A model of nitrogen efficiency in contrasting grass-based dairy systems.

Nitrogen (N) efficiency is one of the key drivers of environmentally and economically sustainable agricultural production systems. An N balance model was developed, evaluated, and validated to assess N use efficiency and N surplus and to predict N losses from contrasting grass-based dairy production systems in Ireland. Data from a 5-yr study were used to evaluate and validate the model. Grass-based and high-concentrate production systems combined with 3 divergent strains of Holstein-Friesian (HF) dairy cows-high-production North American (HP), high-durability North American (HD), and New Zealand (NZ)-were evaluated. As concentrate input increased, N surplus per hectare increased and N use efficiency per hectare decreased (23 and 10%, respectively). When the N required to rear replacement animals to maintain the production system was considered, the N surplus of the HP genetic strain was greater (156 kg of N/cow) than that of the HD (140 kg of N/cow) or the NZ (128 kg of N/cow). The model estimated N leaching of 8.1mg of NO(3)-N/L, similar to that measured by others at the same site. The model creates awareness of methods and indicators available to assess the most suitable and environmentally sustainable grass based dairy production systems.