RESPONSE OF NITROGEN USE EFFICIENT SORGHUMS TO NITROGEN-FERTILIZER

Little information is available on the response of grain sorghum [Sorg hum bicolor (L.) Moench] genotypes differing in nitrogen (N) use effic iency (NUE) (g DM g N-1) to added N fertilizer. Such knowledge is impo rtant for reducing the reliance upon fertilizer N. A dryland field exp eriment was conducted in 1993 and 1994 at Mead, NE evaluating the agro nomic responsiveness of 13 sorghum genotypes differing in NUE to three N rates (0, 50 and 100 kg N ha(-1)) and also to determine physiologic al factors that contribute to improved NUE. The experiment was conduct ed on a fine montmorillonitic, mesic, Typic Argiudoll soil. Total N at maturity, dry matter, and grain yield were used to calculate NUE term s. Genotype differences were found for all measured variables both yea rs, but no N rate by genotype effects were significant. Nitrogen ferti lizer enhanced plant N contents and grain yield, but decreased NUE for total biomass and grain production. An early freeze in 1993 markedly reduced the later maturing genotype grain yields which, in turn, influ enced NUE group comparisons. All genotypes in the study attained their full yield potential in 1994. The linear response to N rate of the N non-responsive group was significantly less than the moderately respon sive or N responsive group. High NUE sorghums had greater yields than low NUE types averaged over N levels only in 1994 since Naga White, a high NUE type, did not reach maturity in 1993. There was no difference in the linear response to N between these two groups. A linear increa se in grain yield with increasing N rate was significantly greater for hybrids than lines. The results suggest that specific selection for h igh NUE sorghums will not diminish responsiveness to applied N.