Little information is available on the responses to N fertilizer of adapted, short-season corn (Zea mays L.) hybrids differing in grain N concentration. The influence of three rates of N fertilizer on grain and stover yields, N accumulation, and partitioning of N between stover and grain of three 95- to 100-d relative maturity hybrids AgriPro (AP) 175, Pioneer (P) 3906 and P3737, was studied in two irrigated experiments at Oakes, ND, in 1989 and 1990. The soil was a Maddock sandy loam (sandy, mixed Udorthentic Haploborolls) low in available N. Marked yield responses to N fertilizer were obtained, but the N fertilizer × cultivar interactions were not significant for 11 yield and N characteristics. Hybrid AP175 averaged over N fertilizer rates yielded 9 and 7 bu/acre more than P3906 in 1989 and 1990, respectively. There were no significant differences in grain yields between AP175 and P3737. Grain N concentration of P3906 at maturity in 1989 was 14 and 9% greater than that of AP175 and P3737, respectively. The corresponding increases in 1990 were 15 and 7%, respectively. Differences in grain N concentration due to hybrid did not result entirely from dilution effects. For instance, AP175, the highest grain-yielding hybrid, accumulated 8 Ib/acre less grain N in both years than did P3906, the lowest yielding hybrid. The N use efficiency (NUE) (Ib grain/lb N in above-ground plant parts) was relatively constant for indivdiual hybrids, and averaged over N rates ranged from 68 in both years for AP175 to 61 and 60, respectively, for P3906 in 1989 and 1990. Grain of P3906 in both years was a more concentrated N source than grain of P3737, and especially of AP175, grown under similar conditions. Research Question Interest in irrigated corn production in the Northern Great Plains has increased because of possible diversion of Missouri River water to eastern North Dakota. Little information is available on the responses to N fertilizer of adapted, short-season corn hybrids differing in grain N concentration. The main objective of this research was to compare the N fertilizer responses of three adapted 95- to 100-d corn hybrids differing in grain N content. A secondary objective was to determine the influence of N fertilizer and hybrid on the relative distribution of N between stover and grain. Literature Summary Several studies on the response of diverse corn hybrids to N fertilizer have been conducted in the Corn Belt. There is controversy over whether certain hybrids should receive different rates of N fertilizer, a question of particular importance in irrigated areas because of possible groundwater contamination with nitrate. When grown in southern Wisconsin, an area suitable for later maturing cultivars, hybrids P3906 and P3737 differed greatly in yield, but responded similarly to N fertilizer. Hybrids P3906 and P3737, high and intermediate grain protein types, respectively, are adapted to southern North Dakota, and do not normally display such extreme yield variability here. Study Description Hybrids AgriPro (AP)175 (a low grain-protein type), Pioneer (P)3906, and P3737, treated with three rates of urea-N, were grown under irrigation in each of 2 yr at Oakes, ND. The soil was a Maddock sandy loam. Applied Questions Did the three cultivars respond differently to N fertilizer? Hybrids AP175, P3906, and P3737 had similar N fertilizer requirements even though grain N percentage differed significantly among cultivars. Responses to N fertilizer, averaged over hybrids, were 57 and 45 bu/acre in the two experiments. How did the three cultivars differ in yield and N accumulation? Hybrid AP175, the highest yielding cultivar, had the highest N use efficiency, due partly to less accumulation of grain N (Table 1). Hybrid P3737, which outyielded P3906 by 30 bu/acre in southern Wisconsin, did not display such superiority in North Dakota, an area with a shorter growing season. Hybrid P3906 had the highest grain N percentage, and accumulated more total N in grain on a per acre basis than the two other cultivars. This could be of significance from an animal nutrition point-of-view. Table 1. Two-year averages of selected yield and N characteristics of three corn hybrids. Hybrid‡ Parameter† AP175 P3906 P3737 Yield (R6), bu/acre 168 160 166 Grain N (R6), % 1.17 1.34 1.24 Grain N (R6). lb/acre 96 103 99 Grain + stover N (R5.5) lb/acre 120 127 128 NUE 68 61 63 † R6 and R5.5 indicate physiological maturity and the location of the starch line halfway to the base of the kernel, respectively. ‡ Data for hybrids were averaged over N fertilizer rates.