Course: Grassweed management in cereals | Last Updates: 7th October 2015
It’s an unwelcome but familiar sight on many arable farms, especially those with rotations based on autumn-sown crops with its dark seed-heads waving above crops in summer months.
Blackgrass, one of the most prolific weeds in England, also occurs across much of western Europe where cereal-based crop rotations prevail. A particular problem in cool and damp regions, it tends to be concentrated on heavier, moisture-retentive soils, or where drainage is poor.
The blackgrass plant can grow up to 80cm high. The leaves are hairless and leaf blade is pointed, green and rough in texture. The flowering head – often seen above the crop canopy in the summer months – is a compact spike, 3-8cm long, narrow and pointed in shape with a purple tinge.
More recently, herbicide resistance has added to the blackgrass challenge. In the UK, resistance is found on at least 80% of the 20,000 farms that spray regularly for the weed, with three main resistance types being present in today’s blackgrass population.
Blackgrass reduces crop yields through competition for nutrients, especially nitrogen. While the tillering capacity and competitive ability of the weed depends on the vigour of the crop, average yield losses of 0.4-0.8t/ha can be expected from blackgrass populations of 12-25 plants/sq m.
However, much higher losses of over 2t/ha are seen at higher weed densities of more than 100 plants/sq m.
The 5% yield loss threshold for justifying herbicide treatment is 12 plants/sq m for blackgrass. This is an average figure, so in practice it should be reduced, to as low as 1 plant/sq m, especially in high-risk situations.
There’s no doubt that changes to cropping systems have much to answer for. The increasing area of autumn sown crops, especially cereals, established with minimum tillage and sown early, has provided perfect conditions for the weed to thrive. The straw burning ban is also thought to have had an effect.
An annual grassweed, spread entirely by seed, blackgrass can typically produce between 2-20 heads/plant in winter cereal crops. Each head contains about 100 seeds, so where populations have reached 500 heads/sq m, seed return can be very high.
This helps to explain why weed populations can build up very rapidly, as well as why preventing seed return can be a big help in reducing populations.
Most seeds are shed between June and August, before the wheat harvest. Of these, about 40-60% of seeds will be viable. Seeds have a fairly short period of dormancy lasting several weeks, although this is reduced when they mature in hot and dry conditions.
Survival of buried seeds in the soil is about 20-30% per year, so after three years, only about 1-3% of the seeds will still be viable. As the number of seeds in the soil can be very substantial (greater than 50,000/sq m), even a small percentage surviving can represent a significant number.
The emergence pattern of blackgrass also helps to explain why weed levels have continued to rise, despite best efforts to control it. The weed can only emerge successfully from seeds found in the top 5cm of soil, which is why the greater use of min-till has encouraged blackgrass.
Where there is enough moisture, about 80% of blackgrass emergence occurs in the months from September to November, which usually coincides with the emergence of the autumn sown cereal crop.
This means that spring sown crops are less vulnerable, as only low numbers usually emerge after Christmas. For these crops, most of the weed plants will have emerged before sowing, so can be destroyed by cultivations or the use of glyphosate. In some years, such as 2013, spring emergence can be greater than usual, although this is atypical.
The emergence pattern means that much of the control has to be done in the winter crop, leading to an over-reliance on herbicides. However, the loss of existing herbicides, a lack of new ones and increasing levels of resistance, mean that non-chemical methods are becoming increasingly important.
Pre-emergence herbicides, applied in the right conditions, can still give a good level of control (50-80%) in cereals and are much less affected by resistance than post-emergence products. All pre-emergence herbicides are affected by resistance, but usually only partially, and resistance tends to increase slowly. Their greater use reduces the dependence on higher risk chemistry and targets the weed at its most vulnerable stage.
Post-emergence herbicides have the potential to give high levels of control, but resistance to all of them is increasing. As a result, neither ACCase nor ALS inhibitors should be relied on as the main means of control in successive crops.
Achieving the levels of control needed to contain blackgrass (more than 90%) is becoming increasingly difficult with herbicides alone. The use of herbicide sequences and mixtures will help to improve control, but will not prevent resistance developing.
Herbicide resistance is inherited and occurs through selection of plants that survive herbicide treatment. With repeated selection, resistant plants multiply until they dominate the population. Three main types of resistance are present:
- Enhanced metabolism resistance (EMR) – the most common resistance mechanism found in blackgrass, EMR results in herbicide detoxification. It tends to increase slowly and affects most herbicides to varying degrees. Only very severe cases result in complete loss of control.
- ACCase target site resistance (ACCase TSR) – this only affects herbicides which belong to the “fop”, “dim” and “den” groups. It can increase rapidly and results in very poor control, as it blocks the site of activity specific to these herbicides.
- ALS target site resistance (ALS TSR) – this only affects sulfonylurea and related herbicides and occurs less commonly than ACCase TSR. It is, however, increasing and can result in very poor control.
These three types of resistance may occur independently, in different plants in the same field, or even within the same plant. Where resistance is suspected, having a test carried out will help assess the problem and avoid the use of herbicides which won’t work.
Combining weed control measures will improve the overall control of blackgrass. Non-chemical methods reduce the need for herbicides, but tend to give only moderate levels of control at the field level. Their use needs to be fully integrated to increase control and reduce the reliance on herbicides. The following are some of the non-chemical methods available:
Rotational planning should be at the heart of any strategy. Where non-cereal crops are included in the rotation, blackgrass infestations are reduced and alternative herbicides can be used. More balanced rotations are needed for a number of reasons – grassweed control is just one of these.
New ideas for blackgrass control are few and far between. There has been discussion and speculation about chemical germination stimulants, breaking seed dormancy with smoky water, the use of biofumigants, allelopathy and the return of straw burning, but none is expected to be a complete solution.
A better approach might be to consider manipulating or measuring the autumn peak emergence period of blackgrass, so that drilling can take place afterwards. In practice, however, germination is so dependent on moisture, that it may not be practical in a field situation and requires much more research.
The most effective, proven solutions are delayed drilling and more spring cropping. These are the "uncomfortable truths" that many farmers don’t want to hear, but an increasing number are beginning to accept, albeit grudgingly.
- Ploughing – gives an average 69% control by burying weed seeds and its rotational use has potential benefits
- Delayed drilling – this allows the use of stale seed-beds and helps give an average 31% control of blackgrass. The longer the delay, the better the results, but the greater the risk to the farmer
- Higher seed rates – increasing the crop seed rate creates more competition and gives an average of 26% control. The higher the seed rate the better, but lodging issues have to be considered.
- Competitive varieties – choosing a competitive variety will help smother emerging blackgrass and gives an average of 22% control.
- Spring cropping – an effective method of reducing blackgrass, giving an average of 88% control, but can be difficult to achieve on heavy soils.
- Fallow or grass ley breaks – providing no new seeding takes place, these can give a 70-80%/year reduction in the seed bank, making them valuable techniques. A one year fallow or break is not enough to substantially reduce blackgrass infestations – two or even three years are much better.
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