CHLOROPHYLL METER TO PREDICT NITROGEN TOPDRESS REQUIREMENT FOR SEMIDWARF RICE

Semidwarf rice (Oryza sativa L.) tolerates excessive N fertilizer without lodging. In fact, it is difficult to maximize semidwarf rice yield without applying excessive N fertilizer since most farmers have not found a quick, unbiased, reliable method for assessing the N requirement for rice plants. A portable chlorophyll meter that instantly provides a measure of leaf chlorophyll may quickly monitor the need for additional N fertilizer. The Minolta model SPAD 502 chlorophyll meter was evaluated during a 3-yr period for its potential to determine the need for N fertilizer by semidwarf 'Lemont' rice grown in research plots on several soils and locations in Texas. Preplant N rates ranging from 56 to 280 kg N ha-1 were applied to paired plots to develop plots with a range in leaf chlorophyll. At one of four plant growth stages, one of the pair of plots was topdressed with 50 kg N ha-1 after measuring leaf chlorophyll. At the prepanicle initiation and panicle differentiation stages, the rice yield increase due to N topdressing dropped from about 1600 to 100 kg ha-1 as the chlorophyll meter readings of the most recently matured leaf increased from 24 to 42, respectively. As chlorophyll readings exceeded 40, N fertilizer did not increase yield. However, at the tillering and heading growth stages chlorophyll readings were not well correlated with N fertilizer needs. These data provide evidence that the chlorophyll meter can be used to determine topdress N needs of semidwarf rice at pre-panicle initiation and panicle differentiation.