Solid-liquid phase separation of pig slurry allows to control and reduce the amount of phosphorous released in the environment, an advantage for the many farms in a situation of excess phosphorous. Various methods of slurry phase separation allow a relatively important concentration of the phosphorous into the solid phase, whereas most of the nitrogen stays in the liquid phase. We studied the utilization, as nitrogen sources, of liquid by-products obtained through natural decantation, filtration through woodchips, anaerobic digestion and physicochemical flocculation. The agronomic and environmental performances of these liquid by-products to those of raw slurry and mineral fertilizers have been compared. During three consecutive seasons, nitrogen loss through ammonia (NH3) volatilization and emissions of dinitrogen oxide (N2O, a greenhouse gas), after each field spreading have been measured. Dry matter yield and major nutrients’ uptake of timothy were measured for each type of fertilizer. Raw slurry emitted 3 to 4 times more NH3 than mineral fertilizer and 20 to 25% more than liquid by-products. Slurry emitted significantly more N2O than the mineral fertilizer and the liquid by-product resulting from anaerobic digestion. This liquid by-product, followed by the one obtained through flocculation, also resulted in timothy yields and nutrient uptake similar to those of mineral fertilizer. Slurry generally allowed lower timothy performances than did mineral fertilizers and liquid by-products. Despite the agronomic and environmental advantages showed by solid-liquid separation of slurry, the higher nutrient availability of the liquid by-products commands caution when spreading in the absence of actively growing crops.