Ung Sopheap's scientific contributions

In Cambodia, cassava is the most important upland crop, but the crop is mostly grown without fertilizer for many years. The magnitudes of nutrient balances are not known. While improved technologies are needed for sustainable crop production, their adoption depends on how farmers view cassava against other crops. The objective of this study was to assess nutrient balances for cassava cultivation, and the perception of farmers on growing cassava relative to other crops. The study was conducted in Kampong Cham province, which has the largest and longest history of cassava production in the country. Forty five households in four zones were selected for the study. A farm survey employing a semi-structured interview combined with a field visit was used to collect data on cassava farmers' cultural practices, crop residue management and their perceptions on cassava and other potential crops. Crop cutting was done in 45 cassava fields of the sampled households to obtain root yields and weights of component plant parts, and a composite soil sample was taken from each field for analysis. Data were analyzed for nutrient balances and for farmers' perception on growing cassava against other upland crops. Nutrient balances were calculated for the individual fields based on the nutrient contents of the component sources obtained from the literature. The results show that the balances of all the nutrients evaluated were negative. The losses were most serious for N, K and Ca with the averages of-59.65 kg N,-53.08 kg K and-10.93 kg Ca/ha, but were less serious for P and Mg with the averages of-4.82 kg P and-6.16 kg Mg/ha. These negative balances were the consequence of low nutrient inputs in current practices where only a few farmers applied chemical fertilizers or manure, and only at low rates. With respect to farmers' perception, farmers in this area regard rice and cassava as their top priority crops, and have a greater preference for growing them than other crops. However, rice is grown for home consumption, while cassava is grown as a source of cash income. The marketing aspects of the crop, i.e. "good price" and "easy to sell", were the most important considerations for farmers' strong preference for cassava. Recently, production of cassava has increased substantially, reflecting increased market demand and prices. With the current trend of favorable marketing conditions, farmers will continue to have a strong preference for cassava, and are also likely to adopt improved technologies that will sustain or improve its yield, even if this entails extra input costs.

Research on cassava has only recently commenced in Cambodia as part of the CIAT coordinated Nippon Foundation Cassava Project, focusing mainly on varietal improvement, nutrient management and planting methods. To identify high-yielding varieties that are suitable for Cambodian conditions, many introduced varieties, mainly from Thailand, were tested in replicated and non-replicated trials in four provinces from 2005 to 2008. Different planting methods (vertical, horizontal, inclined, with either one stake or two stakes per hill) and different combinations of NPK fertilizers were also tested in both replicated and non-replicated trials, conducted on farmers fields in 13 sites in four provinces. Preliminary results indicate that four genotypes, i.e. Rayong 72, Rayong 60, Rayong 5, and KU 50, performed very well in terms of fresh root yield and starch content across the various environments. Applying NPK fertilizers generally increased root yields of cassava and there was a significant difference among the fertilizer treatments. However, the root yield increase due to the application of NPK fertilizer was found to be greater in lower yielding than in the higher yielding environments. This indicates that the application of fertilizer is primarily needed in those areas where cassava is grown on poor soils. There was no consistent effect of planting method, spacing and planting method-by-spacing interaction for root yield of cassava in both replicated and non-replicated trials. The most economic planting method is horizontal planting with 1.0 x 1.0 m spacing.

Cassava (Manihot esculenta Crantz) is currently the most important upland crop of Cambodia, but information on yield variations and causal factors which is important for efficiently targeting efforts to increase production is still lacking. The objectives of this study were to determine the yield variations and causal factors for cassava production in Kampong Cham province in Cambodia. Forty five households in four production zones were selected for the study. A farm survey employing semi-structured interviews, combined with field visits, were used for the collection of information on farmers' practices in cassava cultivation, while crop cutting was done to provide estimates of cassava yields. The data were analyzed for yield variations, yield gaps and causal factors. 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. However, for different fields with similar yield levels, the main production constraints sometimes differed. The results clearly indicated that there are opportunities for yield improvement and narrowing of yield gaps through the adoption of field specific improved technologies and management practices.

In Cambodia, cassava is mostly grown with little or no fertilizer inputs, but the magnitudes of nutrient balances are not known. This study was conducted to assess nutrient balances for cassava cultivation in Kampong Cham province in Northeast Cambodia. Forty five households in four cassava production zones were interviewed in relation to their cultural practices and crop residue management, upon which sources of nutrient inputs and outputs were based. Chemical fertilizer, manure, planting materials and rainfall were the defined inputs, while cassava roots and stumps were the outputs. Crop cutting was undertaken in the cassava fields of the 45 households to obtain weights of roots and other plant parts. Nutrient balances were calculated for the individual fields based on nutrient contents of the component sources obtained from the literatures. The results showed negative balances for all the nutrients evaluated. The imbalances were most serious for N, K and Ca with the averages of -64.45 kg N, -52.83 kg K and -10.83 kg Ca ha -1 , but were less serious for P and Mg with the averages of -2.85 kg P and -7.20 kg Mg ha -1 . These negative balances were the consequence of low nutrient inputs in current practices where only a few farmers applied low rates of chemical fertilizer or manure. Continued use of current practices will threaten the sustainability of cassava production in Cambodia. For long term productivity of the crop, the application of organic manures together with lime and chemical fertilizers high in N and K is recommended.

AbSTrAcT: Cassava is one of the most important upland crops of Cambodia. While improved technologies are needed for sustainable production of the crop, their adoption depends on how farmers view cassava against other crops. The objective of this study was to assess the perception of Cambodian farmers on growing cassava, relative to other upland crops. The study was conducted in Kampong Cham province in Northeast Cambodia which has the largest cassava production area in the country. Secondary data on production and price of cassava and other crops were collected, and 45 households in four cassava production zones were interviewed to obtain information on farmer's perceptions on cassava and other crops. The results showed that production of cassava and other upland crops in Cambodia has increased substantially during recent years, reflecting increased market demand and improved prices. Farmers in the study area in Kampong Cham province regard rice and cassava as their priority crops, and have a greater preference for growing them than other crops, including maize, soybean, mungbean, peanut, sesame and rubber. Rice, however, is grown mainly for domestic consumption, while cassava is grown as a source of cash income. The marketing aspects of the crop, i.e., good price and easy to sell, were the most important considerations for farmers' strong preference for cassava relative to other upland crops. With the current trend of favorable marketing conditions, cassava production in Cambodia is anticipated to expand further, while farmers are also likely to adopt improved technologies that will sustain or improve their cassava yields, even if involving extra input costs. These findings can potentially be used as a basis for the further development and extension of technologies for sustainable production of cassava in Cambodia.