Improving Yields and Enhancing Dietary Zinc and Selenium Intake in the Zambian Population Through Agronomic Biofortification of Maize And Wheat

Abstract

Deficiencies of zinc (Zn) and selenium (Se) are major causes of malnutrition-related health problems affecting the world’s population. The problem is most serious in developing countries such as Zambia because cereals, which are the principal source of calories, often have low micronutrient concentrations and other dietary sources of Zn and Se are limited. The problem may be exacerbated by low supplies of nitrogen (N) and sulphur (S) because they may play a role in the uptake and deposition of micronutrients in the grain. Soils in the major grain producing areas in Zambia have low N and Zn, while S deficiency is also widespread. In this scenario, grain nutrient concentrations and intakes of Zn and Se in Zambia are likely to be low. Little attention has been given to Se nutrition in Zambia largely because Se is not essential for plant growth. However, Se is an essential micronutrient for humans and animals and Se deficiency afflicts at least a billion people worldwide, especially in developing countries. There is evidence that many soils in Zambia are low in plant available Se and based on surveys from neighbouring countries, it is likely that the Se concentrations of grain are not high enough to meet consumption requirements of the population. Despite the important role of Se and Zn in human health, information on their concentration in the grain of crops in Zambia is limited. Two surveys were conducted to document the nutrient concentrations in the grain of maize and wheat grown and consumed in Zambia. One survey sampled grain from farms and the second was based on samples from the market place. All samples of maize (n=67) were deficient in S (median concentration=1030 mg/kg), while 75% were deficient in N (median concentration=1%) and 97% were deficient in Zn (median concentration=19 mg/kg). The survey of wheat was much smaller (n=6), but revealed moderate values of S (median concentration=1335 mg/kg) and Zn (median concentration=26 mg/kg) but adequate N (median concentration=2%). All the samples of both crops were very low in Se (median concentration=16 μg/kg in maize and 8μg/kg in wheat) and based on this an intake of 5 μg Se per day per person was estimated. This is slightly lower than that obtained in Malawi (7 μg Se per day per person) and much lower than the daily recommended intake of 50-70 μg Se per day. These low concentrations and intakes of Se and Zn are likely to be a health risk contributing to low resistance to infectious diseases and high mortality rates in Zambia which require measures to address the problem. Two growth room experiments were used to investigate the role of N and S on the vegetative and grain concentrations of N, S, Se and Zn in maize and wheat. Nitrogen application increased the vegetative yields and yields were higher when the N:S ratio was in balance. Sulphur and N nutrition enhanced dry matter and grain yield. No yield reductions were observed due to addition of Se applied as sodium selenate. The concentrations of N, S, Se and Zn increased under adequate S application. Sulphur concentration was also strongly correlated with K and Mo concentrations, nutrients which were also deficient in the maize and wheat samples from Zambia. Selenium concentrations of above 300μg/kg (more than adequate to satisfy dietary requirements) were easily achieved with a low rate of Se of 0.02 mg/kg applied to the soil at planting. The Zn concentration was significantly correlated with N in both experiments and with S in the vegetative tissue in maize, while there was also a significant positive correlation between S and Se in maize. These results suggest that N and S may have a role in Zn and Se uptake and remobilisation from the vegetative parts to the grain. These studies showed that not only is maintaining a sufficient amount of plant available Se and Zn in the soils a pre-requisite to ensure sufficient uptake of Se and Zn, but also adequate supplies of N and S are important to improve the impact of Se and Zn fertiliser applications. The results of both the surveys and experimental work further suggest the importance of S nutrition in enhancing yields and that inadequacy of S could be a limitation to agronomic biofortification of wheat and maize with Se and Zn. Agronomic biofortification with Se could easily be achieved with soil applications of small quantities of Se as sodium selenate. Therefore, increasing N, S, Se and Zn concentration and content in maize and wheat is a food systems strategy that could improve the intakes of these nutrients for the entire population.