Productivity and profitability of upland crop rotations in Northwest Cambodia

The upland cropping region of Northwest Cambodia exhibits limited crop diversity, with maize and cassava dominating rain-fed production systems. Farmers in the Districts of Sala Krau in Pailin Province, and Samlout in Battambang Province, report soil fertility decline in upland cropping systems with associated reduced yields and profitability compared with five years ago. Research was conducted at one site in each of these Districts over a two year period. The purpose of the study was to investigate crop rotations in this system with a focus on better use of soil water throughout the year to increase crop yields and profitability. This included an experiment to investigate increasing cropping intensity from the usual two crops per year to three and evaluate which crop sequences would be feasible. Out of a total 15 crop sequences evaluated, only four were successful in producing viable grain yields. These were the same sequences at both sites and included maize-maize-fallow, maize-maize-sunflower, maize-soybean-fallow and maize-soybean-sunflower. At Pailin, the sequence with the highest overall mean yield (4.3t/ha) and profit was maize-maize-sunflower which returned a gross margin of $USD3700/ha over two years. The sequences with a fallow instead of a third crop produced the lowest financial returns at Pailin. However, in Samlout the maize-fallow was the most profitable sequence with an overall mean yield of 3.0t/ha, returning $1680/ha over two years. The least profitable sequences at Samlout were the two soybean sequences. This study was successful in growing five crops in two years at Pailin, but could not reach the goal of six due to cultivar maturity length. At Samlout four crops over two years were produced, as the pre-monsoon crop was not planted in either year due to lack of sowing rainfall. Seasonal climatic factors had the greatest effect at both field sites on gross margins, which emphasized the importance of matching sowing date to rainfall to make better use of stored soil moisture to optimise yield and profitability. At Pailin, mean seasonal surface soil moisture, hundred seed weight and harvest index also impacted gross margin returns. This research validated the stability of maize in the farming system while sunflower should be further investigated in cropping sequences.