NITROGEN APPLICATION TIMING EFFECTS ON NITROGEN EFFICIENCY OF DRY-SEEDED RICE

Optimum N application timing contributes significantly to successful r ice (Oryza sativa L,) production. In many areas of the southern U.S. R ice Belt, where rice is produced in a dry-seeded, delayed-flood system , optimum fertilizer timing has traditionally been achieved by applyin g 50 to 65% of the total N required onto dry soil immediately prior to establishing the permanent flood and the remainder in one or two appl ications near the beginning of reproductive growth. Field studies were conducted during 1992 and 1993 at two locations to evaluate the optim um timing for midseason N applications. 'Laccassine' rice was grown in microplots (0.58 m(2)) established with galvanized steel collars. Non labeled urea was applied at the four- to five-leaf growth stage at rat es of 0, 67.2, and 134.4 kg N ha(-1) just prior to flooding. Nitrogen- 15 labeled urea was applied at midseason at a total rate of 67.2 kg N ha(-1) in four different treatments: (i) 67.2 kg ha(-1) at panicle ini tiation (PI) (PI-1), (ii) 67.2 kg ha(-1) at panicle differentiation (P D) (PD-1), (iii) 33.6 kg ha(-1) each at PI and PI + 10 d (PI-2), and ( iv) 33.6 kg ha(-1) each at PD and Po + 10 d (PD-2), in addition to a z ero-N treatment. Plant samples were collected to measure total dry mat ter, fertilizer and total N uptake, and grain yields. Fertilizer N upt ake was not affected by midseason N application timing during 1992 and was only slightly affected during 1993. Increasing the preflood N rat e significantly increased midseason fertilizer N uptake efficiency, to tal N uptake, total dry matter accumulation, and grain. Midseason N ap plications increased total dry matter accumulation, total N uptake, an d grain yields compared with no midseason N at one location, but were minimally influenced at the other location, Results suggest that no re al advantage is gained from split application of midseason N compared with a single application at midseason and that it can be made from PI to PD growth stage.