Van De Weyer, L. M., Hendrick, S. and Waldner, C. L. 2010. Serum micronutrient concentrations in beef cows before and after the summer grazing season. Can. J. Anim. Sci. 90: 563Á574. 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 (PB0.001), and Mo (P B0.001) and selenium (PB0.001) were lower in the fall compared with the spring. Cows aged 10Á14 yr had lower concentrations of vitamin A (P B0.001) and vitamin E (spring: P00.010; fall: PB0.001) than cows aged 4Á9 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 (P00.044). 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 (P00.033). Trace mineral supplementation given before calving was associated with higher spring concentrations of Cu (P 00.007). 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.

Prebreeding micronutrient supplementation programs should be carefully managed in herds with poor reproductive performance in areas known to be copper deficient, and evaluation of serum copper concentrations from a subset of cows should be considered before the start of the breeding season.