Soybean Yield Induced by Winter Crops in No Tillage System in Tropical Region.

The no-tillage system is utilized in approximately 30 million hectares in Brazil. However, this system still needs to be better adapted to tropical regions, with warm and dry winters, common in the southeast and central-west of Brazil. To achieve this, the choice of adapted crops to establish the system is of fundamental importance. This selection should take into consideration, besides economic return, the maintenance or improvement of chemical and physical properties of the soil which contribute to improve productive capacity. The purpose of this study was to evaluate the influence of winter crops on soybean yield in no tillage system. The field experiment was established in 2002 and conducted for seven growing seasons (2002-2009) at Jaboticabal, SP, Brazil, (21°14′S, 48°17′W and altitude of 550 m). Climatologically the area belongs to tropical/megathermal zone or Köppen's Aw. The mean annual rainfall is 1417 mm, with an annual distribution peaking in the period of October to March and a relatively dry season in the period of April to September. The soil of the experimental area is a Rhodic Eutrudox. In the 0–20 cm layer, the mean contents of clay, silt and sand is 555, 63 and 381 g kg^‑1, respectively. The experiment was conducted using a randomized block design, with tree replication. The treatments consisted of seven winter crops, sowed in February-March and repeated every year in the same plots: maize, sunflower, oilseed radish, pearl millet, pigeon pea, grain sorghum and sunn hemp. The results showed that the highest soybean yields were obtained after sunn hemp, probably, due to the highest soil P contents observed in the sunn hemp plots. And the lowest soybean yield after sorghum was, probably, due to the allelopathic effect of several components released by sorghum residues during decomposition negatively affecting the following crop.