Research conducted in the Basque Institute for Agricultural Development and Research, Neiker-Tecnalia, and led by the biologist Haritz Arriaga in collaboration with the Universitat Autònoma in Barcelona, has demonstrated that up to 35% of the accumulated nitrogen on dairy farms in the Basque Country can be reduced with a balanced diet in protein content without reducing milk production. The first part of the research was conducted in 64 commercial farms in the Basque Country, in which it was shown that on most of these (70%) the diet of the lactating cows was excessively rich in proteins. The quantity of protein ingested is directly related to the faecal and urinary excretion of N (R2 = 0.7) because 6.25% of the protein is formed by this chemical element. Thus, the greater the ingestion of protein the greater nitrogen losses into the environment, despite the milk production is also higher. So, the purpose of farmers’ should be an adjustment of protein consumption to the nutritional needs of the cattle without reducing the production and quality of milk. In this sense, results demonstrated that up to 35% of the accumulated nitrogen on dairy farms in the Basque Country can be reduced with a balanced diet of proteins. The results also demonstrated that nutritional strategies can reduce the accumulation of nitrogen on high-density dairy farms. The concentration of this chemical element per hectare of available soil can be reduced by 11.2% through the optimisation of protein content in rations. In the second study, the research group of Neiker-Tecnalia analysed the efficient use of nitrogen in the animal through a greater use of commercial concentrates and, thereby, the energy content of the diet. Afterwards, they studied the excretion of N and its concentration in the resulting manure. In this study a comparison was made of diets with low forage content and high concentrate content (ratio 45:55), usually employed on intensified dairy farms, with diets involving greater forage content and less concentrate content (ratio 75:25), considered less energetic but more sustainable from an environmental and feeding perspective. The resulting manures from the different diets were subsequently applied on a grassland in order to evaluate the volatilisation of the nitrogen-based gases, ammonia (NH3) involved in the acidification and eutrofisation of aquatic and edaphic ecosystems as well as nitrous oxide (N2O) and nitric oxide (NO), involved in the greenhouse effect and destruction of the ozone layer. The researchers demonstrated that rations with higher forage content reduce the voluntary ingestion of food, because the animal is satisfied because of the fibre content of the forage. As a consequence, the nitrogen intake in this diet is also reduced and, consequently, the excretion is lower, which contributes to minimise the ammoniacal nitrogen (N-NH4+) in the resulting manure. However, this reduction in the ingestion of food and nitrogen also causes a loss in milk production. The alteration of the nitrogen composition of the manure (N-NH4+) can have environmental implications depending on the handling carried out by the farmer in the fields. Emissions of nitrogen gases (ammonia, nitrous oxide and nitric oxide) to the atmosphere after applying manures obtained with high or low forage content diets are similar when the same N-NH4+ doses are applied on-field. After applying 120 kg N-NH4+, nitrogen gas emissions were 18.7 kg N per hectare in the case of diets with high content of forage (14,8%), while in the case of diets with low forage content, 11.5 kg of N per hectare (9.6%) were emitted. These data confirmed that between 10% and 15% of N-NH4+ applied in the field will be emitted in the form of nitrogen to the atmosphere, mostly (60%) as ammonia. In a third study, the aim was to determine the effect of the concentration of the protein in the ration on the ammonia and nitrous oxide concentration in dairy barns. Both gases, apart from the environmental repercussion previously pointed out, can harm both the health of humans and cattle. This study demonstrated that the concentrations of NH3 from the barn soil ranged between 7.1 mg of NH3 per cubic metre in low protein diets and 10.8 mg of NH3 per cubic metre in rations with higher ingestion of proteins. On the contrary, the amount of nitrous oxide was very similar with an average of 1.1 mg of N2O per cubic metre. However, despite the lack of response to nutritional changes, it was remarkable that the amount of N2O in the barns was greater than the atmospheric concentration (0.5 mg of N2O per cubic metre). The results obtained underlined the importance of fitting the protein content of the rations to the animal requirements (according to production, lactation stage, genetics, etc) with the goal of optimising the efficiency of nitrogen use. This adjustment of the protein in the rations will moreover enable reducing the concentration of gaseous N losses in terms of NH3, N2O and NO from dairy barns and after manure application on grasslands.