Sow-thistle - the new cleavers in oilseed rape?

UK - February 24, 2006

Sow-thistles are becoming a common sight in winter oilseed rape crops and when it comes to yield loss and harvesting difficulties could soon rival cleavers.

Sow-thistles, like cleavers, are largely unaffected by crop competition.  And competition will be low as many winter oilseed crops were sown at rates of just 3-4 kg/ha last autumn. So sow-thistles and cleavers are set to run riot. 

“An increased use of minimum tillage methods to establish winter oilseed rape, combined with very dry and warm conditions after harvest, has led to sow-thistle populations soaring,” explains Dow AgroSciences’ Rene Pollak. 

The weather at harvest led to good wind dispersal and seed survival. While above average soils conditions encouraged rapid germination of sow-thistles before and after crop emergence.  

Sow-thistles produce exceptionally high seed numbers.  One plant can shed over 20,000 seeds, so with almost ideal conditions in minimal tillage systems it is not surprising that there are higher than usual numbers in oilseed crops this season.

Mr Pollak recommends Galera (clopyralid + picloram) at 0.35 l/ha for immediate sow-thistle removal as well as controlling other highly competitive weeds - cleavers and mayweeds - at the same time.  The herbicide should be applied when target weeds are showing new spring growth and at a crop growth stage up to and before, the flower buds are visible above the canopy. 

“Don’t underestimate the yield loss, harvesting difficulties and increased grain moisture content that sow-thistles can cause in winter oilseed rape, almost as severe as cleavers.  Populations of above three plants/m² are worth treating.  Figures shows that five sow-thistles/m² can cost the grower over £40/ha in lost yield on a 4.3 t/ha crop,” says Mr Pollak.