Yield of first-year corn (Zea mays L.) following alfalfa (Medicago sativa L.) on fine-textured soils responds to fertilizer nitrogen (N) about one-half of the time, but current methods of predicting N-responsive sites and the economic optimum N rate (EONR) for N-responsive sites are not reliable. This study was conducted to quantify the responses of grain and silage yields and N uptake to fertilizer N rate in first-year corn following alfalfa on fine-textured soil and determine whether these are influenced by the time of N application. In 2014, 14 on-farm experiments were conducted in Minnesota with five N rates applied near planting or as a sidedress at the five to six leaf-collar corn stage in first-year corn on fine-textured soils. In three of four experiments where grain yield was increased with fertilizer N, the EONR was not affected by N application time and ranged from 114 to 200 kg N ha-1 at a 5.6 price ratio (US$ kg-1 N/US$ kg-1 grain); the EONR at the one location affected by N application time was 91 kg N ha-1 when N was applied near planting compared to 30 kg N ha-1 applied as a sidedress. Maximum grain yield was greater when N was sidedressed compared to applied near planting at three locations and was not influenced by the time of N application at the fourth location. Silage yield was increased with fertilizer N at seven locations. The EONR at N-responsive locations was not affected by N application time and ranged from 101 to >200 kg N ha-1 at a 13.9 price ratio (US$ kg-1 N/US$ kg-1 silage). Silage yield was greater at all N-responsive locations when N fertilizer was applied as a sidedress compared to near planting. Grain and silage N uptake by corn was increased with fertilizer N at 14 and 13 locations, respectively, and was greater with N applied as a sidedress compared to near planting. When N uptake was maximized, apparent grain and silage fertilizer N recovery efficiency was 11 to 53% across N application times. Corn response to fertilizer N was related to weather conditions that limited N availability and crop uptake. These results indicate that sidedressing N fertilizer in first-year corn following alfalfa at the five to six leaf-collar corn stage can mitigate risk of yield loss when there is elevated risk of N loss due to excessive precipitation prior to sidedressing. Improved methods of predicting the response of grain yield to fertilizer N in first-year corn following alfalfa on fine-textured soils are needed to maximize the benefits of alfalfa in cropping systems and reduce N losses. Data from 21 site-years were used to (i) determine how the Illinois soil N test (ISNT) and pre-sidedress soil nitrate test (PSNT) soil concentrations change during alfalfa-corn rotations; (ii) evaluate the ability of the ISNT and PSNT to predict grain yield response to fertilizer N in first-year corn following alfalfa; and (iii) investigate relationships between the ISNT or PSNT and potentially mineralizable soil nitrate-N. Neither the ISNT nor the PSNT changed as a result of increasing alfalfa stand age in five trials, and the PSNT was greater in first-year corn following alfalfa compared to other crop phases in rotations in three of five trials. The PSNT more accurately predicted corn response or non-response to fertilizer N than the ISNT (62 and 52% of site-years correctly classified, respectively). A multiple logistic regression equation was developed to predict first-year corn response to fertilizer N. Measures of fit and model accuracy were improved when the PSNT, but not the ISNT, was included as a predictor variable. Incubation of soil from first-year corn showed that the ISNT explained 30% of the variation in mineralized soil nitrate-N at the end of the 17-wk incubation period, compared to 13% explained with the PSNT. There was a linear relationship between the PSNT and either aboveground corn N uptake (ANU) or total available N (TAN), and between the ISNT and potentially mineralizable soil nitrate-N (PSN). Estimates of ANU, TAN, and PSN were improved when additional predictor variables were included in models. Overall, the PSNT was less related to soil N mineralization, but more closely related to the response of grain yield to fertilizer N in first-year corn following alfalfa, than the ISNT. Future research should seek to increase knowledge about the size and mineralization rate of soil N pools in first-year corn following alfalfa, thereby increasing the ability to predict sites where corn yield will respond to N fertilizer and the EONR for these sites.