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Vegetation composition in parts of the Northern Great Plains (NGP) varies yearly due to abundance or absence of annual bromes. This study was designed to determine the effects of variability associated with year and two dominate ecological sites (clayey and loamy) on the cover of cool and warm season native grasses and annual bromes.� Research was conducted at the 22,257 ha USDA-ARS Fort Keogh LARRL, Miles City, MT, where native vegetation is predominately western wheatgrass (Pascopyron smithii), needle and thread (Hesperostipa comata), and blue grama (Bouteloua gracillis) grasses.� �Average annual precipitation is 315 mm of which 80% is received from April-September.� Three 50m transects were sampled for percent of canopy cover in 6 pastures, in two ecological sites (ES), �clayey or loamy ES, from 2012-2016. Year (5), ES (2), and their interactions were analyzed using Proc Mixed (SAS 9.4) as a 5 x 2 factorial arrangement of treatments. �Needle and thread, and threadleaf sedge (Carex filifolia) cover was greater (P<0.05) on loamy than clayey sites and buffalo grass (Bouteloua dactyloides), western wheatgrass, and Japanese brome (Bromus arvensis)�cover was greater (P<0.05) on clayey than loamy sites.�� Year*ES interacted to influence annual brome and native cool season grass cover.� Annual brome cover (P<0.05) was highest in above average spring rainfall years 2013-2014 on clayey ES (32%�3%) and lowest in 2012, a low spring rainfall year, for both ES (1%�3%).� Native cool season grass cover (P<0.05) was highest in 2012 for both ES (74%�5%) and in 2013 (64%�5%) and 2014 (61%�5%) in loamy ES.� Warm season grass cover was �not influenced by ES or year (P>0.05).� �Abundance of annual bromes in the NGP are highly variably by year, appearing to be reliant on on timing of rainfall events.� Long term monitoring �is needed to better understand the factors influencing annual brome abundance.