1 Introduction

Onion (Allium cepa L., family Alliaceae) is a valuable vegetable crop of significant economic importance. Egypt occupies the fourth rank in onion production, with a cultivated area of approximately 95,760 hectares, yielding around 3 million tons annually (Adam and Fangary 2020). As a major onion-exporting country, Egypt’s onion industry faces a significant challenge in the form of weed infestation. Onion is a poor competitor crop due to its slow growth rate and narrow leaf shoot system, which fails to shade and inhibit the growth of associated weeds (Dhananivetha et al. 2017).

Till now, weed infestation remains one of the most critical issues affecting both the quality and quantity of onion yields. Uncontrolled weed growth can result in yield losses of 55 to 72% compared to the most efficient herbicidal treatment, primarily due to competition for natural resources and allelopathic interactions (El-Metwally and Shalaby 2019; Shehata et al. 2022; El-Metwally et al. 2022). The critical period for weed competition in onion fields has been identified as 50 days after transplanting (Bin Shuaib 2001). Common weeds in onion fields have been found to release allelochemicals through root exudates or foliage leachates, which can negatively impact onion growth (Sadaqa et al. 2010). For instance, Sambucus nigra L. has been revealed to contain 24 distinct phytotoxic substances, including lignans, cyanogenins, phenolic glycosides and flavonoids, which exhibit potent inhibitory properties on onion germination and growth (D’Abrosca et al., 2001). Similarly, over 30 weed species have been found to harbor phytotoxic substances that inhibit the growth of various crops and weeds, with the most prevalent compounds including alkaloids, fatty acids, phenolics, terpenoids, indoles, lignans, cyanogenins, flavonoids and coumarins (Zohaib et al. 2016). Therefore, recent strategies focus on managing weeds at an early stage to prevent the elaboration of these allelochemicals into the soil, thereby protecting the onion crop.

There is no doubt that either pre-emergence or post-emergence chemical herbicides achieved good approaches in onion fields (Islam et al. 2020). But numerous studies have proven the herbicidal harmful effect on animals, humans and environmental surroundings (Marin-Morales et al. 2013). In addition, some weed species become resistant to the herbicidal effect that need higher doses for weeds limitation that reflect on economical crops causing crop injury (Schütte et al. 2017). Recently, weed scientists reward their researches to find alternative safe methods that achieve the balance between weed management and high crop yield with no harmful effect on humans and animals. These safe alternative methods such as allelopathy and mulching approaches achieved good results in controlling weeds (El-Metwally and El-Wakeel 2019; El-Metwally and Shalaby 2019; Shehata et al. 2022).

Allelopathy involves the release of secondary metabolites that can have either positive or negative effects on the germination or growth of neighboring plants (Li et al. 2010). These released compounds, known as allelochemicals, are emitted by plants into the surrounding environment through processes such as the decomposition of plant residues, volatilization, leaching, and root exudation (Scavo et al. 2019). In seeds or seed coats, these compounds include alkaloids, lectins, and phenolic compounds like lactones, tannins, and flavonoids, which exhibit diverse biological activities, including protective functions against microbial attacks by bacteria and fungi (Akl et al. 2020).Recent findings by Ahmed et al. 2022 suggest that seed powder can serve as a natural bioherbicide for weed control. Furthermore, aqueous solvent proved to be more effective in extraction of phenolic compounds that exhibit inhibitory bioherbicidal effect on weeds (El-Wakeel et al., 2019b).

In recent years, there has been a growing interest in the research bioherbicides which involve the utilization of plant materials containing allelochemicals as a promising approach for weed management. One such alternative method that has gained attention is mulching, which involves covering the soil surface using living or non-living materials. While the automated method of laying organic mulch has proven to be highly efficient compared to traditional mulching techniques, the use of mulch-laying machines is more widely accepted in developed nations, with limited adoption in countries like Egypt. However, the application of plastic mulching has begun to be mechanized using specialized machines, primarily in medium to large-scale field areas (Mohamed 2017). Numerous studies have highlighted the effectiveness of mulching as a safe method for controlling weeds in onion fields, particularly when compared to chemical herbicides (El-Metwally and Shalaby 2019; Shehata et al. 2022). Soil mulching offers several advantages, including increased crop yields by up to 17% and reduced water evaporation from the soil surface by up to 41%, thereby maintaining soil moisture in the top 30 cm of the soil profile (Unger et al. 2010).Mulching is considered a significant agronomic practice that involves covering the soil surface, leading to long-term weed suppression by inhibiting the germination of weed seeds in the soil seed bank (Choudhary et al. 2013). This approach not only reduces weed pressure but also contributes to the conservation of soil moisture and the enhancement of crop productivity, making it a sustainable alternative to chemical weed control methods.

It is well-established that management practices have a significant impact on both the microbial community and functional expression by altering the micro-environmental structure of the soil (Souza et al. 2015). Furthermore, research by Zhang et al. 2020 has confirmed that the application of organic mulching has a positive regulatory effect on the bacterial and fungal communities in the soil, leading to enhanced ecosystem functions. Organic mulching plays a crucial role in water conservation by preserving soil moisture, thereby preventing direct evaporation and surface erosion. This moisture retention mechanism is particularly beneficial in regions with high salt content in water resources, where the prevention of salt accumulation can alleviate plant stress (Sharma and Bhardwaj, 2017). Additionally, the decomposition of organic mulches enriches the soil with nutrients, effectively serving as organic fertilizers (Iqbal et al. 2020). Moreover, allelochemicals present in organic mulches inhibit the growth of weed seedlings and reduce the biomass of germinated weeds, contributing to effective weed control practices (El-Metwally and El-Wakeel 2019). By leveraging the multifaceted benefits of organic mulching, including water conservation, nutrient enrichment, and weed suppression, agricultural practices can be optimized for sustainable soil health and enhanced crop productivity.

Since weeds are great consumer of all soil natural resources, managing weeds is a required approach in to improve onion yield and quality. Oilseeds meal industrial wastes are by-products of oils production which are unused and its accumulation leads to environmental pollution (Sadh et al. 2018). So, the weed scientists tried to utilize these economically inexpensive by-products as organic mulches or natural bioherbicides as alternatives to synthetic herbicides (Shehata et al. 2022). Flaxseed and soybeans meals phenolic extracts have been reported to possess different biological activities such as antioxidant, anticoagulant, anticancer (Akl et al. 2020; Król-Grzymała and Amarowicz 2020). El-Metwally et al. 2022 ensured that soybean and flax meal mulches increased managed weeds and subsequently increased sugar beet yield. The current research hypothesized the ability to apply soybean and flaxseed meals industrial wastes either as organic mulches or extracts as safe natural methods to control weeds in the onion field. Although many researchers revealed that many plant materials and oil pybroduct wastes can be applied as ecofriendly approaches in onion field, this investigation aimed to reveal soybean and flax meal mulches and their extracts concentrations compared to recommended herbicidal and hoeing treatments. Additionally, this study aimed to assess the selectivity of these applied approaches had to inhibit weed growth and increase onion yield and quality considering the economic viability.

Weeds are known to be voracious consumers of soil natural resources, making weed management a crucial approach for improving onion yield and quality. Oilseed meal industrial wastes, which are by-products of oil production, are often unused and their accumulation can lead to environmental pollution (Sadh et al. 2018). In response, weed scientists have explored the utilization of these economically inexpensive by-products as organic mulches or natural bioherbicides, as alternatives to synthetic herbicides (Shehata et al. 2022). Flaxseed and soybean meals have been found to contain phenolic extracts with diverse biological activities, including antioxidant, anticoagulant, and anticancer properties (Akl et al. 2020; Król-Grzymała and Amarowicz 2020). El-Metwally et al. 2022 have demonstrated that soybean and flax meal mulches effectively managed weeds, leading to increased sugar beet yields. The current research hypothesizes the potential application of soybean and flaxseed meals, industrial wastes, as organic mulches or extracts, to serve as safe and natural methods for controlling weeds in onion fields. While many researchers have revealed the effectiveness of various plant materials and oil production wastes as eco-friendly approaches in onion cultivation, this investigation aims to compare the efficacy of soybean and flax meal mulches and their extract concentrations with recommended herbicidal and hoeing treatments. Furthermore, this study seeks to assess the selectivity of these applied approaches in inhibiting weed growth and enhancing onion yield and quality, while considering their economic viability.

2 Materials and Methods

2.1 Location Description

Two field experiments were conducted during the winter seasons of 2020/21 and 2021/22at the Agricultural Production and Research Station of the National Research Centre, El-Nubaria, Beheira Governorate, Egypt (latitude 30.8667 N, longitude 31.1667 E and mean altitude 21 m above sea level). Monthly weather data were recorded according to Egyptian Meteorological Authority along the experimental period with precipitation ranged from 21 to 70 mm and max temperature range 16.5–21 °C. The experimental soil was sandy with Organic matter average 0.49%, average pH 8.72 and electric conductivity (EC) average 0.47 dS m− 1 in accordance with soil taxonomy guidelines (IUSS Working Group WRB 2015). The soil is classified as belonging to the order Entisols and the suborder basements in 60 cm depth from the soil surface. However, chemical properties assessed according to Page et al. 1982 as shown in Table 1.

Table 1 Chemical soil analysis
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2.2 Preparation of Oilseed Meals and Extracts

Soybean and flaxseed meals were purchased from a local factory “peacock” Tanta, Gharbeya, Egypt. The meals are the wastes of hydraulic press for oil production. Three ratios 15,30 and 45% of soybean and flaxseed meals (450, 900 and 1350 g) were dissolved in 3 l solvents with ratio 2:1:1 (water: ethanol: methanol) then leave in ultrasonic bath for two hour then leave over night for soaking then centrifuged 30 min.at 3000xg and the supernatant(1) was kept. The precipitated meals were re-dissolved by another amount of solvents and stirring for one hour then centrifuged 30 min. at 3000xg and the supernatant (2) was kept. The two supernatants were collected. The alcoholic solvent evaporated by rotary evaporator to get rid of ethanol and methanol leaving watery extract.

2.3 Experimental Procedure

2.3.1 Experimental Design

The experiments were applied in randomized complete block design with four replicates. The plot area was 10.5 m2 (1/400 of fed. (4200 m2), contained four rows 3.75 m length and 0.7 m width). Spacing was 70 cm between rows and 10 cm between plants. Onion seedlings (cv. Giza red) were transplanted during the third week of December in both seasons. All cultural practices were applied according to recommendations of Egyptian Ministry of Agriculture and land reclamation.

2.3.2 Treatments

In this study, soybean and flaxseed meals were applied as weed control treatments using two different application methods. The mulching treatments for both meals were applied immediately after transplanting and the application of fertilizers. A prepared soil bed was completely covered with a 1–2 cm layer (equivalent to 1 ton ha− 1) of each meal, applied manually using hoes and spades. The second application method involved six treatments of alcoholic extracts, which were sprayed twice at 4 and 6 weeks after transplanting. Three concentrations of both soybean and flaxseed meals extracts (15, 30 and 45%) were applied by foliar spraying of the extracted alcoholic extracts using knapsack sprayers brand Lamsin, model number is HX-D18F and place of origin is China. at a volume of 500 ml/plot. The spray boom consisted of six flat fan spray pattern nozzles, with a liquid pressure (hydraulic atomization) ranging from 2.5 to 4.5 bar.

2.4 Cultural Applications

All cultural applications such as irrigation, fertilization and pest control were applied according to the recommendations of the Egyptian Ministry of Agriculture and Land Reclamation. During the soil preparation, before transplanting and at 70 days after transplanting (DAT) Potassium sulfate (48% K2O) was added at the rate of 200 kg K2O ha− 1. Moreover, 250 kg ha sulfur + 150 units of P of calcium superphosphate (15.5% P2O5), N fertilizer was applied at the rate of 450 units of N ha− 1 in the form of ammonium nitrate (33.5% N) which were divided into six equal portions during the growing season.

2.5 Metrics

2.5.1 Weed Traits

2.5.1.1 Weeds Growth

From each replicate of the investigated treatments weed samples were randomly collected per one square meter in both seasons at 70 and 100 DAT. The weed samples were separated into broadleaf and grassy weeds to be counted per m2. The fresh weed samples were air dried for 10 days then oven dried at 80 ℃ for 72 h up to a constant weight for estimating total biomass expressed in dry weight m− 2.

2.5.1.2 Nutrient Uptake by Weeds

For N, P and K estimation in weeds, the collected weed samples were dried for constant weight at 70º C until weight constant weight then digested according to Cottenie et al. (1982). After digestion, the nitrogen content were determined using the modified micro Kjeldahl method according to Bremner (1960). Phosphorus content was estimated using spectrophotometer at 650 nm wavelength according to Cottenie et al. (1982). However, the potassium content was determined by a flame photometer method as described by Okalebo et al. (2002). For calculation of nutrient uptake, the determined nutrients contents (N, P and K) were multiblied with weeds dry weight at 100 DAT.

2.5.2 Onion Traits

At 100 DAT in both seasons, onion vegetative growth parameters as plant height, number of leaves plant− 1, plant fresh weight and bulb diameter. At maturity (150 DAT), 10 onion plants were randomly uprooted from each plot to estimate bulb fresh weight, bulb dry weight, bulb diameter and yield (Ton/fed). For estimation of mature bulb quality by estimation of flavonoids, total soluble solid was measured according to Eisa et al. (2001) using refractometer (Model PR 32 Brix-reading 0–32 ranges Atago.Co.LTD-Japan), total soluble sugar determined calorimetrically by the phenol-sulfuric acid method as described by Dubois et al. (1956), total phenolic compounds and total flavonoids were determined according to Srisawat et al. (2010) and nutrient quantities (N, P and K) were estimated as mentioned before.

2.5.3 Quantitative Estimation of Allelopathic Compounds in the Investigated Soybean and Flaxseed Meals Extracts

2.5.3.1 Determination of Total Soluble Phenolic Compounds

The content of phenolic compounds was determined according to the method of Fu et al. 2014. 200 µl of the sample was completed to 3 ml distilled water. 2 ml of 10% folin reagent was added then shake well for 5 min. 1 ml of 7.5% sodium carbonate was added then shaked. The mixture was left for one hour in dark then the absorbance at 765 nm was measured using a spectrophotometer (T80 UV vis spectrophotometers).

2.5.3.2 Determination of Total Flavonoids Compounds

The colorimetric determination of soluble flavonoids was performed according to Kanatt et al. 2011. The absorbance at 510 nm was measured using a spectrophotometer (T80 UV vis spectrophotometers).

2.5.3.3 Evaluation of Antioxidant Activity of all Soluble Phase by Dpph Radical-Scavenging

The method described by De Ancos et al. (2002) was utilized to determine the DPPH radical-scavenging. The reduction of the DPPH radical was measured at 517 nm.

2.5.3.4 HPLC Instrumentation for Qualitative Determination of Phenolic Acids

The HPLC analysis was performed as described by Kim et al. (2006) to soybean and flaxseed meals extracts using Agilent Technologies 1100 series liquid chromatograph equipped with an auto sampler and a diode-array detector. The analytical column had a C18 guard column and was an Eclipse XDB-C18 (150 × 4.6 mm; 5 mm) (Phenomenex, Torrance, CA). Acetonitrile (solvent A) and 2% acetic acid in water (v/v) made up the mobile phase (solvent B). The gradient programme was as follows: 100% B to 85% B in 30 min, 85% B to 50% B in 20 min, 50% B to 0% B in 5 min, and 0% B to 100% B in 5 min. The flow rate was maintained at 0.8 ml/min for a total run period of 60 min. The injection volume was 50 l, and peaks for the benzoic acid and cinnamic acid derivatives were concurrently seen at 280 and 320 nm and 360 nm, respectively. Peaks were identified using UV spectra and retention times that were consistent, and they were then compared to the standards which are represented in Fig. 1.

Fig. 1
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Standard phenolic acids concentrations (µg/ml) and areas.

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2.5.4 Economic Profitability

According to the CIMMYT Economics Program (1988), economic analysis was used to compare costs and returns among different weed control treatments.

The average production cost per hectare was obtained from the Bulletin of Statistical cost Production and Net Return (2017) of each treatment were calculated ($ ha− 1) on the basis of local market prices (2017) using Eqs. 1 and 2 as follows:

Gross return = Root yield × price of root yield ($ha− 1) (1).

Net return = Gross returns − fixed and variable cost of crop production ($ ha− 1) (2).

The average prices were taken from the local market where the price of one ton of marketable onion bulbs was $500 and 540 per ton (first and second season respectively); and $500 ha− 1 (dollar per hectare).

2.6 Statistical Analysis

The recorded data were submitted to homogeneity test (Levene 1960) and Anderson–Darling normality test (Scholz and Stephens 1987) before carrying out the analysis of variance (ANOVA).test before to analysis of variance (ANOVA). ANOVA was performed on the data for each season in accordance with Casella (2008) using the Costat software, Version 6.303, 2004. The means of applied treatments were separated using Duncan’s multiple range test (alphabetical lowercase letters) at the 0.05 level of probability.

3 Results

3.1 Weeds Traits

3.1.1 Weeds Growth

The surveyed weeds in the investigated experimental field were eight winter-season weed species; five were annual broadleaf weed species and three were grassy weed species. The broadleaf weeds were common lambsquarter [Chenopodium album L.], white sweet clover [Melilotus alba Medikus.], common vetch [Vicia sativa L.], rough medik [Medicago denticulate Willd.] and scarlet pimpernel [Anagallis arvensis L.]. The grassy weeds were wild oat [Avena fatua L], littleseed canarygrass [Phalaris minor Retz] and darnel ryegrass [Lolium temulentum L].

The analysis of variance indicated a significant impact of the weed control treatments on both the number and weight of grassy and broadleaf weeds at 70 days after treatment (DAT) in both the 2020/21 and 2021/22 seasons, compared to the weedy check (Table 2). Among the treatments, soybean and flaxseed meal mulching, oxyfluorfen herbicide and two hand hoeing were consistently the most effective in reducing weed density (number of weeds) and biomass of broadleaf weeds per square meter in both seasons, with no significant differences observed between them. Following these treatments, soybean seed meal extracts at concentrations of 45 and 30% ranked second in efficacy compared to the weedy check control treatment. In terms of grassy weeds, the treatments that effectively controlled broadleaf weeds also exhibited significant suppression of grassy weeds, except for oxyfluorfen herbicide, which showed greater specificity in controlling broadleaf weeds over grassy weeds. Notably, soybean meal mulching, two hand hoeing, flaxseed meal mulching, and soybean meal extracts at concentrations of 45 and 30% demonstrated the highest reductions in grassy weed biomass in the first season. Conversely, in the second season, soybean and flaxseed meal mulching treatments were most effective in reducing grassy weed biomass, followed by two hand hoeing and soybean seed meal extracts at concentrations of 45 and 30%.

Table 2 Response of weeds growth to weed control treatments at 70 DAT in 2020/21 and 2021/22 seasons
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All weed control treatments significantly decreased both weed density and biomass in the onion field at harvest in both seasons as compared to weed check (Table 3). In the first season oxyfluorfen herbicide, two hand hoeing, soybean and flaxseed meals seeds mulch applications achieved the highest progress in decreasing broadleaf weeds density and biomass with no significant difference between them. These superior treatments decreased broadleaf weeds biomass by about 96.1, 92.0, 86.8 and 88.6%, respectively. These treatments also came in the first rank as the most efficient treatments in controlling broadleaf weeds in the second season with only one difference that soybean meal seeds mulch treatment precedes two hand hoeing application with reduction percentage reached to 89.0, 87.7, 87.5 and 86.4%, respectively. Soybean meal seeds extract 45, 30 and 15% came in the second rank with significant difference after the aforementioned superior treatments. Concerning to grassy weeds, mechanical two hand hoeing, organic mulching application either by soybean or flaxseed meals as well as spraying of oxyfluorfen recommended chemical herbicide exchanged with each other on the highest ranks in controlling grassy weeds with mostly no significant differences between them in both seasons. The reduction percentage of these superior treatments ranged between (91.0-97.8%) and (89.5–93.4%), in both two seasons respectively. The response of grassy weeds was similar to the response of broadleaf weeds that soybean meal seeds extract 45, 30% came in the second rank after these most effective treatments with no significant difference between these two spraying treatments.

Table 3 Response of weeds growth to weed control treatments at 100 DAT in 2020/21and 2021/22seasons
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The overall response of total weeds served as an indicator of the effectiveness of the various treatments on the entirety of weeds associated with the onion crop. Across both seasons, all applied treatments led to a significant reduction in the density and biomass of total weeds infesting the onion fields. At 70 DAT, detailed in Table 4, mulching with soybean and flaxseed meals, along with two hand hoeing, demonstrated the highest suppression of total weed density and biomass, with no significant differences observed between them. Additionally, the foliar spraying of soybean meal extracts at concentrations of 45 and 30% ranked second in inhibiting weed growth traits. Upon harvest, as indicated in Table 4, two hand hoeing and soybean mulch treatments consistently ranked at the top for achieving the greatest reduction in total weed populations in both seasons, with no significant differences between them. Similarly, flaxseed meal mulch and oxyfluorfen herbicide treatments shared the second rank, also without significant differences between them. This study highlights the noteworthy achievement that the two safe mulching treatments were statistically on par with two hand hoeing and oxyfluorfen herbicide, the recommended weed control methods, in effectively managing total weed populations.

Table 4 Response of total weeds growth to weed control treatments at 100 and 70 DAT in 2020/21and 2021/22seasons
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As shown in Table 5 nutrient uptake of macro-elements was significantly decreased as a response for weed control treatments comparing with weed check. Meanly the weedy check scored the highest nutrient uptake of N, P and K. Vice versa, mulching applications scored the lowest nutrients weeds uptake with no significant differences between them. Oxyfluorfen chemical herbicide and two hand hoeing came on the second rank after mulching applications.

Table 5 Weeds nutrients uptake as influenced by weed control treatments at 100 DAT in 2020/21and 2021/22seasons
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3.2 Agronomic Traits

3.2.1 Growth Traits

Onion growth traits i.e., plant height, number of leaves plant− 1, plant fresh weight and bulb diameter significantly responded to the investigated weed control practices in 2020/21 and 2021/22 seasons (Table 6). In both seasons, the application of soybean and flaxseed meal mulches yielded the highest values for plant height. The oxyfluorfen herbicide treatment ranked second, statistically on par with two hand hoeing. Following these effective treatments, soybean meal extracts at concentrations of 45% and 30% successively stimulated plant height, surpassing the weedy check. Notably, two hand hoeing, soybean and flaxseed meal mulching and oxyfluorfen herbicide achieved the highest number of leaves per plant with no significant differences observed between them. A significant achievement was the recording of soybean meal seed mulch, two hand hoeing and flaxseed meal mulching applications, which resulted in the highest onion plant fresh weight, with percentage increases of approximately 150.2, 147.8 and 141.9% as well as 127.0, 126.1 and 97.7% in both seasons, respectively, with no significant differences between them. Oxyfluorfen chemical herbicide followed these safe and effective treatments, with a significant difference. However, two hand hoeing and oxyfluorfen herbicide scored the highest progress in bulb diameter. Soybean seed meal, in both its mulching and extract forms at 45%, as well as flaxseed meal mulch, followed these recommended treatments, compared to the weedy check.

Table 6 Onion growth parameters as influenced by weed control treatments at 100 DAT in 2020/21 and 2021/22 seasons
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3.2.2 Yield Attributes

All the applied weed management practices increased onion yield traits than untreated treatment as presented in Table 7. In both seasons, mechanical two hand hoeing as well as soybean and flaxseed meals mulching were the potent practices for scoring a significantly higher bulb fresh and dry weight than chemical herbicidal treatment. Soybean meal extract application at 45 and 30% came in the second rank after the aforementioned progressive treatments as compared to control. Additionally, two hand hoeing, soybean meal mulching, chemical herbicide and flaxseed meal mulching applications, successively scored the highest bulb diameter with no significant between them and significantly increased than unweeded check. Whereas, all the other investigated treatments increased bulb diameter with no significant difference with unweeded check. Mechanical two hand hoeing, soybean and flaxseed meals mulching applications as well as oxyfluorfen herbicide proved its progress in scoring the highest onion yield in both seasons than unweeded control by about 85.06, 78.16, 71.26 and 57.47% in the first season and 91.36, 76.54, 72.84 and 64.20% in the second season. These most efficient treatments are followed by soybean and flaxseed meals extracts, successively at high concentrations.

Table 7 Onion yield traits as influenced by weed control treatments at harvest in 2020/21 and 2021/22 seasons at (150 DAT)
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3.2.3 Onion Yield Bulb Quality

The various weed management applications investigated in this study had a significant impact on onion yield and bulb quality parameters, including flavonoids, total soluble solids, total soluble sugars, and phenolic compounds (Table 8).In the first season, it was observed that soybean meal mulch, two hand hoeing, flaxseed meal mulch as well as oxyfluorfen herbicides and the two sprayed extracts at the highest concentration (45%) slightly increased flavonoid compounds than in case of unweeded treatment. Whereas, in the second season, only soybean meal mulch and two hand hoeing treatments achieved this low increment with a significant difference than unweeded control. In this regard, two hand hoeing, soybean meal mulch, oxyfluorfen herbicide and flaxseed meal mulch and the two sprayed extracts at the highest concentrations significantly increased total soluble solid and total soluble sugar parameters in both seasons. On the other hand, all weed managements under investigation didn’t stimulate the accumulation of phenolic compounds in the onion bulb as compared to untreated onion plants.

Table 8 Onion bulb quality as influenced by weed control treatments in 2020/21 and 2021/22 seasons
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Concerning to macro-nutrient elements (nitrogen, phosphorus and potassium) in onion bulb, all the applied weed management practices significantly increased these nutrients in onion bulbs that is in turn increased onion bulbs quality (Table 9). In details, soybean meal seed mulching, two hand hoeing, flaxseed meal mulching and oxyfluorfen herbicides application consequently, produced high onion bulbs quality through increasing N and P contents in both seasons. While, the highest K content in onion bulbs was attributed to the mechanical two hand hoeing. This ideal treatment is followed by soybean and flaxseed meals mulching practices which are significantly leveled with oxyfluorfen herbicide in both seasons. In contrast, unweeded treatment contained the lowest macro-nutrient elements content in onion bulbs.

Table 9 Nutrients quantities in onion bulb as influenced by weed control treatments in 2020/21 and 2021/22 seasons
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3.3 Quantitative Estimation of Allelopathic Compounds in the Investigated Soybean and Flaxseed Meals Extracts

3.3.1 Quantitative Estimation of Total Soluble Phenolic Compounds and Total Soluble Flavonoids in the Investigated Soybean and Flaxseed Meal Extracts

It was found that the oil content was 1.8 and 8.4% for soybean and flaxseed, respectively. The analysis in Table 10 revealed that the total phenolic compounds quantity was ranging from 4.32 to 6.18 mg/ml and from 2.45 to 5.68 mg/ml in soybean and flaxseed meals extracts at different concentrations, respectively. Whereas, flavonoids traces quantity was ranging from 0.93 to 1.82 mg/ml and from 0.71 to 2.50 mg/ml in soybean and flaxseed meals extracts at different concentrations, respectively. Also, it showed that with increasing of the ratio from 15 to 30 then 45%, the total yield of these bioactive compounds directly increased in both extracts.

Table 10 Quantitative total phenolics and total flavonoids compounds of three ratios of soybean and flaxseed meals extracts
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3.3.2 The Antioxidant Activities Determined by Dpph Scavenging Activity and Ferric Reducing Power Methods

The antioxidant activities of both meals extracts were determined by two methods of DPPH. The investigated three concentrations of both meal seeds extracts demonstrated the capability to scavenge the free-radical species. As shown in Fig. 2a the antioxidant activities of soybean and flaxseed meals extracts at different ratios by using DPPH scavenging method. The antioxidant effects of flaxseed meal extracts were higher than soybean meal extracts and with increasing the ratio to (45%) it increases the scavenging effect of DPPH. However, Fig. 2b revealed that soybean and flaxseed meals extracts were able to reduce Fe3+ to Fe+ 2, the greatest capacity to do so (0.52and 0.45 mg/g) was noted in both extracts with concentration 45% respectively. These results are lower than obtained by Akl et al. 2020 which showed 4.240 mg/g.

Fig. 2
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The antioxidant activities of soybean and flaxseed extracts at different ratios. (a) By using DPPH scavenging method. (b) By using FRAP method. *DPPH = 2,2-Diphenyl-1-Picrylhydrazyl. *FRAP = Ferric Reducing Antioxidant Power

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3.3.3 HPLC Fractionation of Phenolic Acids in the Investigated Soybean and Flaxseed Meals Extracts

Separation of the phenolic compounds in the soybean and flaxseed meal extracts was achieved by high performance liquid chromatography (HPLC). The HPLC analysis of soybean and flaxseed meals extracts identified varies poly phenols as illustrated in Table 11. It was reported that chlorogenic, vanillic and Apigenin were determined in soybean meal extract where they are absent in flaxseed meal extract Fig. 3. However, flaxseed meal extract contains different phenolic compounds like gallic, gentisic and rutin Fig. 4. Both soybean and flaxseed meal extracts contain another phenolic compounds in major quantities like protocatechuic, p-hydroxybenzoic, chlorogenic, cateachin, caffeic, syringic, ferulic, sinapic, p-coumaric, rosmarinic and apigenin-7-glucoside, cinnamic, quercetin, kampferol and chrysin. It also observed that gallic acid was absent in both extracts. Although, gentistic and rutin were completely absent from soybean meal extract it found in high amount in flaxseed meal extract.

Table 11 HPLC analysis of soybean and flaxseed phenolic extracts expressed as (µg mL− 1
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Fig. 3
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phenolic acids concentrations in soybean meal extract (µg/ml) and areas

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Fig. 4
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phenolic acids concentrations in flaxseed meal extract (µg/ml) and areas

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3.4 Economic Profitability

It was estimated that the two hoeing treatment required 32 workers per hectare, with two application times and a $6.66 and 7.33 (first and second season respectively) cost per day for each worker. As tabulated in Tables 12 and 16, the cost of oxyfluorfen herbicide was $31.8 and 34.9(first and second season respectively). The aqueous extracts of soybean meal extract $15, 30 and 45% ($35.1, 59.3 and 83.7 as well as $38.6, 65.2 and 92 first and second seasons respectively. While, the aqueous extracts of flaxseed meal extract $23.4, 35.5 and 47.9 as well as $25.7, 39.1 and 52.7 first and second season respectively. The application of soybean meal mulches costs reached $266.8 and 293.5 per three ton the first and second season, respectively. While, flaxseed meal mulches costs reached $155.6 and 171.2 per three ton the first and second season, respectively.

Table 12 Economic net return of marketable onion bulb yield under different weed control treatments (2020/21)
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4 Discussion

As chemical weed management is attributed to resistant unaffected weed and environmental pollution. So, there is a dire need to explore modern approaches with variable mode of action to face the weed management problem in agro-ecosystem (Dayan and Duke 2014). The oil factories produce a huge amount of byproduct wastes which can make many environmental problems. Hence, several studies are being conducted to find safe ways to benefit from these wastes (Tubeileh et al. 2019; Ladhari et al. 2020; El-Metwally et al. 2022; Shehata et al. 2022). As this study hypothesized that soybean and flaxseed wastes can be applied as natural herbicides as both contain many bioactive compounds that are known as allelochemicals (Farag et al. 2021; Nile et al. 2022). Additionally, mulching and spraying of aqueous extracts have been reported as safe approaches for controlling weeds (El-Metwally and Shalaby 2019; El-Wakeel et al. 2019a; Acharyya et al. 2020; Ahmed et al. 2022; El-Metwally et al. 2022; El-Wakeel et al., 2023). In all these regards, this study was aimed to apply safe weed control approaches either by mulching or spraying of aqueous extract through the utilization of oil mill by product wastes.

Our field experiments along two seasons achieved the hypothesis of the ability to apply soybean and flaxseed wastes as natural weed management approaches through mulching or spraying extracts. The suppression of weeds to allelochemicals secondary metabolites as alternative approaches to chemical herbicides are in accordance with El-Metwally and El-Wakeel (2019); El-Wakeel et al. 2019a); Bhaskar et al. (2021) who concluded that weeds showed inhibitory response to mulching applications and allelopathic plant materials. Since many studies reported that phenolic and flavonoid bioactive compounds have actions that limit weeds germination and growth (Macías et al. 2019; El-Wakeel et al., 2023). Moreover, the direct relationship between the negative response of weeds and phenolic content concentration has been reported by Perveen et al. (2019). Mulching application after transplanting of onion seedlings decreased the weed density through inhibiting weed seeds germination and reduced weed biomass by suppressing the growth of weed seedlings. The effectiveness of soil mulching applications is contributed with blocking of sunlight penetration to the soil and releasing of allelopathic compounds through leaching or decomposition to the surrounding rhizosphere (Chang et al. 2016; Kato-Noguchi and Kurniadie 2020). Concerning the allelopathic effect of extracts foliar spray and mulching applications, the allelochemicals in the soybean and flaxseed meals ensured its inhibitory effect on weed density and biomass.

The potency of soybean and flaxseed wastes may be attributed with the rich content of phenolic compounds and flavonoids as well as antioxidants. Soybean and flaxseed meal contain abundant amount of phenolic compounds reach about 56–62% and HPLC profiles were in agreement with Alu’dat et al. 2016. Many researchers concluded that the allelochemicals may interfere with the main biological processes of weeds such as respiration, photosynthesis, cell division, protein synthesis and enzymes activity (Duke and Dayan 2006; Ghanizadeh et al. 2014). The variation in physiological response of different weed species is attributed with the effective doses (Anh et al. 2021). Chai et al. 2013 concluded that benzoic, cinnamic and ferulic acids disrupted cell permeability that led to lipid peroxidation and plant tissue harming. Permeability disruption caused nutrient imbalance which negatively reflected on weeds growth parameters (Kobza and Einhellig 1987). Yu et al. 2003 ensured that benzoic and cinnamic acids suppressed respiration that in turn reflected on weeds growth. Cinnamic and ferulic acids also negatively suppress protein synthesis pathway and phytohormones (He and Lin 2001). Additionally, the remained oil traces in soybean and flaxseed meal byproducts had abundant amount of fatty acids that may be negatively affected on weed growth parameters. Esteves et al. 2010 reported that linoleic acid (55%), palmitic acid and (8%), linolenic acid (11%) and stearic acid (4%) were the main detected fatty acids in soybean oil. Shag et al. 2019 revealed that linoleic (26.2%), palmitic (12.9%) and stearic acids (10.7%) were the major fatty acids in flaxseed oil. All these pervious concepts revealed the progress of mulching application on foliar spray that may be related to the continuous releasing of allelochemicals after onion seedlings transplanting along the season that decreased the weed seeds germination and suppress the germinated seedlings.

The superiority of recommended known methods as mechanical hand hoeing and chemical oxyfluorfen herbicide at recommended dose scored high efficiency in controlling weeds. As the uprooting of weeds associated with onion crop decreased competition and enable sufficiency of the environmental resources (El-Metwally et al.,2019). Although hand hoeing is considered as a traditional safe and most efficient method in weed management strategies, manual labors are scarce and expensive. Additionally, oxyfluorfen herbicide providing control of annual broadleaf weeds and some grasses through inhibition of specific enzyme, protoporphyrinogen oxidase, which leads to accumulation of phototoxic heme and chlorophyll precursors, that in presence of light, produce activated oxygen species. oxyfluorfen herbicide suppression was achieved by necrosis of leaves and stem beside membrane disruption and lipid peroxidation in weed seedlings (El-Metwally and Shalaby 2019). Matzrafi et al. (2021) ensured that resistant weed species increase every year reaching to huge number of monocot and dicot weed species. So, the herbicide application should be reduced and safe new sustainable approaches should be investigated (Papapanagiotou et al. 2019).

On the other hand, mulches enhanced onion growth and yield traits. This may be related to the role of mulches to maintain the ideal soil temperature with good aeriation and optimum moisture level. These favorable circumstances enhance the biological growth of crop roots (Lakew et al. 2014; Biswas et al. 2015). Hence, the soil mulching application under investigation supported the efficiency of N, P and K uptake, water content and photosynthesis by decreasing weeds competition (Abd El-Mageed et al. 2016; El-Metwally et al. 2021). Moreover, mulching decomposition increased the releasing of nutrients that in turn reflected on onion growth and yield quantity and quality.

5 Conclusion

The findings of this study proved the ability of managing weeds through safe management approaches. Many researchers proved the progress of many plant materials and by product oil wastes, but this investigation exploited soybean and flaxseed wastes as natural safe approaches for controlling weeds in onion field as alternatives to harmful chemical herbicides. Applying of these suggested byproducts as mulches after transplanting of onion seedlings and foliar spray of alcoholic extracts at 45% are the most efficient treatments in suppressing weeds and scoring the highest onion yield quality and quantity. Moreover, one notable success observed in this study was the superiority of soybean and flaxseed mulching applications on oxyfluorfen herbicide in marketable onion yield. So, the farmers are advised to apply soil mulching of soybean and flaxseed wastes as a safe and eco-friendly approaches in onion field under all experimental conditions.

5.1 Future Works

Applying of soybean and flaxseed wastes as natural approaches to control weeds with different crops under various field conditions and varying range of weather conditions. Study the residual effect of these mulching treatments on soil micronutrients, microorganisms and following crops.