Climate and management in soybean productivity

The definition of a method that makes it possible to distinguish between these two sources of variation in productivity of soybean cultivation is of great use to producers and consultants, as it makes it possible to define the strategies to be adopted to minimize losses and, consequently, optimize agricultural efficiency.

A soybean productivity is the result of the complex interaction between the genotype (cultivate) employee, weather conditions and agricultural management in the areas of production. While solar radiation and photoperiod control the amount of energy available for photosynthesis, air temperature affects the plant metabolism, which in turn regulates both the rate of photosynthesis regarding maintenance breathing, in addition to controlling, together with the photoperiod, the rate of development of the crop, defining the duration of different phenological and cycle phases. Furthermore, temperature also acts on the hormonal balance of soybean plants, with a fundamental role in the reproductive, when high temperatures can cause flower abortion and green beans. Solar radiation, together with temperature, air humidity and wind speed, are responsible for the water demand of crops soybeans, which if not met by rain will cause water deficits, highly detrimental to productivity. The volume and distribution of rainfall are essential to guarantee good levels of crop productivity. Years very dry conditions result in low productivity, however, excessively dry years cloudy and rainy conditions can also lead to reduced productivity due to decreased availability of solar energy for photosynthesis. Being Thus, it can be said that the years with the best productivity are those that combine a good distribution of rainfall throughout the cycle with periods long enough periods of high solar radiation so that plants can make use of soil water in order to maximize the photosynthesis process, generating high productivity.

In Brazil, the effect of climate variability on soybean productivity is quite evident, both between the different cultivation seasons and between the different producing regions and even between the various sowing dates. Between the different harvests, the variability of soybean productivity is mainly conditioned by climatic events such as the El Niño Southern Oscillation (ENSO), whose hot (El Niño) and cold (La Niña) phases affect thermal and rainfall regimes from different Brazilian regions. Thus, while in the state of Mato Grosso there is little variability in soybean productivity in Rio Grande do Sul, more affected by ENSO, this variability is more pronounced, which conditions a greater climate risk for soybean cultivation. In an intermediate situation, meets the MAPITOBA region, where despite the harmful effects of El Niño, productivity does not fluctuate as much as in the South region.

In addition to the effect of climate on soybean productivity, crop management also presents a great influence, since the combination of cultivar/maturation group, season sowing and soil fertilization and crop protection practices (control of pests, diseases and weeds) is fundamental for defining the level of productivity to be achieved. Despite this distinction between the effects climate and agricultural management on soybean productivity, it is not always It is possible to easily distinguish how much productivity loss is caused by climatic adversities (especially water deficit) and by management deficiencies. In this way, the definition of a method that allows the distinction between these two sources of variation in crop productivity soybean is of great use to producers and consultants, as it allows define the strategies to be adopted to minimize losses and, consequently, optimize agricultural efficiency.

 One of the ways to quantify the impacts climate and management of soybean productivity is through the use of techniques that allow simulating the growth and development of soybean crops, in order to potential productivity (maximum to be achieved by the genotype in a given climatic condition, without lack of water and with optimal management) and attainable (maximum to be obtained by the crop in rainfed conditions with management excellent). Once these productivities have been defined, respectively called PP and PA, they can be compared to the real productivity (PR), obtained by producers, thus defining how much of the drop in productivity came from climate limitations (especially water deficit), due to the difference between PP and PA, and how much was caused by management deficiencies, by difference between PA and PR.

Figure 1 presents the productivity data of the champions of the Soy Strategic Committee Brazil (Cesb) in the last three harvests (2014-15, 2015-16 and 2016-17), as well as the PP and PA values ​​for each of the winners' locations. It is observed that PP reached the maximum value of 214 sc /ha for the N/NE champion in the 2014/15 harvest (Figure 1), a fact associated with the greater supply of solar radiation that occurred during the harvest in this region. In other harvests analyzed, the champions of the N/NE region also presented the highest potential productivity, always above 180 sc /ha. On the other hand, the value minimum of 167 sc/ha occurred in the 2014/15 harvest for the champion of region S (Figure 1), probably due to cloudy days and a low supply of solar radiation. Despite this large variation, these values ​​demonstrate the high soybean productivity potential available in the production environments of the Brazil, which, however, is difficult to achieve due to the limitations imposed by periods of water deficit and the sub-optimal management carried out in the field.

When there was the penalty of PP for water deficit, obtaining greater differences in PA between the minimum and maximum values ​​were observed. The lowest PA was 99 sc/ha for the champion of the N/NE region in the 2015/16 harvest (Figure 1), a place that had a loss of 107 sc/ha due to water deficit. The highest BP value was 164 sc/ ha for the champion of the CO region in the 2014/15 harvest, where losses due to water deficit totaled 37 sc/ha (Figure 2). On average, the water deficit was responsible for the reduction of 44 sc/ha in the champions' areas, including irrigated areas, oscillating between a minimum of 14 sc/ ha for the champion of CO region in the 2016/17 harvest and a maximum of 107 sc/ha for the region champion N/NE in the 2015/16 harvest (Figure 1).

For PR it was a variation from 83sc/ha to 149sc/ha was observed. The minimum value was obtained by champion of the N/NE region in the 2015/16 harvest, the area with the greatest deficit reduction water, in the order of 107 sc/ ha, and only 16 sc/ ha due to management. Already the biggest value was achieved in the 2016/17 harvest in the champion area of ​​region S, where there was reduction of only 26 sc/ ha due to water deficit and only 1 sc/ ha due to the management deficit, which indicates the excellent water condition for soybean crop in this area/crop and the exceptional performance of the producer champion in managing your culture. Low handling losses were also observed in the champion areas of region S in the 2014/15 harvest (2 sc/ha), from N/NE region in the 2014/15 harvest (8 sc/ ha), and the SE region in the 2016/17 harvest (6 sc/ there is). On the other hand, there are cases in which management losses exceeded losses due to water deficit, which indicates that there is still room for improvement agricultural practices and, consequently, to increase productivity.

In a way overall, considering an average of all the champions' areas, the largest losses in soybean productivity were due to water deficit, which accounted for an average of 43,4 sc/ha, while losses resulting from the agricultural management were in the order of 28,0 sc/ ha. This shows that 61% of total losses of crop productivity were due to the water deficit, while 39% due to management deficit.

The results of CESB show that currently, in producing areas, there is a greater effect of climate in limiting soybean productivity. However, when analyzing the average national productivity of the last three harvests of the National Company of Supply (Conab), of the order of 51 sc/ha, and this data is compared to the average values ​​of the Cesb champions (115 sc/ha), it is clearly observed that this effect is much greater, which is probably associated with the limited volume of soil available for the growth of the root system, which makes the water deficits are more pronounced than those that occur in areas of champions, where the soil profile is invariably deeper. Along these lines, actions that maximize productivity are required, in order to minimize impacts of water deficit on soybean productivity. Among these actions, the choice of sowing dates with lower climate risk, as recommended by the climate risk zoning of the Ministry of Agriculture, Livestock and Supply (Map), the selection of cultivars best adapted to the region, especially in relation to drought tolerance, and investments in soil management actions (chemistry, physical and biological) to increase the exploitation profile of the root system, in order to increase the water available for plants, some of which will allow Brazilian producers to achieve high productivity in short-medium term.

Article published in issue 228 of Cultivar Grandes Culturas, May, 2018.