Abstract
The replacement of maize with grain sorghum is a promising practice for enhancing climate change adaptation strategies in the drought-prone areas of Central Europe. The refinement of the agrotechnics of commercial hybrids contributes to the development of sustainable agriculture. A field experiment was conducted between 24 May 2023 and 27 September 2023 in Keszthely (Hungary) in order to evaluate the effects of plant density (D1 = 240,000 plants ha–1, D2 = 280,000 plants ha–1) and equidistantly increasing N doses (0–40–80–120–160–200 kg N ha–1) on the grain yield, biomass weight and leaf area index (LAI) of 4 grain sorghum cultivars (G1 = KWS Nemesis, G2 = RTG Huggo, G3 = GK Erzsébet, G4 = ES Foehn). Another aim was to examine the effect of treatments on weed coverage. According to the results, plant density and N treatment had a general significant effect on grain yield (p < 0.05), biomass weight (p < 0.05) and LAI (p < 0.05; p = 0.015), regardless of the applied cultivar. Positive correlations were observed between biological and grain yield (D1: r = 0.9, p = 0.02; D2: r = 0.91, p = 0.01) as well as between LAI and grain yield (D1: r = 0.89, p = 0.6; D2: r = 0.91, p = 0.07). Significant differences were also observed between the grain yield (p < 0.05), biomass weight (p < 0.05) and LAI (p < 0.05) of the cultivars. In general, grain yields peaked around 120 kg N ha–1 at the plant density of 240,000 plants ha–1 and around 160 kg N ha–1 at the plant density of 280,000 plants ha–1. Among the experimental conditions RTG Huggo yielded the best results. In general, weed coverage was significantly lower (p = 0.002) in denser stands. The enhancing effect of N on weed coverage could not be detected (p = 0.318). In conclusion, a plant density of 280,000 plants ha–1, a N dose of 120–160 kg N ha–1 and RTG Huggo cultivar proved to be the best under the experimental conditions.
Introduction
Considering the future impacts of climate change, there is a great potential in the expansion of grain sorghum (Sorghum bicolor L. Moench) production in Hungary. Due to the enhancement of the greenhouse effect, the climate of Central-Europe is likely to become warmer and drier (MIKA, 2002). The amount of precipitation is expected to decrease in the summer period (NÉMETH, 2017), which primarily jeopardize the safe cultivation of conventional spring-sown crops, especially maize. Owing to its efficient water utilization mechanism, drought- and temperature stress tolerance, grain sorghum can be used as a suitable alternative to maize in drought-stricken areas (ASSEFA et al., 2010).
Despite its C4 nature and advanced stress tolerance, the grain yield of sorghum still depends mainly on proper nitrogen (N) fertilization (BOLLAM et al., 2021). Insufficient N supply results less developed plants, reduced (biological and grain) yield and grain protein content, while the excessive use of N cause N2O emissions and NO3– accumulation in the groundwater, furthermore the doses above the optimum have less yield benefits (ASHIONO et al., 2005; RAMU et al., 2012; AHMAD et al., 2017). In addition to the proper amount of accessible N, the N uptake of sorghum is also affected by several further factors, such as soil water availability, soil physical, chemical and biological properties, soil organic matter content, previous crop and applied genotype (CSATHÓ, 2003a,b; WORTMANN et al, 2007; GARDNER et al., 1994). Proper site- and genotype-specific quantification of the N requirement is crucial to ensure high biomass production and grain yield and to minimize environmental risks (PANNACI & BARTOLINI, 2018; BARTZIALIS et al., 2023).
Thanks to plant breeding activities, the harvest index of major cereal crops has been maximized, therefore further increases in crop yield can primarily be achieved by the increase of total biomass (HORTON et al., 2021). The determination of optimal plant density is crucial to maximize biological and grain yields and to make sorghum production more economical (ZAND & SHAKIBA, 2013).
The weed control of grain sorghum is still an incompletely solved problem in Hungary. The competition for light, water and soil nutrients reduces the yield and grain quality of sorghum (GRICHAR et al., 2005). If a heavy infestation of grassy weeds occurs in the first 2 weeks after germination, the final grain yield may decrease by up to 20% (SMITH & SCOTT, 2010). Depending on the climatic conditions of the growing season, weeds can cause a yield depression of 15–97% in grain sorghum (PEERZADA et al., 2017), therefore proper agrotechnics is of utmost importance. Plant density and different levels of N can greatly influence crop-weed interference, but these effects are strongly crop- and weed species-specific (BLACKSHAW et al., 2003; ABOUZIENA et al., 2007; PEERZADA et al., 2017).
The primary objective of the research was to examine the single and combined effects of plant density and equidistantly increasing nitrogen rates on the performance (yield and yield determining properties) of four commercial grain sorghum cultivars. Since the weed control of the experimental area has proven unsuccessful, we also aimed to assess the characteristics of weeding. The results helped to identify the highest-yielding, most economical cultivars under the experimental conditions and to formulate recommendations regarding the proper agrotechnics of the tested genotypes.
Materials and methods
Site and arrangement of the field experiment
The research was carried out between 24 May 2023 and 27 September 2023 in Keszthely, Hungary (46° 45′ N, 17° 14′ E, 115 m). Based on plant density, the experimental area (5400 m2) was divided into two main blocks (2700 m2): D1 = 240,000 seeds ha–1, D2 = 280,000 seeds ha–1. N treatments (N0 = 0, N1 = 40, N2 = 80, N3 = 120, N4 = 160, N5 = 200 kg N ha–1) were arranged in strip-plot design within the main blocks, with 4 repetitions (Figure 1). Single and combined treatment effects were examined in the case of 4 grain sorghum genotypes (G1 = KWS Nemesis, G2 = RTG Huggo, G3 = GK Erzsébet, G4 = ES Foehn), which were arranged uniformly in 3 rows per treatment with a row spacing of 75 cm. 192 individual plots (22.5 m2) were set in total (Figure 1).
The arrangement of the treatments, where the markings G1–4 refer to the applied grain sorghum genotypes (G1 = KWS Nemesis, G2 = RTG Huggo, G3 = GK Erzsébet, G4 = ES Foehn) and the numbers (0–200) indicate the applied N doses (kg ha–1)
Citation: Agrokémia és Talajtan 2025; 10.1556/0088.2025.00190
The soil texture of the experimental area is sandy loam and soil type is Ramann-type brown forest soil (humus content: 1.6–1.7%, total N content: 0.12%, AL(P2O5) content: 60–80 mg kg–1, AL(K2O) content: 140–160 mg kg–1, pH(KCl): 6.8–7.0) (CSITÁRI et al., 2021). Ploughing, cultivation and PK fertilization (100 kg ha–1 P2O5 and 100 kg ha–1 K2O) were carried out uniformly throughout the experimental area. Weed control was performed by applying pre-emergent herbicides and inter-row cultivation during stem elongation (BBCH 30–34). Since the experimental area was heavily infested with weed-seeds, weed infestation remained generally high despite multi-tactic interventions. The presence of weeds provided an opportunity to examine the effects of plant density and N dose on the weed coverage of grain sorghum.
Meteorological data collection
Data sets on meteorological elements (Figure 2) were provided by a QLC-50 automatic data logger (Vaisala, Helsinki, Finnland), located at the Agrometeorological Research Station of MATE Georgikon Campus.
The precipitation sum for the experimental period was 281.8 mm. There were 42 rainy days during the 157 days of the growing season and the distribution of precipitation was almost even. The balanced water supply and relatively mild summer ensured proper conditions for vegetative and reproductive development. Except for 7 days, the daily maximum temperature was constantly below 33℃, which is considered to be the critical threshold temperature for grain sorghum (TACK et al., 2017).
Precipitation and temperature conditions of the studied growing season (24 May 2023 – 27 October 2023)
Citation: Agrokémia és Talajtan 2025; 10.1556/0088.2025.00190
Determination of leaf area index
Leaf area index (LAI) was determined as follows: The maximum width and length of the leaves of 5 plants per plot were registered in their fully developed state (BBCH 79). Leaf area was calculated based on the formula of SHIH et al. (1981):
where L= maximum length of the leaf, W= maximum width of the leaf.
LAI was determined using the following equation:
where LAI0= leaf area index (dimensionless), LA0= the area of the average-sized leaf of the given species (m2), LN= the average number of leaves per plant, PN= the number of plants on the experimental area, T= the size of the experimental area (m2) (RICHTER, 2009).
Estimation of weed coverage
Surveys on ground coverage of weeds were carried out during the milk stage of sorghum (BBCH 70–77). First of all, the dominant weed species were identified, then the estimation of their coverage (%) was carried out in 4 m2 squares, using the Balázs-Újvárosy coenological method (REISINGER, 2000). Weed coverage was interpreted as the sum of the estimated coverage of individual weed species. In the case of weed coverage, genotype effect was not examined separately.
Determination of biological and grain yields
Aboveground plant samples were collected just before harvest (BBCH 99). 5–5 replicates were collected per plot. Sample processing was performed according to the following method, which is commonly used in similar researches (MISHRA et al., 2019; AHMED et al., 2017; OGBAGA et al., 2014). The aboveground samples were washed with deionized water, let to dry under greenhouse conditions (30–40℃) for 7 days (to facilitate shredding), cut into 3–5 cm pieces, then oven dried at 70℃ for at least 48 hours (until constant weight) in a Memmert UF450 drying cabinet. Dry weights were measured using an analytical balance. Biological yields were calculated as the product of the treatment’s average dry aboveground biomass weight and plant density (reestimated just before harvest). Biological yields were extrapolated to a dimension of t ha–1.
The grains (~14% moisture content) were harvested (on 27 September 2023) with a C-85 plot harvester (Haldrup, Ilshofen, Germany). The on-board computer of the combine provided accurate information about the grain yields (t ha–1) of the treatments. Since the plot harvester was suitable for grain sampling, separate samples were also collected from each treatment. The moisture content of the grain samples was tested using a NIRSTM DS 2500F feed analyser (FOSS, Hillerød, Denmark). Grain yields were converted to a uniform moisture content of 14%, based on the actual moisture content of the samples.
Statistical methods and softwares
Statistically significant treatment effects (at 95% confidence level) were identified with one- two- and three-way analysis of variance (ANOVA) tests, performed in SPSS Statistics Software version 22.0 (IBM, Armonk, New York). Tukey’s honestly significant difference (HSD) post hoc tests were also applied to examine the differences between the experimental groups. The strength and character of the relationships between the examined variables were determined by regression analysis tests, performed in Excel 2013 Software (Microsoft, Albuquerque, New Mexico, USA).
Results and Discussion
Grain yield
Based on the results of the three-way ANOVA, both plant density (p < 0.05), N treatment (p < 0.05) and genotype (p < 0.05) had a significant effect on the grain yield of sorghum (Figure 3). G2 generally outperformed the other cultivars, especially with higher doses of N and plant density. The results of Tukey’s HSD tests also indicate that the grain yield of G2 is significantly higher (p < 0.05 in all cases) than that of the other cultivars. Compared to D1 (Figure 4a), the densification of grain sorghum (D2) (Figure 4b) resulted an average grain yield increase of 18% in the case of G1, 36% in the case of G2, 27% in the case of G3 and 33% in the case of G4. Almost similar results were obtained by ZAND & SHAKIBA (2013). They examined the yield of grain sorghum at the densities of 80,000, 160,000 and 200,000 plants ha–1, and at N application rates of 40, 80 and 120 kg N ha–1. They also observed that grain yield increased in proportion with the increase of plant density and N application rate. Increases in biological yield and grain yield of sorghum with increasing plant densities were similarly reported by several researchers (TANG et al., 2018; LESTARI et al., 2021).
The grain yield of sorghum as affected by plant density treatment (D1 = 240,000 plants ha–1, D2 = 280,000 plants ha–1), applied genotype (G1 = KWS Nemesis, G2 = RTG Huggo, G3 = GK Erzsébet, G4 = ES Foehn) and N treatment (N0–N5 = 0–40–80–120–160–200 kg ha–1). Different letters upon the bars indicate significant differences among the treatments at P < 0.05 level
Citation: Agrokémia és Talajtan 2025; 10.1556/0088.2025.00190
The grain yield of four grain sorghum cultivars (G1 = KWS Nemesis, G2 = RTG Huggo, G3 = GK Erzsébet, G4 = ES Foehn) at equidistantly increasing nitrogen doses and different plant densities (D1 = 240,000 plants ha–1, D2 = 280,000 plants ha–1). Different letters upon the bars indicate significant difference at P < 0.05 level
Citation: Agrokémia és Talajtan 2025; 10.1556/0088.2025.00190
It can be observed that the N demand has also increased with the increase of plant density. N application caused an increase in grain yield in the case of each genotypes with a peak around 120 kg N ha–1 in D1 and around 160 kg N ha–1 in D2. These findings are in accordance with the results of SIKLÓSINÉ RAJKI & HARMATI (2001), according to which the maximum yield of grain sorghum in Hungary can be obtained most economically with a N dose of 130–150 kg N ha–1. The excess N above the optimum dose cannot be utilized properly (WANI et al., 2021). In addition to causing detrimental environmental impacts, excessive nitrogen additions can also inhibit plant growth and decrease grain yield (ZOTARELLY et al., 2015; LI et al., 2020).
Correlations between LAI and grain yield
The effects of plant density and N application on LA can be diverse and strongly dependent on both years, locations and cultivars (GULER et al., 2008). Depending on the treatment, LAI values varied between 1.85–2.7 in G1, 2.19–3.59 in G2, 0.99–1.69 in G3 and 1.76–2.68 in G4 under the experimental conditions. LAI was significantly affected by both plant density (p < 0.05), N treatment (p = 0.015) and genotype (p < 0.05). Similar effects were also reported by HANDISO & MAMO (2022). Both older (COX, 1996) and contemporary (ZHOU et al., 2024) research results indicate that the proper N supply and densification of crops results in higher LAI values and growth rates, which ultimately determine the evolution of grain yield. It is also important to note, that above an unspecified pivotal point, detrimental effects due to intraspecific competition may occur (LESKOVSEK et al., 2012).
Significant and positive correlations were observed between the LAI values and grain yield both in the case of D1 (r = 0.9, p = 0.02) and D2 (r = 0.91, p = 0.01) treatments (Figure 5).
Correlations between the leaf area index (LAI) and grain yield of grain sorghum (4 different cultivars tested) at equidistantly increasing N doses (0–40–80–120–160–200 kg ha–1) and 2 different plant densities (D1 = 240,000 plants ha–1, D2 = 280,000 plants ha–1)
Citation: Agrokémia és Talajtan 2025; 10.1556/0088.2025.00190
Correlations between biological yield and grain yield
Plant density (p < 0.05), N treatment (p < 0.05) and genotype (p < 0.05) have also had a significant effect on total aboveground dry biomass production (Figure 6). Positive correlations were found between biological yield and grain yield, both in the case of D1 (r = 0.89, p = 0.6) and D2 (r = 0.91, p = 0.07) density treatments. NAHARUDIN et al. (2021) studied the correlations between the biological yield components (plant height, biomass weight, stem diameter, number of leaves) and grain yield of 15 different grain sorghum genotypes. They also observed positive and significant correlations between the biomass weight and grain yield of sorghum. According to their results, the improvement of any of the tested attributes would lead to the increase of grain yield.
Correlations between the biological yield and grain yield of grain sorghum (4 different cultivars tested) at equidistantly increasing N doses (0–40–80–120–160–200 kg ha–1) and 2 different plant densities (D1 = 240,000 plants ha–1, D2 = 280,000 plants ha–1)
Citation: Agrokémia és Talajtan 2025; 10.1556/0088.2025.00190
Weed interference
The dominant species of the weed flora were Panicum miliaceum, Setaria glauca, Echinochloa crus-galli and Digitaria sanguinalis, Portulaca oleracea, Convolvulus arvensis, Ambrosia artemisiifolia, Setaria viridis and Mentha arvensis have also occurred on the field in smaller quantities.
Based on the results of the two-way ANOVA, plant density treatment had a significant effect (p = 0.002) on the weed coverage of grain sorghum. Weed coverage was generally lower in D2 than in D1. In their study, BRAZ et al. (2019) examined the effects of plant density on the weeding characteristics of two grain sorghum populations (180,000 and 270,000 plants ha–1). They also concluded that the densification of sorghum plants reduces the density and biomass of weeds. This may be due to competitive advantages resulting from plant quantity and shading.
Although the effect of N treatment was statistically undetectable (p = 0.318), a slightly ascending, then descending trend of weed coverage can be observed as nitrogen doses increase (Figure 7). N stimulates the development of both crops and weeds, but it has also been documented that if the N supply is below the optimum level, weeds have a competitive advantage over crops, because they absorb N and other nutrients faster (SWEENEY et al., 2009). Weed coverage generally increased up to the levels of 80–120 kg N ha–1. At higher levels of N, sorghum was probably able to reach its potential size and weed suppression ability.
Weed coverage in grain sorghum when applying different doses of N and plant densities (D1 = 240,000 plants ha–1, D2 = 280,000 plants ha–1)
Citation: Agrokémia és Talajtan 2025; 10.1556/0088.2025.00190
Conclusion
The results demonstrate several positive aspects of grain sorghum densification. Biological yields, grain yields and LAI values of the tested cultivars were generally higher at the plant density of 280,000 plants ha–1 than at the plant density of 240,000 plants ha–1, under the favourable conditions of the 2023 growing season. The significant and positive correlations between the growth parameters (LAI, biological yield) and grain yield indicate the need to maximize biomass production, which can be achieved primarily by densification. The weed suppression ability of the population was also stronger in denser stands, regardless of the N application rate. Since grain yields peaked around 120 kg N ha–1 at the plant density of 240,000 plants ha–1 and around 160 kg N ha–1 at the plant density of 280,000 plants ha–1, the densification of sorghum entails additional fertilizer costs and environmental risks too. Among the experimental conditions, RTG Huggo used the available N most efficiently. Choosing the most appropriate cultivar is crucial to achieve high and stable yields, therefore further efforts are needed to assess the yield potential of commercial grain sorghum hybrids under domestic conditions. Although this paper presents the results of a one-location study, the results can also be interpreted at an international level. It can be assumed that similar results would be obtained in areas of other countries with similar climatic conditions and soil properties. Since the effect of crop year can also modify the results, it would be expedient to repeat the experiment in several consecutive years to obtain more reliable information about the site-specific requirements of the tested cultivars.
Acknowledgement
The authors would like to extend their sincere gratitude to the colleagues of the MATE Tangazdaság Nonprofit Ltd. for providing agricultural machinery and technical help to conduct this experiment.
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