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He who has bread may have many troubles; he who lacks it has only one.

(Byzantine proverb)

A peasant must stand for a long time on a hillside with his mouth open before a roast duck flies in.

(Chinese proverb)

The origin of the word herbicide is a combination of the Latin words herba (noun, herbaceous plant) and caedere (verb, to kill; R.L. Zimdhal, Weed Science17, 137–139, (1969).

It is now 30 years since the first edition of this book was published and so it is timely to reflect on herbicide use and the environmental consequences of their use. While we now have fewer products in our agrochemical armoury, farmers and growers have become dependent on fewer active ingredients, and glyphosate dominates the global market. This third edition aims to update the reader on how herbicides contribute to modern agriculture, how they are discovered and developed, and how they interact with plant growth and development and the environment. Since the publication of the second edition in 2010 there have been many advances in our understanding of plant physiology, especially regarding how plants function in tune with their ever‐changing environment and how post‐translational modifications provide regulatory control of most plant processes. Modern agriculture, however, still faces many challenges.

1 The global food challenge. To some, the continuing expansion of the human population will inevitably outstrip the growth of our food supply, resulting in global starvation. To others, an expansion of arable land, a growing global food trade and the increases in crop yield predict a more optimistic future. Yet is it inevitable that a growing food supply will continue to meet demand?

2 The problem. More than half the global population will suffer some form of malnutrition by 2030 unless urgent action is taken to increase access to food of high nutritional quality. The Food and Agriculture Organization of the United Nations (FAO et al., 2017) estimated that 815 million persons were hungry in 2016 (11% of the global population), an increase of 35 million since 2015. While 155 million children have stunted growth owing to poor nutrition, 2 billion persons suffer from hunger, while 1.9 billion adults and 41 million children are either overweight or obese. In addition, the human population is expected to grow by about 80 million per annum to an estimated 10 billion in 2050 (Oerke and Dehne, 2004). Furthermore, the global impact of the current covid‐19 pandemic could result in at least a further 200 million undernourished persons. How can we expect to produce 70% more food to feed them all?

3 Food security. Food security is an increasing global problem in the face of climate change, combined with increasing populations and volatile food prices. With the global population growing at 230,000 persons each day and 60% of us now living in cities, the pressure on farmers to increase crop yields is ever present. At the same time, in the UK as an example, the land available per head of population has decreased from 0.8 to 0.2 hectares in the last 50 years. Every year 12 million hectares is degraded globally owing to drought, de‐forestation and desertification, an area roughly the size of Nicaragua, the largest country in Central America. Furthermore, global freshwater supply is becoming increasingly limited and unreliable to an estimated 700,000 persons, notwithstanding the fluctuations in weather as a result of climate change. We are experiencing greater extremes of weather, such as flooding or drought, and so we need to use the available fresh‐ and artesian‐water more wisely. This is especially so whether we grow crops or raise animals. For example, it is estimated that 70 litres of water are needed to produce one apple, whereas 15,000 litres are needed for one kg of beefsteak! It is interesting to note that the carbon footprint of beef and lamb is three times that of pork, five times that of chicken, over 30 times that of bread wheat and 50 times that of potatoes. Urbanisation and increasing incomes generate a higher demand for animal protein, yet beef production requires four times more land than dairy, per unit of protein consumed. In addition, beef is seven times more resource intensive than pork and poultry, and 20 times more so than pulses. Not forgetting that animal production results in increased greenhouse gas emissions. It is a further uncomfortable fact that about a third of all food produced never reaches the table. This value is higher for fruit and vegetables, and such losses are even higher in the developing world, owing to the lack of effective storage and/or transport (IFPRI, 2016).As available land for farming is in ever shorter supply and extremes in climate become more evident, many scientists predict an increased degradation of soils and a need for increased attention to land management. In recent years in the UK, for example, farmers have seen above average rainfall with increasing soil erosion, degradation and run‐off. Palmer and Smith (2013) noted that 75% of fields planted with maize or potatoes in the south‐west of England were severely damaged by soil degradation, with one in five sites experiencing serious rill and gully erosion. Some 60% of fields growing winter cereal crops, such as wheat and barley also displayed high to severe soil degradation. Techniques to avoid soil compaction, such as topsoil lifting or sub‐soiling, are options to loosen soil layers, but the use of increasingly large and heavy machinery increases the risk. These authors concluded that soils with good agricultural properties are over‐exploited in crop production and, as a result, can become highly degraded. Conversely, chalk and limestone soils degrade less.A further definitive study, by Challinor et al. (2016), has predicted that gradually rising temperatures in Africa, and more droughts and heatwaves caused by climate change, will have a profound impact on maize yields. Higher temperatures reduce the length of time between planting and harvesting, which results in less time to accumulate biomass and yield. They also predict similar shortening of time to yield for maize crops across the tropics and suggest that maize breeding systems must adapt to increasing temperatures to ensure positive yields in the decades ahead. A further interesting area of research would be to investigate how the major weeds of maize crops may also adapt to increasing growth temperatures, with especial attention to weeds exhibiting C4 photosynthesis.

4 Greater intensification? In a comprehensive and thought‐provoking study, Fischer et al. (2014) concluded that greater crop yields are possible through a greater intensification of agriculture, especially in Sub‐Saharan Africa. This assumes more agricultural research, development and training in the developing world and a more efficient use of inputs, such as plant protection products. What is certain is that global governments will need to invest substantially in agriculture to achieve the yield increases necessary to feed the world. Greater meat consumption and an expanding human population implies that crop productivity needs to double by 2050.

5 Sustainability. One widely used definition of sustainability is ‘meeting the needs of the present without compromising the ability of future generations to meet their own needs’ (DEFRA, UK). Another considers that ‘a sustainable agriculture is ecologically sound, economically viable and socially just and humane’ (Alliance for Sustainability, www.afors.org). How can humankind tackle climate change, reduce population growth, cut out waste, educate consumers in the developed world to eat less, conserve freshwater and eat more plant products and fewer animals? Political awareness, more education and an alert and informed media may provide the answer. Indeed, Ehrlich and Harte (2015) ‘urge policymakers around the world to move the issue of food security to the top of the political agenda’. And they conclude that ‘anything less is a recipe for disaster’.

6 Can organic agriculture feed the world? Regarding crop production, organic practices infer that the use of inorganic crop nutrition products is not allowed, genetically modified crop cultivars are not permitted and the use of chemical plant protection products for the control of pests, weeds and diseases is forbidden. Such practices are becoming widely accepted in the developed world, as they are seen as a more ‘natural’ means of crop production. Those that espouse organic farming practices often say that it is better for the environment, since it requires fewer inputs, but it is generally agreed that organic farming is less productive per hectare than conventional, intensive agriculture. More land would be needed for equivalent yields, and conversion to organic practice would release more organic carbon into the environment. Furthermore, it is doubtful that legume cover crops could replace the nitrogen fertiliser needed to give higher yields (Connor, 2008). Perhaps in the future, nitrogen fixation from the atmosphere will be possible using genetically engineered crop plants?The EU Farm to Fork Strategy, launched on 20 May 2020, has ‘aspirational targets’ for 2030 for a 50% cut in pesticide use and a commitment to dedicating 25% of agricultural land to organic farming. One wonders if the consequences of reduced yields and higher prices, to name but two, have been thought through. Furthermore, the aspiration to replace pesticides with ‘biocontrol agents’ appears idealistic. Although regularly promoted, an agreed definition of biocontrol remains elusive and, at the present time, commercial agents are expensive, are unproven in the field and no viable weed biocontrol methodology exists. At a time of imminent global recession owing to the covid‐19 pandemic, a global scarcity of food is predicted, made worse by plagues of locusts in Africa, Arabia, Iran and Pakistan adding to pressures on food security. Can food producers realistically promote organic agriculture and cut pesticide use in such uncertain times? Instead, can we use our existing practices more effectively?

7 Plants for the future: genetic diversity. In 2016, the Royal Botanic Gardens, Kew, UK, released a report on the state of the world’s plants. It noted that an estimated 31,000 plant species have a documented use for medicines, food and materials. There are an estimated 391,000 vascular plants known to science of which 369,000 species are flowering plants. A further 2000 new vascular species are described each year. Many are wild relatives of known crops that can be a source of genetic variation to improve our crops in the future, such as tolerance to drought or possess a unique metabolism. Some 21% of the world’s plants, however, are currently threatened with extinction, especially in declining rainforests. Humankind must preserve this genetic biodiversity in seed banks at all costs for future generations (www.stateoftheworldsplants.org). The human population derives 50% of its calorific intake from only three species, namely rice, wheat and maize. The rest is derived from only 20 species. Having so few staple crops means that we lack diversity in our diets and have an over‐reliance on the chosen few. Can conventional plant breeding produce the advances in yield needed to feed our global population? Perhaps it can, with the application and more widespread adoption of gene editing techniques developed in the last decade.

8 Can we survive without plant protection products? Currently, global agriculture is heavily reliant on plant protection products, such as herbicides, plant growth regulators, fungicides and insecticides, to maximise crop yields. Without an equivalent process, yields would be reduced by at least 20–40%, so an increase in food prices would inevitably follow, with public unrest and food volatility. The reader is encouraged to note Oerke and Dehner (2004) and Pesticides in Perspective (n.d.) for further details.I note with concern that the EU is planning to withdraw as many as 75 active ingredients from the crop protection armoury. In addition to yield losses, this will erode farmers’ margins and reduce farm productivity across the EU, and over a million jobs are at risk of being lost. There are no current viable alternatives to the use of agrochemicals. Their judicious use should be promoted and these agents preserved if we are to feed the world and ensure future food security. In order for plant protection products to be used effectively, it is imperative to have an understanding of the biology of the target organisms and how an active ingredient works in both the plant and the environment. Thus, an understanding of weed biology, soil science and plant physiology underpins herbicide choice, use and effectiveness.

9 Is science and technology the answer? Scientists and technologists consider that appropriate scientific and technological developments might come to the rescue of humankind. Why such optimism? The answer lies in recent research findings reported in the plant sciences literature, some examples of which are noted below. The first reports a rice cultivar that has been engineered to have fewer stomata, which has resulted in an increased tolerance to drought and water availability, giving equivalent or increased rice yields. The importance of this finding is the knowledge that 2500 litres of water are typically required to produce 1 kg of rice. The authors consider that rice plants with fewer stomata should perform better when limitations on water supply threaten food security (Caine et al., 2018).A second innovation is the use of gene editing to understand how plants are able to perceive and respond to environmental signals at the cellular level and respond by alterations in gene expression. In this way, plant scientists are able to understand how biotic and abiotic stimuli, such as responses to disease or environmental change, can alter growth, development and crop yield. This advance is largely due to the generation and testing of mutants that can be incorporated into plant breeding programmes. Examples include resistance to drought, resistance to salinity, temperature and water‐logging, insect and disease resistance, potatoes free from late blight, enhanced concentrations of omega oils and vitamins, fruit and vegetables that do not turn brown on impact, and low‐gluten wheat, to name but a few.A third example is the RIPE project – Realising Increased Photosynthetic Efficiency –for sustainable increases in crop yield (www.ripe.illinois.edu). This is a collaboration of US, Australian, Chinese, German and UK universities that began in 2012 with an aim of increasing global agricultural production. Several research strategies have been developed with successful investigations that include:relaxing mechanisms of photoprotection;by‐passing photorespiration;optimising enzyme activity in the photosynthetic carbon reduction cycle;increasing the efficiency of RuBisCo; andoptimising canopies for photosynthesis.Also of note the C4 Rice Project (www.c4rice.com) that is jointly funded by the Bill and Melinda Gates Foundation, in which researchers from seven institutions in five countries are working together to develop high‐yielding rice cultivars. Their aim is to use gene editing to introduce C4 photosynthetic machinery into rice, a C3 crop, which currently accounts for 19% of all calories consumed in the world. If successful, rice plants could be 50% more productive.

10 Finally, the ‘Hands Free Hectare’ project in the UK, demonstrated in 2016 that it is possible to drill, tend and harvest a crop of spring barley without operators of machines or agronomists in the field. It proves that there is no technical barrier to automated field agriculture. Weed control is achieved by aerial sensors that ensure that only weed‐infested areas of a field are sprayed, rather than the whole field, thereby reducing inputs. It is assumed that unmanned automation will become an increasingly important part of agriculture in the future. Achieving precision spraying with dedicated robots fitted with associated sensors is a current engineering challenge (Ghaffarzadeh, 2017). Flavell (2016) has argued that we need to generate clear plans to increase the confidence of investors and society in the future of the plant sciences. Our collective challenge is therefore to see technological advances in the engineering and plant sciences lead to new concepts, products and innovations that will improve the efficiencies of agriculture in the future.

11 A key conclusion of the 2019 cross‐sector review of weed management, commissioned by the UK Agriculture and Horticulture Development Board and the British Beet Research Organisation, was that the approach to Weed Management in the UK needs to be overhauled, and a major investment is required. The review noted that, inter alia: (a) essential information on weed management could be lost to the industry without appropriate key sources of references and an archive; (b) coordinated programmes of research and knowledge transfer are necessary to make the best use of depleted national funding; and (c) the plant protection industry needs to be more unified and strategic to maximise the chances of such methods and research results making an economic difference to farms and growers. I hope that the contribution of research institutes, colleges and universities are to the fore in any future update in the training of the next generations of plant protection personnel.

12 As we have entered a new decade, agrochemical inputs are becoming increasingly under scrutiny and some would argue that agrochemical technology is reaching its limits (Altieri, 2019). Why is this?Large‐scale crop monocultures occupy about 80% of the 1.5 billion hectares currently used in global agriculture.Approximately 2.3 billion kg of pesticides are applied each year to keep weeds, fungal and insect pests at bay.However, less than 1% of pesticides reach the target weed or pest, so that most ends up in the soil, water and the air, leading to declines in biodiversity, especially pollinators, and the natural enemies of pests.Monoculture agriculture leads to pesticide resistance.It follows that the removal of pesticides and herbicides will restore biodiversity and a renewed interest in the biological control of pests. Biodiversity can also be enhanced using cover crops, inter‐cropping, rotations, agroforestry and the introduction of livestock into crop fields. Surrounding these fields with hedgerows and corridors also generates more complex habitats, as field margins are reservoirs of the natural enemies of crop pests, and provide over‐wintering sites for wildlife. In this way, it is thought that replacing monocultures with more complex agricultural systems will contribute to yield advantages via improved biodiversity, enhanced soil quality and resilience to climate change. Such arguments are ecologically persuasive, but more evidence, including detailed cost/benefit analysis, is required before extrapolation to weed control by herbicides. Nonetheless, the observed global increase in weed resistance to herbicides in recent decades is clearly linked to monoculture, and shows no signs of decline.

13 So how can politicians, growers, farmers and the agrochemical industry become more ecologically aware and promote more sustainable practices?The industry should recommend and use technologies for a more precise application of agrochemicals that will reduce application volumes and cumulative dosage.Greenhouse gas emissions can be reduced and soils preserved by promoting minimal tillage and fewer, but more targeted agrochemical applications.More informed farming practices that are sustainable for the use of agrochemicals should be encouraged by continuing professional development and re‐education of farmers and growers.Biodiversity should be encouraged by returning to more complex agro‐ecosystems.We have the tools and knowledge to defeat hunger and malnutrition, but do we have the political will and commitment to do so?Despite these reservations it is important to remember that without herbicides and the sustainable intensification of agriculture we would not be able to feed the existing and growing global population. We must remain alert, however, to the environmental consequences of their use. Furthermore, it is vital that independent research in the plant sciences continues to be supported by national bodies in universities and research institutes. New discoveries and current understanding of how plants are adapted to their ever‐changing environments will continue to drive agrochemical research and development in the years to come.The starting point of this book is weed biology. Subsequent chapters consider the modern plant protection products industry, how herbicides are discovered and developed, how they gain entry into the plant and move to their sites of action, and the basis of herbicide selectivity. Detailed and updated accounts follow of how herbicides interact with the major physiological processes in plants, leading to weed control. This begins with the inhibition of photosynthesis, followed by pigment biosynthesis, interactions with the plant growth regulator, auxin, lipid biosynthesis, amino acid biosynthesis, cell division, cellulose biosynthesis, the plant kinome, herbicide resistance, the development of genetically modified herbicide‐resistant crops and a consideration of some new targets for the future development of new herbicides.In the dozen years since the last edition was written, there have been many advances reported in the plant physiology literature. There has been continuing progress in our understanding of the Arabidopsis genome and our model plant species, and gene editing techniques are now commonplace. It is fascinating to recall that 10 years ago gene editing techniques had not been published. We now understand more about the mechanisms whereby environmental change and protein synthesis are in tune with both biotic and non‐biotic stresses, enabling plant physiology to adapt to an ever‐changing plant environment. Consequently, much of this text is new and many recent references have been added. Note, however, that many older references and figures have been retained because they remain relevant in demonstrating how our understanding has developed, and that the work of previous generations of plant scientists is not forgotten. Of course, the errors are still mine and hopefully will be remedied in time.It is with regret that the co‐author of the second edition of this book, Dr John Reade, has been unable to contribute to this volume, owing to other commitments. He continues to teach the next generations of plant scientists at Harper Adams University and supervises research students with his trademark enthusiasm and intelligence.

And finally,

I think it must be rather nice

to live by giving good advice;

to talk of what the garden needs

instead of pulling up the weeds. (Reginald Arkell, 1882‐1959)

Andy Cobb

July 2021