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Cleavers, Galium aparine, is extremely competitive against wheat: only 2 plants per m2 can cause 5% yield loss and 100 plants per m2 in autumn can lead to 40% yield loss. On an individual plant basis, it is the most competitive weed commonly found in wheat in the UK. It grows large and can choke wheat crops by growing over the top of the canopy. Furthermore, this weed can interfere with combine harvesting of the crop by blocking the mechanism of the combine and can cause crop lodging pre-harvest. Germination is favoured by winter cropping systems, and the weed has spread north and west following the trends in winter cropping. There is a second flush of germination in the spring, requiring secondary control methods. Cleavers seeds can contaminate harvested crops – a particular issue with oilseed rape due to similar seed size and shape. Some 3% of oilseed rape samples are rejected because they contain more than 4% of cleavers seed. The seeds can also be difficult to remove from cereal samples and can cause losses of grain.
There is increasing evidence that cleavers found as weeds within fields have different characteristics to those found in hedgerows although the exact nature of this relationship is unclear. The use of inorganic fertilisers within fields encourages cleavers and its vegetative growth responds to increased nitrogen fertilisation more effectively than does that of winter wheat.
Shed seeds tend to be dormant so relatively few germinate on stubbles immediately after harvest. Most cleavers tend to emerge in a prolonged flush shortly after autumn crops are sown with a second, smaller flush, in the following spring. Cold winter conditions may promote more spring germination. However, emergence patterns vary between fields and years for reasons that are still poorly understood.
Autumn emerging plants are the most significant from a competition perspective but those emerging in spring, while having relatively less effect on crop yield, may produce a significant amount of seed thus maintaining the weed population. Although cleavers is highly competitive, its damaging effect on cereal crops occurs later in the cropping year (end of April onwards) than is the case with many other weeds (e.g. chickweed and black-grass). One beneficial consequence is that there is a long window of opportunity (from autumn to spring) for successful control.
Cultivations tend to encourage germination and seedlings can emerge from a depth of 10 cm or more within the soil meaning that ploughing is a less effective technique for control than it is with annual grass-weeds such as black-grass and rye-grass.
In contrast to most other broad-leaved weeds, cleavers seeds have relatively short persistence in the soil with a typical annual decline rate of 60% for buried seeds. An important consequence is that, provided new seed return is prevented, infestations can be reduced substantially within a few years - potentially by over 95% within four years.
There are a number of commercially available chemical herbicides for controlling cleavers e.g. amidosulfuron, florasulam or metazachlor. There are currently no known resistant populations of cleavers in the UK. Autumn germinating cleavers are more damaging to yield than spring germinating weeds and they should be prioritised for control with early spring treatments.
Cleavers can be partly controlled by cultural methods - mechanical and tine weeders can be used to remove a proportion of emerged cleavers within the wheat crop. However, ploughing may actually increase seedling emergence, as seeds are brought to the soil surface where they can germinate.
Herbicides and resistance
- During the last 20 – 30 years a range of effective herbicides has become available for control of cleavers, at least in the major arable crops.
- In cereals, amidosulfuron, florasulam, fluroxypyr, mecoprop and mixtures containing these herbicides can give excellent control of cleavers. The label of the Corteva herbicide containing the two synthetic auxin herbicides, halauxifen-methyl (‘Arylex’) + fluroxypyr also lists cleavers as being susceptible.
- Some pre-emergence herbicides applied primarily for black-grass control in winter cereals, such as prosulfocarb, can also give useful control of autumn emerging cleavers but are unlikely to adequately control plants subsequently emerging in spring.
- In oilseed rape, clomazone, metazachlor+quinmerac, clopyralid+picloram and halauxifen+picloram can all give good control of cleaves although correct timing is crucial.
- Clomazone and/or mixtures based on this active can be used for control of cleavers in peas, beans, potatoes and wide range of minor crops.
- No cases of herbicide resistant cleavers have been detected in the UK so far and no cases have so far been reported in any other European country.
- However, cleavers resistant to ALS inhibiting herbicides (sulfonylureas) has been reported in Turkey and China. In addition, there is a report of resistance to fluroxypyr in China on the HRAC International Survey of Resistant Weeds website (http://weedscience.org/) but it has not been possible to obtain any further information.
- Resistance to ALS inhibiting herbicides has been detected in the closely related species G. spurium in Canada and G. tricornutum in Australia. In addition, some Canadian populations also show resistance to the synthetic auxin quinclorac.
- Consequently, cleavers can be considered a ‘low resistance risk’ weed based on global experience but should not be consider a ‘no resistance risk’ weed due to the high dependency on herbicides for control.
- If resistance is suspected, collect a seed sample and have it tested. The very competitive nature of cleavers and fecundity, mean that detecting resistance at an early stage is vital.
See other information sheet (Broad-leaved weeds: occurrence, agro-ecology and management) for best approaches to integrated weed management of cleavers. The limitations of these mean that control of cleavers is largely dependent on herbicides on most farms, so maintaining the availability and efficacy of a wide range of herbicides is essential. It is worth emphasising that, as explained above, prevention of seed return over a period of years should greatly reduce the threat from cleavers as the seedbank should decline much more rapidly than for many other major broad-leaved weeds (e.g. charlock and poppy).