United Kingdom
August 21, 2009

Source: HGCA CropResearch News 70

Results from HGCA-funded research (Project 3336), carried out by ADAS, have shown that dormancy in black-grass seed samples this autumn is high. Seeds for this project were collected by farmers, ADAS, AIC members, AICC and Rothamsted Research.

Based on 37 geographical samples from 2009 the actual mean figure was 23% 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 to 2008 funded by HGCA. In all years samples were collected from across the country.  All these years' results are summarised in the table below.

 

Year	Mean % black-grass seed germinating	Conditions during black-grass seed maturation
2001	62	Hot and dry
2002	19	Cool and damp
2003	57	Hot and dry
2004	28	Cold and wet
2005	59	Hot and dry
2006	56	Hot and dry
2007	23	Cold and wet
2008	15	Cold and damp
2009	23	Average

(See below for Appendix Fig. 1)

Weather conditions during maturation influence the dormancy level of black-grass, this period generally occurs during the second half of June through to mid July. The weather in 2009, during this period, has been the most variable of all the years we have studied. Generally the weather has been slightly warmer than the long-term average with a lower number of rain days, but there were 50% of stations wetter than average and 50% drier.

As in previous years, despite an average high dormancy, some samples tested 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?

Again this was a hot topic of conversation and some herbicide applications were delayed to cover this.  As part of this dormancy project large containers of soil were sown in the autumn with a known population of black-grass and emergence monitored through to April.  Over the two years of the project, autumn emergence averaged 29% and a very low level of spring emergence did occur (3%).  Spring emergence can also occur when cloddy seedbeds breakdown or where soil is cultivated or moved again in the late winter or spring. (See below for Appendix Fig. 2)

If only 29% emerged where are the un-emerged seeds?  Unfortunately we do not know precisely where the 68% 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, 31 may have grown, 54 (80% of 80) may have rotted or been predated and that would leave 14 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.
 
The results of 2009 are most similar to 2002, 2004, 2007 and 2008. 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:

Before drilling 

• 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.

At drilling

• 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.

Herbicide strategies 

• 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

Appendix Fig. 1.  Dormancy patterns in black-grass (2001-2009)

This is a box plot. For each year the graph shows the most extreme values in the data set (maximum and minimum values), the lower and upper quartiles, and the mean (written in).

The darkened boxes indicate the quartiles, the first quartile of a group of values is where 25% of the values fall at or below this value. The third quartile of a group of values is where 75% of the values fall at or below this value.

Appendix Fig 2. Black-grass - percentage emergence (2007 and 2008)