Weather conditions during maturation influence the dormancy of black-grass. However, this year there was a complication with the dormancy testing.  Many of the seed samples (77%) were found to have orange wheat blossom midge (OWBM) present and this has significantly affected the germination of many samples (Figure 1).

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                                        Figure 1. The effect of OWBM on black-grass germination.

This may mean that despite high black-grass populations, OWBM may have killed a high proportion of seed and emergence may be low as a result.

Based on seven geographical samples from 2018 (which were unaffected by OWBM), the actual mean germination was 80%. As in previous years some samples tested showed lower and higher levels of dormancy indicating that local conditions were still important, but there were no obvious regional differences in 2018.

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                                                       Figure 2. Dormancy of black-grass. 

For each year the box plot 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. The ‘weather’ is a simple classification of the mean temperature between 11 and 30 June compared to the long-term mean.

Dormancy only applies to freshly shed seed.  The effect of dormancy may only be minimal depending on the amount of seed that is already in the seedbank.  The key factor affecting germination this year will be moisture availability.  Figure 3 shows what to consider when planning cultivations.

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                                               Figure 3 Factors affecting black-grass germination

What to do in a Low dormancy year

• Freshly shed black-grass seed will emerge rapidly where adequate moisture is present.
• Leave stubbles uncultivated, this allows seed to be predated and to germinate.  Cultivations will bury seed, removing it from predators and can induce seed into a deeper dormancy. 
• Use minimal or non-inversion tillage, keeping seed within the top 5cm where it can germinate. Try to minimise bringing up old seed from previous years.
• Delay drilling (no later than the end of October) to allow maximum weed emergence, as crop establishment will be compromised.
• Spray off any black-grass that emerge before drilling with the correct dose of glyphosate
• Apply a robust pre-emergence herbicide with a residual component within 24-48 hours of drilling.
• Apply a robust post emergence herbicide programme.  Post-emergence sprays need to be timed after most black-grass has emerged, applied at the correct growth stage and should also include a residual element.

Author Sarah Cook Adas