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Grassland weed control

Course: Grassland management | Last Updates: 11th October 2015

 
Mark Palmer
Technical Manager
Agrovista UK
Biography >>
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High levels of weeds in grass swards not only reduce pasture’s nutritional value, but restrict grazing areas. Most weeds reduce the nutrition of the sward and provide excessive competition for valuable grasses, particularly in newly sown leys.

Nettles, thistles and ragwort discourage grazing and can make hay and silage unpalatable. A number of weeds are listed in the Injurious Weeds Act including common ragwort, spear thistle, creeping or field thistle, broad-leaved and curled dock. These require management to prevent their spread. Poisonous weeds must also be a priority.

Problem weeds

Buttercups

Creeping buttercup is the most common species and a problem in heavily grazed, poached or wet pastures. Animals tend not to graze areas infested with buttercup as it has an acrid taste and affects grass yield and reduces hay value.

Common chickweed

The most common annual weed, which can persist in rotational grass and establish in long-term pastures where there are gaps in swards due to poaching or slurry injection.

With a prostrate habit and fast growth, chickweed restricts tillering of establishing grass and clover. Up to a 25% reduction in silage yield has been recorded after failure to control it. A fall in herbage dry matter and slow wilting may also occur, causing increased loss of soluble nutrients in the field and as effluent in clamps.

Chickweed may also upset fermentation, resulting in reduced silage quality. Large chickweed populations may cause digestive upset in grazing lambs and calves.

Docks

broadleaved-dockA substantial taproot, large leaves and prolific seeding mean docks are among the most important weeds in grassland. Broad-leaved docks are most common, but curled-leaved docks can be trouble on lighter soils.

Docks thrive in high-nitrogen conditions and grow vigorously in dense swards where other weeds fail to establish. Research has shown the effect of docks on grass is directly proportional to about a 1% loss in grass dry matter for each 1% of ground cover by docks.

See also: Improve grassland by controlling docks

Nettles

Favouring high-fertility sites, nettles spread through tough roots forming clumps. Grazing stock avoids mature nettles, but they may be grazed when young and have some feed value in hay, as they are high in protein and calcium.

Ragwort

ragwortPotentially deadly to livestock, ragwort is listed in the Injurious Weeds Act, which occupiers can be required by law to control.

More than 90% of the complaints Defra receives about injurious weeds concern ragwort. Under the Ragwort Control Act (2003), a code of practice was developed giving guidance on identification, priorities for control, control methods, environmental considerations and health and safety issues.

Ragwort is a danger to all stock, but particularly horses, cattle, free-range pigs and chickens. Alkaloids cause cirrhosis of the liver and there is no known antidote. Although largely unpalatable, ragwort may be eaten when green, particularly when other grazing is sparse. It is palatable when dead or dying because of the release of sugars, so contamination of hay or silage is very dangerous.

Thistles

creeping-thistleCreeping thistle is the most widespread and troublesome, spreading by creeping roots. It can persist for many years at a depth of 1-2m.

Patches can spread 6m in a year. Yield losses of up to 15% have been recorded from as little as two shoots/sq m. In comparison, biennial spear thistle has a strong taproot with a rosette of prostrate leaves in the first season and a tall, flowering shoot in the second. Prolific seeding, wind dispersal and a readiness to germinate in mild conditions make spear thistle widespread.

Control

Spraying grassweeds -Tackling grassland weeds early, at establishment of a new grass ley, will help improve sward palatability, productivity and reduce costs.

In newly drilled pasture, docks readily germinate from seed within the soil. Herbicide choice for controlling seeding docks is affected by timing. Weed control in new grass depends on whether or not clover is in the seed mix.

The weed control options in newly drilled grass in the first year that are clover safe are few. Tribenuron has a  recommendation for use on new grass and is clover safe. 2, 4-DB, has a label recommendation for use in a grass clover mix and is useful in tank-mix with tribenuron.

In established grassland, amidosulfuron will suppress docks while retaining clover and can also be used in young grass leys.

Where clover is not part of the grass mix, hormone mixes with low-rate mecoprop – P + dicamba+ MCPA – may be used from the three-leaf stage. Other options include low-rate MCPA + 2,4-D or low-rate fluroxypyr.

On established grass, aminopyralid + triclopyr sets the standard for dock, thistle and nettle control and also has ragwort on the label. But it is not without restrictions; it must only be used on grass for grazing, for example not on grass for silage or hay production. Any manure produced by the grazing animals must be returned to grassland on that holding and any hay or silage produced in the year following spraying must be kept on the farm. There are further restrictions producers should check before using aminopyralid + triclopyr.

Where aminopyralid + triclopyr is not suitable, herbicide choice will depend on the weed spectrum, level of control required and budget.

See also: Learn from the organic industry when it comes to weed control

Pyridine herbicides are not as costly as aminopyralid + triclopyr. They both give good control of established docks as they translocate to the long taproots. They both control a range of annual weeds such as chickweed and mayweed and do not have the same restrictions as aminopyralid + triclopyr.

Worth considering are hormone herbicides, which can be used on established grassland and newly sown leys for the control of a wide range of annual and perennial broad-leaved weeds, including docks, daisy, buttercup, nettles and chickweed. Established perennials are difficult to get a 100% kill in, though, and any control that is achieved is likely to be less long-lasting.
spraying-grassweeds

 

Controlling difficult weeds

Ragwort

Aminopyralid + triclopyr provides effective long-term control of ragwort, however it can only be used on grass for grazing. Phenoxy herbicides such as MCPA, 2,4-D and Dicamba can also provide control of ragwort. But this needs a targeted programme applied at the rosette stage in autumn and again the following year before flowering. Where allowed on the product label, spot treatment to the rosette can be effective for low populations. Measures must be taken to avoid stock grazing on any dying or dead ragwort present.

Dicamba and 2,4D is effective on ragwort and when applied in conjunction with an adjuvant oil offers the highest level of control possible (susceptible rating).

As an alternative to the phenoxy hormones for ragwort control it is possible to spot treat small populations using citronella oil. Citronella oil is a non-toxic natural oil and is best applied at the rosette stage, although it can be used up to flowering . Ragwort foliage is totally killed after application. It is spot treated as the grass will also be killed.

Thistles

On newly seeded grass with clover, 2,4-D + MCPA will give some control of seedling spear and creeping thistle. Pyridine herbicides will give good control. In rotational or permanent grassland the choices are the inexpensive hormones 2,4-D, MCPA and dicamba alone or in mixes, or more expensive pyridine combinations that offer more long-term control.

Stewardship

Active ingredients used in grassland sprays have been found in water (for example, 2,4D, MCPA, mecoprop and clopyralid). All grassland sprays should be applied in accordance with the Voluntary Initiative guidelines. These are designed to reduce the risk of pesticides reaching water by paying close attention to detail on filling, spraying and disposal and making sure there is no chance of any spills or spray reaching the water.

Injurious weeds

  • Dock, broad-leaved
  • Dock, curled
  • Ragwort, common
  • Thistle, creeping
  • Thistle, spear

Poisonous weeds

  • Bracken
  • Broom
  • Buttercup
  • Horsetails
  • Foxglove
  • Hemlock
  • Ragwort
  • Sorrel

Cultural controls

Common chickweed

Rapid establishment and include clover in the mix. Mob-graze with sheep in the autumn. Harrow in autumn.

Annual meadow grass

Choose vigorous grasses. Avoid early grazing with sheep, as this tends to shorten and open sward.

Docks

Avoid feeding hay or silage contaminated with dock heads to avoid seed spread. Frequent cutting reduces vigour, but does not provide complete control.

Creeping thistle

Frequent topping will reduce vigour, but will not provide complete control.

Nettles

Frequent topping will reduce vigour, but will not provide complete control. Where patches have been controlled, redrilling with a vigorous grass will suppress reestablishment of the nettles.

Buttercups

Improve drainage. Good grass and clover coverage.

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