Best time to sow soybeans in lowlands

Choosing the soybean sowing date does not entail costs for the producer and modifies the productive potential and water availability throughout the development cycle. Furthermore, it is a strategy to minimize the risk of loss of productivity due to stress.

To obtain high productive potential from soybeans, it is necessary to match the climatic conditions according to the needs of soybeans at different stages of development. This can be adjusted by appropriately defining the sowing time, as well as the Relative Maturity Group (GMR) of the cultivars.

However, the highest yields are not always obtained in crops sown at the time previously considered ideal. This occurs due to the numerous stresses that can occur throughout the cycle due to the variation in time between years. Particularly, lowland areas have flat relief and a low rate of water infiltration into the soil, which makes water excess frequent, reducing seed germination, plant population and biological nitrogen fixation. This environment also harms root growth and consequently reduces the volume of soil explored, intensifying water deficiency.

At this point, you must be asking yourself: How can you avoid these stresses and obtain high productivity at the end of the harvest? This is not a simple and objective answer, as weather conditions are different every year. However, knowledge of patterns, that is, what normally happens, can help in establishing management strategies aimed at minimizing risks, including defining the sowing time.

So, how do we know what normally happens if data collection and soybean cultivation in lowlands became more relevant a few years ago? This was exactly the focus of this work, that is, obtaining data on water excess and deficit that occurred in the cycle for different sowing dates in more than 45 agricultural years based on mathematical modeling of soybean development, soil data, daily water balances and probability analyses. In this way, the magnitude of the risks of occurrence of water deficit and excess was determined depending on the sowing date for two regions of great importance, Central and Southern RS.

In general, the risk of water deficit occurring during the soybean cycle shows a decreasing trend throughout the sowing calendar in both locations studied (Figure 1). The risk of drought was lower in Pelotas than in Santa Maria, mainly due to the lower evapotranspiration of the crop in the south of RS. In general, this decreasing trend can be considered similar to the other soybean growing regions in RS, but with specific differences in magnitude according to rainfall characteristics, the water storage capacity of different soils and atmospheric demand. for water in each region.

The risk of water deficit was also analyzed, especially for two critical soybean sub-periods. For the sub-period between the beginning of flowering (R1) and the beginning of grain filling (R5), it appears that the greatest risk of occurrence of water deficit coincides with October sowings, mainly for levels of 25, 50 and 75 mm of water deficit, and from November 01st there is a linear decrease in probability values ​​(Figure 2A, 2C). For the sub-period between R5 and physiological maturation (R7), there is a decreasing trend from 21/Sep to 31/Dec, considering risk levels of 25, 50 and 75 mm in both regions (Figure 2B, 2D) .

The risk of excess water during the development cycle is greater for soybeans sown early (Figure 3). Thus, for example, sowings on 21/09 in Santa Maria have a probability of 78% of the years that there will be 35 or more days of excess water throughout the soybean cycle, reducing this risk to 35% of the years for sowing on 21 /dec (Figure 3A). This interpretation can be used for all other sowing dates. For the Pelotas region, there is a lower risk of excess water in the sowing cycle from November 21st (Figure 3B).

The risk of excess water occurring in the subperiods between sowing (S) and emergence (EM) and between EM and the first trifoliate leaf (V2) is greater in the first sowing dates (Figure 4). For the level of risk of occurrence of at least four days of excess water, simulated sowings until November 01st presented a probability above 80% of the years. Thus, with the exception of years in which there is no excess water in these sub-periods (around 25% of the years), in eight out of every ten years in which soybean is sown on these dates, there will be four or more days of excess water in the soil, which is extremely harmful to the germination process and may lead to the need for reseeding. On the other hand, from November 11th the risks are significantly lower, including the risk of having 8 or 12 days with excess water (Figure 4), with greater chances of obtaining an adequate initial establishment of soybeans.

Final considerations

In general, there are opposite situations that make it difficult to define the ideal sowing date for soybeans in lowlands. If, on the one hand, there is a decrease in productivity potential in sowings carried out from the beginning of November, on the other hand, it is at the beginning of the soybean sowing calendar that the risks of stress, both due to water deficit and excess, are greater.

Considering the data from this work and data from the literature, it can be stated that the preferred sowing window, with high production potential and lower water risk, would be between 15/10 and 15/11. Sowings carried out before October 15th, despite having high productivity potential in years without lack or excess of rain, should be avoided in order to reduce the risk of losses due to water stress, which is very high. Sowing from 01/Nov significantly reduces risks in the Southern Half of RS. It is also worth noting that sowings carried out in the second half of October should only be considered by producers with already consolidated management practices, in areas with a good drainage system and without risk of flooding due to rising river levels.

Mateus PossebonBortoluzzi,

University of Passo Fundo (UPF)

Arno Bernardo Heldwein,

Federal University of Santa Maria