Zinc Application Enhances Biomass Production, Grain Yield, and Zinc Uptake in Hybrid Maize Cultivated in Paddy Soil

Abstract

Zinc (Zn) fertilization is widely used in maize (Zea mays L.) production to alleviate Zn deficiency and improve biomass and grain yield. However, limited research exists on Zn management in maize cultivated in high-pH paddy soils following rice-based systems, where altered soil chemistry may affect Zn availability and plant uptake. This study aimed to evaluate the effects of Zn application rates on growth, yield, and Zn uptake in two hybrid maize varieties under such conditions. Field experiments were conducted during the 2019 and 2020 dry seasons in Phetchabun Province, Thailand, using a randomized complete block design with a 4 × 2 factorial arrangement and four replications. Treatments included four Zn rates (0, 5, 10, and 20.6 kg of Zn/ha), applied as Zn sulfate monohydrate (ZnSO₄·H₂O, 36% Zn) by soil banding at the V6 stage, and two hybrid varieties, Suwan 5731 (SW5731) and Suwan 5819 (SW5819). In 2019, significant Zn × variety interactions were observed for biomass, crop growth rate (CGR), and grain yield. SW5819 at 10 kg of Zn/ha produced the highest biomass (31.6 t/ha) and CGR (25.6 g/m²/day), increasing by 15.3% and 39.1%, respectively, compared to its own no Zn treatment. In contrast, 20.6 kg of Zn/ha reduced SW5819 biomass by 6.6% and 13.1% relative to SW5731 and its own no-Zn treatment, respectively. Grain yield in SW5819 peaked at 14.7 t/ha under 5 and 10 kg of Zn/ha, significantly higher than SW5731 under 0 and 5 kg of Zn/ha by 16.7%, while SW5731 showed no significant response. In SW5819, shoot and grain Zn uptake significantly increased under 5 and 10 kg of Zn/ha by up to 36.8% and 33.3%, respectively, compared to no Zn treatment. The lowest shoot Zn uptake was found in SW5819 under 20.6 kg of Zn/ha (264.1 ± 43.9 g/ha), which was lower than all its Zn treatments and all SW5731 treatments, showing a reduction of 19.4–43.6%. Zn application improved soil Zn availability, and Zn partitioning among plant organs varied with Zn rate and season. A moderate Zn rate (10 kg of Zn/ha) optimized maize performance under high-pH, rice-based conditions, emphasizing the need for variety-specific Zn management.

1. Introduction

Maize (Zea mays L.) is one of the most significant crops grown in Thailand. Recent data indicate that maize production in Thailand is expected to rise to approximately 5.4 million tons in the 2024/25 marketing year, supported by favorable weather and expanded planting areas, particularly driven by the demand from the feed industry [1,2]. Currently, only 9.5% of maize planting areas are utilized during the dry season. To address water scarcity and the increasing demand for maize driven by the expanding livestock sector, a policy has been implemented to increase the dry-season maize planting area by 50% [3]. This will be achieved by converting paddy fields from a rice-rice cropping system to a rice-maize cropping system during the wet and dry seasons. While this shift presents an opportunity to improve maize yields, it also introduces new challenges in soil nutrient management, particularly for micronutrients such as Zn, which typically shows low availability in paddy soils and under high soil pH conditions [4].

Zn is a micronutrient that plays catalytic, structural, and regulatory roles in numerous biochemical pathways. In plants, Zn-containing or Zn-activated enzymes are crucial for photosynthesis, carbohydrate metabolism, protein synthesis, auxin regulation, pollen formation, and reproductive development [5,6,7]. Zn is absorbed primarily as Zn²⁺ from the soil solution, and its uptake is mediated by Zn-regulated transporters [8,9]. However, Zn solubility in soil is strongly affected by chemical speciation, which is in turn influenced by factors such as soil pH, redox potential, calcium carbonate fixation, organic matter, and phosphorus content. These factors often lead to low Zn bioavailability in soils [4].

Zn deficiency in agricultural soils is a global issue that significantly reduces crop yield and nutritional quality [10]. Maize is particularly sensitive to Zn deficiency. Approximately 308 g/ha of Zn is removed with the grain, with about 60% of the Zn taken up by the plant allocated to the grain [11]. In previous research, the critical threshold for Zn deficiency is generally reported as 0.54 mg/kg in soil and 25–100 mg/kg in leaf tissue [4]. However, in maize cultivation, Zn is the fourth most essential nutrient limiting yield, after nitrogen, phosphorus, and potassium [12]. The advantages of applying Zn include enhanced photosynthesis, increased chlorophyll levels, and a greater kernel count and weight, especially in the apical region, effects often linked to improved pollen viability during the tasseling phase under field conditions [13,14,15,16]. Several studies have shown that Zn fertilization significantly enhances maize grain yield and Zn content, although optimal soil concentrations and application rates vary by soil type and production system [17,18,19,20,21,22].

Currently, various methods of Zn application are used in maize production, especially in soils with low Zn availability. These methods have been shown to significantly increase total biomass and maize grain yield by more than 22% and enhance Zn concentration in the grain [23,24]. The main application methods include seed soaking, foliar spraying, and soil application. Among these, soil application allows for higher Zn input, offers residual availability in soils that do not strongly fix Zn, and helps establish a long-term nutrient reservoir [25,26]. Liu et al. [19], for example, reported that in a field experiment conducted in alkaline soil (pH 8.0) in China, a rate of 50 kg of ZnSO₄·7H₂O/ha (equivalent to 4.7 mg Zn/kg soil) was optimal for enhancing maize yields. Similarly, Botoman et al. [21] found that 30 kg of Zn/ha increased grain yields by 11% compared to no Zn application, and Ruffo et al. [18] showed that 11.21 kg of ZnSO₄/ha increased yield by 990 kg/ha. In Thailand, field experiments in calcareous soils (pH 7.8) demonstrated that applying 2–4 mg of Zn/kg improved Zn uptake and maize grain yield [27].

The yield of maize is particularly influenced by Zn availability during key growth stages. The number of kernels per plant (NKP) is determined from germination to the blister stage, while grain filling—which affects thousand kernel weight (TKW)—occurs during the milking stage [14,15]. Zn translocation from root to shoot after uptake is influenced by many factors, and accumulation in the grain depends on both remobilization from shoots and continued uptake during grain filling [28,29]. The Zn harvest index (ZnHI), defined as the proportion of total Zn allocated to the grain, reflects this translocation efficiency. In maize, up to 60% of total Zn uptake is allocated to the grain, contributing to yields as high as 12.0 tons/ha [11].

Increasing soil Zn concentrations through fertilization improves Zn use efficiency. Takkar et al. [30] reported critical Zn concentrations in soil for maize ranging from 0.38 to 1.40 mg/kg, while Zare et al. [31] identified a deficiency threshold of 1.50 mg/kg. Barbieri et al. [32] and Cuesta et al. [33] used modified arcsine–logarithmic calibration curves to determine critical Zn levels of 1.02 mg/kg and 1.06 mg/kg, respectively. In contrast, studies by Kumar et al. [22] and Liu et al. [19] indicated that optimal Zn concentrations for high yield were 2.76 mg/kg and 4.70 mg/kg, respectively, 1.8 to 3.1 times higher than the critical deficiency thresholds.

Despite considerable research on Zn management in maize, there is still limited understanding of how Zn fertilization performs in high-pH soils following flooded or rice-based cropping systems. Such soils may present distinct chemical conditions that influence Zn availability and plant uptake differently from typical upland systems. These uncertainties highlight the need for further investigation to inform fertilizer strategies suited to similar transitional agroecosystems. To address this gap, the present study aimed to evaluate the effects of varying Zn application rates on the physiological responses, Zn uptake, and grain yield of hybrid maize grown in high-pH paddy soils in Thailand. It was hypothesized that increasing Zn application rates would enhance physiological performance, Zn uptake, and grain yield by improving Zn availability and uptake efficiency during critical growth stages.

2. Materials and Methods

2.1. Experimental Sites Description

The two-year field experiments were conducted during the dry seasons of 2019–2020 and 2020–2021, following rice cultivation. The study site was located in Lom Sak, Phetchabun province, Thailand (16°45′0″ N, 101°10′17″ E), where a rice–maize rotation system has been practiced continuously for over five years. In this system, rice is cultivated during the rainy season and maize during the dry season. Prior to maize planting, rice residues were incorporated into the soil after harvest by plowing. Although residual nutrient levels from the previous crop were not analyzed, the chemical properties of the soil prior to maize cultivation were assessed to evaluate the nutrient status. It was found that the surface soil (0–30 cm depth) was classified as clay loam, consisting of 34% sand, 24% silt, and 42% clay. The chemical characteristics of the soil were as follows: 16.3 g/kg organic matter, 26.84 mg/kg available phosphorus, 60.45 mg/kg available potassium, 0.53 mg/kg DTPA-extractable Zn, and a pH of 7.8, indicating slightly alkaline conditions.

Daily meteorological data, including precipitation and maximum and minimum temperatures, were recorded throughout the experimental period (from planting to harvesting) using a weather station installed at the field site (Figure 1). From February 2019 to April 2020, the total precipitation was 170.33 mm, the maximum temperature reached 44.6 °C in April 2020, and the minimum temperature was 11.4 °C in December 2019 (Figure 1a). Between December 2020 and April 2021, the total precipitation was 272.2 mm, with a maximum temperature of 40.1 °C in March 2021 and a minimum temperature of 12.2 °C in January 2021 (Figure 1b).

2.2. Experimental Design

The paddy soil was prepared with conventional approaches, including disc harrow plowing after rice harvesting. The field trials were arranged as two factorials in randomized complete block designs with four replications. Zn fertilizer was applied in the form of Zn sulfate monohydrate (ZnSO₄·H₂O), containing 36% elemental Zn, at four rates equivalent to 0, 5, 10, and 20.6 kg of Zn/ha per crop season. These application rates correspond to 0, 1.5, 3, and 6 times the baseline soil Zn concentration (0.53 mg/kg, DTPA-extractable), calculated to represent a gradient of Zn availability relative to the existing soil Zn status. Zn fertilizer was applied at the V6 stage by banding to the soil surface. Two new hybrid maize varieties, Suwan 5731 (SW 5731) and Suwan 5819 (SW 5819), were used. These varieties were developed and introduced by the National Corn and Sorghum Research Center, Faculty of Agriculture, Kasetsart University, Thailand, specifically for cultivation in paddy soils. The experiment plot size was 28 m² (5.6 m × 5.0 m) with eight rows per plot. The seeds of each maize variety were sown using a manual planting machine with two seeds per hill in eight rows per plot and thinned at two weeks after planting (WAP) to maintain one plant per hill. The density was 71,500 plants/ha, with a row spacing of 0.7 m and a plant spacing of 0.2 m. All plots were applied as follows: 105 kg N/ha (as urea), 35 kg P₂O₅/ha (as diammonium phosphate), and 65 kg K₂O/ha (as potassium chloride). The N-P-K fertilizer program was implemented as follows: prior to planting, 35 kg of N/ha, 35 kg of P₂O₅/ha, and 65 kg of K₂O/ha were applied as a basal dose. An additional 35 kg of N/ha was applied at the V6 growth stage, followed by a final application of 35 kg of N/ha at the tasseling stage. After sowing, sprinkler irrigation was applied immediately and continued weekly until the maturity stage. Acetochlor and Atrazine were used for herbicide control at 21 WAP. Emamectin benzoate and Spinetoram were applied to control fall armyworm outbreaks detected at 30 (V6), 40 (V10), and 63 (VT) days after planting. At the VT stage, pesticides were applied with a spraying drone. The Nacdrone EASY 50 plus, equipped with a nozzle propeller, was sprayed at a rate of 20 L/ha.

2.3. Sampling and Measurements

2.3.1. Plant Sampling, Dry Matter, and Grain Yield

During the growing period, destructive measurements were conducted to evaluate the agronomic characteristics. The selected agronomic characteristics were measured at the VT and PM stages. Three plants per plot were randomly sampled. Plant samples were separated into leaves, stalks, tassels, and grains and placed in a hot air oven at 65 °C for drying until they reached a constant weight. The dry weight of each plant part was then collected and expressed as tons per hectare. The maize grain yield was determined from an 8.5 m² harvest area by using the two central rows of each plot. When maize reached the PM stage, the ears were harvested from the harvested area, as mentioned above. Five ears were randomly selected from the total harvested ears in each plot to calculate the grain yield at 15% moisture content and expressed in tons per hectare. The dry matter (DM) values were used to determine the accumulation and efficiency according to the following equations:

Crop growth rate (CGR) was determined using the following formula from Watson [34].

$$\mathit{CGR} (\mathrm{g}/{\mathrm{m}}^2/\mathrm{day})={\displaystyle \frac{(W_2-W_1)}{(T_2-T_1)}}$$

(1)

where W₁: dry weight of plant at time T₁ (VT); W₂: dry weight at time T₂ (PM); T₁ and T₂: time interval in days.

The relative yield and harvest index (HI) were determined using the following formula from Lui et al. [19].

$$\mathit{Relative}\mathit{yield}={\displaystyle \frac{\mathit{Yield} \mathit{treatment}}{\mathit{Yield} \mathit{max}}}\times 100\%$$

(2)

where Yield treatment was the yield of each plot, and Yield max was the highest mean yield among the four treatments.

$$\mathit{HI}={\displaystyle \frac{\mathit{Grain} \mathit{yield}}{\mathit{Aboveground} \mathit{DM} \mathit{at} \mathit{PM}}}$$

(3)

2.3.2. Zn Concentration Determination

The dried samples were separated into individual leaves (leaf blades), stalks, tassels, and grains. The samples were then weighed, ground into fine powder, and digested in a mixture of nitric acid (HNO₃) and perchloric acid (HClO₄) (1:1) [35]. Zn concentration was determined by atomic absorption spectrophotometry (AAS; Perkin-Elmer Analyst 300; Waltham, MA, USA). The Zn uptake (g/ha) in plant parts was calculated as Zn concentration in each plant part multiplied by its dry weight. The following parameters were calculated according to Lui et al. [36].

Shoot Zn uptake (g/ha) = Summation of Zn uptake in all plant parts

(4)

where the term of plant part was leaf, stalk, tassel, and grain (PM).

Post-anthesis shoot Zn uptake = shoot Zn uptake at PM − shoot Zn uptake at VT

(5)

$$\mathit{Zn}\mathit{harvest}\mathit{index} \left(\mathit{ZnHI}\right)={\displaystyle \frac{\mathit{Grain} \mathit{Zn} \mathit{uptake}}{\mathit{Shoot} \mathit{Zn} \mathit{uptake} \mathit{at} \mathit{PM}}}$$

(6)

$$\mathit{Zn}\mathit{distribution}\mathit{ratio}={\displaystyle \frac{\mathit{Zn} \mathit{uptake} \mathit{in} \mathit{plant} \mathit{part}}{\mathit{Summation} \mathit{of} \mathit{Zn} \mathit{uptake} \mathit{in} \mathit{plant} \mathit{part} }} \times 100\%$$

(7)

2.3.3. Soil Sampling and Analysis

Bulk soil samples (0–20 cm) were collected in each plot at VT and PM stages in each year. The soil DTPA-Zn was measured, according to Lindsay and Norvell [37], and determined using atomic absorption spectrophotometry (AAS; Perkin-Elmer Analyst 300; Waltham, MA, USA).

2.4. Data Analysis

Different data were subjected to analysis of variance (ANOVA) appropriate for a factorial randomized complete block design. Pearson correlation analysis was used to examine the associations between plant parameters and soil parameters. Statistical analysis was performed with STAR (Statistical Tool for Agricultural Research) software, version 2.0.1. At a significance level of 0.05 [38], mean values were compared using Fisher’s least significant difference (LSD) approach. The response of relative grain yield and grain Zn uptake to the soil Zn concentration was described by polynomial regression using Microsoft Excel.

3. Results

3.1. Biomass, Crop Growth Rate, and Grain Yield

The biomass of maize at the VT and PM stages, under the interaction between Zn application rates and maize varieties during the 2019 and 2020 dry seasons, is presented in

Table 1. Effect of Zn application rates and maize varieties interaction on biomass, crop growth rate, grain yield, and harvest index in 2019 and 2020.

Year	Variety	Zn Application Rate	Biomass (t/ha)		Crop Growth Rate (g/m2/day)	Grain Yield (t/ha)	HI
			VT	PM
2019	SW5731	0	8.7 ± 0.34	25.5 ± 1.82 b	20.4 ± 1.92 cde	12.6 ± 0.51 b	0.48 ± 0.02
		5	9.0 ± 0.70	26.0 ± 1.34 ab	21.5 ± 2.76 bcd	12.6 ± 0.43 b	0.50 ± 0.04
		10	8.5 ± 0.77	27.8 ± 1.31 a	23.8 ± 1.07 abc	12.9 ± 0.36 ab	0.48 ± 0.05
		20.6	8.3 ± 0.70	26.1 ± 1.91 ab	21.9 ± 2.64 abcd	13.3 ± 0.22 ab	0.50 ± 0.04
	SW5819	0	10.6 ± 0.74	27.4 ± 1.63 b	18.4 ± 2.42 de	13.8 ± 0.49 ab	0.53 ± 0.05
		5	10.7 ± 0.74	29.0 ± 1.72 b	24.9 ± 1.26 ab	14.7 ± 0.18 a	0.53 ± 0.05
		10	10.8 ± 0.49	31.6 ± 1.83 a	25.6 ± 2.12 a	14.7 ± 0.67 a	0.45 ± 0.03
		20.6	9.6 ± 0.44	23.8 ± 2.92 c	17.6 ± 3.69 e	14.3 ± 0.25 ab	0.60 ± 0.08
		Mean	9.5 ± 0.30	27.1 ± 0.97	21.7 ± 1.82	13.6 ± 0.21	0.51 ± 0.03
Source of variation
Zn application rate (Zn)			ns	**	**	ns	*
Variety (Var)			**	ns	ns	**	ns
Zn x Var			ns	**	**	**	ns
2020	SW5731	0	8.9 ± 0.87	25.8 ± 1.44	20.8 ± 0.98	10.5 ± 0.47	0.41 ± 0.02
		5	9.0 ± 0.27	25.6 ± 1.42	20.4 ± 1.70	11.3 ± 0.48	0.45 ± 0.02
		10	8.9 ± 0.20	26.6 ± 1.49	21.8 ± 1.59	11.0 ± 1.02	0.42 ± 0.03
		20.6	8.4 ± 0.15	26.1 ± 1.77	21.9 ± 2.27	10.9 ± 1.04	0.42 ± 0.02
	SW5819	0	9.2 ± 0.39	26.4 ± 1.33	21.2 ± 1.43	10.7 ± 1.09	0.40 ± 0.03
		5	9.2 ± 0.25	28.6 ± 1.25	24.1 ± 1.58	11.1 ± 0.32	0.39 ± 0.03
		10	9.9 ± 0.58	28.1 ± 2.06	22.5 ± 2.62	10.5 ± 0.33	0.38 ± 0.03
		20.6	9.3 ± 0.89	26.1 ± 1.40	20.7 ± 2.09	10.2 ± 1.42	0.38 ± 0.04
		Mean	9.1 ± 0.25	26.7 ± 0.72	21.7 ± 2.73	10.8 ± 0.42	0.41 ± 0.01
Source of variation
Zn application rate (Zn)			ns	ns	ns	ns	ns
Variety (Var)			ns	ns	ns	ns	ns
Zn x Var			ns	ns	ns	ns	ns

Means in a column followed by different letters are significantly different at p < 0.05 according to Fisher’s LSD test. ns indicates no significant difference. *, and ** indicate significant difference at p < 0.05, and < 0.01, respectively.

. Significant differences in biomass, CGR, and grain yield among treatment interactions were observed in 2019 but not in 2020.

In 2019, a significant interaction between Zn application rate and maize variety was observed for biomass at the PM stage, while no significant differences were detected at the VT stage. At the VT stage, biomass ranged from 8.3 to 10.8 t/ha across treatments without significant variation. At the PM stage, the highest biomass was observed in SW5819 under 10 kg of Zn/ha (31.6 ± 1.83 t/ha), representing a 15.3% increase compared to SW5819 without Zn application (27.4 ± 1.63 t/ha). Similarly, in SW5731, biomass reached 27.8 ± 1.31 t/ha under 10 kg of Zn/ha, which was 9.0% higher than SW5731 without Zn application (25.5 ± 1.82 t/ha). In contrast, the lowest biomass was recorded in SW5819 under the 20.6 kg of Zn/ha treatment (23.8 ± 2.92 t/ha), corresponding to a 13.1% decrease relative to SW5819 without Zn application and 6.6% lower than SW5731 without Zn application.

For CGR, treatment effects varied depending on both Zn application rate and maize variety. The highest CGR was found in SW5819 under 10 kg of Zn/ha (25.6 ± 2.12 g/m²/day), representing a 39.1% increase compared to SW5819 without Zn application (18.4 ± 2.42 g/m²/day). In SW5731, the CGR under 10 kg of Zn/ha was 23.8 ± 1.07 g/m²/day, which was not significantly different from SW5731 without Zn application (20.4 ± 1.92 g/m²/day). The lowest CGR was found in SW5819 under 20.6 kg of Zn/ha (17.6 ± 3.69 g/m²/day), which was significantly lower than most other treatments, but not significantly different from the treatment without Zn application in either variety.

Grain yield was significantly influenced by the interaction between Zn application rate and maize variety. In SW5819, the highest grain yields were found under both 5 and 10 kg of Zn/ha (14.7 ± 0.18 and 14.7 ± 0.67 t/ha, respectively), which were not significantly different from SW5819 without Zn application (13.8 ± 0.49 t/ha). In SW5731, grain yield ranged from 12.6 ± 0.43 to 13.3 ± 0.22 t/ha, with no significant differences observed among Zn application rates. However, SW5819 under both 5 and 10 kg of Zn/ha produced significantly higher grain yield than SW5731 under 0 and 5 kg of Zn/ha, representing a 16.7% increase in grain yield compared to both. HI was significantly affected by Zn application rate, while maize variety and the Zn × variety interaction were not significant. The mean HI across all treatments in 2019 was 0.51.

Overall, results from 2019–2020 showed that Zn fertilization in 2019 enhanced PM-stage biomass, CGR, and grain yield with variety-specific responses. The 10 kg of Zn/ha rate provided the greatest benefit, particularly in SW5819, while SW5731 showed limited responsiveness. In contrast, the 20.6 kg of Zn/ha rate reduced performance in SW5819. The absence of significant effects in 2020 highlights seasonal variability.

3.2. Zn Uptake in Shoots and Grain by Maize

Zn uptake and partitioning in maize were significantly influenced by the interaction between Zn application rate and variety, with distinct responses observed across Zn treatments in 2019 and 2020 (

Table 2. Effect of Zn application rates and maize varieties interaction on shoot Zn uptake at VT and PM stage, grain Zn uptake, post-anthesis shoot Zn uptake, and Zn harvest index in 2019 and 2020.

Year	Variety	Zn Application Rate	Shoot Zn Uptake at VT (g/ha)	Shoot Zn Uptake at PM (g/ha)	Grain Zn Uptake (g/ha)	Post-Anthesis Shoot Zn Uptake (g/ha)	ZnHI
2019	SW 5731	0	62.8 ± 8.0 c	327.5 ± 18.7 d	213.7 ± 32.4 cd	264.8 ± 21.2 abc	0.65 ± 0.04
		5	72.6 ± 14.3 c	368.4 ± 38.3 cd	223.4 ± 45.6 cd	295.7 ± 52.6 abc	0.61 ± 0.06
		10	76.2 ± 9.2 c	405.9 ± 44.8 bc	234.8 ± 10.2 bc	329.7 ± 53.5 a	0.58 ± 0.03
		20.6	131.2 ± 27.4 b	353.8 ± 55.9 cd	224.9 ± 35.3 cd	222.6 ± 36.8 bc	0.63 ± 0.00
	SW 5819	0	118.9 ± 30.0 b	342.4 ± 37.7 d	215.9 ± 42.0 cd	210.1 ± 67.6 c	0.63 ± 0.03
		5	135.9 ± 40.4 ab	456.7 ± 49.0 ab	287.9 ± 62.7 a	307.4 ± 84.0 ab	0.63 ± 0.05
		10	154.9 ± 33.9 a	468.5 ± 79.4 a	261.9 ± 53.5 ab	340.2 ± 69.9 a	0.56 ± 0.03
		20.6	123.0 ± 79.4 b	264.1 ± 43.9 e	188.7 ± 45.6 d	107.8 ± 43.2 d	0.71 ± 0.04
		Mean	107.8 ± 10.9	373.4 ± 21.1	231.4 ± 12.9	259.8 ± 24.0	0.63 ± 0.02
Source of variation
Zn application rate (Zn)			ns	**	*	**	*
Variety (Var)			**	ns	ns	ns	ns
Zn x Var			**	**	**	**	ns
2020	SW 5731	0	99.7 ± 12.6 b	375.6 ± 78.3	268.5 ± 43.5 c	275.9 ± 72.0	0.72 ± 0.02
		5	108.9 ± 17.7 b	382.7 ± 86.2	268.3 ± 41.9 c	273.7 ± 70.6	0.71 ± 0.05
		10	93.0 ± 13.7 b	343.6 ± 23.2	263.8 ± 15.5 c	250.6 ± 10.5	0.77 ± 0.02
		20.6	110.7 ± 34.0 b	466.3 ± 60.1	324.2 ± 42.5 abc	355.6 ± 81.2	0.70 ± 0.03
	SW 5819	0	110.1 ± 15.0 b	361.2 ± 79.1	283.5 ± 51.6 bc	251.1 ± 73.6	0.79 ± 0.02
		5	192.7 ± 50.1 a	479.9 ± 60.7	358.9 ± 41.6 a	287.2 ± 92.4	0.75 ± 0.01
		10	121.9 ± 12.1 b	425.8 ± 32.3	356.9 ± 28.4 a	303.9 ± 27.1	0.84 ± 0.01
		20.6	115.4 ± 19.4 b	431.4 ± 36.9	344.9 ± 32.3 ab	315.9 ± 18.7	0.80 ± 0.02
		Mean	119.3 ± 9.0	408.3 ± 20.0	308.7 ± 12.7	289.2 ± 18.4	0.76 ± 0.02
Source of variation
Zn application (Zn)			*	ns	ns	ns	ns
Variety (Var)			*	ns	**	ns	**
Zn xVar			*	ns	**	ns	ns

Means in a column followed by different letters are significantly different at p < 0.05 according to Fisher’s LSD test. ns indicates no significant difference. *, and ** indicate significant difference at p < 0.05, and < 0.01, respectively.

).

In 2019, the highest shoot Zn uptake at the VT stage was observed in SW5819 under 10 kg of Zn/ha (154.9 ± 33.9 g/ha), which was significantly higher than that in SW5819 without Zn application (118.9 ± 30.0 g/ha), indicating a 30.3% increase. This treatment also resulted in significantly greater shoot Zn uptake than all Zn treatments in SW5731 (62.8–131.2 g/ha), with increases ranging from 18.1% to 146.7%. At the PM stage, shoot Zn uptake in SW5819 remained highest under 5 and 10 kg of Zn/ha (456.7 ± 49.0 and 468.5 ± 79.4 g/ha, respectively), showing significant increases of 34.4% and 36.8% compared to its own no-Zn treatment (342.4 ± 37.7 g/ha). SW5819 under 5 and 10 kg of Zn/ha also exhibited significantly greater shoot Zn uptake than all SW5731 treatments (327.5–405.9 g/ha), with differences ranging from 15.4% to 43.1%. In contrast, the lowest shoot Zn uptake was observed in SW5819 under 20.6 kg of Zn/ha (264.1 ± 43.9 g/ha), representing reductions of 22.8%, 42.2% and 43.6% relative to the 0, 5 and 10 kg of Zn/ha rates, respectively and lower than all SW5731 treatments, with differences ranging from 19.4% to 34.9%. Grain Zn uptake followed a similar trend. The highest grain Zn uptake was found in SW5819 under 5 and 10 kg of Zn/ha (287.9 ± 62.7 and 261.9 ± 53.5 g/ha), corresponding to increases of 33.3% and 21.3% over SW5819 without Zn application (215.9 ± 42.0 g/ha), respectively. These treatments also exceeded all SW5731 treatments by 22.6% to 34.7%. ZnHI was significantly affected by Zn application rate, whereas no significant effects were observed for variety or the Zn × variety interaction. The mean ZnHI across all treatments in 2019 was 0.63.

In 2020, a significant interaction between Zn application rate and maize variety was observed for shoot Zn uptake at the VT stage, but not at the PM stage (Table 2). The highest shoot Zn uptake at the VT stage was found in SW5819 under 5 kg of Zn/ha (192.7 ± 50.1 g/ha), which was significantly greater than all other treatments in both varieties, with increases ranging from 58.1% to 107.2%. Grain Zn uptake was also significantly influenced by the interaction between Zn application rate and variety. In SW5819, grain Zn uptake under 5 and 10 kg of Zn/ha (358.9 ± 41.6 and 356.9 ± 28.4 g/ha, respectively) was significantly higher than all other treatments, except for SW5731 and SW5819 under 20.6 kg of Zn/ha (324.2 ± 42.5 and 344.9 ± 32.3 g/ha, respectively). Post-anthesis shoot Zn uptake showed no significant effects of Zn application rate, maize variety, or their interaction. The mean post-anthesis Zn uptake across all treatments in 2020 was 289.2 g/ha. The ZnHI was significantly affected by maize variety, but not by Zn application rate or the Zn × variety interaction. The overall mean ZnHI across all treatments in 2020 was 0.76.

Collectively, the results from 2019–2020 indicated that Zn uptake and partitioning were influenced by year, with stronger interaction effects between Zn rate and variety observed in 2019. SW5819 consistently exhibited greater responsiveness to Zn fertilization than SW5731, particularly at application rates of 5 and 10 kg of Zn/ha.

As shown in

Table 3. Analysis of variance as it relates to Zn application rates, maize varieties, and their interaction effects on Zn distribution ratios in different plant parts in 2019 and 2020.

Source of Variance	df	2019
		VT Stage						PM Stage
		Stalk		Leaf		Tassel		Stalk		Leaf		Tassel		Grain
		MS	p	MS	p	MS	p	MS	p	MS	p	MS	p	MS	p
Zn	3	335.82	**	116.27	*	74.34	**	155.87	*	44.12	ns	0.22	ns	111.66	*
Var	1	0.94	ns	8.65	ns	3.88	ns	2.70	ns	2.50	ns	9.09	**	9.41	ns
Zn x Var	7	272.23	**	95.12	*	61.56	**	99.91	ns	45.04	ns	1.42	**	63.23	ns
		2020
		VT stage						PM stage
		stalk		Leaf		Tassel		Stalk		Leaf		Tassel		Grain
		MS	p	MS	p	MS	p	MS	p	MS	p	MS	p	MS	p
Zn	3	1.17	ns	478.48	**	433.14	**	15.07	ns	22.86	**	1.27	ns	58.57	ns
Var	1	238.98	**	198.90	ns	1.83	ns	87.82	*	0.04	ns	55.91	**	290.99	**
Zn x Var	7	42.92	ns	268.87	**	206.48	**	20.97	ns	13.46	**	9.06	**	71.43	*

MS: mean sum squares; df: degree of freedom; ns indicates no significant difference. *, and ** indicate significant difference at p < 0.05, and < 0.01 based on the LSD test, respectively.

, analysis of variance showed that Zn uptake in different plant parts was significantly affected by Zn application rate, maize variety, and the interaction of Zn × variety. At the VT stage, there was a significant effect on Zn application rate in leaf and tassel in both crop seasons, except that stalk was only significant in 2019 (Table 3). In 2019, the distribution ratios of Zn in the stalk, leaf, and tassel were 26.7–52.2%, 35.0–48.9%, and 12.8–24.4%, respectively. These values indicate a relatively balanced Zn allocation among these organs. In contrast, in 2020, stalk Zn distribution was lower (14.0–22.6%), while leaf and tassel Zn distribution were higher (46.2–71.9% and 14.1–34.9%, respectively) (Figure 2a,c). During the PM stage, the application rate of Zn significantly affected the uptake of Zn in the stalk and grain in 2019, as well as in the leaves in 2020. In 2019, the interaction between Zn application and variety was significant for the tassel, while in 2020, it was significant for the leaves, tassel, and grain (Table 3). The highest Zn distribution ratio in grain was 80.3% at a Zn application rate of 10 mg Zn/kg in 2020, while the highest rate of 20.6 mg Zn/kg was in 2019 (Figure 2b,d).

3.3. Zn Availability in Soil

Zn availability in soil at VT and PM stage in both crop seasons was significantly increased by Zn application rates (

Table 4. Analysis of variance as it relates to Zn application rates, maize varieties, and their interaction effects on Zn availability in soil at VT and PM stage in 2019 and 2020.

Source of Variance	df	2019				2020
		VT Stage		PM Stage		VT Stage		PM Stage
		MS	p	MS	p	MS	p	MS	p
Zn application (Zn)	3	1.19	*	9.65	**	10.53	**	9.89	**
Variety (Var)	1	0.54	ns	0.02	ns	2.24	ns	0.33	ns
Zn x Var	7	0.97	**	4.23	**	5.08	*	4.77	ns

MS: mean sum squares; df: degree of freedom; ns indicates no significant difference. *, and ** indicate significant difference at p < 0.05, and < 0.01 based on the LSD test, respectively.

). In 2019, the highest soil-Zn availability at the VT and PM stages was 1.49 and 3.33 mg/kg, respectively, and in 2020, it was 3.56 and 3.54 mg/kg, respectively (Figure 3). Increased Zn application rates correlated with increased Zn availability in the soil; plots without Zn application showed the lowest levels. The Zn availability in soil without Zn application was the same as the soil Zn availability before planting (0.53 mg/kg).

3.4. Response of Maize Variety on Relative Grain Yield and Grain Zn Uptake to Zn Concentration in Soil

As shown in Figure 4, during the PM stage, two maize varieties showed no significant changes in relative grain yield or grain Zn uptake in response to varying soil Zn concentrations. However, SW 5731 exhibited a positive response, with a significant increase in grain Zn uptake as soil Zn concentration increased (Figure 4c).

A strong positive correlation (r = 0.79, p < 0.01) was observed between shoot Zn uptake at physiological maturity (PM) and grain Zn uptake, suggesting that efficient translocation of Zn from shoot tissues to the grain plays a critical role in enhancing grain Zn concentration. In addition, post-anthesis shoot Zn uptake was significantly correlated with total shoot Zn uptake at PM (r = 0.85, p < 0.01), indicating that Zn uptake during the reproductive stage contributes substantially to the overall Zn pool available for remobilization to the developing grain. These findings emphasize the importance of maintaining adequate Zn availability during the post-anthesis period to support both shoot Zn retention and grain Zn accumulation, which is particularly relevant for improving the nutritional quality of maize grain (

Table 5. Pearson correlation coefficients between biomass, grain yield, harvest index, Zn uptake in maize, soil Zn concentration in 2019 and 2020.

	BM- VT	BM- PM	GY	HI	CGR	ZnUpVT	ZnUpPM	GZnUp	pZnUp	ZnHI	SZn- VT	SZn- PM
BMVT	1
BMPM	0.32 **	1
GY	0.26 *	0.28 *	1
HI	0.04	−0.50 **	0.65 **	1
CGR	−0.05	0.89 **	0.19	−0.52 **	1
ZnUpVT	0.32 *	0.20	0.06	−0.07	0.10	1
ZnUpPM	0.10	0.42 **	0.01	−0.28 *	0.44 **	0.35 **	1
GZnUp	0.03	0.24	−0.31 *	−0.43 **	0.28 *	0.38 **	0.79 **	1
pZnUp	−0.09	0.35 **	−0.06	−0.31 *	0.44 **	−0.14	0.85 **	0.62 **	1
ZnHI	−0.10	−0.17	−0.51 **	−0.32 **	−0.14	0.14	−0.05	0.57 **	−0.12	1
SZnVT	−0.10	−0.19	−0.36 **	−0.17	−0.15	0.15	0.10	0.28 *	0.04	0.29 *	1
SZnPM	−0.19	−0.20	−0.04	0.09	−0.10	0.15	0.04	0.19	−0.04	0.23	0.54 **	1

BMVT, biomass at VT; BMPM, biomass at PM; GY, grain yield; CGR, crop growth rate; ZnUpVT, shoot Zn uptake at VT; ZnUpPM, shoot Zn uptake at PM; GZnUP, grain Zn uptake; pZnUp, post-anthesis shoot Zn uptake; ZnHI, Zn harvest index; SZnVT, soil Zn concentration at VT; SZnPM, soil Zn concentration at PM. *, ** indicate significant difference at p < 0.05 and <0.01, respectively.

).

4. Discussion

4.1. Zn Application Effects on Biomass, Crop Growth Rate, and Yield

The result clearly indicated that the Zn application significantly impacts maize biomass accumulation at the maturity stage and CGR in 2019. SW 5731 at 5, 10, and 20.6 kg of Zn/ha and SW 5819 at 10 kg of Zn/ha showed the highest biomass when compared with the without Zn application (Table 1). SW 5731 at 10 and 20.6 kg of Zn/ha, and SW 5819 at 5 and 10 kg of Zn/ha, showed the highest CGR compared to the no Zn application. In 2020, there was no interaction effect between Zn application rate and maize variety for the biomass at VT and PM stage and CGR (Table 1). Hossain et al. reported that adding Zn fertilizer to first-year crops (maize and rice crops) had a residual effect on the crops in the following year. As a result, there was no significant difference observed among the various Zn application treatments [24]. Similarly, Wang et al. found that the residual effects of Zn fertilization after one year were comparable to the effects of applying Zn consistently over three years, with both approaches leading to a similar rate of yield increase [39]. Zn can hinder growth at both deficient (low) and toxic (excessive) levels [40]. The role of Zn as a vital micronutrient, participating in various enzymatic activities, including those of carbonic anhydrase, aldehyde dehydrogenases, superoxide dismutase, and RNA polymerase. Carbonic anhydrase, for example, is essential for photosynthetic carbon dioxide fixation and serves as an indicator of physiologically active Zn in leaf tissue [41,42]. Zn also contributes to the stabilization of biomembranes through interactions with phospholipids and sulfhydryl groups in membrane proteins, and it is involved in processes such as protein synthesis, carbohydrate metabolism, lipid and nucleic acid synthesis, chlorophyll formation, photosynthesis, and respiration. Srivastav et al. [43] reported that maximum increases in chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids in maize occurred at 200 mg/L ZnO NPs. Photosynthesis, a primary reaction affected by metabolic stress, is positively stimulated by Zn accumulation, which enhances chlorophyll and carotenoid synthesis, improves photosynthetic efficiency, and increases biomass. This ultimately enhances leaf photoassimilates and grain yield [5,7,44,45].

At the leaf development stage, with increasing Zn applied as basal fertilizer, the net photosynthesis increased with Zn application rates of 30 and 45 kg of ZnSO4.7H2O/ha, while in the inflorescence emergence and grain development, the net photosynthesis increased with Zn application rate at 30 kg of ZnSO₄.7H₂O/ha [13]. Kumar et al. [22] reported that fertilizer treatments significantly improved maize grain and straw yields compared to those without Zn application, achieving the highest yields of grain, straw, and total maize with a Zn application of 10.0 kg of Zn/ha.

In SW5819, the highest grain yields were found under both 5 and 10 kg of Zn/ha (14.7 t/ha, respectively). In SW5731, grain yield ranged from 12.6 to 13.3 t/ha, with no significant differences observed among Zn application rates. However, SW5819 under both 5 and 10 kg of Zn/ha produced significantly higher grain yield than SW5731 under 0 and 5 kg of Zn/ha, representing a 16.7% increase in grain yield compared to both (Table 1). Although maize grain yield did not significantly increase with Zn fertilizer application compared to no Zn application, the Zn-treated crops exhibited an increase in grain yield of 400–600 kg/ha in 2019 and 200–600 kg/ha in 2020. Numerous studies have reported a positive response of maize grain yields to Zn fertilizer application [13,14,18,19]. Liu et al. [19] found that grain yield increased by 80 to 780 kg/ha (0.88 to 8.6%) with Zn application. Similarly, the application of Zn at 30 kg of ZnSO₄·7H₂O (6.3 kg of Zn/ha) produced higher grain yields than those without Zn application [13]. The increase in crop yields due to Zn application is attributed to enhanced enzyme activity, growth and development, and yield-promoting factors within crop plants [22]. However, this study found that the adverse effect of the highest Zn rate (20.6 kg/ha) on biomass implies potential phytotoxicity, which could manifest as reduced growth, consistent with previous findings [13,46,47,48]. Interestingly, the highest Zn rate (20.6 kg of Zn/ha) led to an elevated HI in 2019 (Table 1), potentially indicating a stress response that altered biomass partitioning, in favor of reproductive structures despite stunted vegetative growth; however, this mechanism warrants further exploration [49].

4.2. Zn Uptake Dynamics

The significant Zn and variety interaction effects observed for various uptake parameters (Table 2) further support this, indicating that the varieties possess different optimal Zn requirement ranges or differential sensitivity to Zn deficiency/toxicity. Specifically, SW 5819 showed a stronger positive response to the 5 and 10 kg of Zn/ha rates in 2019. However, various other factors, including the environment, soil, climate, farming practices, and agronomic practices, can also impact the final Zn content and bioavailability post-harvest [9].

In 2019, Zn uptake into shoots at PM, grain, and post-anthesis uptake was significantly enhanced at the 5 and 10 kg of Zn/ha rates compared to zero and high Zn application (Table 2). Analysis of Zn distribution within plant parts revealed dynamic allocation patterns influenced by Zn rate, variety, and year (Table 3, Figure 2). The application of Zn led to an increase in Zn absorption across all parts of the maize plant. In 2019, Zn uptake was observed in maize with the application of 5 and 10 kg of Zn/ha in 2019, while in 2020, higher shoot uptake was noted during the VT and PM stages, along with grain Zn absorption at 5 kg of Zn/ha (Figure 2). Additionally, Post-anthesis Zn uptake in the shoot was also significantly increased by Zn application of 5, 10 kg of Zn/ha. This aligns with the biomass results and indicates efficient absorption and translocation at these optimal rates. The highest grain Zn distribution ratio occurred at 10 mg Zn/kg in 2020,but required the highest rate (20.6 mg/kg) in 2019 (Figure 2), suggesting complex interactions governing Zn partitioning to the sink tissues, potentially involving source-sink relationships affected by environmental conditions [50]. These results are consistent with those found by Botoman et al. [21], which indicated that 30 kg of ZnSO₄.7H₂O/ha significantly improved grain Zn concentration by 15%. Furthermore, the grain Zn uptake varied between 170 and 218 g/ha. Likewise, Kumar et al. [22] reported that Zn uptake by maize grain and straw increased with most of the Zn treatments when compared to the absence of Zn treatment. The highest Zn uptake in grain occurred with the application of Zn at rates of 7.5 and 10 kg of Zn/ha. The Zn in the grain is transferred directly to the kernel during the grain-filling stage. Thus, the source of Zn in grain reflects post-anthesis shoot uptake and depends on the remobilization of Zn from vegetative parts, which depends on the availability of soil Zn [16,29,36]. To improve Zn uptake, enhance micronutrient levels at the root–soil interface. Key traits for improving uptake include releasing hydrogen ions (H⁺) and compounds that bind metals. After absorption, these micronutrients must be efficiently transported to the edible plant parts, either directly from roots or from vegetative tissues following the reproductive phase [51].

ZnHI measures the efficiency of the crop in loading Zn into grain. The ZnHI results show a decrease at higher Zn rates and higher values for SW 5819 (Table 2), indicating that SW 5819 is more efficient at partitioning accumulated Zn into the grain. According to Liu et al. [36], the ZnHI of maize decreased from 0.74 to 0.52 when Zn fertilizer rates were increased from 2.3 to 34.1 kg of Zn/ha. This suggests that excessive Zn supply may impair translocation to the grain relative to total uptake, potentially due to toxicity effects inhibiting phloem loading or sink activity [52]. Significant differences were observed between the two maize varieties. SW 5819 consistently exhibited higher shoot Zn uptake at the VT stage (Table 2) and tended towards higher grain Zn uptake compared to SW 5731, particularly in 2019. This suggests potential genetic differences in Zn uptake efficiency (root absorption capacity), translocation mechanisms (e.g., xylem loading), or utilization efficiency within the plant between the varieties, as has been documented for various micronutrients in maize and other cereals [53]. According to Moa et al. [54], SW 5819 showed a significant accumulation of nitrogen, phosphorus, and potassium, greater than SW 5731 at the VT stage, as well as nutrient uptake in grain. This indicated that SW 5819 demonstrates greater efficiency in nutrient accumulation than SW 5731.

4.3. Soil Zn Availability and Plant Response

Soil generally proves more effective than foliar sprays in enhancing both yield and grain Zn [13]. Zn can exist in various forms in soil, including water-soluble, organically adsorbed, exchangeable, chelated, and solution Zn. Plants access soil Zn primarily as Zn²⁺, ZnOH⁺, or Zn complexes with soluble organic materials, which can fluctuate due to high carbonates, bicarbonates, pH levels, phosphorus content, and imbalances in macronutrient fertilizers [55,56]. Our results reported that the soil pH in this experiment was 7.8 (slightly alkaline soil) and the Zn concentration in the soil was 0.53 mg/kg, which is less than the critical level of Zn in soil [32,33]. Zn availability greatly depends on pH levels. When pH exceeds 6, Zn availability is typically very low. In alkaline soils, the solubility of soil Zn decreases, leading to reduced Zn availability [57]. A high soil pH restricts Zn desorption in clay and organic matter, thereby diminishing Zn²⁺ phytoavailability [58]. The NH4⁺-based nitrogen fertilizer was applied, which led to a reduction in soil pH by 0.54 in unlimed and 0.15 in limed soils. This alteration enhanced nutrient availability, uptake, and maize growth in both acidic and limed acidic (alkaline) soils, even when there were adequate plant-available nutrient concentrations [59]. Zn application is of greater importance in low-Zn soils rather than in high-Zn soils, and the response of grain Zn concentration to Zn application tends to be more pronounced in low-Zn conditions compared to higher Zn soils [60,61].

Optimizing Zn application rates (5–10 kg of Zn/ha) significantly enhances maize growth and Zn uptake (Table 1 and Table 2), improving biomass and potentially yield and grain nutritional quality. The highest Zn application rate (20.6 kg of Zn/ha) showed reducing biomass and hindering efficient Zn translocation to the grain (Table 1 and Table 2). Future studies should explore the physiological and molecular mechanisms underlying the Zn efficiency of hybrid maize varieties and refine the critical soil Zn concentrations for optimal yield and grain Zn accumulation in various environmental conditions.

The application of Zn fertilizer led to a notable increase in plant-available Zn concentrations in the soil during both the VT and PM stages across the two years, with higher application rates correlating to elevated soil Zn levels (Table 4, Figure 3). This validates the effectiveness of the application method in enhancing the Zn supply. However, when examining relative grain yield and uptake across both varieties, no strong, significant correlation with soil Zn concentration at the PM stage was observed (Figure 4). Suggesting that while sufficient soil Zn availability is essential and correlates positively with uptake, attaining optimal yield and grain Zn concentration likely hinges on meeting specific critical thresholds within the soil. Factors such as plant genotype, uptake efficiency, translocation capacity, and prevailing environmental conditions may impose limiting effects [5,49]. Despite using Zn sulfate in nutrient solutions, the amount of Zn plants absorb is considerably influenced by the Zn concentration in the soil. Plant Zn nutrition relies heavily on the initial Zn availability in the soil, which becomes particularly critical in Zn-deficient situations.

On average, cumulative Zn uptake is less than half in soils with Zn concentrations below 0.5 mg/kg compared to those with higher levels. Conversely, the initial Zn concentration in the soil shows a positive correlation with crop uptake, elucidating the more significant decline in Zn levels as crop Zn absorption increases [62]. The optimal application rate of 10 kg/ha achieved soil Zn levels of approximately 3.3–3.5 mg/kg at the PM stage, which are comparable to or fall within ranges previously suggested to support high yield (4.7 mg/kg of Zn in soil) and high grain Zn uptake (7.6 mg/kg of Zn in soil) [19]. Over three years, Zn concentration in the soil from Zn-applied treatments increased alongside the amount of Zn applied, while untreated soil remained at about 0.4–0.5 mg/kg [19]. The low Zn availability in the soil is affected by the high pH (pH = 7.8) observed in our field experiment, and the Zn availability in the soil remained at about 0.46–0.62 mg/kg (Figure 3), which is slightly alkaline and significantly reduces soil Zn availability. The elevated soil pH (>7) can be linked to high pedogenic CaCO₃ content, excessive liming, elevated salt contents, and reducing conditions [4]. Additionally, in calcareous soils, only 1–5% of the applied Zn fertilizer is utilized by plants within the same year [63].

The critical Zn concentration in soil required to achieve 97% relative yield was found to be approximately 1.02 mg/kg [33] and 1.06 mg/kg [32]. However, this study did not specify a critical level of Zn for high yield. It established that grain yield increases with increasing Zn concentrations in soil, particularly when the levels exceed 1.0 mg/kg. Without Zn supplementation, the Zn concentration in the soil is below the critical threshold, resulting in low grain yield (Table 1 and Figure 3). The influence of soil Zn application on maize yield and grain Zn uptake primarily depends on the available Zn concentration. An increase in soil Zn can significantly improve Zn levels in maize grain if it meets the criteria for both high yield and Zn accumulation. Therefore, applying Zn fertilizer directly to the soil is essential for optimal maize production and nutritional quality.

5. Conclusions

This study found that moderate Zn application rates (10 kg of Zn/ha) significantly improved maize biomass, CGR, Zn uptake, and grain yield in slightly alkaline clay loam soil following rice cultivation. Among the evaluated maize genotypes, SW5819 exhibited superior Zn uptake efficiency and grain Zn accumulation. The correlations between post-anthesis shoot Zn uptake, ZnHI, and grain Zn content highlight the importance of Zn remobilization for grain enrichment. These findings highlight the importance of optimizing Zn fertilizer rates to enhance maize yield and nutritional quality, as soil Zn availability positively affects maize performance in the rice-maize cropping system. Understanding these traits may contribute to developing new genotypes that are efficient in Zn uptake.