Influence of Stocking Rate, Grazing Season Length and Dairy Cow Genetic Strain on the Nitrogen Balance of Grass Based Dairy Production Systems

Stocking rate (SR), grazing season (GS) length and Holstein-Friesian (HF) strain can have a profound effect on the productivity of grass-based milk production systems. The optimum SR is that which gives the maximum sustainable economic output of product per unit area. Increasing SR increases output per ha, but also requires increased inputs of feed and fertilizer. Increasing the proportion of grazed grass utilized for milk production improves the economic sustainability of the system (Dillon et al. 2008). Nitrogen (N) use efficiency is one of the key drivers of environmentally and economically sustainable agricultural production systems. Achieving the optimum balance between profitable agriculture and environmental protection is challenging. When production is maximised and output is near equilibrium, all further N inputs are lost to the environment (Rotz et al. 2005), and can result in contamination of waters and contributions to greenhouse gas emissions. A N balance model was used to assess the N use efficiency of components of spring calving grass-based dairy production systems; those components being SR, GS length and HF strain