Nitrogen effect on zinc biofortification of maize and cowpea in Zimbabwean smallholder farms

In sub‐Saharan Africa (SSA), people that rely on plant‐based diets are prone to dietary zinc (Zn) deficiency. Results from producer surveys suggest that Zn concentrations can be increased by adding Zn fertilizer or applying nitrogen (N) fertilizer. Thus, the objective of this study was to determine if the application of N fertilizer increase the Zn concentration in two economically important crops in the region: maize/corn (Zea mays L.) and cowpea (Vigna unguiculata L.) Field experiments were conducted in the Hwedza and Mutasa Districts of Zimbabwe during the 2016–17 and 2017–18 cropping seasons. The treatments comprised two crops (maize and cowpeas), three N rates (0, 45 kg ha−1 and 90 kg ha−1 for maize; 0, 15 kg ha−1 and 30 kg ha−1 for cowpea), 2 compositions of N (mineral or organic N), and 3 application strategies (mineral, organic, and a combination of mineral plus organic). Seven treatments were employed on each crop in a randomized complete block design with four replicates. The soils were Lixisol sandy soils and contained low plant‐available soil Zn concentrations. All treatments received 10 kg elemental soil applied Zn ha−1 plus two split applications of 0.3% w/v zinc sulphate foliar fertilizer. Maize grain Zn concentrations increased from 27.2 to up to 39.3 mg kg−1 with variable responses to N and Zn fertilizers between the two study sites. At 45 kg N ha−1, mineral N fertilizer increased maize grain Zn concentration more than organic N fertilizer from cattle manure, or a combination of mineral and organic N fertilizers. At 90 kg N ha−1, the three N fertilizer application strategies had a similar impact on maize grain Zn concentration. Co‐application of N and Zn fertilizer was more effective than Zn fertilizer by itself at increasing the Zn concentration in maize grain. Cowpea grain Zn concentration increased from 39.8 to 52.7 mg kg−1 from N and Zn co‐applications. Techniques to increase Zn concentration in cowpeas were less clear. Future cost:benefit analyses on the potential for agronomic biofortification need to include information on added benefits of agro‐fortified grain, complex farmer management decisions, including cost and access to both N and Zn fertilizers as well as understanding of the spatial and site‐specific variation in fertilizer response. This article is protected by copyright. All rights reserved