In Brazil, soybean is the agriculural main crop and driving of gross domestic product (GDP). However, the chemical composition of the grains, such as oil, protein and carbohydrates, exhibit variations depending on the environment to which it is exposed, may affect the industrial performance. In this sense, the present study was performed in order to study the behavior of soybean cultivars in different sowing times, at different times of sowing, as the potassium use efficiency, productivity and carbohydrate content, in order to ethanol production in the Center-South region of the State of Tocantins. In agricultural year 2013/14, were held four soybean cultivars competition trials in Palmas - TO, two being installed on December 5, 2013 and two in January 23, 2014. In each planting season, the cultivars were spread under conditions of high and low potassium (200 kg ha-1 de k2O e 40 kg ha-1 de k2O respectively). The experimental design used in each test was a randomized block with four replications and seven treatments: BRS 333 RR, BRS 33871 RR, BRS 325 RR, M 9144 RR, P 98Y70 RR, TMG 1180 RR e TMG 1288 RR. The total reducing sugar content characteristics were studied and yields of total reducing sugars, as well as efficiency for the use of potassium. The season of sowing and potassium fertilization influenced the behavior of cultivars for content and yield of total reducing sugars. High temperatures and low rainfall index, the grain filling stage, favoring the accumulation of total reducing sugars in the grain. The cultivars TMG 1288 RR and TMG 1180 RR present as potentially promising for ethanol production, the latter being also efficient as the potassium use.
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