The human battle against organisms that destroy or compete with crops goes back to the beginning of farming history. In the past it has been mostly a losing battle. Today the enemy is largely contained; the techniques for control of weeds, pests and diseases of crops put into practice over the past fifty years have to a great extent determined the success of the latest agricultural revolution. Some of the substances used, derived from the success of synthetic organic chemistry, are highly effective but, wrongly used, may pose a threat to the natural environment; others depend on sophisticated biological understanding and need skill to be effective. In this chapter we hope to give a balanced view of the development of these powerful tools of husbandry and to evaluate their importance.

Weeds

Until 1940 control of weeds was largely by rotation, by cultivation and by the production of crop seed free from weed contamination. The whole arable cycle was determined more or less by the need to control weeds. Indeed a farmer's skill would be judged by the success of his weed control methods as shown by the appearance of his farm. There is no doubt that farmers successful in weed control were generally successful in financial terms, but there was no absolute relationship and it was possible to go bankrupt with a clean and well-cultivated farm when control of weeds took precedence over careful management of marketing and finance.