Weeds reduce crop yields by competing with crop plants for space, light, water and minerals. The modes of action of three different types of herbicide are summarised in Table 4.1. Table 4.1 type of herbicide | mode of action | year of first widespread use --- | --- | --- photosystem II inhibitor | prevents photophosphorylation | 1960 ALS inhibitor | prevents synthesis of the amino acids isoleucine, leucine and valine | 1980 glyphosate | prevents synthesis of the amino acids phenylalanine, tryptophan and tyrosine | 1990 Fig. 4.1 shows the cumulative number of species of weeds that have become resistant to these three types of herbicide since 1960. [Figure 4.1]