Associative effects between slow-release urea (SRU) and different types of corn grains [i.e., steam-flaked corn (SFC) vs. high-moisture corn (HMC)] need to be investigated to improve N utilization and lactational performance of dairy cows. This experiment was performed to test a hypothesis that supplementing SRU (Optigen, Alltech, Nicholasville, KY) in HMC-based diet would improve microbial fermentation and nutrient utilization by lactating dairy cows due to a better synchronous supply of dietary N and ruminally fermentable carbohydrates. Eight multiparous Holstein cows (32 ± 8.2 DIM) were used in a duplicated 4 × 4 Latin square design, and within each square, cows were randomly assigned to a sequence of 4 diets during each of the 4 21-d periods (14 d of treatment adaptation and 7 d of data collection). The 4 dietary treatments included: SFC without SRU, SFC with SRU, HMC without SRU, and HMC with SRU. Data were analyzed with a model that included the fixed effect of type of corn grain, SRU supplementation, and their interaction with cow and period as the random effects. Supplementing SRU with SFC but not with HMC increased intake of DM and CP, resulting in tendencies for an interaction between corn grain (CG) and SRU supplementation (CG × SRU; P = 0.06). Milk yield did not differ among treatments (39.0 ± 0.37 kg/d). Milk true protein concentration increased with SFC with SRU, whereas it decreased with HMC with SRU (P = 0.01). Dietary treatments did not affect ruminal concentration of volatile fatty acids and ammonia-N. Feeding HMC tended to increase feed efficiency for milk production (P = 0.09) and N utilization efficiency for milk N (P = 0.10), but supplementing SRU did not influence the efficiency parameters. Supplementing SRU with either SFC or HMC resulted in minor effects on ruminal fermentation and lactational performance possibly due to relatively less dietary concentrations of the corn grains (13.6% DM on average) in high-forage diets (60% of the diet DM). However, feeding HMC improved feed and N utilization efficiency for milk production.