The optimum plant density for each crop will differ and will depend on growing conditions, time of sowing and economic viability. In unfavourable conditions (eg delayed sowing, poor soil conditions) the growth of individual plants becomes limited, so higher plant densities may improve competitive ability and yield.
At any sowing time increasing sowing rate can result in earlier crop canopy closure and greater dry matter production, improving weed suppression and the effectiveness of other weed management tactics.
High crop sowing rates reduce weed biomass and weed seed production. Increasing crop density can reduce weed biomass, translating into reduced weed seed-set and seedbank replenishment. In addition, crop yields in the presence of weeds usually increase with crop density.
High sowing rates increase crop competitive ability by:
Promoting early canopy closure and increased dry matter production
Better use of resources (water, nutrients and light) in competition with the weeds.
In turn, improved crop competition increases the effectiveness of other weed management procedures used, and suppresses weed seed-set by survivors.
Crop yield and grain quality may improve with increased sowing rates while benefiting weed control. Most small grain comes from secondary tillers. At higher plant populations there is a greater reliance on primary tillers. However, in some situations high sowing rates can lead to yield decline and/or increased grain screenings.
If using higher sowing rates to improve competitive ability of a crop, remember to optimise the sowing rate for grain yield and quality potential. Using high sowing rates (within the optimum range for the region and target grain yield) will not only ensure maximum grain yield, but also tend to minimise small grain screenings in years with average rainfall during grain filling. Sowing rates in excess of the optimum can increase screenings in some cases (and in a few cultivars) but the economic importance of this is likely to be relatively small.
In situations where terminal stress is likely, choose a cultivar that has good average grain size and stability of grain size.
Article: CulturalWeedControl171 (permalink) Date: 9 January 2013; 7:45:57 PM AEDT