Course: Grassweed management in cereals | Last Updates: 7th October 2015
Given fewer selective herbicides and increased grassweed vigour, cultivation is set to become an increasingly valuable method for controlling weed populations. However, methods must be matched to specific grassweed threats for best results.
What effect do cultivations have?
Soils contain reserves of weed seeds driven by many factors including cropping, rotations, soil type, and climate. These reserves are modified by cultivations in several ways according to their type and timing.
Deeper cultivation means deeper seed banks, and weed seeds kept dormant at depth can aid control.
Shallow work and soil firming for good seed/soil contact can trigger germination in most species, provided they are not dormant. But the impact of subsequent cultivations must always be borne in mind.
Depending on cultivation type, weed seeds can be buried totally (by good ploughing), mixed throughout the profile (by poor ploughing, ploughing with trash boards, tine and/or disc cultivation), or left close to the surface (by minimal soil disturbance with shallow low-lift tines, shallow sweeps, or surface scratching/rolling).
Each cultivation determines the fate of the seed bank of seeds shed this season, plus any previously incorporated, but returned to the surface or to a depth where, given enough vigour and moisture, they can germinate.
Many herbicides' efficacy and weed germination is affected by the state of the soil surface, the ideal in most cases being a fine, level and firm finish.
Unfortunately, this may not be what is best for weather-proofing, so a balance is required. However, consolidation of a loosened profile is vital to create the best combination of water passage and moisture retention.
What influence does dormancy have?
Dormant seeds are those which remain viable although metabolically inactive. Most weed seeds remain dormant below depths that differ between species (see table).
Some grasses, notably blackgrass, also suffer from delayed germination that depends on the weather at the time of seed maturation. If conditions during mid-June to mid-July are warm and dry, blackgrass will exhibit a low dormancy (seeds germinate rapidly when moisture allows), which is good news for control.
But if it is cold and wet, germination can be delayed by six to eight weeks, even if you've created ideal conditions for a weed flush in the intercrop period.
This makes it more difficult to remove the weed seed bank through cultural methods and puts greater dependency on selective herbicides.
What are stale seed-beds?
Stale seed-beds provide the means to germinate volunteers and weed seeds (provided they want to germinate) before the next crop is established. This allows those that have grown sufficiently to be controlled by non-selective herbicides relatively cheaply.
This normally reduces the burden and pressure on selective herbicides applied later.
The type of seed-bed required depends on many factors. Early stale seed-beds will have only limited impact when tackling blackgrass in high dormancy years.
Key points to consider:
Straw chopping and spreading is effectively the first cultivation. If inconsistent, it leads to variable weed (and crop) emergence and so undermines the effectiveness of stale seed-beds.
Should the work aim to be as shallow as possible, deep or full inversion? To some extent this is also driven by soil structure problems that need addressing.
Fine and well consolidated is the theoretical ideal. But for weatherproofing, a corrugated pressed profile is better than a flat one for water movement, and it can ease a return to the land after rain.
The passage of the cultivator can encourage the germination, depending on dormancy. So timing is best chosen according to the weeds present, their potential dormancy, the following crop requirements (early or late drilling) and the general workload and time available on the farm as a whole.
What should you aim to achieve?
A shallow stale seed-bed leaving a fine surface tilth with evenly spread residues maximises the effects of residual herbicides and the broad spectrum types applied to remove weeds and volunteers before drilling.
Deeper cultivations to remove compaction may be carried out at the same time but should not leave cloddy, uneven surfaces.
Depending on requirements, one aim could be to combine the use of minimal cultivations with subsoiling plus occasional, rotational ploughing to completely invert the soil and bury the surface seed bank.
When low dormancy weed seeds and volunteers proliferate, creating a good stale seed-bed 50-75mm deep, consolidated and corrugated over a well structured sub-surface will ideally prepare for spraying off with pre-drilling glyphosate.
High soil disturbance may be needed to deplete herbicide residues from the previous crop, for example after oilseed rape. So the machinery pool should be flexible enough to achieve this – it may simply mean having easily changed tine points and depths.
How do you get results from zero and min-till?
Zero-till minimises soil disturbance, but depends on excellent soil structure to work properly, contrary to what is often the case in practice. The technique also needs good sub-surface drainage to permit easy root growth.
Provided surface tilth is not lost during drilling this approach can be successful. Where the surface is poor because of ruts, compaction, loss of soil structure, or heavy trash, zero-till results will also be poor.
Minimum tillage, provided it does not disturb the surface when correcting a deep problem, can complement ploughing and zero-tilling where appropriate.
Whatever the cultivation approach, a well-chosen cropping sequence can assist herbicides in controlling persistent grassweeds.
Choice of technique depends on knowledge of the soil, its condition, the type of weeds, prevailing weather and the crop in hand.
Having equipment flexible enough to be adapted to specific requirements is the key. This should not mean daily fiddling with different options, more simply adjusting the machine to the general conditions at the time for the upcoming crop, taking into account the most significant factors outlined above. The ability to combine rolling or pressing with cultivation when appropriate can also help.
- Assess the main weed threat
- Try to understand specific weed biology
- Make best use of stale seed-beds
- Take care when ploughing to invert and bury seeds properly
- Leave straw mats in the field
- Be prescriptive on cultivations
- Shun potentially costly rotational ploughing if it can help
- Rely on stale seed-beds to germinate highly dormant weed seeds
Eliminate straw mats blocking light and ensure good seed to soil contact to encourage chit. Conditions at seed maturation also influence germination. Plough down high populations
Meadow, rye or soft brome
Expose to light and warmth to encourage chit
Shallow cultivation or a good covering of chopped straw encourages chit. High populations should be ploughed down deep
Cultivate immediately after harvest to encourage chit. Peak germination usually occurs two to three weeks after soil surface becomes thoroughly moist
Good ploughing provides the best means of grassweed control.
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