Field experiments in 1986, 1987, and 1988 were conducted at three sites in Iowa to determine the performance of the corn hybrids, selected for variation of relative maturity ratings, at varying rates of fertilizer N in the presence or absence of N-Serve. Sites 1 and 2 were under a continuous corn cropping while Site 3 was a corn-soybean rotation. The response of 4 corn hybrids to level and form of N was studied in a factorial of 3N x 2N-Serve x 2 methods of N application at Site 1. All pre-plant N was compared to the split application of N, with the second half of the N and N-Serve rates incorporated during V9-V12 stage.;At Site 1, 5 corn hybrids were used in a factorial of 3K x 3N x 2N-Serve at 3 plant populations in 1986 and 2 plant populations in 1987. In 1986, half of the N from (NH[subscript]4)[subscript]2SO[subscript]4 was applied just before anthesis (81DAP) with wheel point injector fertilizer applicator (PIFA) with or without N-Serve. In 1987, 77 kg N ha[superscript]-1 from urea was broadcast over the entire field before planting. Additional rates of N from UAN was applied 56 DAP using PIFA, with or without N-Serve.;At Site 3a, 3 corn hybrids were used in a factorial of 6N x 3 methods of N application. For Site 3b, the response of P3471 was evaluated in a factorial of 3K x 3N x 2N-Serve x 2 methods of N application. At Site 3c, 2 corn hybrids were evaluated using 3 sources of N, namely: UAN, NP, and P. Urea ammonium nitrate and NP were applied with or without N-Serve using PIFA.;Of all the hybrids used in this experiment, the commercial corn hybrid P3377 and experimental corn line XC754 were found to have good potential to produce higher yield with most of the treatment or the factors used. At Site 1, where N and N-Serve were applied early, N-Serve increased grain yield and decreased stalk lodging. In 1986, at Site 2, where N was applied at anthesis, N-Serve decreased grain yield and stalk lodging increased, probably because of immobilization of the N. At Sites 3a and 3b, the insignificant response to the added fertilizer N and N-Serve may have been due to the large amount of fixed N by soybean crop residue which was returned to the soil.