Weeds and weed management in carrots- a review

Information about key weeds, new weeds or species that have recently spread, critical period of competition, weed management programmes in Integrated and Organic production, approved herbicides and those currently undergoing registration for use in carrots (Daucus carota L.) grown under field conditions in Croatia (HR), Denmark (DK), Finland (FIN), France (F), Germany (D), Greece (G), Hungary (H), Italy (I), Morocco (MA), The Netherlands (NL), Poland (PL), Portugal (P), Slovenia (SLO), Spain (E), Sweden (S), Switzerland (CH), Turkey (TR) and United Kingdom (UK) was collected. In 2001, the world production of carrots was about 20Mt on about 926000 ha (Asia 45%, Europe 32%, Africa 8%, North America 7%, Central and South America 6%, Oceania 1%). In the surveyed countries, production was about 4.9 Mt on about 144000 ha (PL 34500 ha, F 17000 ha, I and UK 13000 ha, TR 12000 ha, MA 10000 ha, D 9000 ha, NL 8000 ha are the major producers). About 95% of the hectarage is treated with herbicides. Organic production is on about 200 ha in DK, 90 ha in FIN, 250 in NL, 130 ha in S and 500 ha in I; reliable data for the other countries are not available but a general increase of organically-grown carrots has been estimated in the last years. In UK, F, NL, I, E, P, MA and TR, carrots are year-round crops, whereas in the other countries carrots are mainly sown in spring (March to June) for summer-autumn harvests. Growth cycle is 3 to 7 months, mainly depending on root sizes (i.e. the larger the root, the longer the crop cycle) and season (i.e. spring-summer crops: 4-5 months; winter crops: 5-7 months). Carrots are usually grown on sandy soils but some major areas of production are also on clay-loam to silty-loam soils (e.g. in I and MA) or on organic soils (e.g. in UK and FIN). Different growing systems are applied in the diffrent countries: flat field, single rows 0.25 - 0.50 m apart in HR, DK, FIN, F, D, G, HU, I, MA, PL, P, SLO, E, S, TR; “flat bed” 1-2 m wide with 3-8 single rows 0.20-0. 30 m apart in F, I, NL, PL, CH, UK; “band” with 2-3 rows 5-8 cm apart, 0.30-0.45 m between the bands in DK, F, I, PL, CH, UK; on “ridge” 0.45-0.5 m apart, a single row per ridge in NL, PL, CH; on “ridge” 0.6- 0.75 m apart, 2-3 rows or a band per ridge in FIN, NL, PL, S, CH. Carrots are grown for fresh market (roots and bunches), storage and processing. Carrots for “baby” food are grown (without herbicide) in F and PL mainly. Crop density depends on type of carrots and growing system varying 30 to 250 plants m-2. The weed communities are commonly very rich of species and their composition is highly variable in relation to climate, soil and sowing period. The most important and frequent species, mostly key weeds, are Amaranthus spp., Capsella bursa-pastoris, Chenopodium album, Cirsium arvense, Convolvulus arvensis, Fallopia convolvulus, Fumaria officinalis, Galeopsis spp., Galium aparine, Galinsoga parviflora, Lamium spp., Matricaria spp., Polygonum spp., Portulaca oleracea, Senecio vulgaris, Solanum nigrum, Stellaria media, Urtica spp., Veronica spp. among the broad-leaved weeds, and Digitaria spp., Echinochloa crus-galli, Elymus repens, Poa annua and Setaria spp. among the grass weeds. Cuscuta spp. and Cyperus spp. are key weeds in E, P, I and MA, whereas Reseda lutea in UK (mineral soils), HU and TR. Some Umbelliferae species (i.e. Ammi majus, Daucus spp., Scandix pecten-veneris, Torilis spp.) are dominant species in I and MA where carrots are frequent in the crop rotation. Volunteer potatoes can be a problem in some area of UK, NL and I. IWM can control the most of weeds but some species are becoming important: Abutilon theophrasti, Datura stramonium, Hibiscus trionum in HR; C. arvense, Galeopsis spp., Rorippa sylvestris, Sonchus arvensis in FIN; D. stramonium, Ambrosia artemisiifolia, Sorghum halepense in HU, C. arvense and Rumex spp. in I; Anthemis spp. and R. sylvestris in PL; C. arvense in SLO. Carrots are characterised by slow emergence and initial growth, and by a relatively open canopy and so weeds are not or hardly tolerated. Uncontrolled weeds can result in total yield loss and they are able to produce a lot of seeds. Therefore weed threshold densities are not used. The maximum weed-infested period is about 4-5 weeks after emergence, whereas the minimum weed-free period is from emergence till around 80% crop canopy closure (i.e. 1/3-1/2 of growth cycle). However, further research is needed on this subject because the critical period of competition probably depends on cultivar, sowing time and growing system. The availability of herbicides approved for use in carrots is very different among the surveyed countries. In details, taking into consideration only the “key” active ingredients, aclonifen is authorised in DK, FIN, I and S and under registration in D; fluorochloridone is approved in HU, I, PL and TR; linuron in all the countries except in D and S; metoxuron in F, NL, CH and UK but it will probably banned after July 2003 in EU countries; metribuzin in FIN, G, I and S, off-label approval in UK, and under registration in D and NL; pendimethalin in HR, DK, D, G, HU, I, PL, P, CH, UK and under registration in FIN and NL; prometryne in HR, G, HU, I, Pl, P, SLO, TR and UK; trifluralin in HR, G, HU, I, PL, SLO, E, TR and UK. A range of post-emergence graminicides is registered for grass-weed control in all the countries. Both ioxynil and isoxaben have off-label approval in UK, the latter with activity against Senecio vulgaris, but with potential risk of crop damage. Conventional weed control is generally applied with narrow distance between rows and involves a pre-sowing (trifluralin) or pre-emergence treatment (linuron, pendimethalin, aclonifen, metoxuron) followed by one to three post-emergence treatments (linuron, metoxuron, CIPC, prometryne, aclonifen, graminicides) at low rate with adjuvant (Low Dosage System, LDS) on just emerged weeds and crop at 1 to 5 true leaves. IWM generally involves: 1) false seedbed technique followed by glyphosate, gluphosinate-ammonium or diquat; 2) sowing in single rows 0.45-0.50 m apart; 3) pre-emergence treatment (possibly band-application); 4) post-emergence LDS and/or repeated inter-row hoeing or rotary cultivation combined with ridging for in-row weed control. Common strategy for organic production are: 1) false seedbed technique; 2) sowing in single rows 0.45-0.50 m apart; 3) pre-emergence flaming (50-80 kg gas ha-1); 4) possible in-row brush weeding when the crop has 2-3 true leaves; 5) repeated (often 5 to 8 passes through the growing season) intra-row mechanical control (i.e. hoeing, rotary cultivation, finger weeding) as close to the row as possible, sometime combined with ridging; 6) hand-weeding (100-500 hours ha-1, “weed beds” for 8-12 persons are normally used for that purpose). Further and more detailed information can be found in the WG web site (www.ewrs.org).