Black-grass

Key Points

  • Propyzamide is a residual herbicide with root uptake that requires even distribution in the soil profile
  • Applications should be made to firm, moist soils with affine tilth
  • Applications must be made to small Black-grass plants, before they tiller
  • Increased rates should be used under warm autumn/winter conditions
  • Increased rates should be used where Black-grass resistance has been confirmed
  • Crop shading can interfere with even distribution to the soil
  • Strategic use is recommended in tank mix or sequence with graminicides.

Introduction

Black-grass — Alopecurus myosuroides — is the most important grass weed infesting cereal and oilseed rape rotations in the UK. It is capable of producing extremely high amounts of seed and has the potential to spread rapidly. Dramatic yield reductions can occur due to very high numbers of plants and great competition with the crop. High populations are perpetuated because Black-grass seed is shed before the crop is harvested.



Biology

The most significant feature of Black-grass as a weed is its ability to produce a very high number of seeds. One seed head can produce 200 seeds and heavy populations of 2000 plants per square metre are not uncommon. One Black-grass plant can produce 20 tillers and if each of these has only 100 seeds this single plant will still produce 2000 seeds.

Black-grass is a particular problem in cereal rotations with very high seed numbers produced. The soil seed bank is replenished as seed are shed before harvest. Around 80% of shed seed germinate in the autumn, though in wet years germination may be delayed until spring.

The weed is distributed throughout the UK but is most prevalent in the south and east of England. During spring growth the tall, narrow grass can reach heights of 80 – 90cm. The leaf blade has a pronounced groove in the middle, with the upper side shiny and under side dull and smooth. The ligule is long and irregular. The leaf sheath can be reddish in colour towards the base, particularly in more mature plants. Dark reddish-purple seed heads appear in May and can be from 2cm to 12cm in length.

An additional reason Black-grass infestation has become such a problem is the now widespread occurrence of resistance of the species to many of the most commonly used herbicides. Two mechanisms of resistance are well-documented and at least one other is suspected.
Enhanced Metabolism Resistance is the most widespread at the moment. Resistance to an applied herbicide tends to be partial and occurs because the Black-grass plant is able to detoxify the active ingredient before it takes effect. This metabolism occurs across a wide range of herbicides but to differing degrees.

Target Site Resistance in Black-grass prevents ‘fop’ (aryloxyphenoxypropionates) and ‘dim’ (cyclohexanediones) herbicides from working successfully. The active ingredients in ‘fop’ and ‘dim’ herbicides normally block the activity of a plant enzyme involved in the production of fatty acids, a component of cell membranes. Resistant Black-grass plants have a modified enzyme, making ‘fop’ and ‘dim’ herbicides ineffective. Target Site Resistant plants are as competitive with the crop as unmodified plants and don’t suffer any lack in fitness. A fitness value is a numerical quantity which describes the survival and subsequent reproductive success of one kind of an individual relative to another as a consequence of natural selection. Black-grass populations can contain plants that exhibit Enhanced Metabolism and Target Site Resistance.

The risk of resistance developing can be managed by employing cultural techniques and not relying solely on the use of herbicides. The cropping system should be based on a good rotation including both winter and spring drilled crops of different species. Monoculture, especially of winter cereals, is likely to accelerate the development and build up of herbicide resistance. Ploughing in year one, followed by minimum tillage in subsequent years, will help reduce Black-grass numbers as the seeds are relatively non-persistent in the soil. Delay winter drilling, where practical, and use a stale seedbed to control emerged Black-grass plants. If there is sufficient soil moisture for germination of Black-grass seeds they can be controlled with glyphosate or cultivation machinery.

Use of herbicides should include sequences and/or mixes of products with different modes of action. In oilseed rape crops ‘fop’ and ‘dim’ herbicides should not be relied on alone to give high levels of control of Black-grass. Herbicides for control of Black-grass that are little or unaffected by resistance issues include Treflan (active ingredient trifluralin) and Kerb (active ingredient propyzamide).

Product Choice

For growers of winter oilseed rape the Black-grass and broad-leaved weed control program can begin in the autumn with a soil incorporated treatment of Treflan* (trifluralin). Incorporation should be carried out simultaneously or within 30 minutes of spraying to maximise efficacy. Treflan is a residual herbicide for the control of broad-leaved weeds and annual grasses. The following weeds will be controlled by Treflan applied at 2.3 litres/ha.

Annual meadow-grass
Black-bindweed
Black-grass(a)
Common chickweed
Common hemp-nettle
Fat-hen
 Henbit dead-nettle
Knotgrass
Pale persicaria
Redshank
Scarlet pimpernel
Speedwell

(a) Strains of some annual grasses (e.g. black-grass, wild-oat, Italian rye-grass) have developed resistance to herbicides which may lead to poor control. A strategy for preventing and managing such resistance should be adopted. Guidelines have been produced by the Weed Resistance Action Group and copies are available from HGCA, CPA, agrochemical distributors, crop advisors and product manufacturers.

Learn more about Treflan

Once the oilseed rape crop is evenly established, and has reached at least the three true leaf stage, applications of Kerb* 50W, Kerb Flo, Menace* 80EDF or Precis* can be made. All these products contain propyzamide as the active ingredient. Kerb 50W is a wettable powder formulation containing 50%w/w propyzamide. Kerb Flo and Precis are liquid suspension concentrate formulations of 400g/litre propyzamide and Menace 80EDF contains 80%w/w propyzamide as a water dispersible granule.

Propyzamide is a residual herbicide that requires soil moisture for successful root uptake by the target weeds. Best residual activity, and hence results, is achieved from applications to moist soils of fine tilth. Applications can be made under frosty conditions unless run-off from the soil surface is likely.

Best results are achieved when growth of weeds, especially Black-grass and volunteer cereals, is slow but transpiration continues. In mild autumns and winters emerged weeds may take longer to be controlled, the residual effect will be shortened and overall control may be reduced.

Established and well-tillered Black-grass is classified as moderately susceptible to propyzamide. Where populations of Black-grass and/or volunteer cereals exceed 50/m2 applications should be made either in tank mix with an approved graminicide, in accordance with the manufacturer’s instructions, or following an effective approved graminicide to ensure optimum weed control. The majority of Black-grass, 80%, germinates in the top 0.5cm of the soil profile. This is where propyzamide is most active. Deeper germinating Black-grass within the soil profile could reduce product efficacy.

Where partial resistance to the partner graminicide is known to exist a higher dose of propyzamide should be applied. This can be done if applications are made early in the season, under warm conditions and an increase in the duration of residual control is required.

There is currently no known strain of Black-grass exhibiting any degree of resistance to propyzamide. Where a high level of resistance to the partner graminicide occurs there is no advantage in adding the graminicide to applications of propyzamide. Even the higher rate of propyzamide will not give acceptable levels of control of established Black-grass under these circumstances.

Visit the product pages for more comprehensive information about Kerb 50W, Kerb Flo, Menace 80 EDF and Precis.