Researchers at IACR-Long Ashton and Rothamsted have been working on weed competitiveness for several years. Clearly, results vary according to season, site and soils. However, the table above gives an indication of the competitiveness of individual weed species, showing how many weeds per sq m will reduce wheat yields by 2%. It is a useful indicator to gauge whether to treat with a herbicide, but not an exact spray threshold.
New IACR work is intended to obtain a more reliable way of assessing weed damage, explains Dr Peter Lutman. "We are looking at three weeds on five sites and measuring yield responses." The work takes into account the size of the weed and crop growth.
There are site characteristics which influence some weeds and not others, he explains. "Residual N is particularly relevant for blackgrass. And heavier soils have more competitive weeds than sandier soils. This may be linked to moisture; we think weeds may root to a shallower depth than winter wheat.
"We aim to take pictures of different levels of weed infestation and ultimately produce cue cards that can be used to trigger herbicide applications," he says.
Meanwhile competitiveness is being turned on its head at Long Ashton where researchers are investigating the competitiveness of the crop against weeds. Two old wheat varieties, Avalon and Spark, are being used, explains Kathryn Wright, to discover what plant characteristics are needed to compete successfully against cleavers.
So far the work shows that Avalons height and canopy structure – wider leaved and denser – is more suppressive than Sparks upright, spikier leaves and open canopy.