Andrew H. Cobb - Herbicides and Plant Physiology

On a global basis only about 250 species are sufficiently troublesome to be termed weeds, representing approximately 0.1% of the world’s flora. Of these, 70% are found in 12 families, 40% alone being members of the Gramineae and Compositae. Interestingly, 12 crops from five families provide 75% of the world’s food and the same five families provide many of the worst weeds ( Table 1.1). This implies that our major crops and weeds share certain characteristics and perhaps common origins.

Table 1.1 Important plant families which contain both the major crops and the worst weeds of the world.

Source: Radosevich, S.R. and Holt, J.S. (1984) Weed Ecology: Implications for Vegetation Management . New York: Wiley. Reproduced with permission of John Wiley & Sons.

Most plants grow in communities consisting of many individuals. If the resources available (such as space, water, nutrients and light) become limiting then each species will be forced to compete. Weeds are often naturally adapted to a given environment and so may grow faster than the crop, especially since the crop species has been selected primarily for high yield rather than competitive ability. A unit of land may therefore be regarded as having a finite potential biomass to be shared between crop and weeds, the final proportion being determined by their relative competitive ability.

The most obvious problem caused by weeds is the reduction of yield through direct competition for light, space, nutrients and water. Weeds can have many further effects on the use of land, as illustrated in Table 1.2.

Table 1.2 Problems caused by weeds.

Source: Naylor, R.E.L. and Lutman, P.J. (2002) What is a weed? In: Naylor, R.E.L. (ed.) Weed Management Handbook, 9th edn. Oxford: Blackwell Publishing/BCPC. Reproduced with permission of John Wiley & Sons.

Crop losses approaching 100% are recorded in the literature ( Table 1.3; Lacey, 1985). Such yield losses will, of course have a profound effect on a national economy in terms of both the need to import foodstuffs and the costs of weed control. Despite the many methods of weed management that are now available worldwide, it is estimated that approximately 13% of crop losses are still due to weeds alone ( Table 1.4). Indeed, in 1974 the annual cost of weeds to agriculture in the USA was estimated at US$10 billion, with 50% owing to yield reductions and 50% owing to the cost of weed control (Rodgers, 1978).

In the tropics, parasitic weed species from the genera Cuscuta (dodders), Orobranche (broomrapes) and Striga (witchweeds) can have a profound effect on a range of crops. They absorb nutrients directly from the crop plant, which may not set seed at all in the case of cereals such as sorghum.

Weed control techniques are therefore aimed at the reduction in the competitive ability of weeds in a crop and the prevention of weed problems in a future crop. The former is increasingly based on chemical use, and the latter also requires suitable cultural and agronomic practices.

Yield loss may be usefully related to the number of weeds per unit area causing a defined yield loss in a defined crop, that is, as a Weed Threshold ( Table 1.5) or as a Crop Equivalent (the amount of resource an individual weed uses expressed as the number of crop plants this resource would support, although in practice it is the biomass of the weed and the crop which is measured). Generally, these figures have only been determined for weed interaction with major crops, but they give a good indication of the ability of a particular species to compete with all crops.

Table 1.3 Examples of yield losses owing to weeds.

Source: Lacey, A.J. (1985) Weed control. In: Haskell, P.T. (ed.) Pesticide Application: Principles and Practice . Oxford: Oxford University Press, pp. 456–485. Reproduced with permission of Oxford University Press.

* From Moss (1987).