ABSTRACT
Weeds, pests and diseases are the three major opponents to successful agricultural output and productivity. All of them are types of living organisms, unwanted by the farmer because they either compete with, or directly injure, the commodities he is trying to produce. The weed population changes in magnitude from site to site according to weather, soil conditions and the timing of cultivations. Weeds usually cause the greatest depression of final yields if they develop rapidly before the crop itself is fully established. The effects of weeds on crop yields tend to be more consistent than those of pests and diseases, but even so there is much variation in losses. Many pests are mobile and can move to their target under their own power; others are more passive, and almost all disease organisms need transport assistance from wind, water or intermediate animal hosts. Pests may be preyed upon by other organisms so that their numbers are reduced to innocuous levels.
