Herbicides and Plant Physiology. Andrew H. Cobb

decline. With an ever‐growing focus on farming in an environmentally sensitive way, it is likely that there will become a greater emphasis on justifying why a non‐crop species should be removed rather than justifying why they should remain.

Table 1.11 Ranking of the competitive effects of selected weed species and their value for birds and invertebrates.

      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.

Species	Competitive index	Value for birds	Value for insects
Alopecurus myosuroides	***		—
Avena fatua	****	—	—
Lolium multiflorum	****
Poa annua	**	**	***
Aethusa cynapium	**		—
Anagallis arvensis	*		—
Aphanes arvensis	*
Brassica napus	***	**	—
Chenopodium album	**	***	***
Cirsium spp.	***	*	***
Convolvulus arvensis	***
Epilobium spp.	**
Fallopia convolvulus	*	***
Fumaria officinalis	**	*	—
Galium aparine	****	—	***
Geranium spp.	**		—
Lamium purpureum	**	—	**
Legousia hybrida	*
Myosotis arvensis	**	—	—
Papaver spp.	***		*
Persicaria maculosa	**	***	**
Polygonum aviculare	**	***	***
Ranunculus spp.	**
Senecio vulgaris	**	**	***
Sinapis arvensis	***	**	***
Sonchus spp.	**	*	***
Stellaria media	**	***	***
Tripleurospermum inodorum	***		***
Veronica spp.	**		—
Viola arvensis	*	**	—

      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