Field experiments of the first phase of the project were conducted in 8 collaborating countries for 2 cropping seasons on wheat, rice and maize. Wheat experiments were carried out by 4 countries (China, Pakistan Turkey and India); rice experiments by 4 countries (China, India, Turkey and Thailand); and maize experiments by 5 countries (China, Brazil, Zimbabwe, Turkey and Mozambique).

The reaction of cereal crops to Zn fertilization showed large variation between the countries and even within a given country in terms of grain yield response. In some locations of  Pakistan, China and Turkey, wheat grain yield was significantly  increased (up to 20 %) by soil Zn applications while in most of the locations of the target there was either very little or no effect of Zn fertilization on grain yield.

In contrast to the large variations regarding the effects of Zn fertilization on grain yield among countries/locations, the results with the effect of Zn fertilization on grain Zn concentrations were consistent. Grain Zn concentrations were significantly increased by foliar Zn applications while soil Zn applications was less effective. Wheat has been found to be the most promising cereal crop for increasing Zn in grains through foliar Zn fertilization. Highly significant increases were also obtained in grain Zn concentrations of rice, but the magnitude of the increases was not as big as in wheat.  In contrast to wheat and rice, the reaction of maize to Zn fertilization was highly variable among countries in terms of changes in grain Zn following Zn fertilization.

In case of wheat, particular increases in grain Zn concentration after foliar Zn spray were observed in each country. For example, foliar Zn spray increased grain Zn concentration from 25 to 61 mg kg-1 in one location in Punjab, from 29 to 60 mg kg-1 in one location in Pakistan and from 12 to 29 mg kg-1 in one location in Turkey.  The Zn treatments containing both soil (e.g., 25 or 50 kg ZnSO4) and foliar Zn application (around 2 to 4 kg ZnSO4 per ha) showed the largest increases in grain Zn concentration.

Additional field trials conducted showed that the timing of foliar Zn application is a critical issue in maximizing grain Zn concentration (Cakmak et al., 2010, J. Agric. Food Chem. 58: 9092-9102). Foliar spray of Zn late in growing season resulted in much greater increases in grain Zn concentration when compared to the earlier applications of Zn.  Increases in concentration of whole grain Zn through soil and/or foliar Zn applications were also well reflected (proportionally) in all grain fractions analyzed (e.g., embryo, aleurone and endosperm fractions), especially in the endosperm, the part predominantly consumed in food products in target countries (see Figure below). Foliar Zn spray was also very effective in reducing the phytate/Zn molar ratio in the endosperm part (e.g., from 112 to 45). The phytate/Zn molar ratio is an indicator for bioavailability of Zn in diets. Reducing this ratio is an important step in improving Zn bioavailability in human body.

Figure: Changes in Zn concentrations of the endosperm part of wheat seeds from plants which were treated by ZnSO4 at different growth stages under field conditions measured by using LA ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry). The diagrams show the Zn concentrations in the endosperm between the aleurone layers of the seed surface and the crease (for more detail see: Cakmak et al., 2010. J. Agric. Food Chem. 58: 9092-9102)