Grassland weed control

Friday 27 March 2009 16:00

Managing weeds in grass for grazing or conservation is crucial to maximise its nutritional value and ensure peak animal performance. Mark Ballingall from SAC and Mark Palmer from Agrovista round up some essential advice.

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 and thistles discourage grazing and can make hay and silage unpalatable. A number of weeds are listed in the Injurious Weeds Act, which requires management to prevent the spread of common ragwort, spear thistle, creeping or field thistle, broadleaved and curled dock. 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, where it has an impact on 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.

A prostrate habit and fast growth restrict tillering of establishing grass and clover. Up to a 25% cut 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 A substantial tap root, large leaves and prolific seeding mean docks are among the most important weeds in grassland. Broadleaved docks are most common, but curled leaved docks can be trouble on lighter soils.

broadleaved dock

Broadeaved dock

curled dock

Curled dock

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

Nettles Favouring high-fertility sites, nettles spread through tough roots forming clumps. Grazing stock avoid 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 Potentially deadly to livestock, ragwort is listed as an injurious weed, which occupiers can be required by law to control. More than 90% of complaints that 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 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 or preventative. 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 thistle is the most widespread and troublesome, spreading by creeping roots. It can persist for many years at a depth of 1-2m.

creeping thistle Colonies can spread 6m in a year. Yield losses of up to 15% have been recorded from two shoots/sq m. In comparison biennial spear thistle has a strong tap root 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.

spraying grassweeds

Tackling grassland weeds early will help improve sward palatability and productivity.

Herbicides In 2008 the approval for sale, supply and use of amino-pyralid-containing products (such as Forefront) was suspended but it is now available for use in established grassland grazed by cattle and sheep (not horses). 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 the product.

Aminopyralid-containing products had set the standard for dock control in grassland. Herbicide choice will depend on the weed spectrum, level of control required and budget. But Doxstar and Mircam Plus are good alternatives for dock control.

The second change is to "straight" mecoprop-p-containing products (eg, Duplosan KV, Compitox Plus). After successful inclusion on Annex 1, the labels were reviewed and updated. The outcome is that the new approval does not permit use on new-sown leys or grassland, with the exception of grass-seed crops or amenity.

Growers will still be able to use mecoprop-p-containing products that are co-formulated with other active ingredients, eg, High Load Mircam (with dicamba) or Mircam Plus (+ dicamba and MCPA). Adding other active ingredients broadens the weed spectrum, and Mircam Plus could be used to replace "straight" mecoprop-p use on reseeds for chickweed control, for example.

Pastor is a useful alternative where mayweed is also present. Neither is suitable when clover is an important constituent of the sward. Alistell may be an alternative on grass/clover reseeds.

High Load Mircam can be used on established grassland and newly-sown leys for the control of a wide range of annual and perennial broadleaved weeds, including docks, daisy, buttercup, nettles and chickweed. Established perennials are difficult.

Controlling difficult weeds

Docks In newly-drilled pasture docks readily germinate from seed within the soil. Herbicide choice for controlling seeding docks is affected by timing and the presence of clover. In the first year with clover the options are very limited especially with the revocation of MCPA + MCPB mixtures. Options include 2,4-D + MCPA. This mixture will also control a range of broadleaved weeds but expect clover damage. In established grassland sulfonylurea chemistry, such as amidosulfuron (Squire Ultra), will suppress docks while retaining clover and can also be used in young grass leys.

Without clover the mix options are wider in the first year, with mecoprop-p, MCPA, dicamba combinations such as Mircam Plus. Pyridine herbicides (Doxstar, Pastor), although more costly, often give better control in established grassland, as they are better translocated to the long tap root.

For optimal control of established docks apply herbicide early at the rosette stage in spring or autumn post-cutting and grazing.

Ragwort Phenoxy herbicides (MCPA and 2,4-D and Dicamba) provide the best control of ragwort. But this needs a targeted programme applied at the rosette stage in autumn and again the following year before flowering. Spot treatment to the rosette can be effective for low populations. Measures must be taken to avoid stock grazing with dying or dead ragwort present.

Magneto, a unique combination of two active ingredients (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). In Agrovista trials Magneto has proved more effective than other herbicides 12 months after application.

Thistles On newly seeded grass with clover 2,4-D + MCPA will give some control of seedling spear and creeping thistles. Where clover is absent the more expensive 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 such as Thistlex or Pastor, which offer more persistent control.

Stewardship Active ingredients often used in grassland sprays have been found in water (eg, 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 placing close attention to detail on filling, spraying and disposal and making sure there is no chance of any spills or spray reaching the water.