Unless you are one of a growing number of no-tillers or you plough every year, it is difficult to be prescriptive when it comes to post-harvest cultivation strategy.
There is residue management to consider, the potential requirement of remedial work on soil structure, as well as trying to get the perfect seed-bed prepared for the next crop.
However, with many farms grappling with problem weeds – particularly grassweeds – correct identification and understanding of the biology of each species is vital in order to adapt stubble management strategy to each situation.
That's the view of Hutchinsons Technical Manager Dick Neale, who says growers are often too dogmatic in what they do once the combine has left the field, often to the detriment of weed control.
He gives one example of a farm he visited recently, which was experiencing high levels of meadow brome across 120ha of its winter barley area.
When cultivation strategy was discussed, it turned out that the farm always cultivates everything within 48 hours of a crop being cut, with no consideration of the potential agronomic consequences.
As many meadow brome seeds are thrown out of the back of the combine at an immature stage, it requires about one month's exposure to sunlight and diurnal temperature fluctuations to finish maturation, break dormancy and germinate. It can then be sprayed off ahead of drilling.
As the grower was burying the meadow brome seed in a dormant state, it is likely that it retained that dormancy until it was brought back up to the surface, where it germinated in the crop.
"Some growers will continue doing the same thing because that's the way it has always been done – and if you don't have any grassweed problems, that's fine, as you can pretty much do what you like," says Mr Neale.
"But where there are grassweed problems, they need to be identified correctly and each species' biology understood to get stubble management right."
There has been a trend towards earlier post-harvest cultivation over recent years, with the sole purpose of encouraging germination of freshly shed weed seeds, but there is little evidence to show this is the right approach in all situations.
In fact, a research review commissioned by the AHDB and carried out by Sarah Cook at Adas suggests that it might be far wiser to leave seeds of most weed species, including black-grass, on the soil surface.
However, former Niab agronomist and weed specialist Jim Orson says the situation is more nuanced and for many (but not all) weed species, it is a question of providing the moisture conditions to encourage seed germination.
So, where the soil is bone dry after harvest and there is no substantive rain forecast, seed on the soil surface (possibly under a straw cover) may experience more moisture from dews and any light showers than if the soil is lightly cultivated.
This means not cultivating at all provides better conditions for germination.
"On the other hand, where there are moist soil conditions directly after harvest, the weed seed will experience better conditions for germination after a shallow cultivation," says Mr Orson.
"It is not only common sense, for black-grass, this conclusion is supported by field trials," he says.
However, he points out that some weeds must be retained on the soil surface in all weathers, including meadow, soft and rye brome, which should be left on the soil surface for a month.
In addition, wild oats should be left on the soil surface for as long as possible in the autumn.
Where high numbers of sterile or great brome seeds are on the soil surface, ploughing to a depth of 15-20cm will cause suicidal germination, while shallow surface cultivation will encourage a chit and the opportunity to spray off with glyphosate.
Good straw cover, where it is being chopped and spread, may also provide adequate darkness to trigger seedling growth.
Cereal volunteers – provided there is adequate moisture in the soil – should also be shallow-cultivated into the soil to encourage a flush.
Conversely, oilseed rape seeds should be left on the surface for four to six weeks, or growers will risk creating future problems.
Incorporating freshly shed oilseed rape seeds into the soil straight after harvest makes them dormant, resulting in greater volunteer emergence in future seasons.
It is clear that there is some conflicting advice in the area of post-harvest stubble management and the industry needs comprehensive, multi-year trials to identify the best approach in a range of situations.
However, independent grassweed expert Stephen Moss believes flexibility is the key, as conditions vary so much from field to field and from year to year.
He agrees that leaving the soil undisturbed for a period of time is best when soil conditions are dry, encouraging surface germination and depletion of weed seeds.
However, there is a better case for cultivating early when soil moisture is present, as this can trigger germination of freshly shed seed on the soil surface and help chit older seed, thus depleting the seed bank.
"Also, the notion of 'as little tillage as possible' to avoid encouraging black-grass germination is sometimes oversold," says Dr Moss.
It's certainly a valid concept, but not at the expense of the overriding priority, which he says should be seed-bed quality, both to maximise crop emergence and pre-emergence herbicide efficacy.
"Growers are increasingly dependent on residual herbicides for grassweed control, but getting the seed-bed right is something you have at least some control over."
When and how to cultivate?
So, what is the practical advice on the best time to cultivate – and how to do it?
Clearly, much will depend on the crop being established, with oilseed rape and winter barley fitting into an earlier drilling window and leaving much less time for stale seed-bed preparation.
But leading into winter wheat, where delayed drilling is a key tactic in tackling grassweeds, Mr Neale sees little point in cultivating until mid-September, when the main blackgrass germination period kicks in. Other species such as ryegrass have similar germination periods.
When planning cultivations, he says, a minimum of two weeks should be allowed between cultivation passes and between cultivator and drill to allow a fresh weed flush for destruction.
For example, if aiming for a 15 October drill date, counting back two weeks will determine a final cultivation timing and a further two weeks back for the first pass.
Glyphosate can be applied ahead of the second cultivation, if necessary, with a final application going on ahead of the drill. Mr Neale also advises growers to work shallow when working in a relatively tight window.
“If you work to depth, you bring up raw, unweathered soil – and for that, you have to build in more weathering time. If you stay shallow – about 4-5cm – you don't need the weathering to produce the final seed-bed," he explains.
Cultivator setup is also an important consideration, with the first pass needing to be well-consolidated to achieve weed seed-to-soil contact, but not too fine, allowing the soil to maintain its aggregate stability.
"If you work the soil too fine too soon, even on the second pass, it will become sticky, unstable and prone to capping if it becomes wet and it just won't dry out before drilling," says Mr Neale.
"Using a very light movement with something like a stubble rake might be more appropriate to avoid overworking," he adds.
Key Points for Post-Harvest Stubble Management
- Evidence suggests no significant reduction of most annual weeds by early stubble tillage in dry conditions, so consider leaving soil uncultivated to allow predation and germination of fresh seed
- In damper conditions, cultivation after harvest can increase germination of freshly shed seeds where they are moved into moisture and older seeds exposed to light
- Volunteer cereals can be cultivated immediately after harvest, particularly if moisture is present
- Oilseed rape seed losses at harvest should be left on the soil surface for 4-6 weeks to avoid seeds becoming dormant
- Always prioritise seed-bed quality over other factors to ensure rapid crop establishment and good pre-emergence herbicide efficacy