In a survey in 2007/08 of 28 farms in Southern England (Lutman et al., 2009), 16 of the 20 commonest weeds were broad-leaved weed species. These were: Cleavers, Common field-speedwell, Scentless mayweed, Groundsel, Charlock, Cut-leaved cranesbill, Chickweed, Field pansy, Sowthistles, Black bindweed, Fat-hen, Shepherd’s purse, Creeping thistle, Fool’s parsley, Docks and Knotgrass.
They vary greatly in competitiveness. Cleavers are severely competitive (CI* = 3.0); charlock, volunteer oilseed rape, scentless mayweed and common poppy are highly competitive (CI = 0.4); black bindweed, chickweed, field forget-me-knot, fat-hen, knotgrass, sow thistles, speedwells, red dead-nettle, crane’s bill, groundsel and fool’s parsley are moderately competitive (CI = 0.06 – 0.3); while scarlet pimpernel, field pansy and parsley piert are weakly competitive (CI = 0.02 – 0.05). (CI* = competitive index = % yield loss per weed plant/m2). For information on other species, including grass-weeds, see the information sheet on the CROPROTECT website: ‘How competitive are different weed species? A summary.’
Competitive indices are useful as an indicator of the relative competitiveness of different species but the actual infestation level in terms of number of weeds/m2 will often be more important in determining crop losses. Thus a high infestation of a moderately competitive weed species may cause substantially larger losses than low populations of highly competitive species.
As well as reducing crop yields, high populations of broad-leaved weeds may slow harvesting and cause contamination of grain and straw. The prevention of seed return is particularly important as most broad-leaved weeds produce seeds that are much more persistent in the soil than grass-weeds (“one year’s seeding means seven years (or more) weeding”).
Many broad-leaved weed species emerge at specific periods of the year. Some tend to be autumn germinators (e.g. cleavers, parsley piert, poppy, scentless mayweed, ivy-leaved speedwell), others spring germinators (e.g. black bindweed, black nightshade, charlock, fat-hen, fool’s parsley, hemp-nettle, knotgrass, , redshank, scarlet pimpernel) while others germinate in both autumn and spring (e.g. chickweed, common field-speedwell, field pansy, fumitory, groundsel, shepherd’s purse).
Seeds of most broad-leaved weeds are more persistent in the soil than those of grass-weeds. Whereas few seeds of black-grass, brome, rye-grass and wild-oats survive for more than three years in the soil, many major broad-leaved weeds (e.g. charlock, poppy, speedwells) produce seeds that can easily persist for 10 years or more in the soil.
Seedlings of most broad-leaved weeds can only emerge successfully from seeds within the top 3 cm soil, although species with larger seeds (e.g. cleavers) will emerge from greater depths of up to 15 cm. Depth of emergence is largely dependent on seed size.
Infestations of broad-leaved weeds usually consist of mixture of many different species, at varying densities, making management more complex than is often the case with grass-weeds, which often involve a single dominant species (e.g. black-grass).
Preventing seed production is important in long-term weed management but this is likely to have less immediate impact compared with control of grass-weeds because of the greater seed persistence of most broad-leaved weeds. For the same reason, total eradication of most broad-leaved weed is unrealistic and reducing infestations to manageable levels is a more practical goal.
- Consider whether you have become over-dependent on herbicides for broad-leaved weed control. Assess the risk posed by herbicide-resistant broad-leaved weeds and take action to reduce that risk (see other information sheets).
- Do not rely solely on herbicides – integrate their use with non-chemical methods wherever possible. This is difficult as there are fewer effective non-chemical options for control of broad-leaved compared with grass-weeds.
- Know which broad-leaved weed species are present, their distribution and densities within each field so that the most appropriate herbicide can be chosen. Be aware that the species composition may vary considerably between autumn and spring sown crops, even within the same field – a reflection of the diverse germination patterns of different species.
- Many ALS inhibiting herbicides (e.g. sulfonylureas such as metsulfuron) are highly effective on a wide range of weed species but are very resistance-prone. Use other modes of action in mixture, sequence or in rotation wherever possible to reduce the risk of resistance – especially on high resistance risk weed species (see other information sheets).
- Be aware that using several herbicides with different modes of action may not prevent resistance if certain weeds are only being controlled effectively by one active ingredient, despite many others being applied. Try to ensure that high resistance risk species (chickweed, poppy, mayweed) are controlled effectively with at least three different herbicide modes of action over the course of a crop rotation.
- Apply post-emergence herbicides according to label recommendations. The best control will normally be achieved by treating weeds while they are small.
- Pre-emergence herbicides used for control of grass-weeds often give good control of many broad-leaved weeds, but much will depend on emergence patterns. Autumn applied pre-emergence herbicides are unlikely to effectively control spring emerging broad-leaved weeds.
- Rotations which include spring as well as autumn sown crops help prevent any one broad-leaved weed species dominating. Inclusion of non-cereal break crops enables the use of a wider range of herbicides.
- Minimum tillage and direct drilling for autumn sown crops tends to reduce broad-leaved weed infestations (although they tend to favour grass-weeds such as black-grass and ryegrass).
- Establishing spring crops by direct drilling or with minimal soil disturbance should reduce the numbers of broad-leaved weeds and grass-weeds emerging in the crop.
- Hoeing in row crops and harrowing can be effective in controlling many broad-leaved weeds, especially if carried out while weeds are small.
- Many broad-leaved weeds are very susceptible to competition, especially at the early growth stages, and a strongly competitive crop will assist other control measures.
- Prevent importation and spread of seeds in combine harvesters, grain, balers, seed, straw, manure and cultivation equipment. Seeds of some broad-leaved weeds are more likely to survive anaerobic digestion (AD) than grass-weeds, although this needs verification.
- Although less appropriate for most broad-leaved compared with grass-weeds, rogue plants, or spray off patches, in early summer wherever possible. This should be done before seeds start shedding, which varies with species.
- Be aware that many of the non-chemical methods of control used against grass-weeds such as black-grass (such as stale seedbeds, stubble cultivations, later autumn drilling and ploughing) are ineffective for control of most broad-leaved weeds.
- Broad-leaved weed control is largely dependent on herbicides on most farms, so maintaining the availability and efficacy of a wide range of herbicides is essential.
Three weed species have evolved resistance to ALS (e.g. sulfonylurea) herbicides in arable cropping situations in the UK:
- common poppy (Papaver rhoeas)
- chickweed (Stellaria media)
- scentless mayweed (Tripleurospermum inodorum)
Resistance to metribuzin and metamitron has also been recorded recently in:
- groundsel (Senecio vulgaris)
Resistant biotypes of all four species have also been found in several other European countries too so these can be considered ‘high resistance risk’ species. Common poppy is the most important resistant weed in Europe as a whole.
Worldwide experience shows that some weed species are more resistance-prone than others but why resistance has evolved widely in these species in Europe, but not in others major weeds such as cleavers (Galium aparine) and speedwells (Veronica spp.), is unknown.
It should not be assumed that resistance cannot, or will not evolve in other species and continued vigilance is required especially where control of a specific weed is dependent on a single, or very limited range of herbicides. Bear in mind that glyphosate was used extensively worldwide for over 20 years before resistance was first detected (in 1996) but there are now 43 glyphosate resistant species in 29 countries worldwide (but not any in the UK - yet).
Resistance in most broad-leaved weeds appears to be almost exclusively by ALS target site mechanisms only, with no evidence of enhanced metabolic resistance which is so common in resistant grass-weeds. An important consequence is that there is not the extensive cross-resistance to other modes of action which commonly occurs in black-grass so using alternative (non-ALS) herbicides can be an easy solution to resistant broad-leaved weeds – provided effective alternative are available.
However, there is increasing evidence from France that other mechanisms may play a more important role than previously thought in resistance in poppy and other broad-leaved weeds, although the practical implications remain unclear.