The aims were to assess the day-to-day variation in trace minerals (TM), macro-minerals, dry matter, and physical effectiveness factor in grass–legume and corn silages and to evaluate the variance partition. Grass–legume and corn silage samples were collected in nine Canadian dairy herds during two episodes of five consecutive days at 4 weeks apart by the same individual. All variables were analyzed in duplicate. The proportion of variation due to the farm was more variable within TM than macro-minerals. Using TM software reference values of silages for formulating rations can lead to important errors. Except for physical effectiveness factor, the within-farm variations between sampling episodes were more marked for mixed grass–legume than corn silage. For most of the minerals and nutrients analyzed, the sampling + day-to-day variations were the main source of variability, accounting for over 50% of the within-farm variance for both silage types. The remaining within-farm variance was explained by subsampling and laboratory analyses. The high within-herd variation suggests that a silage sampling over more than 1 day can be useful to get a representative sample for TM analysis. Accurate nutrients and TM values when formulating cow diets is essential to cow health and productivity.