Distinguishing between yield plateaus and yield ceilings: A case study of rice in Uruguay

Rice yields in Uruguay have increased rapidly (159 kg-1 ha-1 y-1) between 1990 and 2013. There is evidence, however, of an incipient yield plateau in recent years. The aim of this study was to determine if the recent slowdown in yield gains is because average yield (Ya) has approached the yield potential (Yp) ceiling, which makes it increasingly difficult for farmers to sustain further yield gains. We followed the methodology developed by the Global Yield Gap Atlas to estimate Yp and associated yield gaps for irrigated rice supported by data from high-yield experiments to calibrate the rice simulation model Oryza (v3). Subsequently, the model was used to simulate Yp using long-term daily weather data from seven locations, representing 90 % of total rice area in Uruguay. The exploitable yield gap (Yeg) was calculated as the difference between 80 % of Yp and Ya. Estimated national average Yp was 13.9 Mg ha-1, with relatively small variation across sites, from 13.1 to 15.1 Mg ha-1. Average Ya was 8.3 Mg ha-1, ranging from 7.9 to 8.5 Mg ha-1 across sites, and representing 60 % of Yp. Our analysis suggests there is still room to further increase rice yields in Uruguay, because the Yeg is 2.8 Mg ha-1, which means the current yield plateau is not due to Ya approaching Yp, as has occurred in other high-yield irrigated rice systems in China and California, USA. The approach followed here can help determine whether yield plateaus are occurring due to a small Yeg or other factors.