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Introduction

Livestock plays an important role in most developing countries and it's production represents nearly 20% of the agricultural output value in Vietnam (Ly, 2001). Pigs are predominantly kept on smallholder farms in Vietnam. Pig production, with its main product-pork, is the main animal protein source for the Vietnamese, as it accounts 70% of the meat consumed. However, among the major constraints limiting the development of pig production in Vietnam is shortage of feed resources in terms of both quantity and quality.

Sweet potato (Ipomoea batatas (L) Lam) is a tropical crop with a relatively short vegetative cycle, the tubers of which are usually for both human and animal consumption (Woolfe, 1992). It is among the five most important food crops in developing countries (Horton, 1988) and is the third most important crop after rice and maize in many areas in the north of Vietnam. Since the early 1990s, when paddy production increased in productivity, Vietnam has become one of the biggest rice exporters, and the sweet potato is now used mainly as feed for animals. In fact nearly 100% of the sweet potato crop is used as animal feed in the Red River Delta, and 80% in the highland and mountain areas (Statistical Year Book, 1999).

The productive potential of certain varieties of sweet potato can reach from 24 to 36 tonnes/ha/crop of root (Morales, 1980 cited by Dominguez, 1992) and the foliage production varies from 4.3 to 6.0 tonnes dry matter per ha (Ruiz et al, 1980).

Sweet potato can be harvested twice per year, in the summer and spring-winter seasons, and both forage and tuber have been used widely as an alternative feed for livestock in tropical countries. The roots have low protein, fat and fibre content, but high nitrogen-free extractives, which thus indicates their potential value, mainly as an energy source. The vines have a low carbohydrate content but are higher in fibre and protein, and their principal value is as a source of vitamins and protein.

Traditionally, farmers in most of Northern and central Vietnam boil sweet potato roots and vines to make the feed for their pigs, a process that is time consuming. The farmers, usually women, must spend several hours every day chopping the ingredients, gathering fuel and doing the cooking. Sometimes they feed their pigs fresh sweet potato, in which the presence of trypsin inhibitors can lead to problems .

Thus research on the processing and utilization of sweet potato in diets for pigs in different seasons based on traditional diets under smallholder conditions is necessary.

The objectives of this research therefore were to develop and evaluate appropriate methods to process and preserve sweet potato vines and roots as feeds for fattening pigs in different seasons and to determine the optimum level of sweet potato roots and vines (in dried and in silage form) in traditional diets for F1 crossbred fattening pigs under smallholder conditions.