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Weed Science Research: Past, Present and Future Perspectives

Robert J. Froud-Williams

'Russets', Harwell, Oxon, UK

Introduction

Plants popularly referred to as weeds have been described by Sir E.J. Russell (1958) as 'The ancient enemy'. In his text on agricultural botany, Sir John Percival (1936) made the observation that the idea of uselessness was always present in the mind when weeds are being spoken of, while, in the editor's preface to Weeds and Aliens by Sir Edward Salisbury (1961), weeds are likened to criminals - when not engaged in their nefarious activities both may have admirable qualities: 'an aggressive weed in one environment may be a charming wild flower in another'. Our relationship with weeds certainly is as old as agriculture itself and the concept of weediness was recognised from biblical abstracts, for example the gospel according to St Matthew (Ch. 13 v. 7, the parable of the sower): 'Other seed fell among thorns, which grew up and choked them'. Yet weed science as a discipline is less than one hundred years old, albeit Fitzherbert (1523) in his Complete Boke of Husbandry recognised the injurious effect of weeds on crop production: 'Weeds that doth moche harme' included kedlokes, coceledrake, darnolde, gouldes, dodder, haudoddes, mathe, dogfennel, ter, thystles, dockes and nettylles'. These are recognised today as corncockle, charlock, darnel, corn marigold, dodder, cornflower, mayweed, stinking mayweed, fumitory, thistles, docks and nettles, several of which are now greatly diminished in abundance.

A major development in weed removal from within crops was achieved with the development of the seed drill by Jethro Tull c. 1701. Initially, the objective of this invention was to enable cereals to be sown in rows, whereby a horse-drawn hoe could be used to pulverise the soil in the inter-row. Tull conjectured that such 'pulverisation' would release nutrients beneficial to the crop, but coincidentally enabled weed removal, whereby 'horse-hoeing husbandry' became standard practice, reducing weed competition and the necessity of fallow, a serendipitous discovery.

Despite the efficacy of technological advances in weed control, weeds still exert great potential to reduce crop yields. Weeds are considered the major cause of yield loss in five crops (wheat, rice, maize, potato and soybean and a close second in cotton) (Oerke, 2006). Estimated potential losses due to weeds in the absence of herbicides were 23, 37, 40, 30, 37 and 36% for the six crops respectively, while weed control reduced these losses to 7.7, 10.2, 10.5, 8.3, 7.5 and 8.6%, albeit with considerable regional variation (Oerke, 2006). Efficacy of crop protection practices varied between geographic regions, but whereas efficacy of disease and pest control was only 32 and 39% respectively, efficacy of weed control was almost 75%. The greater efficacy of weed control was attributed to the ability to employ both physical and chemical methods. Possible reasons for the apparent mismatch between weed control efficacy and actual yield losses were ascribed to changing cultural practices such as monoculture, multiple cropping, reduced rotation and tillage and the introduction of more vulnerable crop cultivars dependent on increased fertilisation.

Weeds have a major impact on human activities for not only do they adversely affect economic crop yield indirectly through interspecific competition (see Bastiaans & Storkey, Chapter 2) directly as a result of parasitism (see Vurro et al., Chapter 11) and allelopathy, but also they affect human health and the well-being of livestock through physical and chemical toxicity. Additionally they may negatively impact environmental quality and functionality, such as that posed by alien invasive species including aquatic weeds (see Bohren, Chapter 10).

The objective of this preliminary chapter is one of scene setting. It seeks to associate 'man's' controversy with weeds as a consequence of their detrimental as well as beneficial relationships. Our changing perception of weeds is examined in terms of a shift in emphasis from that of pragmatic weed destruction to one of management and rational justification for their suppression.

Agronomic practices greatly influence weed population dynamics and these are outlined with particular attention to the UK weed floras. The history of weed science is explored as a discipline, together with a brief history of weed control technology including the discovery and development of synthetic herbicides. The origins of the Weed Research Organization (WRO) are discussed, together with the subsequent formation of the European Weed Research Society.

Weed science as a discipline originated at Rothamsted in England, the first agricultural research institute to be established in the world, with the pioneering work of Winifred Brenchley on the classic long-term continuous winter wheat experiment, Broadbalk, where she investigated the impact of various agronomic factors such as manuring, liming and fallow on the arable weed flora.

Factors Influencing the Weed Flora

Succession

The British flora is not an event, but a process that is continuing both with respect to accretions and diminutions (Salisbury, 1961). Vegetation is never static and weed populations are probably subject to greatest fluctuation as their habitat is continually disturbed. Two types of change within plant communities may be recognised: fluctuating and successional. Arable plant communities are subject to fluctuations as a consequence of direct intervention. Weeds are fugitives of ecological succession; were it not for the activities of man they would be doomed to local extinction and relegated to naturally disturbed habitats such as dune and scree. Weeds have been described as the pioneers of secondary succession, of which the weedy arable field is a special case (Bunting, 1960).

Successional change is less likely within ephemeral communities, although potentially capable in systems of prolonged monoculture and non-tillage. Two types of successional change may be recognised - autogenic and allogenic. Autogenic succession occurs in response to changes within the habitat, as species better adapted to a changing habitat oust previous inhabitants. A classic example of autogenic succession is Broadbalk Wilderness, whereby climax vegetation was achieved 30 years after the abandonment of an arable crop (Brenchley & Adam, 1915). Allogenic succession occurs in response to modified environmental factors such as fertiliser and herbicide input.

Prior to the advent of selective herbicides in 1945, weeds were kept in check by a combination of rotation, cultivation and clean seed, the three tenets of good husbandry. Previously, weed control was strategic, but the availability of herbicides enabled a tactical approach. However, the realisation that some weed species are of beneficial value to the arable ecosystem rendered the pragmatic destruction of weeds other than those that were most intransigent less acceptable; maximisation of yield was not necessarily synonymous with maximisation of profit.

Clean Seed

The use of clean seed as a consequence of the development of threshing machinery was greatly assisted by improvements in seed screening and legislation such as the 1920 Seeds Act designed to reduce the number of impurities. Regular inspection by the Official Seed Testing Station (OSTS) provides testament to the merits of seed certification. Early casualties of improved sanitation were the mimetic weeds such as Agrostemma githago L. (corncockle)*, a formerly characteristic weed of cereals which could be separated by seed screening. Prior to 1930 it was a frequent grain contaminant, as witnessed by records of the OSTS; the last authenticated record of its occurrence was documented in 1968 (Tonkin, 1968). A further factor contributing to its demise was the fact that its seeds are of short persistency in soil and require continual replenishment for survival. A survey of cereal seed drills in 1973 indicated considerable contamination by weed seeds including wild oats (Avena spp.) and couch grass Elymus repens (L.) Gould) as well as Galium aparine L. (cleavers) and Polygonum spp. (Tonkin & Phillipson, 1973). EU legislation designed to reduce the incidence of weed seed impurities in crop seed has certainly reduced this as a source of infestation, with, for example, only a single wild oat seed permitted per 500-g sample, provided that the next 500-g sample is entirely free of contamination.

Rotation

The season of sowing is the greatest determinant of weed occurrence (Brenchley & Warington, 1930). Hence, in the 1960s when spring barley predominated, spring-germinating species were prolific, the most significant of which was Avena fatua L., but also a diverse array of broad-leaved species, the periodicity of which is predominantly or entirely in the spring. The shift to autumn cropping in the 1980s disadvantaged spring-germinating species as a consequence of crop competition. Avena fatua exhibits a bimodal pattern of germination such that it was not necessarily disadvantaged, but it is possible that the related Avena sterilis ssp. ludoviciana (Durieu) Gillet & Magne., which is entirely autumnal in germination periodicity, may have supplanted it as the dominance...