Changes in Element and NO3–N Concentrations in Grass Due to Nitrogen Fertilisation and Their Consequences for Animal Nutrition

N fertilisation affects the nutrient content of grasslands, and thus animal health. The effect of fertiliser treatments with calcium ammonium nitrate at doses of 0, 100, 200 and 300 kg N ha–1 year–1 was investigated on grassland nutrient content in a long-term field experiment. The dilution effect due to biomass growth was analysed separately from other effects. The biomass of the grass increased 3.4-fold, up to 5.82 t ha–1. N fertilisation significantly reduced the concentrations of P and Mo and increased the concentrations of N, NO3–N, Na, Mn, and Cu in the grass. From a grazing or feeding perspective, N treatment adversely increased the NO3–N concentration, which exceeded the risky level of 1500 mg NO3–N kg in the 200 kg N ha year–1 treatment and decreased the P concentration. The treatments favourably increased the Na, Cu and Zn concentrations, reduced the Mo concentration, and improved the tetany index and the K:Na ratio. The 100 kg ha–1 year–1 N dose can be recommended under conditions similar to those in the experiment. It is important to analyse the element content of grass when using mineral fertiliser in order to minimise animal health risks.