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Managing low black-grass dormancy

The hot dry weather of recent weeks is expected to mean a ‘low dormancy’ year for black-grass. Independent weeds researcher, Dr Stephen Moss, explains what impact this will have and provides his advice for effective weed control strategies for the season ahead.

The hot dry weather of recent weeks is expected to mean a ‘low dormancy’ year for black-grass. Independent weeds researcher, Dr Stephen Moss, explains what impact this will have and provides his advice for effective weed control strategies for the season ahead. 

Following 40 years of studying the notorious weed, Dr Moss explains that the dormancy level of the seed will impact on how quickly it germinates. 

“Weeds have evolved to stagger germination once seeds have shed to give a better chance of at least some surviving,” he says.

“Most black-grass seeds will have shed prior to the harvest of winter wheat, but typically only 50 percent shed prior to winter barley harvest. However, they then have a period of innate, or naturally occurring dormancy, which can last several weeks.”

Dr Moss explains that temperature during the ‘critical phase’ of seed maturation, which typically occurs in May and June, heavily influences this dormancy period.

“Research I did at Rothamsted showed that hot, dry weather leads to lower dormancy, whereas cool, wet weather typically results in higher dormancy,” he explains.

“Low dormancy means that seeds will germinate more quickly if the conditions are right, whereas high dormancy means germination is likely to be more protracted.” 

He adds that annual black-grass seed dormancy trials are undertaken by ADAS, who then provide a germination percentage following testing of freshly shed seeds. 

“Although the 2018 results are not yet available, with the weather being so hot and dry in May and June, low dormancy black-grass should be expected this season. 

“Therefore, if conditions are favourable, there could be a pronounced flush of early black-grass germination,” he says. “However, this will only happen if there is sufficient rainfall. If it stays dry, seeds won’t germinate, regardless of dormancy status.”

Advice for this season

Dr Moss stresses that any advice for the control of black-grass in light of anticipated dormancy levels, must take into account the bigger picture, and be implemented as part of an integrated crop management (ICM) strategy.

“If there has been a high level of seeds shed then no, or minimum, cultivations and delayed drilling will allow many seeds to germinate, once rain arrives.

“It’s vital that as many seeds as possible are encouraged and given time to germinate and the resulting seedlings destroyed prior to drilling.”  

He adds that even if a good flush of early emerging weeds is seen, a number of seeds will remain dormant until after drilling even in a low dormancy year.

“Therefore, the implementation of a robust pre-emergence herbicide strategy will still be vital to substantially reduce weed populations.

“A pre-emergence herbicide programme, which includes a baseline of straight flufenacet such as Sunfire or System 50, is a good starting point,” he says. “But, any herbicide stack built around this must be tailored to the individual farm, to maximise performance and increase overall control.

“The efficacy of pre-emergence herbicides is dependent on soil characteristics, which vary greatly, so it should come as no surprise that the same herbicide can work better in one field than another. 

“Farmers have the ability to see what works best on their farms by using the information available from simple trials, and advice from their agronomist.” 

ADAS trials

Senior weed scientist at ADAS, Dr Sarah Cook, has worked on all aspects of weed control for over 30 years, leading black-grass dormancy trials at ADAS for over 17 years.

“Initially funded by LINK and AHDB, our research indicated that weather during the ripening phase of the weed affects the innate or initial dormancy of black-grass seed. 

“Every year we receive black-grass seeds from across the UK,” Sarah explains. “From this we undertake a simple test, which includes four replicates of 50 seeds. These are incubated for two weeks at a constant temperature and the germination percentage gives us an indication of dormancy. 

“Although trial results for 2018 are pending, this year we expect low dormancy due to the hot, dry weather. This means that black-grass will germinate quickly once it rains and should result in a good year for black-grass control prior to drilling.”

Black-grass dormancy – points to consider

Dr Moss highlights three important facts when considering black-grass dormancy within weed control strategies for the autumn.

  • There needs to be moisture for seeds to germinate. If it remains dry, then enforced dormancy will prevent germination regardless of the anticipated dormancy level
  • Weather-induced dormancy only affects freshly-shed seeds and in most black-grass situations there will be older seeds in the soil from previous years to manage
  • Cultivation significantly alters the impact of dormancy. Through ploughing, fresh seeds will be buried, and older seeds will resurface, in which case the current dormancy level is less relevant