Praising weed diversity

Watch out for different weeds to those you are used to if you are embarking on a new management approach, such as no-till establishment.

That is one of the key messages from one of the work streams of the EU-funded IWM Praise project (see panel) which is specifically looking at the effect of weed management strategies on the composition, diversity and abundance of weed communities in no-till systems.

The starting point of the project was to reduce reliance on any one type of control, but especially herbicides as resistance and scrutiny of their impact increases, in favour of an integrated approach, says Dr Jonathan Storkey, principal agroecology research scientist at Rothamsted Research.

“The idea is if you have a more diverse approach to weed management you avoid the problem of selecting for one or two dominant species.

“But that raised the problem that reduced tillage systems are very reliant on glyphosate, so there was a dedicated work package around how to reduce herbicide use in conservation agriculture,” he adds.

To assess the impact of cultivation system and rotation on weed populations, Rothamsted is using its two Large Scale Rotation Experiment sites at Harpenden and Bury St Edmunds.

These compare reduced tillage with inversion tillage systems in different rotations using different approaches to weed management – for example, delayed drilling in a no-till system and in a plough-based system in the context of a three-, five- or seven-year rotation.

“What we’ve found is different weed species are favoured by different systems.

In reduced tillage systems we’re seeing more perennials, such as docks and perennial sow thistle, and wind-dispersed species like willowherbs and annual sow thistles,” Dr Storkey says.

Selecting for different weed species is not necessarily a problem, as long as you have diversity of management so you are not selecting for one or two dominant weeds that we’ve tended to do with herbicides, he adds.

SPECIES There are more than 100 species that are quite common in arable fields, according to John Cussans, weed biology specialist for NIAB.

“What’s almost certain to happen in no-till systems is that we will reduce the dominance of the most aggressive species currently – black-grass, bromes and wild oats – and other species that are already in arable fields will emerge.”

While evidence so far suggests that conservation agriculture is creating a more even and diverse weed flora, there are one or two warning signs for weeds to pay attention to in a no-till system, he stresses.

“We’re seeing more examples of alien or invasive weeds being introduced in game cover and cover crop seed, as the UK doesn’t, perhaps only in the short term, have enough domestic supply of these non-crop seeds.”

Examples include parasitic weeds such as Cuscuta (dodder) and Orobanche (common broomrape) for which there is not effective weed management other than not to grow a compatible host species, mostly legumes, in the rotation.

“The most common example of a contaminant of game cover mixes is barnyard grass.

It’s less of a risk in current systems, but could be in more diverse rotations with crops like chickpeas or soya, not least because there are examples of herbicide resistance,” Mr Cussans says.

UBIQUITOUS Of greater immediate threat are some ubiquitous native species that have been excluded from arable agriculture by cultivation.

These include the broadleaf bur chervil and the grass Vulpia or rat’s tail fescue.

Both are incredibly aggressive and successful in no-till systems.

“They’re almost ubiquitous in semi-natural and natural habitats and highly invasive of reduced cultivation systems.”

Vulpia has come from nowhere, Mr Cussans says.

“But now we’re seeing regular occurrences all associated with no-till agriculture.”

Studies in the US and elsewhere have suggested it can be difficult to get control with pre-drilling glyphosate, requiring much higher doses than for other annual grassweeds.

Recent research in Germany has also found populations of Vulpia with reduced sensitivity to glyphosate, while Danish researchers found increased metabolic enzyme activity in the weed which helps explain its inherent reduced susceptibility to the chemical.

“We have all the ingredients for a significant problem in the future.”

Spring cropping is reasonably effective at reducing the threat from bur chervil, and currently sulfonylurea herbicides give very effective control.

“If I was a betting man, I think we will have sulfonylurea resistance in bur chervil.”

Those risks make it crucial to create a system where these weeds are not the dominant problem, Dr Storkey reiterates.

“The whole philosophy is you look at not just your tillage, but rotation and other cultural controls and combine them in a way you don’t get those dominant weeds occurring.”

Avoiding that will help reduce the need to reach for herbicides all the time, and relying on them to get you out of jail.

“The opportunities to do that are going to become fewer and fewer.”

Unfortunately, that might be easier said than done, or require some tough decisions around ideology.

For example, effective but infrequent tillage might be one solution no-till growers have to contemplate to manage species such as bur chervil, vulpia and Italian ryegrass (see panel), Dr Storkey adds.

New tools are probably need ed also, Mr Cussans says.

“Inter-row cultivation as a substitute for herbicides in a conventional system is a very poor investment, but as a tool to help the transition to no-till, or in a no-till system to reduce the frequency you invert soil, it becomes much more attractive.

“Similarly with approaches like harvest weed seed control, as soon as you change the system, the economics of adoption become more attractive.

There are emerging tools that could help with the transition,” he adds.