Risco de ocorrência de excesso e déficit hídrico na soja em terras baixas

Abstract

The objective of this study was to determine the duration of the soybean development cycle as a function of the sowing date and to obtain the probability of water excess and deficit in Planosols of the Vacacaí and Piratini river basins in the southern half of Rio Grande do Sul. Soybean development was simulated considering three sets of cultivars of relative maturity groups (RMG) between 5.9-6.8, 6.9-7.3 and 7.4-8.0, in 11 sowing dates comprised between September 21 and December 31. Daily meteorological data were used from 1971 to 2017 obtained from the Pelotas agroclimatological station and from 1968 to 2017 from the main climatological station of Santa Maria. Water excess (days) and water deficit (mm) were obtained from the calculation of evapotranspiration and daily sequential crop water balance. Data of duration of subperiods and soybean development cycle, deficit and water excess in the subperiods and cycle for each RMG were demonstrated through the BoxPlot exploratory analysis and subjected to analysis of variance and mean comparison by the Scott-Knott test at 5% error probability. Data were also subjected to the probability distribution analysis, in which the exponential, gamma, lognormal, normal and weibull probability density functions adjustments were tested using Chi-square and Kolmogorov-Smirnov adhesion tests, with a 10% significance level. The development cycle duration is higher in Pelotas than in Santa Maria and there was a decrease from the first to the last sowing date in both locations. The climatic risk of occurrence of water excess in the soybean cycle decreases throughout the sowing date calendar. There is no water excess in about one every four years during the soybean crop establishment, but this frequency tends to reduce starting from mid-November. The mean frequency of water excess is two every three years for the period between the beginning of flowering (R1) and beginning of grain filling (R5). There are small differences in the risk of occurrence of water excess between the sowing dates of the R1-R5 subperiod, but there is a reduction starting from November 11 for the soybean initial establishment. The risk of water deficit decreases from the beginning of November for the subperiod R1-R5 and for the total soybean cycle. The sowing date must be defined specifically for each farm location. However, the sowing procedure should be planned to start from November 1st when considering jointly acquiring high soybean yield potential and the lowest historical risks of occurrence of water excess and deficit.