3.1. Effect of Production Factors on Maize CMR
Factors influencing the relative chlorophyll content (CMR) of maize were examined (
Table 3). A significant effect was found in terms of crop year, genotype, as well as fertiliser treatment (
p < 0.001) and phenophase (
p < 0.01) on the values. Of the interactions, genotype × crop year (
p < 0.01), phenophase × crop year, and fertiliser treatment × crop year (
p < 0.001) had significant effects on the relative chlorophyll content of maize. Phenophase × genotype and genotype × fertiliser treatment interactions had no effect on CMR. Of the different factors, phenophase had the highest influence on CMR of maize based on the SS value.
The CMR—averaged over the examined years, genotypes and fertiliser treatments—was measured in three different phenological phases (V6, V12, R1) during the growing season. The lowest value (40.23 ± 5.57) was measured in the V6 phenological phase, which was significantly different (
p < 0.05) from the other two phenophases according to Duncan’s test. The relative chlorophyll content was statistically confirmed to be the highest at the 50% silking growth stage (49.91 ± 8.41). The obtained results indicate that the relative chlorophyll content of maize, and thus nitrogen accumulation, increased steadily between the V6 and R1 growth stages (
Figure 2).
When examining the effect of fertiliser treatment on CMR, it was found that increasing nitrogen doses, averaged over years, genotype, and phenological stages, also increased the relative chlorophyll content of maize. The non-fertilised plot (control, A
0) had the lowest value (41.29 ± 6.30), which was significantly different (
p < 0.05) from the other treatments. The treatment A
60 at the V6 phenophase supplemented with 30 kg N ha
−1 dose (V6
90) had a significant (
p < 0.05) CMR increasing effect (+2.81). Top-dressing applied at higher doses of basal fertiliser (A
120), V6 and V12 phenophases (V6
150 and V12
180) had a CMR-increasing effect (+1.99; +0.62), but the increase in value was significant (
p < 0.05) only in the V12
180 treatment. The highest CMR (49.00 ± 8.94) was significant in the V12
180 fertiliser treatment (
p < 0.05) (
Figure 3). Based on the obtained results, the highest increase in CMR was provided by the basal fertilisation treatment (
p < 0.05). A
120 fertiliser treatment increased the relative chlorophyll content by 5.11 compared to no fertiliser treatment, 1.67 more than A
60 treatment. Top-dressing applied to the basal fertilisation in phenological phases V6 and V12 resulted in a smaller increase in CMR.
Examining the effect of fertilisation on CMR by year and genotype, averaged over the different phenophases, the obtained results show that, in 2016, 2017, 2020, 2021, 2022, the CMR was the lowest in the non-fertilised treatment for both examined genotypes (
Table 4). These lowest values were significant (
p < 0.05). In 2018, the least significant CMR (39.66 ± 5.11,
p < 0.05) for hybrid Fornad was in treatment A
0. For the Armagnac hybrid, the lowest relative chlorophyll content (40.81 ± 8.91) was measured in treatment A
60, but this was not significant. In the 2019 growing year, the lowest CMR for the Armagnac hybrid were in treatments V6
90 and V12
120 for the Fornad hybrid (47.71 ± 6.66; 47.74 ± 9.25), which were not significant.
Increasing doses of fertiliser increased CMR. The most significant difference for both genotypes (56.77 ± 3.27 and 56.39 ± 5.10; 50.77 ± 5.31 and 53.12 ± 6.57) was observed in the V12180 treatment (p < 0.05) in 2016 and 2020 (rainy crop years). Compared with the non-fertilised treatment (A0), the lowest fertiliser dose (A60) increased the relative chlorophyll content of the Armagnac hybrid by 6.94% on average over the years, while the increase was 9.71% for the Fornad hybrid. The treatment A60 had a significant effect on increasing the relative chlorophyll content of Armagnac in 2016 and both genotypes in 2022. The higher dose of the basal fertiliser (A120) increased CMR by 12.37% compared to the A0 treatment, averaged over the examined years. Based on Duncan’s test, the CMR increasing effect of A120 treatment was significant for both genotypes in 2016 and for the hybrid Fornad in 2022.
Increasing the 60 kg N ha−1 basal fertilisation (A60) in the V6 phenophase by an additional 30 kg N ha−1 (V690), averaged over the years, the authors observed higher CMR by 6.45% for the Armagnac hybrid and by 6.10% for the Fornad hybrid, with a significant increase in 2018 for the Armagnac hybrid (p < 0.05). In the phenological phase V6, the top-dressing applied to the fertiliser treatment A120 (V6150) had an increasing effect of 3.95% and 4.63% on CMR for the hybrid Armagnac and Fornad, respectively, with no significant increase in values. For the treatment V12120, a decrease in CMR was observed for the hybrid Armagnac in 2017, 2018 and 2021 and for the hybrid Fornad in 2019, 2021 and 2022 compared to the treatment V690. For the V12180 treatment, the authors observed a decrease in CMR for both genotypes in 2019 and 2021, and for the Armagnac hybrid in 2017 and 2018 compared to the V6150 treatment. The obtained results confirm that higher doses of nitrogen application and the splitting of nitrogen as a basal and top-dressing nutrient will be effective under optimal water availability.
The effect of nitrogen fertilisation on the average of genotypes was examined in relation to years and different phenological stages (
Table 5). The lowest CMR were in most cases in A
0 treatment, in 2016 and 2022 in all threephenophases (2016—V6: 44.63 ± 3.05; V12: 41.61 ± 2.74; R1: 47.08 ± 3.86; 2022—V6: 36.23 ± 2.64;V12: 35.82 ± 2.22; R1: 32.44 ± 3.62;
p < 0.05). In 2017, 2018, 2020 and 2021, the lowest relative chlorophyll content was observed in the growth stages V12 and R1 after A
0 treatment (2017—V12: 40.72 ± 2.94; R1: 42.16 ± 3.16; 2018—V12: 41.66 ± 4.19; R1: 42.73 ± 6.23; 2020—V12: 40.84 ± 2.20, R1: 46.71 ± 4.41; 2021—V12: 39.23 ± 2.55; R1: 40.41 ± 4.47;
p < 0.05).
The highest CMR in 2016 and 2020, i.e., the rainy crop years, were also provided by the V12180 treatment in V6, V12 and R1 phenological phases (2016—V6: 53.51 ± 2.83; V12: 54.77 ± 1.41; R1: 61.47 ± 1.57; 2020—V6: 47.03 ± 1.64; V12: 49.44 ± 1.39; R1: 59.35 ± 2.50; p < 0.05). In the R1 growth stage, which is the most important for yield formation, the highest CMR was observed in the V12180 treatment in 2017 (55.52 ± 0.69; p < 0.05), 2021 (52.72 ± 5.12; p < 0.05), and 2022 (47.68 ± 0.94; p < 0.05). The obtained results indicate that in all years, the highest CMR was observed in R1 phenological stage due to the effect of higher amounts of basal fertilisation or subsequent top-dressing.
Compared to the A0 treatment, A60 basal fertilisation increased CMR by 10.29% in 2016, 10.34% in 2017, 4.94% in 2018, 0.29% in 2019, 8.94% in 2020, 6.32% in 2021 and 20.20% in 2022, averaged over the phenological phases.
Compared to control plots, A120 treatment increased CMR by 19.32% in 2016, 12.55% in 2017, 12.06% in 2018, 2.85% in 2019, 9.97% in 2020, 16.78% in 2021, 15.26% in 2022, on average over the phenological phases.
Increasing the basal fertilisation rate of 60 kg N ha−1 (A60) by an additional 30 kg N ha−1 in the V6 phenophase (V690) has a significant effect (p < 0.05) on increasing CMR in 2016 in R1, in 2017 in V12, in 2018 in V6 and R1 and in 2021 in V12. In the phenophase V12, applying +30 kg N ha−1 active ingredient (V12120) resulted in no significant increase in CMR.
The 120 kg N ha−1 basal fertilisation (A120) in the V6 phenophase increased by +30 kg N ha−1 (V6150) in 2017 and 2018 in the R1 and in 2020 in the V6 growth stages showed a significant increase in CMR. An additional dose of 30 kg N ha−1 applied at the 12-leaf stage (V12180) showed a significant (p < 0.05) increase in relative chlorophyll content in the phenophase V6 in 2016 and 2020 and in the pheonphase R1 in 2022.
In 2016–2022, the highest CMR were measured in the R1 phenological phase in all fertiliser treatments during the growing season, except in 2022 for treatments A0, V690, V12180 where the highest relative chlorophyll content was observed in the V12 phenological phase.
3.2. Effects of Fertiliser Treatments, Crop Year and Genotype on Maize Yield, Correlation Analysis between Yield and CMR
The greatest effect (
p < 0.001) of crop year and Fertiliser treatment on maize yield was statistically confirmed (
Table 6). Among the interactions, genotype × crop year, fertiliser treatment × crop year, and genotype × fertiliser treatment also had significant effects on yield (
p < 0.001).
Examining the effect of fertiliser application on yield by genotype and year, it was found that in all examined years, the control treatment (A
0) had the lowest yield for both maize hybrids, which was a significant effect (
p < 0.05) (
Table 7). In 2016, for the Armagnac hybrid, the V12
120 treatment provided the highest yield (18.61 ± 0.12 t ha
−1), but the highest significant difference (
p < 0.05) was obtained in the V6
150 treatment. For the hybrid Fornad, the V6
150 treatment gave the highest yield (19.74 ± 0.07 t ha
−1;
p < 0.05).
In the 2017 growing year, the highest yield was observed in the V12180 treatment for both genotypes (Armagnac: 14.92 ± 0.10 t ha−1; Fornad: 13.44 ± 0.72 t ha−1), with the highest yield in the A120 treatment for Armagnac and in the V6150 treatment for the Fornad hybrid.
In the subsequent year (2018), the highest yields were measured in the V12180 (Armagnac, 14.83 ± 1.27 t ha−1) and V12120 (Fornad, 13.61 ± 0.72 t ha−1) treatments, with the highest significant difference in treatment A120 for both hybrids.
In 2019, the highest yields were observed in the Armagnac hybrid in treatment A120 (14.53 ± 0.51 t ha−1; p < 0.05) and in the Fornad hybrid in treatment V6150 (14.02 ± 0.55 t ha−1 p < 0.05).
In 2020, the highest (p < 0.05) yields were obtained with treatment A120 (Armagnac: 13.55 ± 0.11 t ha−1; Fornad: 14.19 ± 0.55 t ha−1). The highest yields for the Armagnac hybrid (13.878 ± 1.121 t ha−1) were obtained with treatment V12120.
In the penultimate examined year (2021), the V6150 treatment had the highest yield of the Armagnac hybrid (12.06 ± 0.89 t ha−1), but the A120 treatment had the highest yield, which was significant (12.02 ± 0.56 t ha−1; p < 0.05). For Fornad, the treatment A120 had the highest yield (10.88 ± 0.15 t ha−1), which was significant (p < 0.05).
In 2022, the A120 treatment gave the highest yields for both genotypes (Armagnac: 9.45 ± 0.60 t ha−1; Fornad: 8.15 ± 1.32 t ha−1; p < 0.05).
The A60 treatment increased the yield by 11.27% in 2016, 18.72% in 2017, 46.72% in 2018, 33.04% in 2019, 46.80% in 2020, 29.73% in 2021 and 52.79% in 2022 compared to the data for the non-fertilised plots, averaged over the genotypes (p < 0.05).
Compared to the A0 treatment, the higher rate basal fertilisation increased the yield by 23.98% in 2016, 57.06% in 2017, 87.53% in 2018, 44.44% in 2019, 109.03% in 2020, 104.71% in 2021, 102.38% in 2022, averaged over the genotypes (p < 0.05).
Averaged over years and genotypes, the A60 fertiliser treatment resulted in 30.75% higher yields and the A120 basal fertiliser treatment in 66.68% higher yields compared to the non-fertilised treatment. The 60 kg N ha−1 basal fertiliser application (A60) at six leaf phenophases increased yield by an additional 30 kg N ha−1 dose (V690) by 13.33% on average across years and genotypes. The V690 treatment had a significant effect (p < 0.05), resulting in higher yields for both genotypes in 2016, (Armagnac: +4.20 t ha−1; Fornad: +4.80 t ha−1), 2017, (Armagnac: +2.50 t ha−1; Fornad +1.72 t ha−1), 2021, (Armagnac: +2.93 t ha−1; Fornad: +2.06 t ha−1) and 2020 for the hybrid Fornad (+2.83 t ha−1). In the V12 phenological phase, an additional 30 kg N ha−1 of active ingredient (V12120) showed a significant yield increase in the hybrid Armagnac in 2016 (+1.03 t ha−1) in 2017 (+1.63 t ha−1) in 2018 (+4.52 t ha−1) and 2020 (+4.26 t ha−1) and in the hybrid Fornad in the growing years 2017 (+2.37 t ha−1) and 2018 (+3.84 t ha−1). Averaged over years and genotypes, the V12120 treatment provided 10.47% higher yield compared to the V690 treatment.
The 120 kg N ha−1 basal fertilisation (A120) increased with +30 kg N ha−1 active ingredient (V6150) in the V6 phenophase resulted in a 1.06% increase in yield, averaged over the years and genotypes. Significant (p < 0.05) yield increases were observed in 2016 (+5.09 t ha−1), 2017 (+0.92 t ha−1) and 2019 (+1.31 t ha−1) for the hybrid Fornad, and in 2016 (+3.63 t ha−1) for Armagnac. No significant yield increase was observed when an additional 30 kg N ha−1 of active ingredient was applied at the twelve-leaf growth stage (V12180) compared to the V6150 treatment.
For yield, averaged over the applied fertiliser treatments, differences were observed between maize hybrids of different genotypes for the examined years. In the years 2016 and 2020, which were the wetter years for maize production, the hybrid Fornad had a higher yield (2016: +0.45 t ha−1; 2020: +0.49 t ha−1). In the dry years 2018, 2021, 2022, the longer maturing hybrid Armagnac had a higher yield (2018: +0.2 t ha−1; 2021: +0.71 t ha−1; 2022: +0.60 t ha−1). In 2017 and 2019, which were average cropping years based on weather data, the hybrid with a longer maturity also gave a higher yield (2017: +1.48 t ha−1; 2019: +0.64 t ha−1). The differences in yield between hybrids could not be statistically verified.
A correlation analysis was performed between relative chlorophyll content and maize yield (
Table 8). As the phenological stages progressed, different levels of correlation were observed between years and genotypes. The strongest correlations were observed in the V12 and R1 growth stages, except in 2019 for the hybrid Fornad (r = 0.385). CMR measured at this time had the most significant effect on yield. For the genotype Armagnac, the strongest correlations were observed in 2016 (r = 0.733 ***), in 2018 (r = 0.711 ***) at the phenological stage of V12 and in the other years at the 50% silking (R1) stage (2017: r = 0.903 ***; 2019: r = 0.742 ***; 2020: r = 0.791 ***; 2021: r = 0.759 ***; 2022: r = 0.570 *). In the case of Fornad, the strongest correlations were found in the R1 growth phase (2016: r = 0.829 ***; 2020: r = 0.874 ***; 2022: r = 0.625 ***), except for 2017 (r = 0.858 ***), 2018 (r = 0.769 ***), 2019 (r = 0.385
NS), 2021 (r = 0.797 ***).
When examining the effect of crop year, it was found that the V12 phenological stage had the strongest correlation between CMR and yield in all three years, except for the Armagnac hybrid in the average crop year, where the R1 phenological stage showed the strongest correlation (r = 0.762 ***) (
Table 9). It was found that under rainy weather conditions there was a moderately strong correlation between relative chlorophyll content and yield even at phenological stage V6 (Armagnac: 0.624 ***; Fornad: 0.649 ***). Among the hybrids, Fornad showed the strongest correlation except at growth stages V12 and R1 in the average vintage.