Non-chemical weed management: Which crop functions and traits to improve through breeding?

The agro-ecological transition aims at reducing the anthropogenic impacts of crop production on the environment, for instance by decreasing drastically the applications of pesticides, among which herbicides are the most prevalent. In this review, we focus on management of arable weeds in agro-ecological systems, considering a perspective of steady reduction of synthetic herbicides by fostering the breeding of varieties adapted to nonchemical weed management. Diverse strategies of non-chemical weed management are discussed, taking into account agronomic levers and identifying breeding targets. Weed suppression by enhancing crop competition from cash or cover crops, grown in pure stands or as intercrops, is a key strategy that could be considered together with dense canopies and optimal nitrogen management, also in addition to growing varieties that are tolerant to weed competition and/or characterized by low nitrogen requirements. Then, escaping weed competition could be achieved by shifting sowing dates and/or diversifying crop rotations, particularly by targeting varieties of different maturity groups, more productive spring-sown crops and integrating more frequently minor crops in the rotation. Weeds can be also suppressed by mechanical control that requires varieties tolerant to mechanical weeding. Allelopathy is a less applied strategy that deserves further studies e.g. the screening of allochemical composition among varieties of cash and cover crops. For each crop-related agronomic lever contributing to integrated weed management, we identify the functional crop traits to target, i.e. the set of morpho-physiological traits associated with an effective weed management, to be screened within the commercial variety panels or to be integrated in a genetic improvement scheme. For all the functional traits and according to the crop species, the potential availability of genetic resources, as well as the ability of varieties to meet the required genetic variability have been explored while, where relevant, the development of appropriate phenotyping methods and trait assessment procedures have been considered. Finally, we propose a set of nonchemical weed management strategies, functional effect traits and agronomic practices associated, as well as their synergies and antagonisms with the other cropping practices for cash and cover crops. We conclude that, to better combine a set of agronomic levers with crop varieties or reinforcing the efficacy of these levers, there is a need to complete classical agronomy and weed science approaches by plant genetics and breeding when designing and evaluating non-chemical weed management strategies.