Summary
Four planting periods, spaced at 15 days intervals on December 4, were used in a field plot at the INPA-ARIPUANÃ Experiment Station — Mato Grosso do Norte with the object of studyng the effects of planting periods on several agronomic characteristics of the varieties "Jupiter", "Prata", "Oriente" and "Pelicano" during the farming season of 1976/77. The split plot design was repented four times, using planting periods and varieties as main plot and sub-plots, respectively. The results obtained showed the following: With a delay in planting there is a decrease in the plant cycle. This is evident in the vegetative period and/or reproductive period. This is a consequence of photoperiod. Along with this decrease of plant cycle there is a decrease in plant height, stem diameter, number of pods per plant and an increase in first pod insertion. The variety "Jupiter" showed resistence to bacterial pusture (Xanthomonas phaseoli var. sojensis). The best planting date both varieties was December 18, which presented a yield of: "Oriente" = 2,795.83 kg; "Jupiter" = 2,245.83 kg; "Pelicano" = 1,733.33 Kg and "Prata" = 1,145,08 kg/ha. For the planting of soybean Aripuanã-MT, the variety "Oriente" in December and variety "Jupiter" in December until the middle of January are recommended. As this is a pioneer experiment in this region, the choosing of the planting period was dependent on climatic conditions of the region using varieties of various other that are less sensitive to the photoperiod.