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Dormancy this autumn is the highest we have ever seen reliance on a compet

Dormancy this autumn is the highest we have ever seen - reliance on a competitive crop and good herbicide choice important.

Results from HGCA-funded research have shown that dormancy in black-grass seed samples this autumn is very high. Seeds for the project were collected by farmers, ADAS, AIC members, AICC and Rothamsted Research.

Based on 40 geographical samples from 2008 the actual mean figure was 16% germination.  This compares with the results for about 20 samples tested in each of the years 2001 to 2005 as part of a LINK project (LK 0923) and from tests in 2006 and 2007 funded by HGCA.  All these years' results are summarised in the table below. 

The % black-grass seed germinating in 2008 was slightly lower than 2002, 2004 and 2007, these were after cooler and wetter summers.  In 2008 the weather has been much cooler than the long-term average with an average number of rain days.  In all years samples were collected from across the country.

Year   Mean % black-grass seed germinating  Conditions during black-grass seed maturation 
2001  62  Hot, dry
2002  22  Cool, damp 
2003  57  Hot, dry 
2004  28  Cold, wet 
2005  59   Hot, dry 
2006  53  Hot, dry
2007  25  Cold, wet 
2008  16  Cold, damp  
    
As in previous years, despite an average high dormancy, a few samples showed lower levels of dormancy indicating that local conditions were still important.

High dormancy is likely to mean a more protracted germination period for black-grass.

Does high dormancy in the autumn mean a spring germination flush?
This was a hot topic of conversation last year and some herbicide applications were delayed to cover this.  As part of this dormancy project large containers of soil were sown in September with a known population of black-grass and emergence monitored through to April.  Emergence in these containers was slow and only 20% of seeds germinated.  There was no evidence of spring emergence.  However, spring emergence might occur when cloddy seedbeds breakdown or where soil is cultivated or moved again in the late winter or spring.  At ADAS Boxworth we have seen high populations of spring germinating black-grass in spring crops but none in autumn sown crops where seedbeds were reasonable.

Unfortunately we do not know where the 80% un-emerged seeds are or what they will do.  However, we would normally expect an 80% seedbank decline in undisturbed soil.  If that were the case for every 100 seeds shed last year, 20 may have grown, 64 may have rotted or been predated and that would leave 16 with the potential to emerge this autumn.

What do these dormancy results mean?
The prediction this year is that black-grass germination will be slow even with adequate seedbed moisture.
 
2008 is similar to 2002, 2004 and 2007. This is unlike 2005 and 2006 where seeds had a lower dormancy and germinated readily when moisture was available.

The following management principles are likely to apply:

  • Spray off any black-grass that does emerge before drilling - although this is likely to be less than in a low dormancy year.
  • Where there are high black-grass populations consider burying weed problems by ploughing.  However, after high levels of control in the current crop avoid ploughing-up more seeds than you bury.
  • Aim to get a competitive crop, perhaps by early drilling.
  • Avoid cloddy seedbeds to maximise herbicide performance and reduce risk of late emerging black-grass.
  • Do not be tempted to reduce crop seed rates where high populations of black-grass are expected.
  • Use a robust pre-emergence herbicide with a residual component to cover the protracted period of emergence.
  • Post-emergence sprays could need a residual element to cover some late emergers.  Based on last years monitoring, high dormancy did not, in winter crops, lead to a spring flush of black-grass.  It is also worth remembering that late emerging plants are much less competitive, especially in a well established crop.  Consider the following important aspects in determining the optimum post-em timing: 
    • Apply herbicides in conditions most suited to their activity
    • Small black-grass plants are easier to control.  This is especially the case where enhanced metabolism resistance is, or may be, present.

This project was sponsored by HGCA (Project Number 3336). For further information please contact Sarah Cook, ADAS Boxworth (e-mail sarah.cook@adas.co.uk; Tel. 01954 268215).

Appendix 1.  Dormancy patterns in black-grass (2001-2008)

Appendix 2. Black-grass emergence patterns(Autumn 2007 - Spring 2008)

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