The diet of the female ricefield rat, Rattus argentiventer, influences their breeding performance in a mixed rice cropping ecosystem in An Giang province, the Mekong Delta, Vietnam

Context. The ricefield rat, Rattus argentiventer, is the main rodent pest of rice in Vietnam, causing annual pre-harvest rice losses of around 10%. Although its diet is reported to consist of mainly rice, other components might also be important, and it is not known whether its breeding performance is affected by the quality and/or quantity of food. Furthermore, if its breeding is synchronised with the growth of rice crops, the rats will be able to makefull use of the food supply to produce offspring. This knowledge is necessary for developing improved strategies for managing this pest. Aims. This study aimed to determine whether the breeding performance of female R. argentiventer is responding to food quality (increased percentage of rice in stomach) and/or quantity (increased weight of stomach content) or whether its breeding is synchronised with the growth of rice crops. Methods. The stomach contents and breeding condition of 296 adult female rats were sampled by snap trapping once a month from October 2008 to December 2009 over three consecutive irrigated lowland rice crops fields to cover a range of rice growth stages in An Giang province, Mekong Delta, Vietnam. Key results. Overall, rice formed 70.5% +/- 2.0 s.e. in the diet, followed by green material (25.9% +/- 1.9 s.e.), insects (3.1% +/- 0.6 s.e.) and mung bean (0.5% +/- 0.3 s.e.). The probability of breeding increased with increasing mean percentage of rice in the diet: Pr(Breed) = 1/(1 + exp(0.6395-0.0090 x Rice)). However, this relationship was not consistent because breeding performance was high at the heading stage when the percentage of rice in the stomach was low and because breeding intensity was low at sowing when the percentage of rice in stomach was high. The rice growth stage is a better predictor for the breeding performance than the percentage of rice in the diet. Conclusions. Breeding was driven by a functional response of rats to the abundant supply of food quality (rice grain), modified by some unknown mechanisms that modifies this response of rats so that rats can anticipate food supply. Understanding the nature of this mechanism may provide insights into population processes that can be exploited in controlling rats in rice crops. Implications. Farmers could slow the reproductive rate of female R. argentiventer by reducing the food quantity through minimising the amount of spilt grain left in the field and through more synchronous cropping.