Effect of Nitrogenous Fertilizer Treatment on Mineral Metabolism in Grazing Yaks

To assess the impact of N fertilization on contents of mineral elements in herbage and the effect of increased forage S on the copper metabolism of grazing yaks, study was conducted during the summer grazing season (2005, 2006, and 2007). Pasture replicates (20 ha; n=3 per treatment) received the same fertilizer treatment in each growing season, consisting of i) 90 kg N ha−1 from quickly available nitrogen, ii) 90 kg N ha−1 from ammonium nitrate, iii) 90 kg N ha−1 from ammonium sulfate, and iv) control (no fertilizer). Forage sampling was collected at 60 days intervals following fertilization (10 samples per pasture) for Cu, Mo, Mn, Se, Fe, Zn, Ca, and P. To determine the effect of fertilizer treatment on mineral metabolism in grazing yaks, liver and blood samples were collected at the start and end of the study period in 2005, 2006, and 2007. Ammonium sulfate fertilization increased (P < 0.01) forage S concentration. Plant tissue N concentrations were increased by N fertilization, regardless of source in 2005, 2006, and 2007. Yaks grazing S fertilization pastures had lower (P < 0.05) liver and blood Cu concentrations at the end of the study period in 2005, 2006, and 2007, compared with urea, ammonium nitrate, and control. Nominal increases in forage in vitro organic matter digestibility were realized by fertilization, regardless of N source in each year.