* From Moss (1987).
Table 1.4 Estimated percentage crop losses owing to weeds, 1988–90 (from Oerke et al., 1995).
Source: Oerke, E.C., Dehne, H.W., Schonbeck, F. and Weber, A. (eds) (1995) Crop Production and Crop Protection: Estimated Losses in Major Food and Cash Crops. Amsterdam: Elsevier.
| Estimated loss owing to weeds (%) | |
|---|---|
| Africa | 16.5 |
| North America | 11.4 |
| Latin America | 13.4 |
| Asia | 14.2 |
| Europe | 8.3 |
| Former Soviet Union | 13.0 |
| Oceania | 9.6 |
| Average | 13.1 |
Table 1.5 Relative competitive abilities of a number of common weeds found in winter cereals.
Source: Lutman, P.J., Boatman, N.D, Brown V.K. and Marshall, E.J.P. (2003) Weeds: their impact and value in arable ecosystems. In: The Proceedings of the BCPC International Congress: Crop Science and Technology 2003 1, 219–226.
| Weed species | 5% yield loss (plants m−2) | Weed species | 5% yield loss (plants m−2) |
|---|---|---|---|
| Galium aparine | 1.7 | Poa annua | 50.0 |
| Anisantha sterilis | 5.0 | Epilobium spp. | 50.0 |
| Avena fatua | 5.0 | Polygonum aviculare | 50.0 |
| Lolium multiflorum | 8.3 | Sonchus spp. | 50.0 |
| Alopecurus myosuroides | 12.5 | Taraxacum officinale | 50.0 |
| Brassica napus | 12.5 | Fumaria officinalis | 62.5 |
| Sinapis arvensis | 12.5 | Geranium spp. | 62.5 |
| Tripleurospermum inodorum | 12.5 | Lamium purpureum | 62.5 |
| Cirsium spp. | 16.7 | Ranunculus spp. | 62.5 |
| Convolvulus arvensis | 16.7 | Veronica spp. | 62.5 |
| Fallopia convolvulus | 16.7 | Aethusa cynapium | 83.3 |
| Papaver spp. | 16.7 | Senecio vulgaris | 83.3 |
| Chenopodium album | 25.0 | Anagallis arvensis | 100.0 |
| Myosotis arvensis | 25.0 | Allium vineale | 250.0 |
| Persicaria maculosa | 25.0 | Aphanes arvensis | 250.0 |
| Silene vulgaris | 25.0 | Legousia hybrida | 250.0 |
| Stellaria media | 25.0 | Viola arvensis | 250.0 |
Yield loss may also occur in addition to direct competition for resources. Allelopathy is the production of allelopathic chemicals by one plant species that may inhibit (or, in the case of positive allelopathy, stimulate) the growth of other species. Anecdotal evidence of negative allelopathic effects has been reported for a number of weed species, although supporting research is often lacking. Recent findings have been reviewed by Olofdotter and Mallik (2001) and others (see Agronomy Journal vol. 93). Given the ample evidence of allelopathy exhibited by crop species, it is highly likely that many weed species will also display these effects, and that it is only a
