Blackgrass – Alopecurus myosuroides – is the most important grassweed 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 blackgrass seed is shed before the crop is harvested.
The most significant feature of blackgrass 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 blackgrass plant can produce 20 tillers and if each of these has only 100 seeds this single plant will still produce 2000 seeds.
Blackgrass is a particular problem in cereal rotations with very high seed numbers produced. The soil seed bank is replenished as seeds are shed before harvest. Around 80% of shed seeds 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 90 cm. 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 2 cm to 12 cm in length.
Blackgrass control across the rotation
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. The risk of resistance developing can be managed by employing cultural techniques and not relying solely on the use of herbicides. Ploughing can reduce blackgrass survival. However it can have big downsides, namely expense, adversely affect timeliness and more importantly for effective activity of propyzamide, can result in cloddy seedbeds and blackgrass roots establishing below the herbicide activity layer. Using min-till techniques when establishing oilseed rape will mean the blackgrass will germinate from shallow depth and is best placed for root uptake of propyzamide. Min-till can create opportunities for stale seedbeds prior to drilling. If there is sufficient soil moisture for germination of blackgrass seeds they can be controlled with glyphosate or cultivation machinery.
An additional reason blackgrass 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.
Enhanced Metabolic Resistance is the most widespread. Resistance to a herbicide tends to be partial and occurs because the blackgrass 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 blackgrass prevents herbicides from working successfully. Blackgrass populations can contain plants that are susceptible to herbicides and plants that exhibit Enhanced Metabolism and Target Site Resistance. With no reports of herbicide resistance, propyzamide is a valuable tool in the control of blackgrass.
Once the oilseed rape crop is evenly established, and has reached at least the three true leaf stage, applications of Kerb® Flo 500 and ASTROKerb can be made. Kerb Flo 500 is a liquid suspension concentrate formulation of 500 g/litre propyzamide and ASTROKerb is a liquid concentrate containing 500 g/litre propyzamide and 5.3 g/litre aminopyralid for control of both grassweeds and key broad-leaved weeds such as poppy and mayweed in oilseed rape.
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 soils of fine tilth. Propyzamide is most active in the top 5 cm of the soil profile. Deeper germinating blackgrass within the soil profile could reduce product efficacy. Priority must be given to avoid run-off into water-courses at all times.
For optimal performance applications should be made when soil temperatures have got down to 10°C and falling and soils are sufficiently moist. 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 blackgrass 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.
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. Consider sequencing and or mixing in a “fop” for improved control of blackgrass and/or volunteer cereals.
Dow AgroSciences does not believe that a sequence or tank mix of carbetamide and propyzamide is a responsible herbicide strategy given the unacceptably high levels of propyzamide and carbetamide that are found in surface waters (used for drinking water). Our data shows that a correctly-timed treatment of Kerb Flo 500 or ASTROKerb, applied with cycloxydim, provides comparable control to programmes of carbetamide.
Ensure at least 4 of the following are met before application:
- Field drains not running and not likely for 7 days or no field drains
- Field slope less than 5%
- Not bordered by a watercourse or field has a 6 m grass buffer strip
- Not sub-soiled below plough layer
- Not mole drained in the past 6 months
- No risk of rain in the next 48 hours