Nitrogen Application Timing Effects on Nitrogen Efficiency of Dry‐Seeded Rice

Abstract

AbstractOptimum N application timing contributes significantly to successful rice (Oryza sativa L.) production. In many areas of the southern U.S. Rice Belt, where rice is produced in a dry‐seeded, delayed‐flood system, optimum fertilizer timing has traditionally been achieved by applying 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 applications near the beginning of reproductive growth. Field studies were conducted during 1992 and 1993 at two locations to evaluate the optimum timing for midseason N applications. ‘Laccassine’ rice was grown in microplots (0.58 m2) established with galvanized steel collars. Nonlabeled urea was applied at the four‐ to five‐leaf growth stage at rates 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 initiation (PI) (PI‐1), (ii) 67.2 kg ha‐1 at panicle differentiation (PD) (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 PD + 10 d (PD‐2), in addition to a zero‐N treatment. Plant samples were collected to measure total dry matter, fertilizer and total N uptake, and grain yields. Fertilizer N uptake was not affected by midseason N application timing during 1992 and was only slightly affected during 1993. Increasing the pre‐flood N rate significantly increased midseason fertilizer N uptake efficiency, total N uptake, total dry matter accumulation, and grain. Midseason N applications increased total dry matter accumulation, total N uptake, and grain yields compared with no midseason N at one location, but were minimally influenced at the other location. Results suggest that no real 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.