Rice cultivation plays a major role in the economy of Vietnam. Besides being a staple food, the growing of rice is an important source of employment in the country, with approximately 80% of the population relying on agriculture for their livelihoods. Rice field rats are an important pre-harvest pest of irrigated lowland rice farming systems, accounting for between 10% and 15% of pre-harvest yield losses each year, but with losses up to 100% in some circumstances. Traditionally, rodent control is mainly applied by individual farmers and control measures are often too late for economic benefits to be achieved, due to the rice plants inability for compensatory growth.
The pattern of population dynamics of the rice field rat (Rattus argentiventer) have been reported to be different between the Mekong River and the Red River Delta, and even varied among provinces in the Mekong River Delta. Further, little is known about diet and its influences on breeding of the species in the rice field environment. A better understanding of the biology and ecology of the most common rodent species found in the rice fields of Vietnam is necessary to help improve control initiatives.
The general objectives of this thesis were to achieve a better understanding of the major factors that control or regulate population growth of the rice field rat in the lowland irrigated rice ecosystem in the Mekong Delta. A computer simulated model of rice production systems was developed and used as an aid to decision support for rat management. The studies were conducted in lowland irrigated rice systems in An Giang province, in the Mekong Delta between October 2008 and December 2009.
Changes in population abundance, reproduction and habitat use of R. argentiventer were investigated throughout an entire rice cropping season (from land preparation through to harvesting of the crop), including a 2-month pre-sowing fallow period under flooded conditions. Population abundance of R. argentiventer was significantly related to crop growth stage and rice bank habitat. The breeding performance of adult females responded to rice crop growth stage. Adult females in their burrows during the day when rice cover was sparse, but remained in the rice fields both day and night when vegetative cover became abundant. The rats immediately returned to their burrows after harvest.
Following the population study, an experiment was designed to investigate whether the breeding performance of female R. argentiventer responds to food quality and/or food quantity, or whether the breeding is synchronized with rice crop growth stage. The main finding of this experiment was to confirm a significant link between the breeding by adult female R. argentiventer and rice crop growth stage. Rice was the dominant food category in the stomach of adult females. They ate more green material when rice grain was unavailable, indicating that the species was able to respond to changes in the availability of different types of food.
The relationships between rodent damage and total grain losses in reference to the different rice crop growth stages were also characterised, to help with the timing of appropriate control measures. For up to 50% tiller damage during the tillering stage of crop development, the rice crop was able to completely compensate for rodent damage by increased tillering. The ability of the rice crop to compensate at later growth stages progressively declined.
The findings of the study were then used in collaboration with farmers, extension workers and researchers, in an examination of the use of the APSIM rice crop simulation model as a potential alternative approach to the management of rodents. The application of the model to help identify the optimum time and best rodent management options, proved feasible. The use of the model became an integral component of the learning cycle for farmers and extension workers, in achieving a clear understanding of the rationale for changing traditional rodent control strategies. Farmers subsequently changed the timing of rodent management to the tillering stage of crop development instead of later in crop growth after most damage has been done.