Effects of N-fertilization and Off-season Crops on Soybean Yield and Grain Protein and Oil Concentrations in a Tropical Climate

In the no-tillage system in Brazil, grasses are commonly grown for grain production or soil cover in the soybean off-season with no or low nitrogen (N) fertilization rates. The soybean sowing in soil containing high amounts of grass straw can lead to mineral N temporary immobilization at the beginning of the crop cycle. Some farmers apply N fertilizers at soybean sowing, often combined with seed inoculation with Bradyrhizobium spp. to circumvent that limitation. The objective of this work was to evaluate the effects of N fertilization at soybean sowing cultivated after different off-season crops or after fallow on soybean yield and grain protein and oil concentrations. The field experiment installed in Londrina, Paraná, Brazil, lasted seven years. The treatments were plots unfertilized or fertilized with 30 kg ha−1 N at soybean sowing with five land uses in the off-season: (i) corn for grain production with N (80 kg ha−1) broadcasted, (ii) corn for grain production without N fertilization, (iii) wheat for grain production without N fertilization, (iv) ruzigrass (Urochoa ruziziensis) as a cover crop, and (v) unplanted fallow. Results showed no interaction between soybean N fertilization and off-season crops on any variable. Soybean N fertilization did not affect grain yield (mean of 4064 kg ha−1 without N and 4136 kg ha−1 with N fertilization) in any of the seven seasons, including when the yield average was higher than 4500 kg ha−1, which implies a high N demand for grain production. N applied at soybean sowing did not influence grain protein or oil concentration. Off-season cultivation of ruzigrass and wheat resulted in higher soybean yields (4354 and 4304 kg ha−1, respectively) than off-season cultivation of corn with or without N and fallow (4058, 3942, and 3843 kg ha−1, respectively). Soybean protein concentration (367 g kg−1) was highest after ruzigrass and lowest (354 g kg−1) after fallow. Soybean cultivated after N-fertilized corn yielded the maximum oil concentration (222 g kg−1) and rendered the minimum (216 g kg−1) after wheat. The results indicate that the mineral N application at soybean sowing was unnecessary, even in plots with high amounts of grass straw produced during the off-season.