Serum concentrations of Cu, Mn, Mo, Se, vitamin A, and vitamin E were measured at the start and at the end of the summer grazing season in an observational field study of 791 commercial beef cows from 40 cow-calf herds from five community pastures. Copper and vitamin A were higher in the fall (P < 0.001), and Mo (P < 0.001) and selenium (P < 0.001) were lower in the fall compared with the spring. Cows aged 10-14 yr had lower concentrations of vitamin A (P < 0.001) and vitamin E (spring: P = 0.010; fall: P < 0.001) than cows aged 4-9 yr. Cows with thin pre-breeding body condition scores were more likely to have lower pre-breeding Cu concentrations than cows that were not thin (P = 0.044). Cows that were thin at the end of the grazing season were more likely to have lower vitamin A concentrations at this time than cows that were not thin (P = 0.033). Trace mineral supplementation given before calving was associated with higher spring concentrations of Cu (P = 0.007). Season of sample collection and the physiological status of the animals should be considered when interpreting the serum micronutrient status of beef cows and developing supplementation programs.