Guineagrass (Panicum maximum Jacq.) is a valuable grass in grazing systems in Puerto Rico, but when conserved as hay, its low protein (<6.0%) concentration limits both meat and milk production. This study assessed the effect of applying liquid urea (LU) on hay harvested at 8-weeks regrowth of guineagrass cvs. 'Mombasa' and 'Tanzania' hay. Liquid urea were applied in a fine mist at baling at low (L; 0 lt/ha), medium (M; 204 lt/ha) and high rate (H; 807 lt/ha). Hays were stored for 8 week and core samples (250 g) were taken for determination of chemical composition of crude protein (CP), neutral detergent fiber (NDF) and acid detergent fiber (ADF) using a completely randomized design. There were cultivar effects and LU rate effects (P<0.05) for CP, but there was no interaction. Crude protein averaged 13 and 17.7% for cv. Mombasa and Tanzania, respectively. There was a linear increase (P<0.05) in CP with an increasing rate of LU. Mean CP was 12, 15 and 20% for the L, M, and H rate, respectively. Additive LU did not affect (P<0.05) either NDF or ADF concentration in Mombasa and Tanzania. This study shows that adding LU at baling increases CP, but has no effect on NDF or ADF. Liquid urea at either concentration can be used to increase the nutritive value of low quality hay in Puerto Rico. Forage intake and digestibility and nitrogen balance studies, however, are needed to determine potential losses of N.