Rice cropping in the coastal areas of the Mekong Delta is affected by salt intrusion during the dry season. Construction of sluice gates and dykes for salinity prevention has changed the water regime for rice farming. We focused on a case in which rice production did not increase appreciably by the salinity control projects implemented in Go Cong District, Tien Giang Province, Vietnam. We conducted a socio-economic survey and analyzed water quality in the river and canals in two communes: Phu Thanh (PT) and Phu Dong (PD), located in the lower reaches of the site, where five gates and dykes had been installed along the coastal line in 2001. The annual rice yield (t/ha/year) increased by 31% at the location near gate g1 (PT-N) after the construction of sluice gates and dykes, while at the location far from g1 (PT-F), it decreased by 30%, and in PD the yield did not change significantly. The construction has extended the salt free period, and allowed more farmers to practice double cropping using a modern variety at PT-N. However, at PT-F, the construction exacerbated the acidification in the canal water at the beginning of the rice season, resulting in the reduction of the number of croppings. At PD, the number of croppings did not increase due to the severe water acidity in the canals.
Adoption of direct seeding of rice via dry-seed broadcasting (DS) was surveyed in rain-fed lowland in Northeast Thailand and Northwest Cambodia from 2003 to 2005. Based on interviews with village chiefs, nine villages in the target area were categorized into three types in relation to their field characteristics and rates of DS adoption. Village type I was identified with DS adoption of more than 80% in low and flat areas in Northeast Thailand, where DS was introduced from the late 1980s to early 1990s, and characterized by earlier planting in April or May, low weed infestation, and by high yields. Type II was identified with DS adoption of 6 ∼ 30% in undulating areas in Northeast Thailand, where DS was introduced from the late 1990s to early 2000s. This was characterized by a yield level lower than that in transplanting (TP) and a lack of effective weed control. Type III was identified in Northwest Cambodia, where extensive and low-yielding DS had popularly been practiced in remote, large, low and flat fields. DS is started from April to May like as that in type I, but the harvest lingered from mid-November to mid-January. In this type mid-season tillage was practiced to control weeds according to farmers’ knowledge in ecologically harmonized ways. Furthermore, 23 farmer fields were monitored, and grouped into three types in Northeast Thailand and six in Northwest Cambodia, depending on the planting methods, availability of field water and time of harvesting. In addition, three on-station experiments were conducted to compare yields between DS and TP. As the results, DS yielded comparable or higher than TP under favorable conditions, while DS yielded less than TP in cases with low availability of water and severe weed infestation.