YIELD AND HARVEST-INDEX RESPONSES TO PREFLOOD NITROGEN-FERTILIZATION AT LOW RICE PLANT-POPULATIONS

Rice (Oryza sativa L.) producers frequently have plant populations inadequate for producing maximum crop yields and must be able to alleviate this situation. The objectives of this study were to determine the following: (i) the upper limit of the linear N response at lower-than-optimum plant population densities; (ii) the N response of suboptimal rice plant populations that are near the optimal plant population of 15 plants/sq ft; and (iii) yield component and harvest index responses, which help explain the yield responses. Field experiments were conducted at Keiser, AR, in 1989 and 1990. Seeding rate treatments were 5, 25, and 45 seeds/sq ft, and preflood N fertilization rates were 0, 60, 90, 120, 150, and 180 lb/acre. In 1989, yields responded linearly to the entire range of N rates at 2 plants/sq ft. Yields subsequently leveled off between 90 and 120 lb N/acre at 8 and 14 plants/sq ft. In 1990, yields responded linearly to 120 lb N/acre at 3 and 13 plants/sq ft, to 90 lb N/acre at 25 plants/sq ft, and subsequently leveled off. At very low plant populations, to achieve maximum yields, 30 to 60 lb/acre more N was required than was needed to produce maximum yields at adequate plant populations. At marginally low plant populations, to achieve maximum yields, 0 to 30 lb/acre more N was required than was needed to produce maximum yields at adequate plant populations.