If herbicides controlled blackgrass, it would not be considered the problem it is. Unfortunately, if blackgrass appears in a field there is a good chance that it will be resistant to some or all of the available herbicides.
Recent blackgrass herbicide experiments completed at Teagasc Oak Park found that eight of 18 populations were resistant to ACCase (Falcon and Stratos Ultra) or ALS (Pacifica) and that resistance to both ACCase and ALS herbicides was found in five populations.
Experience from the UK tells us that even when herbicides control blackgrass, this is likely to be shortlived. Therefore the implementation of integrated weed management (IWM) practices is essential for control.
IWM practices range from putting land back into ley/fallow for five years to cultural methods that will help reduce the population in a crop.
The successful implementation of any cultural practices to reduce blackgrass is highly dependent on having a deep understanding of the biology of blackgrass and implementing practices that target the plant when it is most vulnerable.
One of the most significant characteristics of blackgrass is its high rate of seed decline in the soil each year. This is why returning land to ley/fallow is one of the most successful ways of controlling blackgrass.
Even though blackgrass has a 70% rate of decline, a minimum of two years grass/fallow is required to significantly reduce the population and up to five years to get adequate control.
The fallow/ley requires very careful management to prevent seed being returned during this time.
In a comprehensive review of 52 field experiments, mostly from the UK, studying the effects of cultural control methods, it was found that blackgrass can be significantly reduced by using a range of cultural control methods.
Switch to spring cropping
Where changing to grass is not an option, avoiding the key germination period in the autumn and switching to spring cropping will result in the largest reduction in blackgrass germination.
Switching to spring cropping also provides opportunities to use one or more stale seedbeds to break dormancy of seeds shed in the previous crop or to reduce seeds in the soil. Shallow cultivation is vital.
Blackgrass readily germinates up to 4cm but germination is seriously reduced when the cultivation depth is greater than 4cm.
After a switch to spring cropping, changing cultivation practice to ploughing from non-inversion gives the largest reduction in blackgrass populations. However, the results can be variable depending on previous cultivations and seed depth distribution in the soil.
There is a balance between bringing older seed from depth and burying newly shed seed. Rotational ploughing is often the best approach, eg ploughing in year one and not ploughing in year two, then returning to ploughing in year three.
A good crop rotation increases soil health leading to better yields but crop rotations can also play a significant role in controlling blackgrass.
Rotation allows for the use of alternative herbicides with different modes of action, thus reducing the chances of resistance development and achieving better control. Spring break crops facilitate stale beds. Avoid the main germination period and allow alternative herbicides to be used (not oats).
Winter oilseed rape presents an opportunity to control blackgrass. Winter oilseed rape is sown during the main germination window and the herbicide propyzamide (Kerb) is effective at controlling blackgrass.
There are no reports of herbicide resistance to propyzamide. Establishing winter oilseed rape using non-inversion techniques will encourage more blackgrass to germinate and further reduce the weed seedbank.
Delayed sowing until the second half of October will help to bypass the main autumn flush of blackgrass.
UK research shows that seed dormancy is increased by cool wet conditions during seed maturation, so delayed sowing could be less effective in those seasons.
Blackgrass thrives in open spaces. Competitive crops reduce space and light and, consequentially, the amount of seed blackgrass produces.
Aim to increase sowing rates to limit space and ensure that the crop’s nutrient requirements are met.
Research has shown that competitive cultivars like hybrid rye and hybrid barley also reduce seed return.
What not to do ?