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 maizesoybean-sunflower. At Pailin, the sequence with the highest overall mean yield (4.3 t/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 3.0 t/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 Samloutfour 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.

Crop yields are declining in Northwest Cambodia and crop failure in the pre-monsoon season is commonplace with 69% of farmers surveyed stating that drought is a constraint to production. Farmers currently lack knowledge to adopt more sustainable farming practices. A trial was conducted in Samlout District, Battambang Province, Northwest Cambodia to investigate the feasibility of a sowing time two months later than typical local practices. The aim of the shift in sowing time was to increase crop yield and reduce crop failure due to heat and drought stress throughout the season. A secondary aim was to compare sequences of continuous maize ('Zea mays' L.), and maize in rotation with peanut ('Arachis hypogaea'), sun- flower ('Helianthus annus'), sorghum ('Sorghum bicolor'), cowpea ('Vigna unguiculata') or mungbean ('Vigna radiata'). Sunflower and sorghum would be new crop type introductions, whilst the other crops are part of the traditional farming system in this region. The trial was undertaken for four cropping seasons over two years, during which time crops produced successful yields from the new sowing time windows. However, it was the maize-sunflower sequence that produced the highest gross margins. Maize-sunflower returns were $514 per hectare per annum more than the typical planting of continuous maize, and over $1100 per hectare per year higher than the other maize-legume and maize-sorghum rotations. Continuous maize produced the most stable yields across the four seasons and maize-sunflower produced the second highest mean yield. Results from modelling of soil moisture suggest that a shift in sowing time may avoid the extreme heat of the pre-monsoon season, and align crop growth stages with periods of more reliable rainfall. Site specific surface soil moisture data and rainfall was entered into the APSIM model to predict the soil profile moisture throughout the growing season (r² = 0.73). Both the modelling and on-farm research resulted in higher crop yields compared with traditional practices and expectations, and a low probability of crop failure. Crops of maize, sunflower and sorghum grew well from an early October sowing date into the post monsoon season and produced good yields on stored soil water with low plant stress due to mild seasonal conditions. Delayed sowing may prove to be the best option for farmers in the Northwest upland, achieved by a simple shift of sowing dates.