing geranium seedling levels by 85 to 95%, versus only 46 to 88% for herbicides on a tilled soil. In addition, the combination of direct drilling with the intercropping of certain frost-sensitive leguminous species was found to limit weed development. In other intercropping trials, ESA of Angers in western France has demonstrated the benefits of growing winter lupin with triticale.
Longer term efforts to sustainably modify weed populations to reduce herbicide use will require changes to cropping systems, notes the report. However, trial results suggest that no ‘off-the-shelf’ solutions exist. Rather, cropping systems will need to be adapted to particular pedo-climatic and production contexts. Long-term 20-year experimentation is ongoing, involving a diversification of species and sowing dates and a succession of two spring crops in the rotation.
The report notes the increasing use of cover crops between two main crops in France, both for agronomic reasons and to ensure compliance with the nitrates directive. It reports on two long-term trials conducted by Arvalis on the impact of cover crops on weed and crop volunteers. The results show that ground cover generally led to a clear decrease in crop volunteers during the period between crops, but cover crops had no effect on weed infestation in the following crop. Finally, the report examines trials with the introduction of temporary grassland conducted at the SOERE-ACBB experimental centre, which have proved the efficacy of this approach and revealed the mechanisms at work (direct competition with grassland cover and, in the longer term, changes in the weed seed bank). However, the results suggest that grassland must be maintained for at least three years to observe changes in the weed seed bank.
Winnning combinations
Concerning the combination of practices to optimise weed control, the report covers several presentations dealing with the complex interactions between alternative approaches with partial efficacy (sowing techniques and dates, varieties used, associated crops and crop rotations etc.). For example, the GIS GC HP2E working group has analysed an existing database to identify the effects of decreased tillage on weed management, noting increases in herbicide use with direct drilling but lower levels of other pesticides in the absence of tillage.
InVivo Agrosolutions and INRA have examined monitoring data from the DEPHY-Ecophyto farm network. Having determined cropping systems according to major production situations, they identified the strategy resulting in the lowest level of herbicide use. These strategies generally included more diverse rotations in terms of the species introduced and sowing periods and tillage. In situ studies are required to refine the analysis for given situations, and the report contains details of work conducted by Arvalis in Normandy on wheat-based rotations and the PURPAN engineering school in south-west France on maize systems. Arvalis identified, for example, that the combination of tillage and a longer rotation considerably reduced weed density, while a low-input maize system (annual tillage, mixed or mechanical weeding, ryegrass and clover cover between crops, semi-early variety) provided the best compromise between yields and decrease in Treatment Frequency Index (TFI).
Farmers’ experiences
In terms of how these approaches can be used by farmers and the support that can be provided, the meeting heard feedback from real-life experiences. Providing evidence of the importance of networking, the DEPHY 27 group of around a dozen farmers with very diverse cropping systems described how they came together around a shared goal: weed control with a single full dose of herbicide each year. Supported by a common methodology and a decision flow-chart for weed management, cropping systems were identified and adjustments designed collectively and implemented between 2010 and 2014. Across the systems, TFIs are in general well below regional reference values and farmers’ perceptions of both their role and the idea of fields containing weeds as ‘dirty’ had changed. An example of individual voluntary change was provided by cereal farmer Philippe Mouraux, who belongs to the local APAD (sustainable agriculture association). Faced with an impasse concerning simplified cropping techniques and contamination with black-grass and brome grass, Philippe has conducted a 10-year transition to a direct drilling system with a cover crop, continually adjusting his approach and using empirical solutions to overcome unexpected problems.
The report notes that targeted assistance appears to be essential to encourage such developments. This includes documentation and decision support tools, and the strengthening of support networks and structures equipped with standardised tools. The report identifies a larger scale management of weeds, with closer links between arable and livestock farms, as desirable. It also identifies the importance of economic assistance, particularly data concerning the analysis of risks associated with changes in cropping systems.
Future R&D
A final session at the meeting sought to identify new measures for integrated weed management and the research questions they pose. Xavier Reboud (INRA Dijon, pictured right) offered a panorama of these different approaches, listing them in a conceptual framework with five major pathways for managing weed flora (see below). This can be used to assess the 17 measures identified as possible supports to Ecophyto ll.
Distribution between five weed-management pathways of 17 potential measures to support Ecophyto II
A – Occupying niches to ensure that no space is left vacant
B – Preventing the development of plants after their germination
C – Exhausting the weed seed bank and preventing its renewal
D – Presenting an unusual situation to which the species to avoid cannot adapt
E – Optimising practices: explicit coupling of the detection of a species with targeted action
In this framework, says the report, the potential efficacy of a measure can be evaluated by the ‘biological principles mobilised’ approach. In this approach, it is suggested that the effect of introducing a new measure will be greater and more durable if it influences several different management pathways. For example, a cover crop is a pathway B measure (preventing the development of plants after their germination) but it also affects pathways A, C and D, suggesting that it is potentially a very influential measure. Other criteria should also be considered, such as the compromise between feasibility and efficacy, or the degree to which measures can be adapted to existing systems.
The report suggests another approach, based on agroecological principles and seeking to exploit ecosystem services, could also be a major element in future solutions. The challenge here will be to identify effective combinations of agroecological measures and to integrate them into systems with good economic performance. Other issues raised by participants included the importance of pursuing R&D to maintain and improve the efficacy of current weed management methods (herbicide resistance was a recurring theme), the development of precision agriculture and modelling efforts to support predictions of the effects of the combination of current and future measures. Finally, the report acknowledges that we need to improve our knowledge on the biology of weed species.
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