Dry autumns bring double weed troubles

A dry autumn can spell double trouble for arable growers as herbicides don’t work as well and troublesome blackgrass takes longer to emerge.

Most pre-emergence herbicides are vulnerable to dry conditions, while blackgrass germination is reduced by a lack of moisture, according to Jim Orson, weed expert at NIAB TAB.

“If September is dry, you end up with a double weed control problem. The herbicides don’t work as well and the blackgrass takes longer to emerge,” Mr Orson told the British Crop Production Council’s annual weed review meeting, where the UK’s top weed specialists considered the lessons from the past 50 years of weed research.

As the post-emergence grassweed herbicides are losing their efficacy, the recent trend towards dry autumns is a concern, he says. “It means the main component of blackgrass programmes is weakened.”

Fortunately, pre-emergence herbicides can be ranked according to their vulnerability (see table 1), he added.

“Flufenacet and DFF are the most vulnerable and won’t give you the required results in a dry seed-bed. At the other end of the scale, tri-allate is the least affected by drought. So there are things growers can do to help tackle dry conditions,” he said.

It’s not just the dry conditions – drilling date is also crucial in the blackgrass battle. Stephen Moss of Rothamsted Research pointed out more than 80% of blackgrass emergence occurs in the early autumn.

“And over the years, there’s been a 10-fold increase in the amount of September wheat drilling. So we’ve handed that land over to blackgrass,” he said.

This is just one of the reasons why delayed drilling should be considered, he added.

“It means you’re dealing with lower blackgrass populations because you can use stale seed-beds more effectively,” said Dr Moss.

He added that with delayed drilling, there is better control from herbicides as conditions for residual products are better in the late autumn.

Pre-emergence herbicides – tolerance to dry seed-beds 

Least vulnerable Most vulnerable 


Flufenacet and diflufenican (DFF) 

Blackgrass control according to drilling date   

Drilling date  Control of plants (%)  Control of heads (%) 
16-21 September  79  50 
3-8 October  87  72 
21 October – 5 November  89  84 
The good news on herbicide resistance is that the enhanced metabolism resistance known to exist to Atlantis does not confer cross-resistance to other herbicides, Dr Moss said. “It is specific to Atlantis, so there’s been no increase in resistance to actives such as flufenacet.”

However, all of the pre-emergence herbicides are affected by resistance to some extent.

“It is only partial resistance and it does appear to increase very slowly. That’s illustrated by a study of 375 field trials that has shown the level of control from the flufenacet-based products Crystal and Liberator has not fallen substantially in the past ten years,” Dr Moss added.

Spring crops

Spring crops can make a huge contribution to blackgrass control, reducing populations by 88%, reported independent agronomist Andrew Cotton.

“They need to be competitive crops, such as spring oats, barley, wheat or oilseed rape, not thin, open crops such as linseed. And they should be sown at high seed rates for maximum competitiveness, later on in the spring, where possible,” he added.

No spring cultivations should be used on over-wintered seed-beds and glyphosate should be applied both pre- and post-drilling, he recommended.

Spring wheat can still yield very well from April sowing, Mr Cotton stressed. “A client had some Mulika drilled in April this year. It went on to yield between 8-9t/ha,” he added.

Spring oats were the best margin crop for another one of his clients. “He sold them early for £180/t and got more than 6t/ha in yield. The input costs were very low,” he said.

More on this topic

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