Soybean Yield Gaps in Brazil - Climate x Crop Management Causes

Many are the concerns related to the food, fibers and energy production around the world. The demand by agricultural products has increasing in a rate greater than the offer and there is an expectation that crop yields, in general, may increase by 70% till 2050. The changing climate and the limitations of other natural resources in addition to the growing worldxs population has stimulate several studies about how to increase agricultural production without devastating forests and other biomass. It is estimated that 80% of crop production increases in developing countries will come from yield, while only 20% will come from the expansion of the agricultural lands. So, the increase of crop yields will remain a critical component for a global strategy to achieve food and energy security while also protecting natural resources for the next generations. Based on that, the studies related to the concept of yield gap, which is the difference between attainable and actual yields, and to the increase of crop yields in order to supply enough products for worldxs demand are of very high priority. For improving crop yields, the first point to be considered is the magnitude of the yield gap and its main causes. Such information will make possible to bring solutions to reduce yield gap and production costs. In such context, soybean is one of the main crop to be evaluated in terms of yield gap due its use as human and animal food, and biodiesel. In Brazil, as well as in other countries like U.S., Argentina, Russia and China, soybean is the main crop, with a huge economic and social importance; however its yield is not increasing in a rate high enough. Therefore, the aim of this study was to propose a procedure to estimate de yield gap for soybean crop in Brazil, based on the use of a crop simulation model, in order to identify its magnitude and causes for different production regions in the country. The yield gap was determinate by the difference between attainable and actual yields, which was used to demonstrate how much of the yield gap comes from the crop management, and also between potential and attainable yields to understand how much comes from water deficit. Soybean potential yield (Yp) was estimated by the FAO Agro-Ecological Zone Model, which has as inputs solar radiation, photoperiod, air temperature and soybean crop parameters. The attainable yield (Yat) was estimated by penalizing Yp by the water deficit along the different crop phases, considering the water response factor (Ky). Yp and Yat were simulated for the soybean sowing window of each of the following production regions across the country (from the south to the north): Cruz Alta (RS); Campos Novos (SC); Campo Mourão (PR); Dourados (MS); Presidente Prudente (SP); Uberaba (MG); Jataí (GO); Formosa (GO); Primavera do Leste (MT); Tapurah (MT); Correntina (BA); Peixe (TO); Bom Jesus (PI); Santana do Araguaia (PA); and Balsas (MA). Actual soybean yield (Yac) data were obtained from the official surveys of the Brazilian Institute of Statistics and Geography (IBGE) for the period of 1980 to 2011. These historical Yac data had the technological trend removed for the analysis. The general results showed that there are differences among soybean Yp, Yat, Yac and yield gaps across the country. In more than 50% of the years Yp is greater than 4700 kg ha-1, while Yat is 3428 kg ha-1 and Yacf is 3028 kg ha-1. The yield gap relative to crop management is, in average, 464 kg ha-1, while the yield gap relative to water deficit is greater, 1378 kg ha-1. The maximum yield gaps observed were 2048 and 4053 kg ha-1, respectively, for crop management and water deficit, which occurred in the regions were drought frequencies are higher. In relative terms, the soybean yield gaps were, in average, of 28.8 and 13.6%, respectively, by water deficit and crop management. In such case, the main factor that reduced soybean yield in Brazil was the climatic stresses caused by water deficit along the crop cycle. About the crop management, the main causes of yield gap are related to soil physics and fertility. Pest (like Helicoverpa spp.), diseases (like Phakopsora pachyrhizi) and weeds (like Conyza bonariensis) are of less impact but the abusive control can reduce the incomes. These results allowed us to conclude that for reducing soybean yield gap in Brazil in order to achieve the growth demand the use of irrigation will be of major importance, as well as more investments in precision agriculture for soil preparation, correction and fertilization. In addition to these aspects, the use of drought tolerant cultivars, crop climatic risk zoning, for defining the best sowing dates, and on farm experiments for comparing alternative crop managements, as crop rotation for improving soil structure and nutrient cycling, should be considered.