INTERNATIONAL POTATO CENTER: WORLD SWEETPOTATO ATLAS
VIETNAM
HISTORY AND OVERVIEW
The sweetpotato, Ipomoea batatas, was widely grown in tropical America when Columbus brought several roots with him on his return to Spain from his first voyage in 1492, eighty years before the potato reached Europe (Purseglove 1968). The sweetpotato was carried by mariners from Portugal, and subsequently other European nations, to ports throughout Africa and Asia in the early sixteenth century. However, this dispersal, the "batata line," was not the only means by which the sweetpotato was transferred to the Asia Pacific region. A separate "camote line" (from the Nahuatl word comotli ) was transferred directly from Mexico via Spanish galleons to the Philippines, also in the sixteenth century (Zhang et. al. 2004, p. 116). The sweetpotato was introduced to Vietnam probably in the late sixteenth century via the Luzon Islands of the Philippines, and/or Fukien, China (Hoa et. al. 2000, p. 189).
Adding to the complexity of this story is evidence of a transfer to the western Pacific predating European exploration. Carbonized sweetpotato roots found in northern New Zealand have been dated back approximately 1,000 years (Yen 1982, cited in Zhang et. al. 2004, p. 116). Whether this transfer occurred via natural dispersal or by human voyagers has remained a point of speculation for several decades. Research by Zhang et. al. (2004) compared genetic associations among seventy-five sweetpotato landraces, drawn from several countries of the Asia Pacific region, as well as Peru, Ecuador and Mexico. That research demonstrated a relatively weak association between sweetpotatoes from the Asia Pacific region and those of Peru and Ecuador, suggesting a Mesoamerican origin prior to introductions via European exploration.
Whatever its origins and the means of its introduction, sweetpotato has become well established in Vietnam due to its high yield relative to both land and labor, its capacity to grow in relatively poor soils, and its high content of carbohydrates and vitamins. Sweetpotato has long been important to Vietnam as an insurance crop against famine, but in recent years has become an important source of feed for livestock, especially pigs. It is widely grown throughout Vietnam, usually by small scale farmers under low-input conditions, with some regional variations (ibid.).
For several years following the reunification of Vietnam in 1975, Vietnam's sweetpotato crop helped provide a measure of food security in the face of unstable agricultural production. The destruction caused by war was followed by several years of devastating weather, including a drought in 1977 and typhoons and flooding in 1978 (US LOC: Agriculture). International sanctions following Vietnam's invasion of Cambodia in December 1978 further exacerbated economic difficulties and food security (US LOC: Foreign Relations: Laos and Cambodia), while the government's policies of agricultural collectivization and centrally managed production and marketing generally failed to meet production goals (US LOC: Agriculture).
Sweetpotato and other "minor crops" (such as cassava and beans) which accounted for less than ten percent of Vietnam's production of grain (principally rice, also maize) in 1976 accounted for over twenty percent of the food value of grain production by the end of the decade (US LOC: Agriculture; FAOSTAT). Sweetpotato production increased from 1.5 million tons to 2.5 million tons in less than five years (FAOSTAT), but declined in the 1980s when grain production recovered and food security stabilized.
Since the mid 1980s, following policy reforms and improvements to the country's physical infrastructure (especially irrigation), rice production has increased steadily with more area under cultivation and higher average yields made possible in part by the adoption of modern varieties (FAOSTAT). Since the 1990s, Vietnam has emerged as a major exporter of rice and other agricultural commodities, including coffee and aquaculture products. Yield of sweetpotato roots has increased from around 5.5 tons per hectare (T/HA) in the early 1980s to above 7 T/HA since 2002. However, with decreasing area under cultivation, overall production declined from a peak of 2.6 million tons in 1981 before stabilizing at about 1.6 million tons per year since 1995 (FAOSTAT).
Sweetpotato nevertheless remains an essential component of agricultural productivity, currently ranking third among all crops (behind rice and maize) in terms of area cultivated and production. In recent years, following consistent gains in overall agricultural productivity, the role of sweetpotato has shifted from staple crop and famine reserve to an efficient source of high quality feed for livestock, especially pigs, a major source of protein and farm income in Vietnam. An estimated seventy to eighty percent of the sweetpotato crop is utilized for livestock feed, usually in close proximity to the farms where it is grown, with most of the remainder grown for the cash market (Keith Fuglie, personal communication). Sweetpotato vines, as well as roots, are important as livestock feed in Vietnam. Since some varieties are selected and grown primarily for their vines as a source of forage, with little or no harvest of roots, the importance of the crop is underestimated by production data which include only root harvests.
- For a chart of Vietnam's sweetpotato production over the past several decades, please see Production Charts.
GEOGRAPHY AND PRODUCTION ZONES
Vietnam, although not a very large country, extends approximately 1,650 kilometers from 8º N to 23º N, an extensive range of latitude that accounts for a wide diversity of climatic conditions affecting agriculture. The country is characterized by tropical lowlands, hills, and densely forested highlands. Except for coastal river deltas, little of Vietnam is level (US LOC: Geography). Although sweetpotato is widely grown throughout Vietnam, production is somewhat concentrated in the north, centered on the Red River Delta and along the North Central Coast. Production in the Mekong River Delta area is relatively minor, but more intensively produced, with higher yields on less area.
The Red River Delta, comprising an area of 3,000 square kilometers, is the area of highest population density and most intensive agricultural development. The region is surrounded by forested highlands, but the delta is generally no more than three meters above sea level, much of that within one meter. Since the area is subject to frequent flooding, water control has long been an integral feature of the delta's culture and economy. An extensive system of dikes and canals contains the Red River and provides irrigation, primarily double-cropping of rice (US LOC: Geography).
The delta is characterized by a monsoon climate of cool winters and hot summers. Annual rainfall, averaging about 1,700 millimeters, is concentrated in the months of May through October. Agriculture in the winter months is generally irrigated. Soils are alluvial, generally light loam to clay loam.
The delta is uniquely well suited to the production of high value crops during the cool months of winter. Although many farmers are responding to economic changes by shifting some production to higher value crops, many continue to cultivate some sweetpotato for direct consumption, but increasingly for animal feed. Most of the crop is rotated with rice in a spring rice - summer rice - winter sweetpotato system.
Major constraints include:
- a relatively short growing season which can be further limited by late rainfall which delays the rice crop and thus disrupts the rotational system;
- restricted access to irrigation;
- limited availability of sweetpotato cultivars well adapted to low temperatures and cloudy conditions (ibid.).
The Central Coast, for example the province of Quangnam-Danang along the southern central coast, is not as cold as the northern provinces, but is vulnerable to considerable crop damage from strong northeast winds. Climatic extremes of drought, flooding, and typhoons are fairly frequent. The rainy season occurs from September to January, and the dry season from February to August can be characterized by hot temperatures and sometimes severe drought.
Soils are more diverse, and often determine locally variable cropping patterns. However, the general pattern is for sweetpotato to be grown in rotation with rice such as: summer and autumn rice - winter and spring sweetpotato and sesame - spring and summer sweetpotato and rice. Sweetpotato is sometimes grown over the summer and autumn, or autumn and winter, in order to supply vines for livestock feed (mostly pigs) and to maintain planting material (Hoa et. al. 2000, p. 198).
Sweetpotato is generally more concentrated closer to the coast on sandier soils. Since 1980, sweetpotato cultivation in this area has declined as more land is converted to rice production due to improvements in irrigation systems (ibid.).
The Mekong River Delta of the south, the country's greatest concentration of rice, accounts for relatively little area under sweetpotato cultivation, but by far the highest average yields (roughly double the national average), due in part to hotter weather and a longer growing season. Sweetpotato is more commonly grown as a direct food crop in this region, in some areas as a cash crop for the Ho Chi Minh City market. Data from the General Statistics Office and FAO indicate a consistent pattern of declining area of cultivation, from approximately 30,000 hectares in 1977 to 12,500 hectares in 1994, but generally stabilizing through 2004 (FAOSTAT).
- Several maps of more specific classifications — for example by soil types, agro-ecological zones, and land use — are available from the European Digital Archive of Soil Maps, Vietnam. Most images are digitized from paper maps, of variable clarity.
- A map of sweetpotato distribution in Vietnam is included on the Maps Page.
PRODUCTION SYSTEMS AND CONSTRAINTS
Cropping Patterns
Cropping patterns and land preparation practices are regionally variable. Where sweetpotato is grown intermittently with rice, fields are thoroughly prepared using buffalo or cattle-drawn plows, typically to a depth of 12 to 20 centimeters (cm), after which the land is formed into ridges and furrows. On sandier soils of the Red River Delta or Central Coast, plowing is usually omitted. Sweetpotato is usually grown on raised ridges, typically a meter apart and 30 to 50 cm high in the Red River Delta, but higher and more widely spaced along the Central Coast, where farmers are more concerned with retaining soil moisture and protecting the crop from excessive heat (Hoa et. al. 2000, p. 207).
Planting material usually consists of vine cuttings obtained from a previous crop or nursery garden. In areas adjacent to higher elevations, typically the Central Coast area, vines for the winter-spring crop are multiplied at hilly sites with good drainage to prevent rooting, then distributed to farms on the plains at planting time (ibid., p. 209). Vines are selected for overall health (free of apparent pests and disease), lack of adventitious roots at the nodes, with short internodes, thick leaves, and short petioles. Terminal cuttings have been demonstrated to produce higher yields than those taken from the middle or basal portion of the vine.
Planting vines on prepared ridges can be done using any of several techniques (Hoa et. al. 2000, p. 210-211):
- vertical planting of two or three nodes into small holes, a method which saves labor, but often at the cost of less yield of roots;
- "slantwise planting," in which a hoe is used to prepare one side of a ridge, against which a vine cutting is placed, then the other side of the ridge is formed;
- horizontal planting, in which cuttings are placed along the furrow in the middle of the ridge, with ends facing in the same direction, then buried to a depth of three or four cm along approximately three fourths the length of the vine; this method is more typical in sandy loam soils and typically uses vines of longer length to assure an adequate crop;
- "U " or "V" shaped planting, in which cuttings are placed in the ridge and buried at the middle, leaving both ends exposed above ground.
Planting density is variable with soil condition, season, cultivar and overall farming intensity of a given area. At lower levels of fertilizer application, optimal yields as measured in tons per hectare (T/HA) are generally highest at around 40,000 plants per hectare. Lower planting density typically results in lower overall yields, but higher average size roots (ibid., p. 212).
In order to discourage vines from producing adventitious roots which compete with the crop for available energy once they have been planted on ridges, a common practice is to place dry branches on the ground beneath the vines. Under more labor intensive management, farmers might also lift vines out of the ground temporarily, approximately 50 - 60 days after planting, in order to dry them and thus discourage adventitious roots. This practice can increase yields by ten to fifteen percent, although if not done properly can have a negative effect (ibid., pp. 204-212, 222).
Topping, a practice in which the tip (usually about two cm) of a growing vine is cut to promote branching and rapid leaf cover, is also typical under relatively intensive management. Topping is normally done when a main vine reaches about 40 - 45 cm, roughly thirty days after planting. Sometimes topping is practiced earlier, when vines are in a nursery prior to being transplanted (ibid., p. 224).
The Agricultural Calendar
Since sweetpotato production is possible year-round in Vietnam, the calendar has not been very rigidly determined. However, most of the decline in area harvested over the past decade has been of the spring and summer crops, with production becoming increasingly concentrated in the shorter winter season (Keith Fuglie, personal communication). Generally the best time to plant sweetpotato in the Red River Delta is late September to early October, when there is still sufficient moisture and warmth for rapid establishment of the crop. Elsewhere, sweetpotato is more typically a dry season crop planted in January (Hoa et. al. 2000, p. 212).
Fertility Management
Several forms of organic matter are used for sweetpotato cultivation in Vietnam, from animal manure to green manure of several types, [e.g] crop residues, water fern, and rice straw. Dung, commonly used at a rate of approximately 8-15 T/HA, is often used with green manure, incorporated into the depressions of ridges, then covered with a thin layer of soil before vine cuttings are planted (Hoa et. al. 2000, p. 216).
Sweetpotato responds well to applications of inorganic fertilizers, but experiments indicate that application timing is an important factor. Generally, three applications are recommended: a basal application of all organic matter and phosphorous and half of nitrogen and potassium, followed by two sidedressings of the remaining treatment, 25 - 30 and 40 - 45 days after planting (ibid.). Yields of 18 - 22 T/HA have been reported by farmers using both organic and inorganic fertilizer and otherwise practicing careful management ([e.g] lifting vines to discourage adventitious roots) in Than Binh, of the Central Coast area. Here the concern is not only with sweetpotato production, but also soil condition for the subsequent rice crop (ibid.). Information regarding typical (as opposed to recommended) fertilizer application rates are not widely reported, but a survey undertaken in 1998-99 by E. Van de Fliert et. al. indicated that most farmers use at least some inorganic fertilizer, more so in the south.
Occurrence and Control of Pests and Diseases
The most important biological constraint of sweetpotato in Vietnam is the sweetpotato weevil, Cylas formicarius, which can destroy a root crop almost completely. Cultural practices include crop rotation and hilling up (to lessen the exposure of roots to the weevils). The International Potato Center (CIP) has recommended an integrated pest management approach which utilizes additional measures, such as the use of terminal cuttings, irrigation to help keep the soil well sealed and thus limit weevil penetration, pheromone traps, and dipping vines in an insecticidal solution prior to planting (Hoa et. al. 2000, p. 221). Experimental results suggest that maintaining regular moisture can be nearly as effective as use of insecticides (Basudin or Trebon), but is labor intensive and obviously requires the availability of water, often via irrigation when sweetpotato is grown in the dry season. Specific data regarding pesticide application rates are not readily available.
Other pests include: hornworm (Agrius convolvuli ), cutworm (Spodoptera litura), stemborer (Omphisa anastomasalis ), and leaf folders (Pyralidae). Stemborer has been reported as a problem in then Central Coast area, causing losses estimated at five to twenty percent, and was cited as a major constraint in some areas in the survey undertaken by Van de Fliert et. al.
There has been little research to date on diseases affecting sweetpotato in Vietnam. A few diseases have been noted, but as minor problems (ibid., p. 222), though this might simply reflect the status of research rather than the true degree of the constraint.
VARIETIES AND SEED SYSTEMS
Knowledge of Vietnam's sweetpotato genetic resources has expanded greatly since the reunification of the country in 1975. A collection of over 100 accessions was lost in 1967, but following the conclusion of the war, researchers began traveling throughout the county to assess the genetic resources of many crops, including sweetpotato. A later and more comprehensive investigation undertaken from 1991 to 1993 resulted in an initial collection of 633 sweetpotato accessions, of which 358 were retained after the elimination of duplicate samples. These were subsequently classified as 229 landraces, 103 introduced varieties, and 26 breeding lines retained for conservation (Ho et. al. 1996, p. 51).
All sweetpotato accessions are now maintained in the field at the Vietnam Institute of Agricultural Science (VASI), where they can continuously evaluated under realistic conditions, but not without some risk. Varieties can become inadvertently mixed or lost, sometimes due to natural hazards, such as happened with a flood in 1994. Some varieties were recovered via backup collections maintained in vitro, and others had to be re-collected. Varieties are maintained separately, but are still subject to biotic and abiotic stress. As of 1996, twelve cultivars were also being maintained in situ by farmers in Ninh Binh province (ibid., p. 52).
Between 1981 and 2003, 14 new sweetpotato varieties were officially released in Vietnam (Fuglie et. al., 2002). Six of these varieties (HL 518, K4, KL5, K51, NN 31, and KB1) were developed in collaboration with the International Potato Center (CIP). CIP's collaborative breeding effort in Vietnam has focused on developing dual use varieties, [i.e], varieties with high yield of both roots and vines and which can be used for both human consumption and as animal feed. Some of these new varieties yield 75 percent more dry matter content (combined roots and vines) than most traditional varieties (Peters et. al. 2005).
CONSUMPTION, STORAGE, AND MARKETING
Consumption
Direct consumption of sweetpotato in Vietnam is becoming less common than indirect consumption via animals, especially pigs. As food security improves, pork consumption has increased dramatically. In the north, per capita pork consumption more than doubled from 1985 to 1995 (Prain et. al. 1996). Sweetpotato, primarily in the form of vines, has become an important element of high quality animal feed. As noted above, vines are not normally counted as production, even though in Vietnam vines are an important productive resource.
Sweetpotato root can also be used as an animal feed, although it contains relatively little protein. Protein digestibility is also reduced by trypsin inhibitors contained in sweetpotato. Farmers have overcome this constraint by supplementing sweetpotato with other ingredients such as rice bran, fish meal, and soy beans, but another solution involves silage of sweetpotato roots with other ingredients.
More information is contained in:
Storage
Harvested roots are generally sorted so that defective or damaged roots are fed to animals and only those in good condition are put in storage. If roots are stored in fresh form, they are often spread over the floor of a ventilated place and layered with either dry ash or sand. Underground storage is sometimes practiced. Another alternative is to clean, then slice or chip roots, then dehydrate them to approximately ten percent moisture. Data pertaining to the frequency of this practice have not been reported, but it might be fairly common in areas where price fluctuations can be strong. Data from Quangnam-Danang in the Central Coast area, from 1995, estimated fluctuations of about 100 percent, with the highest prices in October (Hoa et. al. 2000, pp. 225-226).
Marketing
The needs assessment survey undertaken in 1998-99 (Van de Fliert et. al., op. cit. ) indicated a very striking difference between northern and southern Vietnam, where the percent of the sweetpotato crop (in fresh root form) which was marketed ranged from as little as ten percent in the north to over ninety percent in the south.
Contact information for the Vietnam Agricultural Science Institute (VASI) is provided by MekongInfo, an interactive site for sharing information about participatory natural resource management in the Lower Mekong Basin.
CONTRIBUTORS
Kelly Theisen is the principal contributor to the initial (2006) Vietnam chapter of the Sweetpotato Atlas.
Keith Fuglie provided information and advice via personal communication.
REFERENCES
FAOSTAT. (Agriculture, Agricultural Production, Crops Primary, Sweet Potato)
Fuglie, K., S. Mahalaya, and F. Suri. 2002. The Economics of Sweetpotato Genetic Resource Conservation and Varietal Improvement in Asia. In: Exploring the Complementarities of In Situ and Ex Situ Conservation Strategies for Asian Sweetpotato Genetic Resources (R. Rao and D. Campilan, eds). Proceedings of the 3rd International Workshop of the Asian Network for Sweetpotato Genetic Resources (ANSWER) held in Denpasar, Bali, Indonesia on 2-4 October, 2001. Serdang, Malaysia: International Plant Genetic Resources Institute Regional Office for Asia, the Pacific and Oceania (IPGRI-APO), pp. 79-122.
Ho, Truong Van; Nguyen Ba Lieu; Nguyen Ngoc Hue. 1996. Sweetpotato Genetic Resources in Vietnam. In: Proceedings of the Workshop on the Formation of a Network for the Conversation (sic ) of Sweetpotato Biodiversity in Asia held in Bogor, Indonesia, 1-5 May 1996. International Plant Genetic Resources Institute (IPGRI). Rome, Italy.
Hoa, Vu Dinh. 1998. Sweetpotato Production and Research in Vietnam. In: Proceedings of the International Workshop on Sweetpotato Production System: Toward the Twenty-first Century. Kyushu National Agricultural Experiment Station (KNAES). Miyazaki, Japan.
Hoa, [V.D]; [D.T] Loc; [T.V] Ho; H. Kim. 2000. Sweet potato in the Postrice Areas of Vietnam. In: Sweet Potato in Tropical Asia. Philippine Council for Agriculture, Forestry, and Natural Resources Research and Development (PCARRD). Los Baños, Laguna. The Philippines.
Peters, Dai; Nguyen Thi Tinh; Pham Ngoc Thach. Sweetpotato Root Silage for Efficient and Labor-saving Pig Raising in Vietnam.
Peters, D., [T.T] Nguyen, [T.H] Mai, [T.Y] Nguyen, [N.T] Pham, and K. Fuglie. 2005. Rural Income Generation through Improving Crop-based Pig Production Systems in Vietnam: Diagnostics, Interventions and Dissemination. Agriculture and Human Values (in press).
Prain, G.; C. Wheatley; Nguyen Doy Duc. 1996. The Potential of Root Crop Processing for Rural Development in Vietnam. Program Report 1995-96, International Potato Center (CIP). Lima, Peru.
Purseglove, [J.W] 1968. Tropical Crops: Dicotyledons. Longman Group Limited. Essex, England.
Trin, Luu Ngoc; Bui Tuyet Mai; Nguyen Ngoc Hue. 1999. Status of Conservation and Use of Sweetpotato Germplasm in Vietnam. In: Conservation and Utilization of Sweetpotato Genetic Diversity in Asia: Proceedings of the Second Asian Network for Sweetpotato Genetic Resources, 3-5 November, 1999. Bogor, Indonesia.
US LOC (United States Library of Congress) Country Studies: Vietnam.
Van de Fliert, E.; [N.T.K] Oanh; [N.T] Son; [T.D] Hoa; [P.D] Thanh; [L.Q] Hung; [L.H] Khoang; [N.T] Lan. The Sweetpotato Production-Postharvest System in Vietnam: Participatory Needs and Opportunity Assessment. CIP Program Report 1999 - 2000.
Yen, D. E. 1982. Sweetpotato in historical perspective. In: Villareal, [R.L] and Griggs, [T.D] (eds.). Sweet Potato: Proceedings of the First International Symposium. AVRDC Publication 82:172, pp. 17-30. Tainan, Taiwan.
Zhang, Dapeng; Genoveva Rossel; Albert Kriegner; Robert Hijmans. 2004. AFLP Assessment of Diversity in Sweetpotato from Latin America and Pacific Regions: Its Implications on the Dispersal of the Crop. Genetic Resources and Crop Evolution 51: 115-120. Kluwer Academic Publishers. The Netherlands.