Comparative Analysis of Soybean (Glycine max (L.) Merrill) Varieties: Implications for Yield and Quality

Studies on yield and quality of soybean varieties of different origin were conducted in 2022 on the fields of S. Seifullin Kazakh Agro Technical Research University located in Northern Kazakhstan. The region is characterised by a sharply continental climate with arid conditions and average heat levels. The average annual rainfall is 240-330 mm, the average annual sum of positive temperatures is about 1900-2000℃, and the length of soybean growing season is about 89 to 101 days. Temperature conditions during the period under study had insignificant differences, while moisture content differed significantly from the long-term average, as conditions were very dry. The hydrothermal coefficient differed insignificantly by year (Table 1).

Table 1. Hydrothermal coefficient during the growing season of soybean

Source: compiled by the authors of this study.

Soils of the experimental plot were represented by southern carbonate chernozem with neutral or slightly alkaline reaction, containing 56.5% of physical clay and 43.5% of physical sand in the arable layer. The humus content was about 4.5-5%, nitrogen content – 28-30%, phosphorus content – 0.13-0.14%, and potassium content – 2.1-2.2%. Soil quality index was 65. The relief was flat, not characterised by forest vegetation [8]. More than 130 soybean varieties and lines of different origins were studied. The standard soybean variety approved for use in the area, namely Ivushka, was used for comparison and evaluation of these varieties. The other varieties were arranged in the list at 10-number intervals relative to this standard. Sowing of the collection nursery was done in optimal terms for the zone according to the methodology of implementation of innovative solutions (IIS).

Soybean grain yield was estimated by harvesting at full ripeness from each variant separately. After threshing, seeds were weighed, and yields were recalculated in kg/ha. The Kjedahl method was applied to carry out biochemical analysis of soybean seeds including protein and fat content.

The Kjeldahl method is a widely used chemical technique for determining the nitrogen content in organic and inorganic substances, particularly in samples like soil, plant material, and food products. Named after its inventor, Johan Kjeldahl, this method involves several key steps. The sample is digested by heating it in the presence of concentrated sulfuric acid. During this process, organic nitrogen compounds are converted into ammonium sulfate (NH₄)₂SO₄. After digestion, the acidic solution is neutralized with sodium hydroxide (NaOH) to convert the ammonium sulfate to ammonia (NH₃). The liberated ammonia is distilled from the neutralized solution using steam distillation. It is then collected in a receiving flask containing a known amount of boric acid (H₃BO₃) solution. The ammonia forms ammonium borate. The collected ammonium borate solution is titrated with a standard solution of sulfuric acid (H₂SO₄). The acid reacts with the ammonium ions (NH₄⁺) to form ammonium sulfate, while the boric acid remains unchanged. By measuring the volume and concentration of the titrant (sulfuric acid) used in the titration, the nitrogen content in the original sample can be determined.

The study carefully selected over 130 soybean varieties of diverse origins to ensure a comprehensive evaluation. A standard regional variety, Ivushka, was chosen as a reference point, with other varieties systematically listed at 10-number intervals relative to Ivushka. This approach aimed to capture a wide range of genetic backgrounds and traits, providing a robust dataset for analysis.

Cultivation and experimentation took place at the fields of S. Seifullin Kazakh Agro Technical Research University in Northern Kazakhstan, a region characterized by a sharply continental climate with arid conditions. This choice aligned with the study's goals of assessing soybean performance in challenging environments. While specific latitude and longitude coordinates were not provided, the description of Northern Kazakhstan's climate and soil characteristics offered sufficient context.

Regarding replication, the methodology does not specify the number of replications for each variety. Soybean grain yield was estimated individually for each variant, implying a potential use of single replications. However, the study's primary focus appears to be the assessment of a wide variety of soybean types rather than statistical replication.

Using the mean standardised values of these biochemical parameters, cluster analysis was performed using Ward’s method. In addition, during the study, correlations between the main elements of yield structure and yield of soybean varieties were established. The results were processed for reliability using multivariate analysis of variance MANOVA using Microsoft Excel software and Statistical 10 software package. Differences in the obtained results are possible at the significance level of P≤0.05 by Student’s t-test.

Current research confirms the importance of increasing soybean production and is being conducted to investigate various aspects of soybean yield and quality. This is important for several reasons [9-11]:

1. Food safety. Increasing soybean production plays a vital role in ensuring food security as soybean is one of the major sources of vegetable protein and vegetable fat in the world. Increased soybean yields provide access to food resources for the population, especially in countries where protein deficiency is an issue.
2. Economy support. Soybean is a major crop that creates economic prosperity in many parts of the world. Increased soybean yields contribute to agricultural development, create jobs, increase the incomes of agricultural producers, and support economic growth.
3. The importance of high-protein crops. Soybean is one of the major high-protein crops that play an essential role in balancing the diet of the population. The protein found in soybean has high nutritional value and is an important source of amino acids for humans. Increased soybean production reduces dietary protein deficiencies and improves nutrition.
4. Improving environmental sustainability. Soybean yield and quality research is also aimed at developing varieties with increased resistance to diseases, pests, and agro-climatic conditions. This helps reduce the use of pesticides and fertilisers and reduce the adverse environmental impact of agriculture.

The weak and negative relationships between yield and protein content, as well as yield and fat content, have important implications for soybean cultivation and its impact on food security and quality. While this study does not delve deeply into the discussion of these relationships, we can explore potential implications and explanations. A negative relationship between yield and protein content suggests that as soybean yields increase, the protein content may decrease [12-14]. This has implications for the nutritional quality of soybean-based products. Lower protein content could impact the suitability of soybeans for use in protein-rich foods and animal feed. This may not align with the goal of improving food security, especially in regions where soybeans are a significant source of protein [15].

Similarly, a negative correlation between yield and fat content implies that as yields increase, the fat content may decrease. The fatty acid composition of soybeans is essential for various applications, including cooking oils and biodiesel production. A reduction in fat content could affect the suitability of soybeans for these purposes. The negative relationships may be attributed to crop management practices. High-yielding varieties may require different agronomic practices that prioritize yield over nutritional quality. For example, increased plant density and nitrogen fertilization may boost yields but potentially dilute protein and fat concentrations. Understanding these trade-offs is crucial for farmers and policymakers when selecting soybean varieties and implementing farming practices.

In summary, the weak and negative relationships between yield and protein/fat content in soybeans have complex implications for food security, nutritional quality, and agricultural practices. Further research and a nuanced approach to crop management and breeding are necessary to address these challenges effectively.

Limitations related to heat, photoperiodic response, and weather conditions can be significant factors affecting soybean production. Kazakhstan has a sharply continental climate characterised by dry and hot summers, cold winters and significant temperature fluctuations throughout the year. Late spring and early autumn frosts can be dangerous for many crops, including soybeans. Under conditions of long daylight hours and insufficient sum of active temperatures during the growing season, it is important to choose soybean varieties with weak photoperiodic sensitivity. Such varieties can adapt to conditions with long daylight hours and start flowering relatively early, allowing seed formation in a shorter growing season [16].

According to Ghani et al. [17], yield and quality studies on soybean varieties of different origins help expand the genetic resource of the crop and ensure adaptation to changing conditions. This is particularly important considering climate change, which requires crops that can withstand extreme temperatures, drought, and other stressful conditions. The development and introduction of new soybean varieties has already led to several results. For instance, soybean varieties have been developed that have increased resistance to drought, pests, and diseases, thereby increasing yields and reducing losses. Some new varieties also show improved quality characteristics such as higher protein and fat content in the seeds. This allows farmers to grow soybean in a wide range of regions, thereby promoting diversity of production and increasing its sustainability [18]. Years of trials and progeny evaluation help select the best soybean varieties with high yields and desirable traits. This is a crucial step in the breeding process that allows discarding less successful variants and focus on developing the most promising varieties.

Adetiloye et al. [19] believe that cluster analysis method is an important tool in the study of yield and quality of soybean varieties of different origins, allowing systematisation of data, identification of patterns and drawing conclusions regarding different groups of varieties. Furthermore, the method of cluster analysis for grouping soybean varieties based on their yield and quality characteristics, as well as determining the correlation relationship between the main elements of yield structure and yield of soybean varieties has been widely applied by different researchers. Thus, in a study by Li et al. [20], in which 40 soybean varieties were clustered based on their yield and quality characteristics such as protein, fat content, and other components, several clusters were identified to classify soybean varieties according to their performance and quality.

A study by Mahmoud et al. [21] also applied cluster analysis method to investigate 25 soybean varieties based on their yield and phytochemical parameters such as isoflavones content and antioxidant activity. Cluster analysis allowed the identification of groups of varieties with comparable characteristics, which helped assess differences in yield and quality among them. According to the study by Matsuo et al. [22], in which data on yield and quality characteristics of 20 soybean varieties were analysed, positive correlations between yield and such elements of yield structure as number of beans per plant and seed weight per plant were found. Furthermore, a negative correlation relationship was found between yield and seed fat content, which also supports the findings of the current study.

Further support for the study can be found in a study by Mofokeng and Mashingaidze [23], where yield data of 30 soybean varieties were analysed. Positive correlations between yield and such elements of yield structure as number of lateral branches, plant height, and number of beans per plant were found. A positive correlation between yield and thousand-kernel-weight was also found. A plethora of studies conducted on the development of high-yielding soybean varieties with increased seed protein content have revealed some inverse correlation between yield and protein content [24-26]. Thus, in the study of Agegn et al. [5], a negative correlation relationship was observed between yield and protein content of soybean seeds (r=-0.068) and the coefficient of variation for this trait was 4.1%, which also resonates with the study conducted.

Staniak et al. [27] suggest that one plausible reason for this inverse correlation may be that when genetic characteristics of a variety are improved to increase protein content, plant growth intensity and yield may decrease slightly. This is because plants may devote more energy and resources to protein accumulation in seeds, to the detriment of other aspects of growth and development. However, Xia et al. [28] note that this inverse correlation is not always observed in all studies and varieties of soybean. In some cases, it has been possible to develop varieties that combine both high yields and increased protein content. This can be achieved through breeding and genetic improvement of varieties and optimisation of growing conditions and agronomic practices. According to Adamič and Leskovšek [1], one of the challenges for researchers and breeders is to strike a balance between yield and protein content in soybean seeds. The aim is to obtain varieties that provide high yields and at the same time have a high enough protein content to meet the needs of the food and feed industry.

Considering the number of scientific studies [29-31], as well as the results of the performed research on the yield and quality of soybean varieties of different origins, it can be stated that the yield and quality of soybean varieties can vary significantly depending on their origin and growing conditions. Studies conducted in Northern Kazakhstan allow assessing the adaptation and productivity of soybean varieties under specific agro-climatic conditions, and help identify the most suitable varieties, considering factors such as climate, soil, moisture, and other conditions

A study, like any other, has its limitations, which may affect its results and the adequacy of its conclusions. One possible limitation is that soybean varieties were sampled from only a limited number of sources. This may lead to the fact that the results do not fully reflect the diversity of soybean varieties and their characteristics. For more representative conclusions, it would be useful to expand the sample to include more varieties from different sources and regions.

Since the study was conducted in a specific region, changes in climatic conditions may affect the results. Changes in temperature, humidity, and other factors can cause changes in plant growth and development, which in turn can affect the protein and fat content of soybean seeds. The results of the study may also be limited by the agronomic practices used during soybean cultivation. Different approaches to tillage, fertilization, and irrigation can affect soybean yield and quality. Optimizing these practices can improve results.

Data quality and measurement accuracy can also affect results. If data sources were not reliable or measurements were made with errors, this can lead to inaccurate results. Differences in the genetic diversity of soybean varieties can also be a factor affecting results. Different varieties may have different genetic potential for growing proteins and fats, and this can be an important factor. Limitations related to environmental conditions and economic circumstances can also affect the results of a study. For example, economic instability or changes in market conditions could affect soybean production.

In summary, the weak and negative relationship between yield and protein/fat content of soybeans has complex implications for food security, nutrition, and agriculture. To effectively address these issues, more research and a nuanced approach to plant cultivation and breeding are needed.