Weed control options and future opportunities for UK crops (research review)

In the UK, growers rely almost entirely on synthetic herbicides to control weeds cost effectively. However, the use of these products is coming under increasing pressure from legislation, climate change and market requirements, such as reduced pesticide inputs and maximum residue levels. This, combined with herbicide resistance, is having a significant impact on arable and horticultural sectors.

This report is a comprehensive literature review of weed control options, on a national and international level, that could benefit UK crop production in horticultural crops, cereals and oilseeds, sugar beet, potatoes, grassland, legumes and maize.

The techniques available for weed control are reviewed in Section 3. The efficacy of these techniques in different crops is then discussed in Section 4. Finally, Section 5 highlights the weaknesses in the biology of key weed species, as these can then be exploited for weed control.

For effective weed control, a knowledge of the weed life cycle is essential. The life cycle is simply the seasonal pattern of growth and reproduction. For the purpose of this review, the life cycle has been split into five sections. Each weed control technique, described in the review, will be effective in controlling weeds at one or more sections of the life cycle. Effective weed control, generally, involves the use of more than one method. This is the heart of integrated weed (pest) management (IWM/IPM).

1.    Prevent seed return

Those weeds that are most difficult to control produce high levels of seed and can establish large viable seedbanks in just one season. Seed heads are often above the crop canopy and seed can be removed/spread from/within the field at harvesting. In other cases, for instance short-term horticultural crops, weeds do not even get a chance to complete their life cycle before the crop is harvested (which can benefit later crops in a rotation).

2.    Deplete seedbank

Soil contains any weed seeds from previous years seeding and this is known as the ‘seedbank’. The number of seeds in the seedbank increases as weeds set and shed seeds, some buried seed will become dormant and survive for many years. Seed numbers decrease over time, as some germinate, some decay and some are eaten by wildlife. Understanding the seedbank is at the heart of effective weed management. Cultivations stir up the seedbank, burying freshly shed seed and bringing seed, from lower down the profile, to the surface. Weeds emerge each year, generally only from the top 5 cm of the soil.

3.    Kill weed seedlings

Weeds emerge at different times during the year and interact with the crop. Most problems occur when weeds and crops emerge at the same time. Knowing when a weed germinates can help determine the most appropriate control methods. Cultivation strategies can be optimised to reduce weed numbers.

As weeds grow they compete with the crop. The damage they cause depends on: the species, density, the competitive ability of the crop, and the growth stage when crop and weeds compete. While some weeds are highly competitive, others pose little threat and may be valuable to wildlife.

4.    Stop seed set

Although by this stage, weeds may have competed with the current crop, there is now a great opportunity to reduce weed seed production, which in turn reduces the weed seedbank for future years. This is most applicable to weeds that are difficult to control (e.g. weed beet in sugar beet and weeds resistant to herbicides) and when weed densities are low. It is often overlooked how important this step can be to stop an early stage infestation becoming a larger problem.

5.    On-farm hygiene

Preventing weed seeds arriving on farm or being moved from an infested field to a clean field is key in the battle to control weeds. This will include preventing ingress of weed seeds from non- cropped areas. This is particularly important for windblown species into perennial crops.

The review is comprehensive and the specific key findings were:

• Herbicides are the most common weed control
• Herbicide use is generally reactive (when weeds are visible)
• Proactive use of herbicides is increasing (increased use of pre-emergence products), particularly where difficult-to-control weeds are This is predominately in arable crops
• Other weed control approaches need to be integrated with an in-depth knowledge of the weed biology to achieve desired outcomes
• Vast amount of information is available on weed biology and control but this is not always accessible, particularly to growers

The way forward for weed control has to be Integrated Weed Management (IWM). IWM is the use of multiple weed control methods to sustainably manage a weed problem. It is a component of integrated pest management (IPM). It can include cultural, genetic, mechanical and biological weed control, in conjunction with the use of herbicides. The aim of IWM is to diversify weed management strategies to reduce the reliance on herbicides, and promote the use of site-specific weed management and target applications to reduce herbicide impacts, where possible.

However, there is a general lack of uptake of IWM to date. IWM is knowledge intensive. One of the main barriers to its uptake is that there is often little visible evidence of immediate success and little idea of the return in investment of time and money. Further reasons include the fact that herbicides are convenient, less complex and are, generally, cheaper and take less time to apply. It appears that non-chemical practices are often only adopted to compensate for reduced herbicide efficiency, which could be when herbicide resistance is already present in weed populations on the farm.

The overall recommendations and priorities resulting from the review findings are to:

Increase access to and use of current knowledge

Many growers are probably unaware of all the weed management information that is available to them. Knowledge is also often kept within the individual sectors.There is much relevant work on weeds that was once funded by MAFF or Defra and it is often hard to find, with only the current researchers knowing of its existence and so it will be lost when they leave the industry. Peer- reviewed information is also unavailable to many potential beneficiaries. Consequently, making better use of existing knowledge is a very high priority. Enabling greater access to it should be a high priority. Eroding barriers between different cropping sectors, through putting the weed biology at the centre of the knowledge, will enable good progress in all sectors. Decision support tools that incorporate up-to-date information on weed management could also be developed. A targeted, central location for weed control, which covers all crop sectors, should also be developed.

Link practical knowledge better with fundamental research

As in many other science disciplines, there is too great a gap between those who undertake fundamental research and those who look to apply their findings in practice. There is huge scope to derive more benefits from research. To do so needs more involvement of those with an in-depth and practical understanding of weed management in the setting of project objectives. A good example would be to better focus research on those areas where gaps in the understanding of weed biology are hindering the development of better control options.

Maximise herbicide availability

The availability of herbicides continues to decline. Further actives will be withdrawn and there are unlikely to be many new herbicides to replace them. Good stewardship of current active substances is vital and requires companies, regulators and users to work together to retain them, through continued support and prevention of bad practice.

Retaining product efficacy, by minimising resistance and ensuring good practice, is something over which agronomists and growers have considerable ‘control’. Much is known about the risks of weeds developing resistance to herbicides. Pro-active identification of the high-risk uses/situations, which could select for resistance, should be a priority. Weed management strategies for these high-risk situations should be agreed and communicated widely. Monitoring of weed species shifts and emerging cases of herbicide resistance, in relation to herbicide use and other integrated weed management strategies, is needed.

Agree funding for Integrated Weed Management (IWM)

Both growers and politicians recognise the need to maximise non-chemical control of weeds and develop integrated weed management. However, research in these areas, typically, does not attract commercial funding. To ensure future development of sustainable weed management solutions, collective funding from farmers/growers and/or those promoting non-chemical approaches is required. The availability of suitable funding mechanisms, to drive what are often too costly and less effective options, is not an industry priority. However, if government and industry can work together it will be possible to make more progress than is currently the case.

Weed research and approaches to control need to be considered more strategically

Reviewing and compiling information for this review has highlighted how the current approach to weed control is very often based on the use of herbicides against specific weeds and/or in specific crops. It is very clear, however, that, as with nutrient and soil management, there is considerable scope for a more strategic approach that is relevant to the whole cropping system, which can then be deployed in specific crops. A key recommendation is that there should be a more strategic approach to weed research and control.

Putting weed biology/weed life cycles at the heart of control strategies will enable more rapid progress across multiple crops. Interventions need to target and exploit the weakest stage of the weed life cycle, while maximising the tolerance of the current and future crops. A cross-sector, multi-annual approach is therefore vital.

Understand selectivity between crops and weeds

All technologies require a differential selectivity between the crop and the weed. Development of appropriate techniques will build on those principles. Selectivity can be achieved by a number of routes:

• Spatial selectivity is a major opportunity for chemical and non-chemical approaches and irrespective of the crop we need to be able to identify one from the other. The wider the row spacing, the greater the opportunities. This could be optical and ground or satellite Additionally, alternative ways of highlighting where the crop is (‘plant marking’) should be considered, such as by seed treatments or genetic. There are now much better systems to detect and locate weeds within fields and that is already very helpful. Agreeing criteria and operating speeds is a key need to enable wider deployment of all technologies.
• Temporal selectivity enables treatments to be made when crops are more tolerant or weeds more Just as pre-emergence herbicides are widely used, such approaches should be considered for non-chemical approaches.
• Crop and weed tolerance is critical for herbicides, but also for non-chemical Information on what it takes to kill a weed and what it takes not to kill a crop will be vital considerations in enabling current and new non-chemical approaches, but also in prioritising herbicide options. The screening of herbicides for minor crops could be advanced, and cost minimised, through a more strategic approach that considers weed and crop tolerance independently and enables a more focused approach to deliver quicker results. In parallel, the regulatory issues of using herbicides on a wider range of crops will need to be addressed and requires a combined grower, regulator and retailer approach.