Agronomic Biofortification Strategies to Increase Grain Zinc Concentrations of Wheat, Rice and Maize: A Systematic Review and Network-Meta-Analysis

Zinc (Zn) malnutrition is a prevalent micronutrient deficiency that can negatively impact cognitive and physical health, especially among women and children in developing countries. One effective strategy to address low Zn levels in staple cereals (e.g. wheat, rice and maize), which are essential for human health, is Zn biofortification. There are two main approaches to Zn biofortification: genetic biofortification, which involves using the genetics of the crops to increase their Zn content, and agronomic biofortification, which uses zinc-rich fertilizers to boost the Zn levels in the edible parts of the crops. This systematic review aims to evaluate the effectiveness of agronomic biofortification strategies in increasing grain zinc concentrations in major cereals, specifically wheat, rice, and maize, through a network meta-analysis. Cochrane Library, Google Scholar, Scopus, and agricultural journals were searched up to the year of 2017 to identify relevant field trials assessing the impact of Zn fertilisation on grain Zn concentration and yield. The search was updated in 2020. Eligible studies were those focusing on wheat, rice, and maize, field-based, addressing Zn fertilisation research question, where grain Zn concentration served as the primary outcome and grain yield as a secondary outcome. Data were extracted and assessed for quality of evidence in the included studies. Outcome data was limited to grain Zn concentration and grain yield. Pairwise and network meta-analysis for direct and indirect comparisons of Zn fertilisation methods were performed. This review summarises evidence from 44 independent field-based studies related to zinc (Zn) fertilisation, of which 27 focused on wheat, 13 on rice, and 4 on maize. These studies involved different application methods, including soil, foliar, and combined soil + foliar Zn fertilisation, compared against control groups with no Zn fertilisation. The design of the studies was randomised blocks, typically with three or four replicates. In total, 21 studies evaluated the effectiveness of soil, foliar, and combined soil + foliar Zn fertilisation methods against a control, while 15 studies focused solely on foliar application comparisons against control groups. The review highlights the prevalence of foliar Zn studies and the few instances of comparisons involving combined fertilization methods. The risk of bias in the included studies is mostly low. Soil + foliar Zn fertilisation significantly increases grain Zn concentration in wheat by 28.7 mg/kg [95%-CI: 22.77 to 34.53], while the increase is smaller for rice (6.8 mg/kg [95%-CI: 5.3 to 8.22]) and maize (7.9 mg/kg [95%-CI: 6.83 to 8.91]). Foliar Zn fertilisation also increases Zn concentration, reaching up to 18.0 mg/kg [95%-CI: 14.26 to 21.79] in wheat, 6.7 mg/kg [95%-CI: 3.25 to 10.04] in rice, and 7.6 mg/kg [95%-CI: 6.51 to 8.75] in maize, although it has minimal effects on grain yields. Conversely, soil Zn fertilisation results in a lower increase in Zn concentration, such as 4.7 mg/kg [95%-CI: 2.30 to 7.12] in wheat, with variable impacts on grain yields. Overall, considering the baseline of 16 mg/kg for rice and 25 mg/kg for wheat and maize, the effectiveness of Zn fertilisation strategies varies across different crops; the combination of soil and foliar Zn application significantly enhances grain Zn concentrations, especially in wheat with an increase of 114.8% over the baseline, while rice shows some statistically significant increase of 42.5% over the baseline, but maize does not show significant increases. Despite the biologically significant effects, there is low certainty of evidence, due mostly to high heterogeneity and unexplained inconsistency between studies. Agronomic strategies can increase wheat grain Zn concentrations, aiding in biofortification efforts to improve human Zn availability. The combination of soil and foliar applications is the most effective approach. Although increase in rice and maize grain Zn concentrations is statistically significant, it remains relatively small; however, even a slight increase (1-mg/kg) in rice could have an impact on human health due to its lower baseline Zn levels. Future research on the genetic variability of Zn concentration in rice could enhance farming practices and breeding programs, potentially incentivised by premium payments for high-Zn grain products.