Sensitivity of soybean planting date to wet season onset in Mato Grosso, Brazil, and implications under climate change

Crop planting dates control the yield and cropping intensity of rainfed agriculture, and modifying planting dates can be a major adaptation strategy under climate change. However, shifts in rainfall seasonality may constrain farmers’ ability to adapt planting dates, and imperfect knowledge of how farmers currently select planting dates makes it difficult to predict how adaptations will proceed. This study analyzes variations in soybean planting and wet season onset dates across the agricultural state of Mato Grosso (MT), Brazil, for 2004 to 2014. It starts by exploring the strength of relationships between planting date and several precipitation-based definitions of the wet season onset, and shows that planting date is better correlated to easily observed onset definitions based on rainfall frequency than to climatological definitions. Next, a regression analysis shows that the sensitivity of planting dates to wet season onset exhibits large variations with cropping intensity and across farm fields, and that planting dates trended earlier over the study period, independently of onset variations. Finally, the results are used to predict soy planting dates in Mato Grosso under the RCP 8.5 climate scenario. Predictions show that planting dates will likely become delayed relative to preferred times, and that this may preclude double cropping in some parts of the state. This study demonstrates that the simple assumptions about farmers’ behavior often used in agricultural forecasting omit important spatio-temporal variations. Improved understanding of planting choices can reduce uncertainty in projected agricultural responses to climate change and highlight important areas for policy and agronomic adaptation.