Soil conditions and cultivations critical to resistant blackgrass control

UK - August 02, 2006

Oilseed rape growers are advised to keep cultivations shallow for the best control of resistant blackgrass using Kerb Flo (propyzamide). 

“The depth of blackgrass seed as well as soil moisture levels at, and shortly after, application is critical,” says Dow AgroSciences’ Rene Pollak.  “Kerb Flo does not leach much further than 5 cm depth at any time after application.  But blackgrass can germinate from as deep as 10 cm, and may not be controlled if the roots are below the level of the Kerb Flo.”  

Mr Pollak emphasises that post-harvest cultivation choice is very important to eliminate any deep germinating blackgrass.  Minimal tillage systems usually operate to a depth of 5 cm and all the blackgrass shed from the previous crop remains within Kerb Flo’s control zone.  Up to 98-99% control can be expected with Kerb Flo, even in the highest blackgrass populations.  

However, conventional ploughing and deep cultivations can spread blackgrass seed throughout the soil profile up to 30 cm deep.  Although blackgrass will struggle to germinate from below 10 cm a high percentage will be left in the 5-10 cm band where control levels from Kerb Flo drop.      

“In five years of trials the difference in control by Kerb Flo between conventional cultivation and minimal tillage systems is between 5-7%.  That may not sound significant, but in a field with over 500 blackgrass seeds/m² some 25-35 plants may be left alive.  These will produce around 30,000 seeds/m²  which is 6,000% more than the original population,” says Mr Pollak.  

To keep blackgrass within Kerb Flo’s Control Zone when using conventional cultivations Mr Pollak advises:

• If ploughing:
  • use skimmers and make sure the furrow is completely inverted to place all seed at the furrow bottom, well below the depth at which it can emerge from;
  • Use a lower speed for good, even inversion;
• Cultivate to a depth of 10 cm after ploughing to stimulate germination of any blackgrass in this zone;
• Create a fine firm stale seed bed at least once, and preferably twice, before drilling oilseed rape or beans;
• Use glyphosate to kill any emerged blackgrass;
• Use Kerb Flo at 2 l/ha for resistant blackgrass, applying only after the soil is cool and has been wetted to at least 5cm.