Integrated weed control management encourages post-emergence herbicide application, and especially use of lower doses due to economic savings and reduction of negative environmental impact. Knowing the effects of herbicides in different stages of development can help to provide more effective control of weeds such as Echinochloa crus-galli L. Beauv. (barnyardgrass), which is a dominant narrow-leafed weed of summer crops in the continental part of Croatia. Since the amount of applied herbicide mostly depends on barnyardgrass development stage it is very important to determine the optimum time for application. The purpose of this thesis was to determine the efficiency of reduced doses (2/3, 1/2, 1/4, 1/3 and 1/8) of topramezone, a new herbicide from the chemical group of triketones, combined with adjuvants (NIS and MSO) and applied on weed species Echinochloa crus-gali in two different stages of development - three developed leaves (BBCH 13) and tillering (BBCH 20-21). Treatment evaluation was determined by visual evaluation of damage (0 - 100%), the number of surviving plants after treatment and determining the dry weight of weeds. The research results indicate that topramezone applied in recommended dose without the addition of adjuvants achieved unsatisfying effect on barnyardgrass in both researched stages of development. The addition of adjuvant MSO and NIS can significantly reduce dose of topramezone. There has been 100% biomass damage visually assessed and more than 90% reduction of the number and weight of the treated barnyardgrass in stage three developed leaves using all reduced doses of topramezone with adjuvants. In contrast, in the tillering stage (BBCH 20 - 21) only increased dose of topramezone (134.4 g. a.t. ha-1) and the recommended dose (67.2 g. a. t. ha-1) combine with adjuvant NIS (73.3%) achieved satisfactory effect on barnyardgrass. Entering the stage of tillering barnyardgrass can not be control by reducing the dose of topramezone even when herbicide application is used with adjuvants MSO and NIS.