Maximum manure N use efficiency by corn (Zea mays L.) requires that N mineralization and subsequent plant availability coincide with N uptake demand. Information regarding the effects of time of manure application and use of a nitrification inhibitor on manure mineralization and corn yield is needed for producers to make sound manure management decisions. Studies were conducted at seven southern Minnesota sites over 3 yr to determine the effects of three manure application times (mid-September, mid-October, and late April), two manure rates, and a nitrification inhibitor, (nitrapyrin [NP] 1.0 lb a.i./acre), on corn yield and soil nitrate N concentration (0 to 12 in.) at the V4 growth stage, Application rates ranged from 3000 to 4000 gal/acre for the low rate and from 6000 to 8000 gal/acre for the high rate. For the four sites receiving liquid dairy (Bos taurus) manure, soil nitrate N was greatest when manure was applied in April and lowest when applied the previous September. The addition of NP increased soil nitrate N at three sites but affected grain yields at only one site when averaged across time and rate of manure application. Grain yields for the April, October, and September applications averaged 146, 142, and 139 bu/acre, respectively, across manure rates and NP treatments, with April exceeding the September application time (P < 0.10). When liquid swine (Sus scrofa domesticus) manure was used, the effects of application time and NP on soil nitrate N concentration and grain yield depended on climatic conditions. At one site where 8.5 in. of rain fell in a 5-wk period in the spring, NP significantly increased both soil nitrate N concentration and grain yield for the October application but had no effect with the April application. Similar effects of NP on soil nitrate N were found at the other sites, but grain yields were not increased. Results from the seven site-yr support about a 5% yield increase with April application compared with fall applications. However, it is unlikely that many farmers will consider this an economic advantage because of logistical challenges often faced with spring applications of manure in the Northern Corn Belt.