Assessing yield and economic impact of introducing soybean to the lowland rice system in southern Brazil

Lowland irrigated rice in southern Brazil is typically grown in monoculture, with one rice crop per year. However, during the past 10 years, some farmers have switched from the traditional continuous rice system to a 2-y soybean-rice rotation. Here we performed an on-farm assessment about the impact of introducing soybean to the lowland continuous rice system in southern Brazil. The goal was to determine how the soybean-rice rotation compared to continuous rice in terms of yield and profit. We used farmer-reported survey data collected from lowland rice-based systems in southern Brazil over three growing seasons. Cropping-system yield, profit, and return-to-inputs were compared between fields following continuous rice versus soybean–rice rotation. In addition to the survey data analysis, we evaluated the long-term economic impact of adopting the rotation using a combination of a crop simulation model and Monte-Carlo stochastic modeling. Average rice yield was 26% higher in the rotation compared to continuous rice. Besides the rotation effect, sowing date, N fertilizer, and weed management explained most of the field-to-field variability in rice yield. Cropping-system yield and gross income were lower in the soybean-rice rotation than in continuous rice as a result of replacing an irrigated crop (rice) by a water-limited rainfed crop (soybean). Despite that yield penalty, there was no difference in net economic return between the two cropping systems due to lower production costs in soybean-rice rotation compared to continuous rice. The rotation also exhibited smaller labor requirement and higher benefit-to-cost ratio and return to labor than continuous rice. Despite these potential benefits, our long-term analysis indicated higher inter-annual variability and economic risk in the rotation compared to continuous rice. Other factors further constrain adoption of the soybean-rice rotation, including the high risk of growing soybean in fields that are prone to excess water and difficulties to change current farm logistics. Findings from this study are relevant to other rice-based systems in the world looking for opportunities to increase or maintain net profit while reducing costs and/or labor.