Course: Grassweed management in cereals | Last Updates: 30th October 2015
Non-chemical means of grassweed control can no longer be considered separately from a herbicide strategy. To keep a lid on resistance it is essential to consider an integrated strategy that makes the best use of every tactic you have at your disposal. How you use these tactics lies at the heart of effective grassweed control.
What are the aims of cultural control methods?
The overall aim should be to manage the weed seed bank over time, taking opportunities to reduce the passage of seed to seedling to mature plant. No one operation can be 100% successful, even if you do a perfect job, and that will go for your autumn herbicide application, too. The objective is to give herbicides as little to do as possible.
In the premier division of weed control are choice of crop and cultivation. Putting in a spring crop will bring about 80% reduction of the overall grassweed seed bank. Other methods of control for autumn-sown cereals, such as using a non-selective herbicide pre-drilling, delaying the drilling date and increasing crop competitiveness by raising seed rate and variety choice, are in the next division down. Combine these with ploughing, for example, and you'll be leaving a much easier job to be done by herbicides (see right).
Minimal cultivations will contribute much less to the removal of weed seeds. This puts proportionately more reliance on other control techniques and you may then be left with a much greater proportion of the infestation to control with autumn herbicides.
How can break crops help?
Cropping choice is your key weapon. For starters it will determine the size of the window you have between crops to control weeds.
A wheat crop going into oilseed rape presents the biggest challenge because there is hardly any opportunity to control weeds, especially for growers further north.
A spring crop is the ultimate choice. This gives plenty of opportunities during the autumn and winter to reduce the weed seed bank. Best results come from encouraging weeds to chit and spraying off with a non-selective herbicide several times before drilling the crop in the spring. Many seeds germinate better in autumn than spring and so this will be more effective for species such as brome and blackgrass, than wild oats, that also germinate in the spring.
Winter beans are drilled much later and often ploughed in – both will help reduce the weed seed bank. One thing to note, however, is that the ploughing usually leaves a poor surface for residual herbicide application.
Although oilseed rape is drilled earlier, its key advantage is choice of herbicides. Grassweeds are generally easier to control in broadleaved crops. There's a good range of graminicides for oilseed rape, and conditions are often favourable for getting good activity from herbicides not affected by resistance.
There are fewer herbicides available for pulses. Sugar beet, though a useful spring crop option, has few graminicide choices apart from "fops" and "dims", a group that should be avoided if resistance is a problem.
How can rotation help?
A well-managed rotation offers good weed control because you are varying drilling date, inter-crop window, crop and cultivations. The very act of changing tactics and herbicide choice reduces the risk of resistance build-up.
The best rotations rely on a good mix of broadleaved and cereal crops. Including an occasional spring crop in an autumn-dominated rotation can significantly reduce the weed seed bank.
Rotational ploughing can offer many benefits, but done badly it can do more harm than good. Ultimately fallow is an option to include in the rotation, and can be cost-effective in high blackgrass situations, especially if included as an Entry Level Stewardship (ELS) option. Make sure your priority remains weed control, however – some ELS options restrict herbicide choice. Be careful you don't supplement the seed bank – weeds left unchecked with no competition bring a far bigger problem than you started with.
How can drilling date help?
Delayed drilling offers up to 82% control. But again manipulating drilling date can do more harm than good. The key is a good knowledge of when your target weeds germinate (see below).
Germination-date-tableAt drilling you will naturally create the ideal conditions for the weeds to germinate, so ideally you want to delay drilling until after the main flush of your target weeds has passed. You will not only get lower numbers in the subsequent crop, those plants that do survive will be weaker. They will be easier to control with herbicide and produce less seeds per head.
The longer the delay in the autumn the better: Sowing cereals in early to mid-October, rather than September, can reduce blackgrass plant densities by about 14%. But waiting until November brings reductions of about 70%. The downside is reduced yield and lower crop competition. Drilling the weediest fields last could be the best compromise.
What about crop competition?
Seed rate and variety choice do have a significant influence. Both can typically achieve up to 30% control. The more crop, the less space there is for grassweeds to develop. A high plant count in the autumn is the aim, but this must be balanced against other agronomic factors, such as lodging and disease.
Cereal cultivars with good early vigour and a planophile leaf will help smother weeds in autumn. This is where the leaf turns over as it emerges and flattens out. Varieties that grow tall will help shade out surviving grassweeds and reduce seed return.
What other options are there?
There are other options that have varied results, depending on time and resource available. Weed rakes have been found to help reduce and weaken grassweeds. Roguing helps reduce specific bad patches that could spread through the field.
These can be removed with a knapsack sprayer in autumn or mature plants hand-rogued nearer harvest. Cleaning the harvester between fields can help reduce the spread of weeds such as wild oats and meadow brome.
- As much as you can to give herbicides as little as possible to control
- Consider using the plough, at least in rotation
- Think about introducing a spring crop if you have an all-autumn rotation
- Drill the weediest fields last
- Plough in a high weed infestation then bring the surviving seeds back up the next year
- Drill just before your target weeds are likely to germinate
- Reduce seed rates in weed-infested fields
- Let grassweeds get out of control on an ELS option plot
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