Efficiency of use of imported magnesium, sulfur, copper, and zinc on Idaho dairy farms.

Six commercial dairies from south central Idaho were surveyed to estimate the whole-farm surpluses of magnesium (Mg), sulfur (S), copper (Cu), and zinc (Zn). Mineral imports and exports were monitored in a 12-mo period and samples from the diets, feeds, feces, urine, and manure were collected at regular farm visits. Soils from manure-amended fields were sampled in the spring and fall. In all cases, the largest import of Mg, S, Cu, and Zn to the dairy was with purchased feeds, from 91 (S) to 97% (Zn) of all imports. The major mineral export item was manure [from 60% (S) to 89% (Cu) of all exports] and forages, in the case of a dairy with a large land base. Export with milk represented on average only 8.6, 25, 2.1, and 11% (Mg, S, Cu, and Zn, respectively) of all exports. Thus, the conversion of the imported feed Mg, S, Cu, and Zn into milk was rather low (on a whole-farm scale): 5.6, 11, 1.4, and 5.2%, respectively. Concentrations of Mg, Cu, and Zn in the lactating cow diets from the participating dairies exceeded National Research Council (2001) recommendations on average by 85, 34, and 73%, respectively, which contributed to the inefficient use of imported minerals. Whole-farm Mg surplus varied from 4 to 54 t/yr (3 to 19 kg/cow per year). The efficiency of use of imported Mg varied from 27 to 88%. Sulfur surpluses were from 9 to 52 t/yr (12 to 40 kg/cow per year). Copper and Zn surpluses were also significant (average of 59 and 585 kg/yr and 0.05 and 0.4 kg/cow per year, respectively). The average efficiency of use of imported S, Cu, and Mg was 44, 62, and 56%, respectively and, as with Mg, varied significantly among the dairies. The results from this study suggest that reduction in the concentration of dietary Mg, Cu, and Zn is potentially the most efficient way of reducing overall excretions and whole-farm surpluses of these minerals.