Van De Weyer, L. M. and Waldner, C. L. 2011. Geographic determinants of copper and molybdenum concentrations in serum at the end of the grazing season and associations with reproductive performance in beef cows from western Canada. Can. J. Anim. Sci. 91: 423–431. Serum concentrations of Cu and Mo were measured at the end of the summer grazing season in an observational field study of 783 breeding females from 66 cow-calf herds located in western Canada. The mean Cu concentration was 0.82 ppm (S, 0.30 ppm) and the mean Mo concentration was 0.056 ppm (S, 0.055 ppm). Serum Cu concentration differed across ecoregions (P=0.013). Serum Mo concentrations varied by ecoregion (P =0.002), soil type (P =0.011), soil color (P=0.018), and total precipitation during the growing season (P =0.004). Copper and Mo serum concentrations at the end of the grazing season were not associated with reproductive outcomes. Herd location is an important determinant of the risk of primary and secondary Cu deficiency and can inform the need for trace mineral testing of feed samples as well as supplementation.