The results showed large variations in yield among farmers’ fields, ranging from 12.7 to 37.2 t ha-1. The fields were divided into five yield categories, with the mean yields of the lower four categories ranging from 76.0 to 34.2% of the maximum yields, with corresponding yield gaps ranging from 8.9 to 24.4 t ha-1. The main yield constraints identified were soil nutrient deficits, short crop duration and weed competition. The highest yielding fields had no production constraints, but the number and/or the level of constraints increased in fields with lower crop yields.