Enhanced grain quality of malt barley (Hordeum distichon L.) in response to mixed use of organic compost and mineral nitrogen rates

Arega Wole; Amsalu Nebiyu; Getachew Agegnehu; Yenus Ousman

doi:10.1371/journal.pone.0343009

Abstract

Declining soil fertility status and poor agronomic management practices are major factors of declining quality for malt barley in the Ethiopian highland area, particularly in the study area. To address these major challenges, a two-year (2022−2023) field experiment was conducted in experimental fields in the Welmera district to evaluate the effects of mixed-use mineral N fertilization and compost rates on malt barley quality parameters. A randomized complete block design with factorial arrangements of five N rates (0, 23, 46, 69, and 92 kg ha^-1) and four compost rates (0, 2.5, 5, and 7.5 t ha^-1) was tested in three replications. According to the results, both compost and mineral nitrogen fertilizer were significantly influenced thousand-seed weight, protein content, malt extract, beta-glucan content, malt friability and germination energy of malt barley grain, with seasonal variations. Increased mineral N levels enhanced seed weight and grain protein content but reduced malt extract yield and malt friability, while compost improved grain protein content and malt beta-glucan. These influences were improved by organic compost and mineral fertilization, which enhanced multiple quality parameters. The results clearly demonstrated that application of 69 kg N ha^-1 and 5 t ha^-1 of compost rate in moderation, which optimized the malt quality parameters, met industry standards without increasing protein concentration or diminishing malt extract yield of malt barley grain. These mixed management approaches not only enhance the quality of malt barley grain for the beer industry but also help soil fertility restoration and guarantee long-term production sustainability for smallholder farmers in the Ethiopian highlands. For robust and wide applicability, subsequent multiple-seasons and multiple-locations studies with additional quality assessments are recommended.

Citation: Wole A, Nebiyu A, Agegnehu G, Ousman Y (2026) Enhanced grain quality of malt barley (Hordeum distichon L.) in response to mixed use of organic compost and mineral nitrogen rates. PLoS One 21(2): e0343009. https://doi.org/10.1371/journal.pone.0343009

Editor: Raed Abduljabbar Haleem, University of Duhok, IRAQ

Received: May 9, 2025; Accepted: January 31, 2026; Published: February 17, 2026

Copyright: © 2026 Wole et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: The data underlying the results presented in the study are available from https://zenodo.org/records/18467511.

Funding: The author(s) received no specific funding for this work.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Barley (Hordeum vulgare L.) has high genetic diversity with distinct geographic population structure. The Western Fertile Crescent is considered the primary center of diversity and domestication of the barley crop, while several reports most strongly support the Eastern part of the Fertile Crescent (Iran or the Himalayas) as the second primary center of barley diversity and domestication [1]. Barley is believed to have been domesticated from the wild crop in Ethiopia, Morocco, and Tibet, although robust traces are pointing towards the Middle East, particularly in the region that is encompassed by Israel and Jordan [2]. Morocco, Ethiopia, Algeria, Tunisia, and South Africa were the top five countries in Africa producing barley grain with predicted production, area coverage, and productivity of 2.8, 2.4, 0.56, 0.43, and 0.33 million tons; 1.5, 0.96, 0.53, 0.51, and 0.1 million hectares; and 1.87, 2.45, 1.06, 0.84, and 3.5 tons/ha, respectively [3]. In Ethiopia barley ranks fifth in the acreage and production of cereal crop, following tef, maize, wheat and sorghum [4], while accounting for approximately 25% of the continent’s total barley production [5].

Barley is a critically important crop in Ethiopia for grain food, animal feed using straw, malt production (brewing industry), and income generation of smallholder farmers predominantly found in the highlands, where its use is partially embedded in traditional recipes such as ‘Injera,’ and other types of food [6,7]. However, its productivity (2.18t ha^-1) is very low compared to the world average (2.89 t ha^-1) and its potential productivity of 6 t ha^-1 [8]. This due to low soil fertility levels, high soil acidity, insufficient nutrient inputs application, limited application of organic amendments, and poor agronomic management practices, which are major causes of declining yields for malt barley in Ethiopia [8–10]. Due to low production and quality capacity and outdated malting factories, Ethiopia is forced to allocate millions of dollars annually to import malt barley to meet brewery demands at the national level [11].

Numerous malt barley quality parameters influence the suitability of malt barley production for an acceptable range of malt and beer factories’ beer production [12]. The crucial malting barley includes malt extract yield, diastatic power, grain size, and grain protein accumulation [13]. Malt extract yield based on grain parameters such as grain size, shape, hardness, moisture content, dormancy and free from physical impurities, as well as biochemical factors like protein concentration, beta-glucan, starch concentration and enzyme activities [12]. While barley grain is accessibility of for malting purposes, its quality is below standard and does not meet the standards of malting quality. These due to different factors such as the specific variety, soil properties, and applied fertilizer like N fertilization [14].

Nitrogen fertilizer can be considered a main factor in enhancing the production of malt barley, as it directly affects both yield and quality, as supported by multiple studies [9,15–19]. Similarly, different scholars discovered the effect of N fertilization on different quality parameters of malt barley grain such as grain protein concentration [20–22], malt extract yield [23], malt β-glucan [24], malt friability [25], hectoliter weight [22] and germination capacity [26]. However, the continuous utilization of artificial fertilizer sources results in SOM decline and decreased soil quality of agricultural fields, and its high dose affects microbial activity, leads to stunted plant growth, and reduces soil fertility [27] and also influences the agricultural production, which leads to reduced returns [28], which influences the production and productivity of crop plants. Similarly, continuous reliance on mineral fertilizers, the high cost, and incorporation with organic fertilizer sources create major issues by adversely impacting the soil nutrient availability, quality, and soil structure [29].

Compost is more effective in increasing barley yield and its components, such as grain weight, straw weight, and total yield, and nutrient uptake [30,31]. Additionally, it plays a significant role in improving soil physical and chemical properties [32,33]. Moreover, their easy accessibility, low cost, and eco-friendly nature make them an advisable choice for enhancing soil fertility and safeguarding the environment [34,35]. Even if organic compost has numerous merits, many farmers are rejecting utilizing it alone due to its slow-release fertilizers, and labor intensive [36–38].

Combined application of organic and mineral amendments can help address the above problems and improve soil properties, ultimately enhancing crop yield as well as quality and finally increasing the economic benefits of malt barley production [10,13]. Although extensive international research has been conducted on mineral N management in barley production, relatively little attention has been given to combined approaches that conjointly use natural and mineral N fertilizer sources under Ethiopian condition. This knowledge gap is particularly critical in the Ethiopian highlands, where compost can provide multiple advantages such as gradual nutrient release, improved soil structure, and restoration of soil organic matter, yet its potential role in enhancing malt barley quality parameters remains unclear. To address this research gap, the current experiment investigates the integrated application of compost and chemical nitrogen to optimize malt barley quality in the Ethiopian highlands, particularly in the study area, while balancing the need for higher yield with the quality standards of the brewing industries. The specific objectives were to (1) evaluate the effects of the integrated use of nitrogen and compost fertilizer rate on malting quality traits and (2) identify the optimum rate of integrated use of mineral nitrogen and compost fertilizer rate that meets the malt quality standard for the malt barley growing area of the Ethiopian highlands.

Materials and methods

Description of the study area

Two sets of field experiments were conducted during the 2022 and 2023 cropping seasons at Holetta Agricultural Research Center (HARC) in the Welmera district of the Oromia Regional State, Ethiopia. The experimental site is located about 30 km west of Addis Ababa at an altitude of 2400 meters above sea level. The experimental site (HARC) is located at 9° 3′ 19.43′′ N, 38° 30′ 25.43′′ E, and 30 km west of Addis Ababa, at an altitude of about 2400 meters above sea level (Fig 1). The area receives an average annual rainfall of1100 mm, with most precipitation occurring from June to September. The average minimum and maximum air temperatures are 6.2°C and 22.1°C, respectively [29]. The dominant soil type at the study area is Eutric Nitisols, and the district is one of the major barley-producing areas of the central Ethiopian highlands.

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Fig 1. Weather data (2022 and 2023) of growing seasons at experimental site.

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Experimental materials and design

Two rows of the malt barley variety Ibon174/03 were used as test crops for this study. Compost was used as an organic fertilizer source in the experiment. Urea [CO (NH₂)₂] (46% N) and triple superphosphate (46% P₂O₅) were used as nutrient sources of nitrogen and phosphorus fertilizers, respectively. The experiment was a factorial randomized complete block design with the treatments studied being five N levels (0, 23, 46, 69, and 92 kg N ha^-1) and four compost levels (0, 2.5, 5, and 7.5 t ha^-1) and three replications for each treatment. A gross plot size of 2 x 3 m was used, and each treatment had 15 rows, each 3m long and 20 cm between rows. The net plot size was 2 x 2.6 m (central 11 rows of 3 m length) leaving the two outermost rows on both sides of each plot and 0.5 m row length at both ends of each plot to avoid border effects.

The spacing between plots and blocks was 0.5 m and 1 m, respectively. Triple superphosphate (46% P) was used as a source of phosphorus fertilizers. To avoid N losses by leaching, urea application was done in two splits, i.e., half at sowing time and the other half at the stages of tillering. The experimental fields were under potato and rapeseed cultivation in the years 2022 and 2023, respectively. The plots were kept free of weeds by hand weeding at different growth stages of the crop. Other agronomic practices were applied based on local research recommendations.

Crop management

The experimental land was prepared by plowing. The 1^st ploughing was done by tractor, and the 2^nd and 3^rd ploughing by using oxen. Before sowing, the field was cleaned and prepared properly to receive treatments. Field layout was done based on the design of the experiment, and treatments were assigned to each experimental plot randomly. Seed was sown on 16 July 2022 and 8 July 2023 by hand drilling seeds in the rows at a recommended rate of 125 kg ha^-1. Phosphorus as TSP (46% P₂O₅) was applied in the rows at the time of sowing, and nitrogen was applied based on the nature of treatments. Compost was applied to all plots (except the control) one month before planting and incorporated into the upper 15–20 cm of the soil layer. Furthermore, during both growing years, no disease and insect infestations were occurred in the experimental fields.

Data collection and measurement

Thousand kernel weights was determined by taking three samples of thousand seeds weight taken from the grain yield of each net plot of experimental field. Seeds were counted using an electronic seed counter and weighed with a sensitive balance, and adjusted to 12.5% moisture content of the grain. Grain moisture content was measured by using apparatus Grain Analysis Computer (GAC) 2100) (perusing protein analysis computer) as described in the AACC (2000) method. Hectoliter weight was determined on dockage-free samples using a standard laboratory hectoliter weight apparatus (grain analysis computer (GAC)) as described in the AACC (2000) method. The protein content analysis was analyzed at the Holetta Agricultural Research Center laboratory using the standard operating procedures were measured by the Kjeldahl method (grain analysis computer (GAC) 2100) (perusing protein analysis computer) as described in the AACC (2000) method. Malt β-glucan was determined using the Mixed-Linkage β-Glucan Assay Kit according to the streamlined procedure, following AOAC Method 995.16 and AACC Method 32−23. Germination capacity was determined from 100 seeds germinated in a Petri dish after 72 hours. The germinated kernels are counted and the result is expressed as a percentage of the total. Germination energy was tested simultaneously with germination ability, and it was carried out on day four. The seeds that sprouted on day four after sowing were counted, and the proportion of sprouted seeds was calculated as a percentage.

Statistical analysis

All collected data were subjected to the analysis of variance (ANOVA) procedure. Prior to analysis, normality of residuals was tested using the Shapiro–Wilk test, and homogeneity of variances was tested using Levene’s test residuals were tested. Once these assumptions were satisfied, the collected data was subjected to the ANOVA using R statistics software (ver. 3.4.1, 2017). A combined analysis of the two-year data was performed after testing the homogeneity of variances using an F-test, as described by [39].

Ethics statement

This study did not involve human participants or animals. No specific permits were required for conducting the field experiments, and the research complied with institutional, national, and international guidelines for the responsible conduct of agricultural field studies.

Results

Thousand seed weight

The analysis of variance demonstrated that thousand seed weight (TSW) was very highly significantly affected by nitrogen fertilizer rate (P < 0.001) and by the two-way interaction of N rate and growing season. Besides, the two-way interaction of compost and growing season had a significant effect (P < 0.01) on the thousand seed weight of malt barley grain. However, the main effect of compost rates, the main effect of growing season, and the remaining two- and three way interaction had a significant effect on thousand seed weight (Table 1). The maximum TSW of (45.75 g) was recorded at N rates of both 92 kg ha ⁻ ¹ and 69 kg ha ⁻ ¹, and these values were statistically similar to the N rate of 46 kg ha^-1 (Fig 2). Moreover, even a small application of 23 kg ha ⁻ ¹ N fertilizer increases in TSW from 42.93 g under zero or nil nitrogen to 44.20 g, indicating that even low nitrogen rate applications positively affect TSW (Fig 2).

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Table 1. The effects of organic compost and nitrogen fertilization rates on selected quality of malting barley grown in the central highland of Ethiopia.

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Fig 2. Main effects of N fertilizer rates on thousand seed weight.

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The two-way interaction effects of mineral N and growing years highly affected (P < 0.001) the thousand seed weight. The highest (45.83 g) mean value of TSW was obtained at 92 kg ha^-1 N fertilization rates during the 2022 growing season, whereas the lowest (41.48g) mean of thousand seed weight was obtained from plots that had no mineral N rates during the 2023 growing years (Fig 3A). The two-way interaction effects of compost rates and growing years were highly affected (P < 0.001) the thousand seed weight. The highest (45.88 g) mean value of TSW was obtained at 7.5t ha^-1 a full dose of compost rates during the 2022 growing season, whereas the lowest (43.12 g) mean of thousand seed weight was obtained from plots that had was produced on plots treated with 5 t ha ⁻ ¹ during the 2022 cropping season (Fig 3B).

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Fig 3. Interaction effects of N and compost rates by growing years on thousand seed weight.

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Moisture content

The moisture content is one of the quality parameters that affect the quality of the product, storage, and processing of malt barley. The analysis of variance for moisture content of malt barley revealed that, excluding growing years, both the main and interaction effects of compost and nitrogen fertilizer rates over growing seasons were not significant (p < 0.05) (Table 1). On the other hand, the three-way interaction effect of N fertilizer rates, compost application rates, and growing seasons was statistically significant at (p < 0.05) on the moisture content of malt barley. The highest mean value of moisture content (11.79%) was recorded during the 2022 cropping years, while the smallest mean value of moisture content (11.50%) was obtained during the 2023 growing years (Fig 4).

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Fig 4. Effect of growing years on grain moisture content of malt barley.

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The analysis of variance revealed that the interaction effect of organic (compost) and N fertilizer rates showed a significant difference in the moisture content of barley grain which means the influence of compost fertilizer rates depends on the application rate of N fertilizer. The maximum mean value of moisture content (12.20%) was recorded with 7.5 t ha^-1 applied compost and 92 kg ha^-1 nitrogen fertilizer rates during the 2022 cropping season, whereas the lowest mean of moisture content (11.22%) was obtained from plots that had the combination of 7.5 t ha^-1 compost and 92 kg ha^-1 nitrogen fertilizer rates during the 2023 growing season (Fig 5).

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Fig 5. Three-way interaction effect of compost, N fertilizer rates, and growing year on moisture content.

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Hectoliter weight

Analysis of variance revealed that hectoliter weight was very highly significant (P < 0.001), affected by the main effect of the growing season and significantly affected by the interaction effect of nitrogen and growing season (Table 1). The maximum (64.66 kg hl^-1) and minimum (62.13 kg hl^-1) hectoliter weights were scored in the 2022 growing season and 2023 growing season, respectively (Fig 6).

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Fig 6. Effect of growing years on hectoliter weight of malt barley grain.

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The combined mean values of hectoliter weight of the growing season and rate of nitrogen fertilizer application show the highest hectoliter weight (65.75 kg hl^-1) was recorded at 46 kg nitrogen fertilizer application during the 2022 growing season, while the lowest (61.03 kg hl^-1) hectoliter weight was recorded at 46 kg ha^-1 nitrogen fertilizer application during the 2023 growing season (Fig 7).

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Fig 7. Two-way interaction effect of nitrogen rates and growing year on hectoliter weight.

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Malt extract yield

The analysis of variance revealed that malt extract had a highly significant (P < 0.001) difference because of the main effects of nitrogen fertilizer, organic compost, and growing seasons. Besides, the malt extract was also significantly influenced by the interaction effects of nitrogen and compost fertilizer (P < 0.01), nitrogen and growing season (P < 0.05), and compost and growing season (P < 0.05) of malt barley. However, this parameter was not significantly affected by the interaction effects of fertilizer rates of nitrogen, compost, and growing season (Table 1).

The highest malt extract (80.44%) of malt barley was measured from the lowest N fertilizer application (0 kg ha^{- 1}), followed by a 23 kg ha^{- 1} nitrogen fertilizer application, which was 80.25%, while the lowest malt extract (78.28%) was observed from the highest nitrogen fertilizer rates (92 kg ha^{- 1}), which was statistically different from all the other nitrogen fertilizer rates (Fig 8A). The highest malt extract (80.13%) of malt barley was observed from the lowest organic compost application rates (2.5 t ha^{- 1}), followed by 0 t ha^{- 1} of compost rates, which was 79.99%, while the lowest malt extract (78.69%) was recorded from the highest compost fertilizer rate (7.5 t ha^{- 1}), which was statistically different from all the other compost rates (Fig 8B). The malt extract of malt barley in the growing seasons was significantly different, with the highest malt extract (80.37%) recorded from the 2022 cropping season, while the lowest malt extract (78.63%) was observed in the 2023 growing season (Fig 8C).

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Fig 8. Main effect of nitrogen, compost rates and growing year on grain malt extract yield.

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The combined use of nitrogen and compost fertilizer rates significantly impacts the malt extract in malt barley grain. The malt extract was significantly influenced by the interaction effects of nitrogen and compost fertilizer (P < 0.01). The highest mean (81.87%) of malt extract was recorded from non-fertilized plots or control treatments, whereas the lowest mean (77.58%) of malt extract was observed from the combination of 92 kg N ha^-1 with 7.5 t ha^-1 (Fig 9A). The two-way interaction effects of nitrogen rates and growing years significantly affect the malt extract in malt barley grain. The highest mean (81.19%) of malt extract was recorded from 23 kg N ha^-1 during the 2022 growing year treatments, whereas the lowest mean (77.16) of malt extract was observed from the application of 92 kg N ha^-1 during the2023 growing year (Fig 9B). The two-way interaction effects of compost rates and growing years significantly affect the malt extract in malt barley grain. The highest mean (81.06%) of malt extract was recorded from 2.5 t ha^-1 of compost rates during the 2022 growing year treatments, whereas the lowest mean (77.84%) of malt extract was recorded from the application of 7.5 t ha^-1 of compost amendments during the 2023 growing years (Fig 9C).

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Fig 9. Two-way Interaction effects of N and compost rates, N rates and growing year, compost rates and growing season on grain malt extract.

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Grain protein content

The grain protein concentration of malt barley grain was highly significantly (P < 0.001) influenced by the sole effect of mineral N, compost rates, and growing years. Besides, the interaction effect of organic compost, nitrogen rates, and growing seasons significantly (P < 0.05) impacted the protein content of malt (Table 1). The grain protein concentration levels are positively correlated with N fertilizer rates, which is increased protein content of the grain with N fertilizer rates. The maximum (11.82%) grain protein concentration was measured from the maximum (92 kg N ha^-1) application of N fertilizer rates, whereas the lowest protein level was recorded from non-fertilized plots (Fig 10A). Plots fertilized with a maximum (7.5 t ha^-1) compost rate gave the highest (11.43%) protein level of malt barley grain, whereas unfertilized plots with compost resulted in the lowest (10.61%) grain protein concentration, which shows that an increase of compost fertilizer levels raises the grain protein concentration of malt barley in the study area (Fig 10B). The highest (11.16%) grain protein content was obtained during the 2023 main cropping season, whereas the lowest (10.82%) grain protein concentration was recorded during the main cropping season (Fig 10C).

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Fig 10. Main effect of nitrogen, compost rates and growing year on grain protein content.

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The maximum malt grain protein concentration (11.44%) in the 2023 growing year was recorded from 7.5 t ha^-1 of compost application, while the lowest (10.40%) malt grain protein content in the 2022 growing season was obtained from 0 t ha^-1 of compost application, which was statistically similar to the grain protein accumulation recorded in 2022 at 2.50 t ha^-1 of compost application rate (Fig 11).

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Fig 11. Two-way interaction effect of compost rates and growing years on grain protein content.

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The highest (12.33%) grain protein concentration was recorded at the highest input application of 92 kg N ha^-1 combined with 7.5 t ha^-1 compost rates during the 2023 growing years while the lowest (9.39%) grain protein content was recorded at nil application of N fertilizer with 2.5 t ha^-1 of compost rates during the 2022 growing year (Fig 12).

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Fig 12. Three-way interaction effect of compost, N fertilizer rates, and growing year on grain protein content.

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Malt friability content

The ANOVA analysis revealed that the main effect of chemical N fertilization and compost fertilizer sources had a highly significant (P < 0.001) effect on the malt friability content of malt barley grain; however, the interaction effects were not significant except for the combination of cropping season and organic compost fertilizer levels (Table 1). The N fertilization levels are inversely correlated with the malt friability content and the highest obtained from the non-fertilized plots with N fertilizer rates (81.13%), followed by 23 kg N ha^-1 (80.73%) and 46 kg N ha^-1 (79.77%), whereas the lowest (77.81%) friability concentration was measured from the highest (92 kg N ha^-1) chemical nitrogen fertilizer rate (Fig 13A). The highest (80.65%) mean malt friability was recorded from the application of 7.5 t ha⁻¹ of compost rate, while the lowest (80.65%) malt friability was obtained from the application of 2.5 t ha⁻¹ of compost rate (Fig 13B).

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Fig 13. Main effect of nitrogen and compost rates on malt friability.

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The maximum malt friability value (80.84%) was observed in the 2022 growing years at 7.5 t ha^-1 of compost application rate, while the minimum (78.12%) was obtained from 2.5 t ha ⁻ ¹ of compost applied in the 2022 growing years. In the 2023 growing year, the maximum value (80.47%) of malt friability was also observed at 7.5 t ha ⁻ ¹, showing that maximum compost rates consistently improved malt friability across both growing years (Fig 14).

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Fig 14. Two-way interaction effect of compost rates and growing years on malt friability.

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Malt β-glucan

The analysis of variance showed that the beta-glucan of malt barley was highly significant (P < 0.001) influenced by the main effect of mineral N, organic compost, and growing year. Additionally, the interaction of growing year and compost fertilizer levels significantly (P < 0.05) affected the malt beta-glucan of malting barley grain. However, other interaction effects did not significantly impact the β-glucan of malt barley (Table 1). The maximum (607.53 ppm) malt β-glucan was obtained from the plots fertilized with 92 kg N ha^-1 which was followed by (544.66 ppm) recorded from 69 kg N ha^-1, whereas the lowest (433.55 ppm) malt β-glucan was observed from control or unfertilized plots (0 kg N ha^-1) (Fig 15A). Regarding the main effect of compost fertilizer rate on malt β-glucan concentration, the highest (572.53 ppm) malt β-glucan content was obtained with the maximum application of compost (7.5 t ha^-1), whereas the lowest (478.36 ppm) malt β-glucan was observed at the (2.5 t ha^-1) compost application rate (Fig 15B). Regarding the effect growing year on malt β-glucan, the highest (535.36 ppm) was recorded in the growing year, while the lowest mean (497.87 ppm) of the malt β-glucan was obtained at 2022 growing year (Fig 15C).

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Fig 15. Main effect of nitrogen, compost rates and growing year on grain malt β-glucan.

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Malt β-glucan content was significantly affected by compost fertilizer rate, with influences varying between growing years. The maximum malt β-glucan content (590.72 ppm) in the second year (2023) was recorded from 7.5 ton ha^-1of compost application, and the lowest (447.89 ppm) malt β-glucan content in the first year of the growing season (2022) was obtained from 2.5 t ha^-1 of the compost application (Fig 16).

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Fig 16. Two-way interaction effects of compost rates and growing years on grain malt β glucan.

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Germination capacity

The analysis of variance results indicated that the main effects of N fertilization and compost rates highly (P < 0.001) and significantly (P < 0.01) affected the germination capacity (GC) of malt barley kernels, respectively. Additionally, the interaction effect of mineral N and compost levels highly (P < 0.001) influenced the germination capacity. However, the other main and interaction effects did not significantly affect the germination capacity of malt barley (Table 1).

The maximum (98.24%) germination capacity was recorded from the highest 92 kg N ha^-1 mineral N fertilization rate, which was followed by 69 kg N ha^-1 (96.90%), while the lowest (94.78%) germination capacity was from the control or unfertilized treatment, which was statistically similar to 23 kg N ha^-1 (Fig 17A). The highest (96.96%) grain germination capacity was recorded from the highest 7.5 ton ha^-1 application of compost level, while the lowest (95.31%) GC of barley grain was measured from unfertilized plots by compost (Fig 17B).

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Fig 17. Main effect of nitrogen and compost rates on germination capacity.

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The maximum (98.92%) germination capacity mean was recorded from the combined application of 92 kg ha^-1 of N fertilization and 7.5 t ha^-1 of compost fertilizer rates. However, the lowest (90.44%) germination capacity of malting barley grain was obtained from a control or unfertilized plot (Fig 18).

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Fig 18. Two-way interaction effects of mineral nitrogen and compost rates on germination capacity.

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Germination energy

The germination energy of malt barley grain was highly (P < 0.001) responsive to the main effect of N fertilization, compost rate, and interaction effects of compost fertilizer rate and growing year, while the main effect of growing year and other interaction effects did not significantly affect the germination energy (GE) of the malt barley grain (Table 1). The highest (97.05%) germination energy was recorded from 92 kg ha^-1 of N fertilization levels and followed (95.60%) by 69 kg ha^-1 chemical N fertilizer rate. While the lowest (92.22%) germination energy was obtained from the control or unfertilized plot (Fig 19A). The current finding showed that the GE of the malted barley fell within an acceptable range for both 69 kg ha^-1 and 92 kg ha^-1 of N fertilization levels. The main effect of compost application rates highly affected the germination energy of malt barley grain. The highest (95.17%) of GE of barley grain was recorded from the highest (7.5t ha^-1) compost rate, while the lowest (94.49%) germination energy was obtained from unfertilized plots or treatments (Fig 19B).

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Fig 19. Main effects of mineral nitrogen and compost rates on germination energy.

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The result of the analysis of variance showed that the combination effect of compost fertilizer rates and growing year highly significantly affected the germination energy of malt barley grain. Specifically, the highest (96.04%) germination energy of malting barley was recorded in the 2023 growing season with the application of (7.5 t ha^-1) compost fertilizer rates and also closely followed by (5 t ha^-1) compost rates in the 2022 growing season. However, the lowest (93.84%) germination energy was obtained in the 2022 growing season with a compost application rate of (2.5 t ha^-1) (Fig 20).

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Fig 20. Two-way interaction effect of compost rates and growing years on germination energy.

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Multiple regression analysis of malt barley quality parameters

Multiple regression analysis for grain protein concentration.

The analysis of multiple linear regression showed a positive effect on the grain protein content of malt barley. There was an expected average protein concentration of 9.73% for malt barley grain under field conditions with no inputs and no change in growing years. An increase of N rate by 1 kg ha^-1 will raise the protein concentration by 0.02% (Fig 21). There was a stronger effect of compost rate than N fertilization rate, which found that an application of each t ha^-1 of organic compost raised the grain protein content by 0.12% (Fig 21). Moreover, the change in the growing year also has a significant effect on the protein concentration of malt barley grain and raises protein content by 0.34% (Fig 21).

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Fig 21. Multiple linear regression of grain protein content as influenced by nitrogen, compost, and growing year.

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Multiple regression for malt extract yield

The regression analysis equation showed that the malt extract levels decrease in boosting N fertilization, organic rates, and change in growing years. The difference in malt extract of the grain affected about 71%, which was indicated by adjusted r-squared (0.69) (Fig 22). There was an expected average malt extract yield of 80.93% for malt barley grain under field conditions with no inputs and change in growing years. The findings show that increasing of N rate by 1 kg ha^-1 reduces the malt extract yield by 0.19%, holding other factors constant (Fig 22). In addition, increasing the compost rate by 1 t ha^-1 reduces the malt extract yield by 0.92% (Fig 22). Moreover, the change in the growing year also has a significant effect on the malt extract yield of malt barley grain and reduces malt extract by 1.45% (Fig 22).

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Fig 22. Multiple linear regression of malt extract yield as influenced by nitrogen, compost, and growing year.

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Multiple regression for malt friability

The regression analysis equation demonstrated that the malt friability percentage was affected by changing N fertilization, organic rates, and growing years. The difference in malt extract of the grain affected about 68%, which was indicated by adjusted r-squared (0.68) (Fig 23). Rising the N fertilization rate has a negative correlation with malt friability, which shows that elevating N fertilization rates decreases malt friability percentage. The results show that increasing the N rate by 1 kg ha^-1 reduces the malt friability by 0.4%, holding other factors constant. Regarding compost rates, increasing the compost rate by 1 t ha-1 reduce the malt friability by 1.36% (Fig 23). Moreover, the change in the growing year also has a significant effect on the malt friability of malt barley grain and reduces malt friability by 2.28% (Fig 23).

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Fig 23. Multiple linear regression of malt friability as influenced by nitrogen, compost, and growing year.

https://doi.org/10.1371/journal.pone.0343009.g023

Multiple regression for beta-glucan of malt barley grain

The multiple regression analysis equation showed that the beta-glucan concentration in malt barley was highly affected by nitrogen fertilization, organic compost rates, and growing years. The difference in beta-glucan content of the grain affected about 86% of the variation, which was explained by these three factors (Fig 24). The regression analysis equation showed that for every 1 kg ha^-1 rise in N fertilization, beta-glucan content increases by 49.93 ppm (Fig 24). Regarding compost rates, increasing of compost rate by 1 t ha^-1 increase the malt beta glucan by 80.31 ppm (Fig 24). Moreover, the change in the growing year also has a significant effect on the malt beta glucan of malt barley grain and increases malt beta glucan by 111.11 ppm, holding other factors constant (Fig 24).

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Fig 24. Multiple linear regression of malt beta glucan as influenced by nitrogen, compost, and growing year.

https://doi.org/10.1371/journal.pone.0343009.g024

Multiple regression for germination energy of malt barley grain

The multiple regression analysis equation showed that the germination energy in malt barley was highly affected by nitrogen fertilization, organic compost rates, and growing years (Fig 25). The difference in germination energy of the grain affected about 71% of the variation, which explained by these three factors (Fig 25). The regression analysis equation showed that for every 1 kg ha^-1 rise in N fertilization, germination energy increases by 1.47% (Fig 25). Similarly, each additional ton ha^-1 of organic compost results in a 2.68% increase in the germination energy of malt barley (Fig 25). Moreover, the influence of growing years has a considerable influence, with an increase of 3.38% of germination energy per growing year showing that environmental conditions across years play a significant role (Fig 25).

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Fig 25. Multiple linear regression of germination energy as influenced by nitrogen, compost, and growing year.

https://doi.org/10.1371/journal.pone.0343009.g025

Correlation analysis of quality parameters on malt barley

Protein concentration of malt barley grain was positively and highly significantly correlated with beta-glucan (r = 0.83***), germination energy (r = 0.71***), and germination capacity (r = 0.55***); but malt extract yield (r = −0.71***) and malt friability (r = −0.43***) were negatively and highly significantly correlated with protein content. Germination energy exhibited a positive and highly significant correlated with beta-glucan (r = 0.75***) and germination capacity (r = 0.49***); however, malt extract yield (r = −0.58***) and malt friability (r = −0.54***) were negatively and highly significantly correlated with germination energy (Table 2). Malt extract yield exhibited a negative and highly significant correlation with beta-glucan (r = −0.71***) and germination capacity (r = −0.44***); however, malt friability (r = 0.24**), moisture content (r = 0.24**), and hectoliter weight (r = 0.29**) were positively and significantly correlated with malt extract yield. Beta-glucan of malt barley grain was positively and highly significantly correlated with germination capacity (r = 0.52***) and significantly correlated with thousand-seed weight (r = 0.27**); but malt friability (r = −0.38***) was negatively and highly significantly correlated with beta-glucan (Table 2).

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Table 2. Correlation among grain quality parameters of malt barley.

https://doi.org/10.1371/journal.pone.0343009.t002

Discussion

Thousand seeds weight

The results indicated that as the N fertilizer rate increases, the thousand-seed weight also tends to increase, with notable statistical significance among the different treatment levels. The positive response of thousand seed weight to increasing N rates shows the importance of nitrogen in improving barley seed development. This might be related to N fertilizer at a higher rate promoting better nutrient uptake (increases nutrient availability) and enhancing photosynthesis (promotes chlorophyll production) and robust vegetative growth (greater leaf area and more developed root systems), resulting in improved seed development and quality of seed in malt barley. The current finding is highly agreed with result of Kassie and Tesfaye [40] revealed that the thousand seed weight of malting barley grain increased as synthetic N fertilizer increased in the Arsi Highlands of Ethiopia. Similarly, Azam et al. [41], who stated that the application of synthetic nitrogen fertilizer of 120 kg N kg^-1 increases the thousand kernel wheat of bread wheat during two growing seasons. The highest thousand grain weight was recorded at the highest application of mineral nitrogen rate on durum wheat under Algerian semi-arid conditions [42].

The maximum thousand-kernel weight obtained, probably due to the response to N fertilizer, can differ significantly between cropping seasons, most likely due to variations in weather conditions such as temperature, rainfall, and soil moisture, which can influence nutrient uptake and plant growth. This result was supported by Cammarano et al. [17] who revealed that the growing years or cropping seasons affected the thousand-seed weight of malting barley at the James Hutton Institute.

The optimum dose of organic amendment positively affects thousand seed weight by improving soil health, nutrient accessibility, and soil physical properties together with favorable weather conditions for plant growth. This result was agreed with Omirou et al. [43], who reported that compost can provide essential nutrients needed for plant growth and development. The difference may result from seasonal variations in weather conditions, such as rainfall and temperature, which can influence nutrient availability and finally affect plant growth and seed weight of the crop. These results corroborate the previous finding of Omirou et al. [43], who suggested that the kind of N fertilizer source influences the accumulation of essential plant nutrients and its effect varies depending on the growing season.