Weeds can be significant components of the vegetation in arable systems and are usually managed intensively to reduce crop-weed competition and increase crop yield. High weed population densities can lead to a serious reduction of crop yield, but the buried seeds and emerged plants are a direct source of energy and nutrition for arable food webs populated by species of decomposers, saprophytes, herbivores and predators that are not sustained by ”cleaned” crop monocultures. These plants can also stabilise soil and reduce leaching by taking up excess fertiliser or nutrients released by decaying crop matter.
Arable weeds provide a greater variety of form, composition, and function than the few crop species that dominate arable land and are therefore important in maintaining diverse farmland habitats.
In the sustainable system at the CSC, the weed control programme targets the dominant or competitive species in each field/crop. The aim is to use specific products to allow an understorey of non-competitive weeds for wildlife benefit, but at levels low enough to avoid impact on yields or harvest.
We carry out weed species population counts in November (winter crops) and in May and July/August (all crops). Quadrat counts are made at GPS locations 1, 3 and 5 on each transect, providing 216 sample points across the site. Crop plant populations are also recorded along with crop and weed % cover.
At the July/August count, an estimate of total biomass is made by harvesting all plant material from a 0.5 x 1 m quadrat at each sample point. Harvested material is divided into dicot and monocot weeds, crop stems and grain, then dried and weighed. Dried material is then milled and weighed out for elemental analysis of C and N.
Preliminary data indicate significantly higher densities of both monocot and dicot weeds under sustainable management in the spring sown crops (potato, spring barley and beans). The grass weeds were more abundant in the winter cereals but there was no difference between treatments in dicot weeds for these crops.
There was no detectable effect of weed biomass on crop yield, suggesting that weed densities are generally low enough to be non-competitive with the crop.
The goal of weed management in the sustainable system is to maintain a high diversity of beneficial (mainly dicot) weeds but at densities below economic threshold. We appear to be achieving this in the spring crops, but need to improve the management of grass weeds in the winter cereals.