The Implications of Extensification for Crop Weed Interactions in Cereals.
Reductions in agrochemical inputs, particularly nitrogen fertiliser and herbicides, may form a major part of any programme of extensification. Little information is presently available regarding the impact of such reductions on interactions of the crop and natural mixed weed populations. This study shows that limiting nitrogen availability can lead to an increase in weed density accompanied by greater species richness. There was no consistent relationship, however, between biomass production and nitrogen rate. Reductions in nitrogen fertiliser inputs would not therefore necessarily decrease weed biomass, but may change the composition away from one dominated by nitrophilous species. From a conservation view point, extensification may promote increased abundance of rare arable and less competitive weeds by reducing competition from nitrophilous species and the crop, allowing these species to survive and set seed to a greater extent. Maintaining such a weed flora could provide sufficient host plants for associated insect fauna and hence more food for gamebird chicks. Herbicides also regulate aspects of the weed flora, having profound consequences on the composition of future generations and the competitive ability of offspring via maternal effects, as seen in Viola arvensis. In the utilisation of resources, the crop is potentially more efficient and the use of more competitive cultivars may be exploited in weed control. The winter wheat Maris Huntsman was found to be more effective at reducing weed biomass than its modern shorter strawed counterpart, Mercia. The crop did not appear, in the cases studied, to suffer any severe loss in grain quality with reduced nitrogen or weed control, except grain protein. In some cases no weed control or half rate herbicide applications gave significantly better yields than commercially recommended doses. Any yield loss incurred via extensive farming would be partially offset by reduced input costs.