The number of asterisks refers to the species’ increased importance to birds/invertebrates or increasing competitive impact; ‘—’, no importance; blank, no information).
Allelopathy, as mentioned in Section 1.4.1, is usually used to describe the negative effect of one plant on another via the release of natural growth inhibitors. However, incidences of positive allelopathy have been reported, where allelopathic chemicals produced by one species have a positive effect on another species. An example is corncockle (now a rare arable plant) that grows alongside wheat. Corncockle produces agrostemmin which increases the yield and the gluten content of the wheat (Gajic and Nikocevic, 1973).
1.8 The ever‐changing weed spectrum
Weed populations are never constant, but are in a dynamic state of flux owing to changes in climatic and environmental conditions, husbandry methods and the use of herbicides. Such change is evident from a consideration of cereal production in the UK. Cereals were traditionally sown in the spring, but MAFF (Ministry of Agriculture, Fisheries and Food) statistics show that by 1985, 75% of the crop was sown in the autumn (Martin, 1987) with major changes in the weed flora. In the 1950s and 1960s the greater area of spring cereals encouraged spring‐germinating broadleaf weeds such as knotgrass (P. aviculare), redshank (P. persicaria), black bindweed (Bilderdykia convolvulus), poppy (Papaver rhoeas), charlock (Sinapis arvensis), chickweed (Stellaria media) and fat hen (Chenopodium album). The move to winter cereals has meant less competition from these spring weeds because the crop is already established. However, winter cereals have encouraged weeds that germinate and establish in the autumn and early spring, particularly cleavers (G. aparine), speedwells (Veronica spp.), chickweed (S. media) and field pansy (Viola arvensis). Similarly, traditional cereal cultivation techniques based on ploughing have given way to direct drilling and minimal cultivations using tines and discs, which are less costly and energy intensive. Indeed, Chancellor and Froud‐Williams (1986) have estimated that 40% of arable land in the UK is now cultivated without ploughing. One consequence is a reduction in the density of many annual broad‐leaf weeds since they require deep soil disturbance to bring buried seeds to the surface; a lack of seed return owing to herbicide use has also encouraged this decline. On the other hand, minimum cultivation has led to an increased abundance of annual grasses, particularly of species which readily establish near the soil surface and which have relatively short periods of dormancy. The main examples are black‐grass (A. myosuroides), meadowgrass (Poa spp.) and sterile brome (Anisantha sterilis). Wild oats (A. fatua) have also flourished in minimum cultivation, even though their seed requires moderate burial. These observations have been quantified in a major, large‐scale survey of weeds present in winter cereals in the UK (Table 1.12).
The frequency of creeping perennials has also increased with minimal cultivation, for example field bindweed (C. arvensis) and Canada thistle (C. arvense). Chancellor and Froud‐Williams (1986) also point to the occurrence of unusual species in undisturbed arable land, particularly wind‐dispersed seeds of the genera Epilobium, Artemisia, Conyza and Lactuca, and suggest that these may be the problem weeds of the future. Current increases in minimum and zero cultivation methods for establishment of cereal and oilseed crops, for environmental and financial reasons, will undoubtedly be mirrored by further changes in the weed spectrum of arable land.
Table 1.12 Main broadleaf weeds and grass weeds present in 2359 winter cereal fields.
Source: Whitehead, R. and Wright, H.C. (1989) The incidence of weeds in winter cereals in Great Britain. Brighton Crop Protection Conference, Weeds 1, 107–112.
| Broadleaf weeds | Fields infested (%) | Grass weeds | Fields infested (%) |
|---|---|---|---|
| Stellaria media | 94 | Poa annua | 79 |
| Veronica persica | 72 | Avena spp. | 42 |
| Matricaria spp. | 67 | Alopecurus myosuroides | 38 |
| Galium aparine | 58 | Elytrigia repens | 21 |
| Lamium purpureum | 47 | Lolium spp. | 14 |
| Viola arvensis | 45 | Anisantha sterilis | 13 |
|
Sinapis arvensis
|
