1. What is the best rotation?
Variation of cropping and flexibility are essential aspects of rotation planning for black-grass management, as there is no one solution to suit all situations.
This may well require a move away from a “block cropping” approach with problem areas treated separately according to weed pressure and site characteristics. Varying cropping allows for different levels of crop competition to be introduced, as well as alternative drilling dates and chemistry.
Spring cropping should play a key role in tackling black-grass, with research suggesting that hybrid barley (see question 6) is particularly effective given its competitiveness and the range of chemistry available, says Mr Lloyd.
In contrast, winter oilseed rape appears less suited to high black-grass pressure situations, as weeds can often be hidden within the crop canopy, he notes.
When making any cropping decisions, experts agree it is vital to consider the financial impact in terms of cost of production and gross margin across the rotation alongside impact on black-grass control.
“Reducing black-grass population is important, but crops must be profitable at the same time,” says Mr Lloyd.
2. Does full-year fallow play a role and how do you see the rotation gross margin?
Fallow is relatively ineffective at managing black-grass, especially under high-pressure situations, according to Mr Hull.
While there will be a natural 70% a year degradation of the seedbank, 30% will remain and with no crop or chemistry to reduce populations, it can create “a real mess” in following crops when there is a high starting population, he says.
There may be an option for temporary short-term fallow before sowing a competitive crop in the spring, he says. This approach is used by Mr Drinkwater as part of the Higher Level Stewardship overwintered stubbles option, but he says “it’s not a winner for black-grass control”.
3. At what point should we top/ burn-off crops?
Ideally the decision as to when to spray-off crops heavily infested with black-grass should be made in the winter, before significant amounts of money are invested on spring fertilisers and fungicides, says Mr Cotton. This can also allow time to establish another crop in the spring.
It can be harder to identify patches of black-grass at this early stage of growth, but Mr Drinkwater says it may be possible to use digital maps generated from later in previous seasons to help inform management decisions. The worst black-grass patches are often in the same areas each season.
Mr Anderson-Taylor favours a “zero tolerance” approach to black-grass, suggesting there is no minimum population threshold that determines when to spray-off crops. However, he recognises the conflict between what’s best for reducing black-grass and the need to generate an income and says decisions must be made on a case-by-case basis.
Mr Bradshaw believes this may mean growers have to accept and “live with” some level of black-grass population.
4. Is it time to revert to traditional mixed farming (with grass in the rotation) for proper cultural control?
Including a temporary three-year (minimum) grass ley in the rotation can significantly reduce black-grass, says Mr Drinkwater, although he acknowledges it is not an option for all farming systems and is reliant on local opportunities to make leys stack up financially.
There may be scope to use agri-environment scheme options to introduce leys for black-grass control, notes Mr Lloyd. Renting grass for livestock grazing or to supply silage for AD are other options to consider, but growers need to be cautious about importing weed seeds if taking manure or digestate in return.
Mr Cotton advises growers it can be hard to establish ryegrass in autumn under very high black-grass pressure and in such cases he suggests delaying ley establishment until the spring, controlling black-grass before sowing.
“If black-grass emerges in the ley cut it for silage rather than hay to avoid it setting seed,” he adds.
He also warns that black-grass in leys grazed by livestock has been seen to set seed heads on very short plants, further highlighting the ability of the weed to adapt to different situations and selection pressures.
5. Does hybrid barley have a role to play in reducing black-grass numbers?
Yes, hybrid barley can be an effective option in high black-grass situations, providing the right variety is grown correctly to maximise benefits from hybrid vigour, says Mr Lloyd.
“It’s surprising to see how effective hybrid barley is at getting ahead of black-grass. You still see some black-grass emerge within the crop, but numbers are radically less, with smaller ears, weaker plants and reduced seed viability.”
It is the competitive ability of hybrids in the spring that gives them the edge, notes Mr Hull. Early nutrition is therefore crucial to get crops going and maximise any benefits.
Six-row hybrids have proved more effective than two-row varieties, and higher seed rates – typically 200/m2 – should be used to help crops outcompete black-grass, he says. Sowing crops around the end of September to first 10 days in October appears to be optimum, he adds.
Mr Drinkwater favours a higher seed rate of 250-300/m2 to maximise black-grass competition and produce a commercial crop.
Black-grass can still emerge within hybrid barley sown during autumn, so it may not be the best option where there is very high seed return, says Mr Hull.
6. Do cover crops really have a useful role to play in black-grass control and if so, what is the best strategy? Is it cost-effective?
If the right cover crop species can be drilled in good time to put on enough canopy cover before winter then Mr Lloyd believes they can have a “useful effect” on black-grass control, especially when compared to leaving land fallow before drilling.
Cover crops can help dry land out and aid spring drilling, especially on heavy land.
However, sowing the wrong cover species at the wrong time or achieving poor establishment can let black-grass in and potentially exacerbate problems, he warns.
“Cover crop management is a massive issue and there’s still a lot to learn.”
Mr Hull urges caution when considering trials that show a reduction in black-grass when cover crops are used ahead of spring cropping, as it may be the spring crop element that is having the greatest effect rather than the cover crop.
“Cover crops are very good for certain things, such as adding organic matter, drying-out soils, or improving structure, but we don’t yet know whether we’re gaining much in high black-grass situations.”