NITROGEN UPTAKE AND USE OF 2 CONTRASTING MAIZE HYBRIDS DIFFERING IN LEAF SENESCENCE

In eastern Canada, the use of fertilizer N has been identified as the most energy-consuming component of maize (Zea mays L.) grain productio n. As the economic and environmental costs of excessive N fertilizatio n rise, there is an increased emphasis on selection of hybrids with gr eater N use efficiency (NUE; defined as the ratio of the amount of N-1 5 recovered in grain or stover dry matter to the amount of fertilizer N-15 applied to the soil in this study). Using an N-15-labelling appro ach, a field study was conducted on a tile-drained Brandon loam soil ( Typic Endoaquoll) on the Central Experimental Farm at Ottawa, Canada ( 45 degrees 22' N, 75 degrees 43' W) in 1993 and 1994. Fertilizer N upt ake and partitioning within the plant in relation to dry matter change s were monitored during development of a current stay-green maize hybr id and an older early-senescing hybrid grown with three fertilizer N l evels (0, 100, 200 kg N ha(-1)). Dry matter, N concentration and N-15 atom% enrichment of plant components were determined at five growth st ages. The current stay-green hybrid, 'Pioneer 3902' had greater NUE th an the old early-senescing hybrid, 'Pride 5', which was associated wit h 24% more dry matter production and 20% more N uptake during grain fi ll for Pioneer 3902. There was no indication of greater allocation of N to the grain in Pioneer 3902. Our data suggest that prolonged mainte nance of green leaf area for photosynthate production during grain fil l and the ability to take up available soil N later in grain filling a re characteristics of maize hybrids with greater NUE.