Weeds pose greater risk to yield in low winter rape populations

UK - March 10, 2006

As farmers adopt lower seed rates to establish winter oilseed rape, the crop’s highly competitive nature is not likely to be sufficient to prevent broadleaved weeds causing yield loss.

When seed rates of 5-8 kg/ha were the norm, dense and highly competitive crops prevented broadleaved weeds threatening yield.  Now the trend is towards lower plant populations that allow greater spring light interception with easier canopy management is leading to significantly higher yields.  However, crops with these open structures will have potentially less competition against weeds. 

To optimise yield potential, winter rape growers are currently being advised to sow in late August to early September using seed rates as low as 2-3 kg/ha for conventional varieties, to give late autumn plant populations of around 50-60 plants/sq m.  Even this low rate can be reduced by as much as 50% when planting very early.  Crops sown in early to mid August with established populations as low as 9 plants/sq m can be maintained to produce a surprisingly high yield!  Hybrid varieties have always had lower seed rates recommended due to the strong and robust vigour from these plants. 

Whilst current advice can lead to optimum yields, the crop faces a number of threats including poor germination and slugs, that can lead to patches of unacceptably low populations that require redrilling. Therefore, seed rates are often a compromise taking account of establishment risk, especially on soils that produce cloddy seedbeds in dry areas. 

Experimental work carried out by Dr Peter Lutman of Rothamsted Research shows that plant vigour between drilling and late autumn is crucial to overall crop density, and therefore competitiveness to weeds.   

“Very low plant populations from low seed rates can produce strong growth from early plantings, resulting in good crop vigour and cover by early winter,” says Dr Lutman. “However, crop vigour tends to be stronger with higher seed rates. Lower seed rates can result in plants with less vigour, particularly when planted later than the optimum time of mid to late August.    

“Our trials found a close correlation between ‘rape early vigour’ (measured in grams dry weight/sq metre in December) and % yield loss from weeds.  The chart illustrates that once dry weight vigour drops below 70 grams/sq m, yield loss from 100 weeds/sq m will be around 10%.  However, when dry weight falls to 40grams/ sq m, the same weed density can produce yield losses of 40%.” 

The most competitive weeds in winter rape in order of descending importance are cleavers, which are three times more competitive than chickweed, then sow thistle and mayweeds.    

Past research has shown that the yield loss caused by one weed/sq m in competitive rape crops (December dry weight= 100 gms/sq m) was 1% for cleavers and 0.3% from chickweed and sow thistle.  However, if the December dry weight halves – as is likely with low seed rates – yield losses will rise by 20%.    

Obviously, a mixture of several weed species at low crop mass will present a serious threat to yield.  A reliable late post-emergence cleaver herbicide for winter rape is Dow AgroSciences’ Galera.  On top of mayweed and sow thistle control, it consistently provides up to 80% suppression of cleavers, and under warm conditions and with the assistance of the crop canopy may provide even higher levels of control. 

Dow AgroSciences advises that Galera must be applied once the spring weed growth has started, and temperatures are rising.  Applications can be made until the crop green buds start to show above the crop.