Surface application of urea can result in high nitrogen (N) losses through ammonia (NH3) volatilization. While management practices aim to increase the efficiency of nutrient cycling and prevent N loss, it is unknown whether the combination of multiple practices will have a synergistic or antagonistic effect. Therefore, laboratory volatilization studies were conducted to determine the effect of five cover crop treatments (surface clover [Trifolium incarnatum L.] and rye [Secale cereale L.], incorporated clover and rye, and bare soil), three N application timings (2, 4, and 8 wk after cover crop addition), and two N sources (untreated and treated urea) on the effectiveness of a urease inhibitor. Soils were incubated according to N application timing treatment, amended with the appropriate N source, and placed in chambers which captured NH3 over 7 d. There were significant interactions between cover crop treatment and N source and N source and N application timing on cumulative NH3 loss, ranging from 29 to 174 kg N ha−1. Losses were highest from treated urea when applied 2 wk after residue addition (75.9 kg N ha−1) or on top of surface residues (85.8 kg N ha−1). There was no significant effect of application timing on cumulative NH3 loss from untreated urea. However, inhibitor effectiveness did increase when residue was applied eight weeks after residue addition (77%) as compared with 2 wk after residue addition (45%). Future research should focus on alternate dosing or application timing to overcome high residue scenarios in these systems.