Farmers are advised to be aware of the presence of parasitic weeds, especially if more diverse rotations are in place and legumes have been included in stewardship mixes or as cash crops.

Parasitic weeds are plants that obtain some or all of their nutrients and water from a host plant, to which they attach at either stem or root level.

A major problem in other parts of the world, they can affect yield and quality.

See also: Advice on managing glyphosate resistance this spring

The best-known example of a parasitic plant in the UK is mistletoe, which is hosted by certain tree species, but native grassland species such as yellow rattle (Rhinanthus) are also parasitic and feed off the roots of nearby grasses.

Others include dodder (Cuscuta), with a wide range of host plant species including clover, and broomrapes (Orobanche), which weaken host plants, but are not toxic or unpalatable to livestock.

Although none of these are very common on UK farms at present, where they do infect a host plant often the only solution is not to grow a compatible host plant in that field again.

Orobanche crenata © John Cussans

Changing rotations

As John Cussans, weed science principal consultant for Adas explains, the rising threat from parasitic weeds is becoming apparent in the changing farm landscape, as most of the hosts are legumes.

“We are using so many more host species in mixes – whether that’s legume fallows, herbal leys, pollen and nectar mixes or companion plants – that we need to develop a better understanding of parasitic weeds and how to deal with an infestation,” he says.

The host legume species include clover and vetches, notes John, as well as peas and beans grown as break and companion crops. 

This means organic farms and those that have embraced the Sustainable Farming Incentive are at greater risk.

Although the focus is on legumes, other hosts include carrot, sunflowers and oilseed rape, he reveals.

Weed recognition

For farmers, the first step is to be able to recognise the weeds, stresses John, as that allows them to be dealt with quickly and removed physically to prevent seeding.

“If they are across a whole habitat, it is more difficult,” he acknowledges. “Like all weeds, they have a seed bank.”

May is a good time of year to do an assessment, as they will be flowering, making identification more straightforward.

John is working with colleagues in Europe to determine the extent of the problem and to understand the level of threat and the crops most at risk.

A survey being done in conjunction with Wageningen University and the Natural Resources Institute at the University of Greenwich is currently running, and farmers are being urged to fill it in (see below).

It has been translated into four European languages, reflecting the spread of the issue across the Continent and the possible role of climate change.

Weed spread

“We are seeing more and more of these parasitic weeds so we’re keen to know how many farms are affected, as well as how the weeds have arrived on farm.

“Some are native, others may have come in a seed mix.

“If farmers could take the time to fill out our survey, we will be able to develop our understanding and issue the right guidance for sites that are affected.”

John reports two different situations he has been made aware of recently.

“One was an organic farm using red clover in a fertility building ley, the other was an arable farm that had seen a problem in an AB15 mix on vetch.”

Neither had been aware of the weeds in question before, he adds.

“We are keeping a watching brief and are keen to hear from anyone who thinks that they may have a problem developing with parasitic weeds.”

Agronomists’ view

Ruth Naudé is an independent agronomist working with farmers across Suffolk.

She recently observed broomrape in an AB15 legume mix that had been planted for a recently introduced Sustainable Farming Incentive (SFI) scheme.

Having never seen the weed before, Ruth contacted her colleagues at Apex Agronomy as well as John Cussans at Adas for assistance with identifying the weed and advice on how to deal with it.

“Having since been in contact with other members of the Association of Independent Crop Consultants, a few of us have now come across broomrape on farms,” she says.

Phelipanche aegyptiaca © John Cussans

“We are very concerned that this and other novel weeds are being introduced to the UK via imported, unregulated seed mixes which are in much greater demand since the SFI was introduced.”

She says weeds such as broomrape could have a massive impact on the ability to grow successful legume crops.

Ruth stresses the need to understand much more about them and the impact they may have on cropping rotations.

“Losing another break crop because of contaminated seed mixes would have a significant detrimental effect on both farming and environmental schemes.”