Treatments to reduce solids content in liquid manure have been developed, but little information is available on gaseous N emissions and plant N uptake after application of treated liquid swine manure (LSM). We measured crop yield, N uptake, and NH3 and N2O losses after the application of mineral fertilizer (NH4 NO3), raw LSM, and LSM that was decanted, filtered, anaerobically digested, or chemically flocculated. The experiment was conducted from 2001 to 2003 on a loam and a sandy loam cropped to timothy (Phleum pratense L.) with annual applications equivalent to 80 kg N ha(-1) in spring and 60 kg N ha(-1) after the first harvest. Raw LSM resulted in NH3 emissions three to six times larger (P < 0.05) than mineral fertilizer. The LSM treatments reduced NH3 emissions by an average of 25% compared with raw LSM (P < 0.05). The N2O emissions tended to be higher with raw LSM than with mineral fertilizer. The LSM treatments had little effect on N2O emissions, except for anaerobic digestion, which reduced emissions by >50% compared with raw LSM (P < 0.05). Forage yield with raw LSM was >90% of that with mineral fertilizer. The LSM treatments tended to increase forage yield and N uptake relative to raw LSM. We conclude that treated or untreated LSM offers an alternative to mineral fertilizers for forage grass production but care must be taken to minimize NH3 volatilization. Removing solids from LSM by mechanical, chemical, and biological means reduced NH3 losses from LSM applied to perennial grass.