In the early 1990s a school principal, concerned about the nutritional status of the learners in his school, approached the Medical Research Council (MRC) for assistance. The MRC contacted the Agricultural Research Council Vegetable and Ornamental Plant Institute (ARC-VOPI) and together they launched a community-based vegetable garden project in Ndunakazi, a rural village in KwaZulu-Natal.
The MRC and ARC-VOPI developed, monitored and evaluated this home-garden project. The Ndunakazi project focused on growing beta-carotene-rich fruit and vegetables, such as the orange-fleshed sweet potato, and providing nutrition education. This proved to be successful in improving the vitamin A status of children aged two to five. A similar project was successfully implemented by the ARC-VOPI in Lusikisiki in the Eastern Cape in 2002.
Our vitamin A-deficient population
Dr Mieke Faber, a nutritionist at the MRC, says that vitamin A deficiency is of public health significance in South Africa. Two national surveys showed that more than half of children aged one to nine years consumed less than 50% of the required amount of vitamin A.
Children living in rural areas were worse off in terms of nutrient intake and vitamin A status than children in urban areas, clearly signalling the need for intervention in poor rural areas through the cultivation of vitamin A-rich crops.
The occurrence of vitamin A deficiency varied among the nine provinces, with KwaZulu-Natal and Limpopo having the highest prevalence (see Figure 1).
Root and consequences
Vitamin A deficiency is caused by inadequate consumption of plant foods containing beta-carotene, which is converted to vitamin A by the body. This has a dramatic impact on the health, growth and development of children, as they are more likely to get infections and to die from these infections than children with adequate vitamin A intake. Severe deficiency can result in blindness, though this is not common in South Africa.
Resisto – the nutritious solution
According to MRC biochemist, Dr Paul van Jaarsveld, orange-fleshed varieties of sweet potato offer one of the highest sources of naturally occurring beta-carotene, as indicated by the vegetable’s deep colour.
In a controlled feeding trial in the Valley of a Thousand Hills in KwaZulu-Natal, Dr Van Jaarsveld found that giving as little as 125g (half a cup) of the orange-fleshed variety Resisto to primary school children for 53 days improved their vitamin A status, compared to a control group that consumed the white-fleshed Bosbok variety. The trial also revealed that children liked the orange-fleshed sweet potato more, and that most of this vegetable’s beta-carotene content was retained during cooking.
A nutritional crop that suits South Africa
Sunette Laurie, a senior researcher at the ARC-VOPI, manages a breeding programme developing high-yielding, dark orange-fleshed cultivars that are acceptable to consumers. These cultivars are widely adapted and have good storability, drought tolerance and disease and pest resistance, which make them ideal for planting by emerging or communal farmers in vitamin A-deficient rural areas.
Several other research projects of the ARC-VOPI are investigating optimum fertilisation, weed-control practices, planting methods, plant spacing and water requirements of different cultivars.
More nutrients with biofortification
The process of developing crops with enhanced nutrient levels is called biofortification. Each year selective characteristics are enhanced in the sweet potato population through the polycross method. This entails selecting between 20 and 30 parents for their specific traits from the ARC’s sweet potato greenhouse collection of local breeding lines and imported clones.
Five plants of each parent are planted randomly in a field. Naturally occurring insects provide cross-pollination, and seed is harvested from March to July. Each year the seeds from the polycross are first sown in trays and later established in the seedling nursery in the field. The selection is very strict. Only between 2% and 4% of plants are selected for further evaluation on the basis of their orange flesh colour, yield, taste and quality.
The next step is to evaluate the lines in larger plots. In the initial evaluation and preliminary yield trial, about 100 to 150 lines are evaluated annually in single plots. Between 20 and 30 promising lines are then evaluated in the intermediate yield trial, with each plot replicated twice.
Thereafter, advanced lines proceed to cultivar trials in four to six target production areas around the country. At this stage trials consist of 15 lines with each plot replicated three times. Based on yield performance and taste acceptability over two seasons, cultivars are registered and recommended for different production areas. Finally, the cultivars are used in the food-based programme to reduce vitamin A deficiency. This whole process takes six to eight years.
Getting your hands on this crop
Disease-free planting materials of popular sweet potato cultivars are maintained in greenhouses at the ARC-VOPI. Cuttings from these mother plants are propagated in nursery bags for sale to field nurseries, from which cuttings can be purchased.
Readers are welcome to contact the ARC-VOPI for a list of vine growers and cultivar recommendations. Order forms for disease-free material are also available. For more information contact the ARC-VOPI on (012) 841 9639.