Integrated weed management systems in vegetables: current status and perspectives

Most vegetable crops are characterised by a low plant density, a wide row distance, a slow initial growth and, as a consequence, by a poor competitive ability. Taking into consideration that most vegetables are high-income crops, the threshold weed densities are very low and the critical periods of weed competition are pretty long. Most vegetables are minor crops, thus the availability of approved herbicides for use is scarce due to the low economic interest by the chemical industries. Special projects for supporting the registration of pest control products on minor or specialty crops (e.g. the IR-4 in USA) or for coordinating scientific and regulatory decisions on pesticides were established to alleviate the problem. In EU the already difficult situation has been worsening by the application of the directive 91/414/EEC concerning the authorization, placing on the market, use and control of plant protection products in commercial form. This directive has already caused the expiration of the authorisation of several herbicides largely used in vegetables and other ones will be withdrawn within few years. Chemical weed control in vegetables shows peculiar environmental and health concerns due to the relatively short growth cycle, fresh edible parts of vegetables, and a coarse soil texture found in the main production areas; moreover, a repeated use of herbicides with similar mode of action may lead to a strong and quick selection of weed flora. So an Integrated Weed Management System (IWMS) in vegetables, like in any other crops, should be based on: 1) weed population management strategies by sound cultural weed control methods, i.e. any aspect of crop management that favours the crop relative to the weeds, reduces the weed seed-bank, regulates weed communities and prevents the build-up of adapted species; 2) an integration of non-chemical and chemical weed control methods characterised by a low selection pressure on weed communities, an environmental sustainability and an economic feasibility. Regarding preventive (indirect) weed control methods, it should be pointed out that: although the crop rotation was crucial for an IWM, in practice a sound crop rotation frequently is not applied due to economic and market constraints; the strategic importance of the cover crops seems low in environments characterised by limited availability of irrigation water or high water cost; intercropping, thanks to new technical solutions for mechanical harvest, is increasing in interest in organic and low input farming systems; a stale seedbed preparation is widely applied in several vegetable crops throughout the world; the breeding of competitive cultivars is not yet enough developed even if experimental results seem to be encouraging; the transplanting instead of the direct sowing is commonly applied in order to give a higher competitive ability to the crop, shorten the critical period of competition and facilitate direct weed control; the increase of crop plant density and the adoption of a narrower row distance or twin rows in order to increase the crop competitivity offers interesting applications but in some crops the cost of transplants, the negative effects of a higher crop density on quality product and the need to have well-spaced crop rows for the application of mechanical weed control limit the use of those cultural practices; the effect of different localised fertilisation and/or irrigation methods (e.g. starter fertilisation, band fertilisation, fertigation) on crop competitive ability against weeds should be better studied. Regarding curative (direct) weed control methods: non-degradable black PE mulches are widely used in several vegetable crops; non-degradable photo-selective coloured plastic mulches, that combine the thermal properties of transparent films with the weed control ability of black films, show a good efficacy but not always an economic feasibility; starch-based biodegradable mulches are more and more used both in conventional and organic farming systems where they show a mulching activity for 2-4 months, that is enough to cover the critical period of competition of most vegetable crops; inter-row weeds are easily removed by inter-row cultivation (i.e. hoeing, harrowing, brushing) while intra-row weeds still constitute a major challenge aimed at minimising laborious hand weeding although new implements (i.e. finger weeder, torsion weeder, split hoe, steering hoe) show a pretty good efficacy if their application is included in a sound IWM programme; physical and mechanical weed control methods are widely used in organic farming systems and in conventional systems where the availability of approved herbicides for use is scarce; at present, biological control does not seem to be applicable on large scale and successfully in European vegetable crops systems characterised by small fields, a high number of crop species, and pluri-specific weed infestations; chemical control still is the main weed control method in conventional and low input vegetable production systems even if concerns about food safety, environmental sustainability, weed population dynamics and application cost are increasing among public opinion and technicians, particularly because the global market shows a huge variability in crop management and regulatory decisions on pesticides.