Herbicide choice and use has also had a profound effect on the weed flora in cereals (Martin, 1987). The use of 2,4‐D and MCPA since the late 1940s has caused a decline in many susceptible weeds, such as charlock (S. arvensis) and poppy (Papaver rhoeas), although more tolerant species, including chickweed (S. media), knotgrass (P. aviculare) and the speedwells (Veronica spp.), have prospered. The introduction of herbicide mixtures in the 1960s with, for example, mecoprop and ioxynil, gave a much wider spectrum of weed control. Other herbicides have since been developed for an autumn‐applied, residual action, including, for example, chlorsulfuron and chlorotoluron for improved control of grasses, and new molecules for the control of specific weeds, for example fluroxypyr for cleavers (G. aparine). However, the same principles will always apply, namely that the selection pressure caused by sustained herbicide use will allow less‐susceptible weed species to become dominant, and their continued use may encourage the selection of herbicide‐resistant individuals within a species, as has now occurred to many herbicides (see Chapter 13).
Climate change is predicted to have significant effects on both the geographical distribution of weeds and the severity of weed infestations. Evolutionary rate (for instance, in the development of herbicide resistance) has been demonstrated to vary dependent upon both temperature and moisture availability. This is probably a result of a combination of factors including generation time, population size and relative fitness of herbicide‐resistant individuals. All of these factors may be affected by increased average global temperatures and subsequent differences in regional weather patterns (Anon., 2000). In addition, milder winters and warmer summers may allow for the survival and population growth of species that were previously unsuited to a region’s climate. Increases in the occurrence of Phalaris grasses in the UK in recent years may be a direct result of this (A.H. Cobb and J.P.H. Reade, personal observations).
1.9 Weed control
It is outside the scope of this book to examine the finer details of weed control. Instead, a broad overview is presented. The reader is referred to other, more comprehensive texts for further information, such as Naylor (2002).
According to Lacey (1985), weed control encompasses:
1 the reduction of the competitive ability of an existing population of weeds in a crop;
2 the establishment of a barrier to the development of further significant weeds within that crop; and
3 the prevention of weed problems in future crops, either from the existing weed reservoir or from additions to that weed flora.
The first two objectives are met primarily by chemical means, and the third relies on agronomy and crop husbandry. Cultural practices are forever changing, along with the weed spectrum, and it is now increasingly recognised that an integrated approach utilising both cultural and chemical practices is necessary for optimal weed control.
1.9.1 Traditional methods
It was recognised in medieval times that the rotation of crops with fallow was the best means to conserve soil fertility and to prevent the build‐up of pests, diseases and weeds. The later use of ‘cleaning’ crops (such as turnips and potatoes) allowed weed control by hand during active growth, and was balanced against ‘fouling’ crops (such as cereals) which could not be similarly weeded. By the mid‐nineteenth century fertility was maintained from clover and livestock manure, and weed control by ‘cleaning’ crops, so that the unprofitable fallow period could be avoided. The advent of chemical fertilisers in the early twentieth century removed the need for clover, and profitability increased by the use of sugar beet as a combined cleaning and ‘cash’ crop. However, after the Second World War, increased urbanisation and industrialisation have reduced the available workforce, and herbicides have gradually replaced the hoe. Similarly, farm practices have become increasingly mechanised, such that the continuous cultivation of one crop (monoculture) has become widespread, and reduced cultivation techniques are now in vogue.
1.9.2 Chemical methods
Chemical weed control is a twentieth‐century technology. Copper sulphate was the first chemical used at the turn of the twentieth century to control charlock (S. arvensis) in oats, and soon after came corrosive fertilisers (such as calcium cyanamide) and industrial chemicals (including sodium chlorate and sulphuric acid). Modern synthetic herbicides first appeared in France in 1932 following the patenting of DNOC (4,6‐dinitro‐o‐cresol) for the selective control of annual weeds in cereals. Further dinitro‐cresols and dinitrophenols soon appeared, but these compounds had variable effectiveness and appeared to kill animals as well as plants. The discovery of the natural plant growth ‘hormone’ auxin in 1934 led to the further discovery of the synthetic growth regulators 2,4‐D and MCPA based on phenoxyacetic acid chemistry. These compounds were the first truly selective herbicides that could reliably kill broad‐leaved weeds in cereal crops, and they developed widespread popularity and use after the Second World War (Kirby, 1980). These compounds truly ‘replaced the hoe’ so that cereals could no longer be regarded as ‘fouling’ crops, and paved the way to the current practice of cereal monocultures.
Since the 1950s an increasing proportion of world cereal crops has become regularly treated with agrochemicals to achieve the control of an ever‐widening variety of weeds. Nowadays, chemical weed control has expanded to probably every crop situation in the world. Modern chemical weed control is not only more economical than traditional methods, but also has important technical advantages as weeds growing closest to the crop, and hence competing most for resources, can be controlled by selective herbicides. Furthermore, less crop‐root disturbance is evident than with mechanical hoeing and fewer, if any, weed seeds are brought to the surface in the process. Finally, farmers now have chemical answers for most weed problems at a reasonable price.
