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Several ecological research studies have indicated that increased plant biodiversity benefits primary production. However, it is not clear whether the results and concepts of basic ecological biodiversity studies apply to manage temperate grazed pastures. An Agriculture and Agri-Food Canada - SPARC study was started in 2001, in which class 3-4 crop land was seeded to two different native mixtures: a simple (S) mix with six different wheatgrasses and one forb and a complex (C) mix containing eleven grasses (warm and cool season species), one forb and two shrubs. In 2005 four pastures (each pasture 2 ha), two S and two C, were grazed continuously at a utilization levels of 50% for the next ten years. Study objective was to determine if increased species diversity resulted in higher primary forage production. Available pasture biomass measurements were taken at the beginning of the grazing season which occurred at the beginning of July (AYD) and peak biomass measurements were taken at the end of July (CYD). Both AYD and CYD measurements were taken over all years (2005 - 2014) and were an average of 10 random quadrat samples per pasture. Results observed no interaction or primary biomass differences (P>0.10) between the S versus C mixtures for AYD or CYD. As expected there were year effects (P<0.0001) for AYD or CYD. Although several drought or below normal moisture periods occurred over the ten years, the level and/or duration of environmental heterogeneity may not have been sufficient to show any primary production benefit with a higher forage diversity mixture and the lower-diversity mixture did just as well.