Course: Weed control | Last Updates: 28th July 2017
Grassweeds and volunteers can be a real problem in arable crops, particularly with growing herbicide resistance and the loss of some active ingredients.
Producers are increasingly relying on tackling weeds in stubble after harvest – but if done incorrectly it will achieve disappointing control and potentially affect yields in the following crop.
It’s, therefore, vital to make effective use of every tool in the armoury – from glyphosate to cultural control – to optimise stubble hygiene. So what are the options?
For some weeds, stubble hygiene really starts before harvest – creeping thistle and couch are best controlled with pre-harvest glyphosate. This also gets rid of any green matter, making combining easier.
Other weeds, like blackgrass, brome, cereal volunteers, Italian ryegrass and – further north – annual meadow grass are best tackled after harvest.
The aim is to get these weed seeds to chit in the stubble, and then spray them off with glyphosate. Drilling a crop into a clean seed-bed sets up the whole rotation: If you have a lot of weeds carrying over into the next crop it can act as a green bridge for diseases and pests, thereby affecting subsequent yields.
In addition, if you are min-tilling into stubble which has a lot of green matter it will be very attractive to slugs.
Seed shed before or during harvest will chit either in the base of the crop or in the resulting stubble – it depends on weed seed dormancy and moisture availability.
There’s no need to cultivate immediately after harvest; just leave the stubble to green up naturally and then spray it off.
Tackling weeds is a numbers game – the fewer you start with the less you’ll end up with. To keep blackgrass at a steady level; neither increasing or decreasing, you need to achieve 97% control.
The optimum number of stale seed-beds is a difficult question to answer and is very weather dependent.
Initial data from research and company sources has shown that best control can be achieved within three spray applications and no more are required. This assumes that the dose is right for the target weed.
Delaying drilling to allow for three stale seed-beds will, therefore, be extremely beneficial. Research into the value of cultivation between sprays is ongoing, and it seems clear that one pass just before drilling is worthwhile, to get a final chit before the third glyphosate application.
Blackgrass seeds germinate in the top 5cm of the soil – if they are buried below that depth they will die or become dormant until they are brought to the surface again.
It’s therefore important, if cultivating to encourage seed chit, not to inadvertently bury the seed.
Drilling will help to kill off any emerging seedlings, but if ploughing before drilling, it’s useful to spray off the final chit as green growth can make ploughing more difficult.
It’s important to get good inversion and bury weed seeds at depth to avoid bringing them back to the surface with the drill.
The gap between spray applications on stubble will depend on the weather: If it’s warm and wet you’ll get a more rapid flush than in cold, dry conditions. What is absolutely key is to use the right dose rate of glyphosate, tailored to the growth stage of the weed.
Glyphosate is a vital part of arable weed control, particularly if you have herbicide resistant blackgrass on the farm. But to work effectively the weed should be actively growing, so a mild autumn is ideal.
The best time to apply glyphosate is when blackgrass has two or three leaves, at a minimum dose rate of 540g of active ingredient per ha.
It’s vital not to use a lower dose rate than this, as leaving any survivors can increase the risk of resistance developing.
Larger weeds will require a higher dose rate – follow label guidance for specific products, target weeds and crops.
Use a medium-coarse spray (200-400 microns) – conventional flat fan nozzles (such as F110-03 or F110-04) are most suitable for seedlings while low drift nozzles can be used on well-tillered plants.
Water hardness and pH will affect the efficacy of glyphosate, so consider adding a water conditioner if you’re in a hard water area.
Adjuvants can also be used to boost the effectiveness of the product, providing they are recommended on the glyphosate label.
Ideal application conditions include an air temperature of 15-25C, and at least six hours before rainfall, as rain will wash off and dilute the mixture.
Avoid spraying before nightfall due to the colder temperatures and longer drying time – and don’t spray in windy conditions as drift onto hedgerows and field margins will kill off beneficial species and open gaps for weeds to flourish.
Any drift will also be at a low concentration, potentially raising the risk of resistance, especially in meadow and sterile brome.
Never spray after drilling as it may kill off the emerging crop.
Several key broad-leaved such as clover, annual nettle, coltsfoot, rosebay willow herb, horse tail and some cover crop species can sometimes be poorly controlled by glyphosate alone.
Glyphosate is available in mixture with 2,4-D and control of these weeds is improved over glyphosate alone.
How to minimise resistance
Blackgrass resistance to herbicides has increased rapidly and by 2013 over 20,000 farms in 35 counties of England had recorded resistance.
Recent changes in legislation have led to fewer herbicides being available to growers and there is no new chemistry in the pipeline, so farmers have to rely on older products.
There is no known resistance to glyphosate in the UK: If we lose that efficacy, or access to it, there will be nowhere else to turn.
The glyphosate resistance project – which is led by ADAS and is just completing two years of its five-year stint – aims to fill in knowledge gaps about the use of glyphosate, and how best to manage it to maximise efficacy and reduce the risk of resistant weeds.
So far, there is no known resistance to glyphosate in the UK, but two years ago the first case of resistance in Italian ryegrass in an arable crop was discovered in Italy.
That was created by using repeated low doses of glyphosate, so it’s vital that we learn from that and act to reduce the risk of resistance developing in this country.
Any weeds that are sprayed with a herbicide and survive have the potential to develop resistance, so aim for complete kill every time, whichever product you are using.
Avoid over-reliance on a single active ingredient and monitor herbicide efficacy after spraying to detect any loss of control.
If resistance is suspected, act quickly to get confirmation via ADAS, your agronomist or chemical supplier.
Consider non-chemical control like ploughing, hand rogueing, mowing and mechanical weeding as a supplement or alternative to herbicide treatment.
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