Blackgrass - Cultural control options

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. Ciaran Collins, Tillage Specialist discusses Cultural control options here

Recent blackgrass herbicide experiments completed at Teagasc Oak Park found that eight of 18 populations were resistant to ACCase (Falcon & 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 that this is likely to be short-lived, therefore the implementation of Integrated Weed Management (IWM) practices are essential for control.

IWM practices range from the very successful practice of 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.     

Blackgrass biology

• Approximately 80% of blackgrass germination occurs from August to October 
• This will vary from year to year as weather conditions during flowering and seed formation will affect seed dormancy.
• Blackgrass will emerge in the springtime, typically from April onwards as air and soil temperature increase.
• Light helps to induce germination.
• Plants can produce up to 1000 seeds (100 seeds per head) but this can be much higher (up to 6000 seeds per plant) in the absence of competition.
• There is a 70% seed decline in the seedbank per year.
• Seed longevity is 1 – 5 years.
• Germination depth is up to 4cm.
• Deeper burial (> 4 cm) will reduce seed emergence of freshly shed seeds.

Cultural control

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 2 years grass/fallow is required to significantly reduce the population and up to 5 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.

• Ploughing reduced blackgrass population by 69% when compared to non-inversion.
• Delaying sowing from September to the end of October decreased weed plant densities by approximately 50%.
• Sowing wheat in spring achieved an 88% reduction compared with autumn sowing.
• Increasing winter wheat crop density above 100 plants m² had no effect on weed plant numbers, but reduced the number of heads m² by 15% for every additional increase in 100 crop plants, up to the highest density tested (350 wheat plants m²).
• Choosing more competitive cultivars could decrease blackgrass heads/m² by 22%.

Switch to spring cropping

Where changing to grass is not an option, avoiding the key germination period in the autumn and a switch 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 >4cm.

Cultivation

After a switch to spring cropping, changing cultivation practice to ploughing from non-inversion gives the largest reduction in blackgrass population, 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, for example ploughing in year one and not ploughing in year two (75% seed dies per year), then returning to ploughing in year three.

Crop Rotation

A good crop rotation increases soil health leading to better yields but, crop rotations can also play a significant role is 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

Delayed sowing until the second half of October will help bypass the main autumn flush of blackgrass.

UK research shows that seed dormancy is increased by cool wet conditions during seed maturation, as such delayed sowing could be less effective is such seasons.

Competitive crop

Blackgrass thrives in open spaces and can produce up to 6,000 seeds/plant when given adequate room. 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 crops nutrient requirements are met.

Research has shown that competitive cultivars like hybrid rye and hybrid barley also reduce seed return. 

What not to do

• Ignore the problem – It will get worse very quickly.
• Continuous autumn cropping - cereals
• Same cultivation type every year, non-inversion is worse
• Sow early
• Grow uncompetitive crops
• Use post emergence herbicides only
• Allow seed return

The article is part of a blackgrass campaign. #Blackgrass21

If you liked this article you might also like to read Beware of wild flower mixes as you might get more than you paid for

Find out more information and advice from the Teagasc Crops team here. The Teagasc Crops Specialists issue an article on a topic of interest to tillage farmers every Thursday on Teagasc Daily.