Sunflower (Heliánthus ánnuus) Yield and Yield Components for Various Agricultural Practices (Sowing Date, Seeding Rate, Fertilization) for Steppe and Dry Steppe Growing Conditions

Abstract

The dynamics of the productivity of the sunflower hybrid “Baiterek 17” were studied for rainfed conditions of the steppe and dry steppe zones of Kazakhstan for two fertilizer levels (without fertilizers and with the application of a nitrogen-phosphorous fertilizer), two sowing dates, and three seeding rates. The growing season duration varied among zones and was affected by sowing dates. An increase in duration (3–5 days) was observed for the early sowing date and fertilized treatments, regardless of the zone. Increasing the seeding rate for all treatments by sowing date and fertilizer application reduced the duration of the growing season by 3–6 days. The fertilizer application did not affect the formation of seedlings in the study areas. In the dry steppe zone, from 2.5 to 4.8 plants/m² were formed before harvesting, with an increase in the number of plants at the high seeding rate (57,000 seeds/ha). In the steppe zone, the same pattern was preserved: from 3.5 to 4.9 plants/m² at a seeding rate of 65 thousand seeds/ha. The maximum diameter and weight of the flower head were found for the early sowing date and fertilized treatments at a low seeding rate, with a strong effect on the yield for both the steppe and dry steppe zones in 2022. On average, for both years, the highest yield for the dry steppe zone was obtained for the sowing date of 15 May and at a seeding rate of 57,000 seeds/ha, while for the steppe zone, the highest yield was obtained for the 10 May planting date and at a sowing rate of 65,000 seeds/ha. Improving plant nutrition and increasing the plant density up to a seeding rate of 65,000 seeds/ha for the early sowing dates (10 May) increased the plasticity in the steppe zone. For the dry steppe zone, the plasticity of the hybrid decreased, but the highest plasticity was also obtained for an earlier sowing date (15 May) and at a seeding rate of 57,000 seeds/ha crop. The study shows that the hybrid “Baiterek 17” has a high ecological plasticity under changing environmental conditions and, with an increase in moisture availability, it requires intensive agricultural practices (fertilization, increased seeding rate, and early sowing dates) to obtain a high yield.

1. Introduction

Sunflower is the main oil crop in Kazakhstan and, in 2022, its total production acreage increased to 1 million ha out of 3,093,000 ha for all oil crops. In 2022, 1.2 million tons of achenes were harvested, which is two times higher than that in 2021. The regions of Pavlodar and Kostanay in east Kazakhstan remain the leaders in sunflower cultivation with up to 60% of the total production. At the same time, the processed achenes for oil are also increasing and Kazakhstan exports parts of processed products from sunflower to foreign markets.

One of the main topics in increasing sunflower productivity, along with the development of improved varieties and hybrids with economically valuable traits, is cultivation technology. Key elements of this technology are optimization for crop nutrition, seeding rate, and sowing dates.

During the growing season, sunflower removes a significant amount of nitrogen and phosphorus and a large amount of potassium from the soil. For the formation of one ton of achenes, sunflower requires about 50–60 kg of nitrogen, 20–25 kg of phosphorus, and 120–160 kg of potassium [1]. During the active growing season, which is from the formation of the flower head to flowering, sunflower needs a sufficient amount of nutrients. During the flowering period, the plants have already extracted 60% of nitrogen, 80% of phosphorus, and 90% of potassium of the total amount required for the growing season. Sunflower is especially sensitive to phosphorus deficiency during reproductive organ formation [1,2].

Among the various nutrients, nitrogen is one of the main nutrients that enhances protein-based metabolic processes, leading to an increase in vegetative and reproductive growth in sunflowers [3]. Since nitrogen is the most limiting nutrient, plants must obtain it from the soil in significant amounts. Nitrogen is readily absorbed by plants in the form of nitrate (NO₃), urea (CO(NH₂)₂), and ammonium (NH4+). It was demonstrated that the sources and doses of nitrogen significantly affected the yield and agronomic characteristics of sunflower [4]. Several other studies have also shown that the use of ammonium nitrate increases the height of plants, the diameter of the flower head, 1000 achene weight, and yield [5,6].

In general, nitrogen and phosphorus fertilizers complement each other in terms of their effect on seed growth, development, and maturity. According to the Grain Farming Research and Production Center after Barayev (Kazakhstan), the optimal dose of fertilizer for the steppe and dry steppe zones of northern Kazakhstan is N30-40P60-90. Depending on the production environment, various studies have provided recommendations on fertilizer application rates that vary widely, ranging from a low dose of N30P30K30 [7], a double dose of N60P60K60, and even higher doses of N130P108K110 [8]. It was also found that the sunflower response to the fertilizer rate (N80P80K80) was tillage dependent and resulted in an increase in yield increase of 10.6–18.7% compared to the control [9]. The fertilizer application effectiveness is also determined by the individual response of the sunflower hybrid. For example, for the variety SPK, the maximum yield was obtained at N30P40-60, but for the early maturity variety Zhaina, this was obtained at N30P20-40 [10]. An assessment of the response to fertilizer rates should be conducted in accordance with the soil and climatic conditions of the production region and the characteristics of sunflower varieties and hybrids [11]. Field trials were conducted to quantify the comparative performances of various sunflower hybrids (Hysun33, Hysun38, and Pioneer-64A93), as influenced by various levels of nitrogen (N) fertilizer (0, 60, 120, 180, and 240 kg N/ha) under different agro-environments, including arid and semi-arid conditions that are similar to dry steppe and steppe zones. The results of the study demonstrated that the productivity of sunflower hybrids varied greatly in response to N fertilization and different environmental conditions [12].

Similarly, the timing of sowing and the seeding rate of sunflower can be varied as well. Pinkovskyi and Tanchyk [13] conducted an experiment to determine the impact of sowing dates and standing density on the growth, development, and productivity of middle–early maturity group sunflower hybrids in the steppe zone. It was established that regulating the timing of sowing and selecting the optimal density of standing plants could affect the growth and development of sunflower plants, bypassing critical periods during cultivation. An experiment conducted with two sunflower hybrids (Vidok and Euroflora) on two sowing dates and with three seeding rates showed that the varieties significantly differed in yield depending on the sowing date and seeding rate [14]. Another experiment aimed to study the effect of sowing dates (5, 20 May and 5 June), and various amounts of nitrogen (control, 15, 20, and 25 kg active element per ha) on sunflower achene growth and final yield. The results showed that the sowing time had a significant effect on the stem diameter, the diameter and the number of achenes per flower head, the 1000 achene weight, and biological yield [15,16]. In a study on the effect of seeding rates (40,000, 50,000, 60,000, and 70,000 seeds/ha) on sunflower growth, development, and yield, the highest yield of 4350 kg/ha was found at a seeding rate of 50,000 seeds/ha under irrigation and 4090 kg/ha, depending on varietal characteristics [17].

For different climatic regions, the optimal planting time for sunflower varies greatly, ranging from August [18] to November [19]. Sowing when the soil temperature at a depth of 10 cm is about 16–18 °C sharply reduces the sunflower yield [18,19]. At that time, the topsoil layer is usually dried up, which prevents the emergence of seedlings, because some of the seeds fall into dry soil and emerge only after the occurrence of rainfall. For these conditions, the timing of seedling emergence is extended, which leads to further uneven plant development [20].

Various approaches have been used by researchers to determine the optimum fertilizer rate, sowing date, and seeding rate for sunflower productivity in different climatic zones and for different sunflower varieties and hybrids. Therefore, it is very important to establish a practical foundation for adaptive cultivation technologies for sunflower based on the assessment of varietal responsiveness to sowing dates, plant density, and plant nutrients. In the dry steppe and steppe zones of northern Kazakhstan, the main limiting factor for plant development is phosphorus. This element promotes the development of the root system and is responsible for the formation of reproductive organs. With an optimal amount of phosphorus, plant growth and development accelerate, moisture is used more efficiently, and more oil accumulates in the achene. This determined the overall approach of this study and defined its overall goal and objectives. The goal of this study was to identify patterns in the implementation of the genetic potential of the new hybrid “Baiterek 17”, under the influence of rainfed environmental conditions and agricultural practices, and to assess its adaptive potential in terms of yield and yield components. The specific objectives were to conduct the study in different soil and climatic zones and for different management practices (sowing dates, seeding rates, and fertilization) of the sunflower hybrid “Baiterek 17”, to determine (a) a response of the hybrid to the hydrothermal conditions of cultivation, and (b) the dynamics of the yield and yield components and overall productivity. The achievement of the study goal will allow for developing practical approaches for the realization of the sunflower productivity potential for specific production conditions.

2. Materials and Methods

2.1. Study Locations

Two field experiments were conducted in two locations in Kazakhstan (Figure 1). The first site was a dark chestnut soil of the dry steppe zone of central Kazakhstan, the Karagandy region at Limited Liability Partnership (LLP) “Naydorovskoye” (Lat 49.4°, Lon 72.41°). The second site was a chernozem soil of the steppe zone of northern Kazakhstan, the North Kazakhstan region at the North Kazakhstan Agricultural Experimental Station (NK AES) (Lat 54.17°, Lon 69.53°).

The dark chestnut soil in the dry steppe zone at the LLP “Naydorovskoye” has a thickness at the upper horizon of 20–22 cm with a humus content that ranges between 2.26 and 3.43% and is characterized by an alkaline reaction of the environment. The bulk density ranges between 1.16 g/cm³ and 1.28 g/cm³ for the top layer (0–30 cm).

The black chernozem soil in the steppe zone at NK AES is calcareous and loamy with a humus content of 4.1–6.0% and has a neutral pH throughout the soil profile. The bulk density of the ordinary chernozem soil is optimal and ranges between 1.1 g/cm³ and 1.2 g/cm³ in the top horizons.

The mobile phosphorus content in the soils for both experimental fields is very low for the top (0–15 cm) soil layer and ranges from 9.57 mg/kg to 13.0 mg/kg. For the deeper horizons, the mobile phosphorus content decreases sharply and, in some places, is absent. The total nitrogen for both soils varies from 0.10% to 0.25% in the upper horizons. The largest amount was found in the black chernozem soil at NK AES. The total nitrogen in the soils of the two fields was low compared to the zonal indicators of these soils.

2.2. Study Object

The sunflower hybrid was “Baiterek 17”, obtained from the LLP “Experimental farm of oilseeds”, Kazakhstan, based on the breeding line of Formula CB567F (maternal line) × SP1459B (paternal line). This is an early ripe hybrid with a growing season duration of 100–101 days. The hybrid has a flat flower head that is 19–20 cm in diameter and green leaves of a large size with large serrations without wings. The average yield in the East Kazakhstan region is 2650 kg/ha, in the Kostanay region is 1990 kg/ha, and in the Almaty region 2560 kg/ha when irrigated. The weight of 1000 achene is 57.2–58.6 g, with the weight of achene head ranging from 65 to 81.8 g and a plant height of 132 to 176 cm. The average oil content of the achene is 45.2%, with an achene oil yield up to 1166 kg/ha. The sunflower hybrid “Baiterek 17” is resistant to drought, lodging, and shedding, and responsive to fertilization. It is also resistant to downy mildew, broomrape, and fungal diseases. Finally, it has a high ecological plasticity [21].

2.3. Experiment Design

Two different experiments were designed to study the adaptive properties and dynamics of the yield formation of the new sunflower hybrid “Baiterek 17” in rainfed environmental conditions, depending on the fertilizer application, sowing dates, and seeding rates in two climatic zones for the 2021 and 2022 growing seasons. In the dry steppe zone at LLP “Naydorovskoye” in the Karaganda region, the experiment was conducted for two sowing dates on 15 May and 20 May. For this zone, the seeding rates for sunflower recommended by the Karaganda Experimental Station are 25–45 thousand seeds/ha with a 25% adjustment considering plant survival prior to harvest. But when studying the adaptive properties of a new hybrid, we must define the optimal limit of seeding rate for yield formation. Thus, three levels of seeding rates, 32, 41, and 57 thousand seeds/ha, were chosen for this experiment. In addition, with these options, 2 nutrition levels were considered: without fertilizer application (control) and with fertilizer application at a rate of P₉₀ recommended for phosphorus. Nutrition was provided by the application of ammonium phosphate fertilizer in the amount of 200 kg/ha at the depth of 16 cm (Chemical formula: Mixture of (NH₄H₂PO₄ + (NH₄)₂HPO₄); N-11%, P-46%) to the fallow field in the fall using a John Deere 942OR. The dose of fertilizer was calculated based on the phosphorus content in the soil (low level) and for the optimal level of the nutrient (24–25 mg/kg soil). Sowing was carried out using a Vaderstad sowing complex. The row spacing was 70 cm with a sowing depth of 5–6 cm. The experimental field is dominated by dark chestnut soils, calcareous, and solonetzic, to varying degrees.

The second experiment in the steppe zone at LLP “NK AES” in the North Kazakhstan region included two sowing dates on 10 May and 20 May. For this zone, the seeding rates for sunflower recommended by the LLP “NK AES” are 35–40 thousand seeds/ha with a 25% adjustment considering plant survival prior to harvest. When sowing highly productive hybrids, the seeding rate should be increased by 10–15%, thus reaching 45–55 thousand seeds/ha [22]. To define the optimal limit of seeding rate for yield formation, we set three levels of seeding rates, 45, 55, and 65 thousand seeds/ha. Similar to the first experiment, two fertilizer levels were considered: without fertilizer application (control) and with fertilizer application of the dose of P₉₀ recommended for phosphorus by the application of ammonium phosphate fertilizer in the amount of 200 kg/ha at the depth of 16 cm (Chemical formula: Mixture of (NH₄H₂PO₄ + (NH₄)₂HPO₄); N-11%, P-46%) to the fallow field in the fall using the machine SZS-2.1. The dose of fertilizer was calculated based on the phosphorus content in the soil (low level) and for the optimal level of the nutrient in the soil. The seeds were sown in rows at 70 cm distances and a 5 cm depth in the soil using a UPS-8 pneumatic seeder. The soils in the experimental field are represented by ordinary chernozems, while some locations in the field have ordinary calcareous chernozems. The granulometric composition of the soils is mainly medium loamy.

A similar experiment design was used in both experimental fields. Factor combinations for all levels resulted in 12 treatments per experiment and each treatment had 3 replicates. The allocation of plots under the treatments of the experiment was sequential and according to a predefined order defined during the experiment design process [23]. The size of each plot under a treatment was 56 × 60 m. The same sequential order of treatments in the field was used each year. The recommended agrotechnology for the zones of northern and central Kazakhstan was used in the experimental fields.

2.4. Meteorological Conditions

To assess the influence of weather conditions on plant growth and development, the duration (in days) of the main interphase periods of the growing season was calculated. To determine the influence of climatic factors, we used measurements of air temperature (°C) and the amount of total daily precipitation (mm). The weather data were obtained from the meteorological stations located at the experimental stations in LLP “Naydorovskoye” and LLP “NK AES”.

To determine the degree of agricultural drought in the field, we used a conditional indicator of humidity, referred to as the Hydro-thermal Coefficient (HTC) [24] and estimated using following equation:

$$HTC=\frac{10\sum p}{\sum t}$$

(1)

where $\sum p$ is the sum of daily precipitation (mm) for a period with a daily average air temperature above +10 °C and $\sum t$ determines the sum of the daily average air temperature (t > 10 °C) for the same period.

The moisture availability for crop growth was estimated on the basis of the HTC values and corresponding agricultural drought classes as follows: less than 0.3—very dry (I), from 0.3 to 0.5—dry (II), from 0.5 up to 0.7—slightly dry (III), from 0.7 to 1.0—insufficient moisture (IV), 1.0—equality of moisture inflow and outflow (V), from 1.0 to 1.5—sufficient moisture (VI), and more than 1.5—excessive moisture (VII), as per Usatov [25].

2.5. Data Collection and Analysis

Observations were conducted according to the “Methodology for conducting variety trials of agricultural plants” [26]. During phenological observations, the dates of the onset of the main development phases, such as emergence, 4–5 pairs of leaves, flower head formation, flowering, and full ripeness of the achenes, were noted. The observations for the development phases were carried out during the entire growing season on the same plants at a distance of 5 m in a row and on 4 sites along the diagonal of the plot. The onset of each phase was determined visually. The day when at least 10–15% of plants entered the phase was taken as the beginning of the phase, the full onset of the phase when at least 75% of plants, and ripeness when most plants (60–70%) were ripe.

In the phase of full germination, the field germination of seeds was determined based on the number of laboratory viable seeds sown and the plant density in the germination phase. This was conducted in two adjacent 10 m long rows and in four places of the plot.

Prior to harvesting, observations and recordings were conducted in fixed sites of each plot on indicators that determine the yield and yield components, including:

• Plant density: the number of plants per unit area (m²) was determined in two adjacent 10 m long rows and in four places. The standing density was recorded along the diagonal of the plot.
• The number of sunflower achenes from one plant was determined by the number of achenes in the flower head after the threshing of 10 flower heads.
• The weight of achenes (g) from 1 flower head as an average from 10 flower heads.
• The weight of 1000 achenes (g) at the moisture content of 12%.
• The yield was harvested in the phase of full ripeness of the achenes at a moisture content of 12% for a designated area of each plot. Plants were threshed and the achenes from the plot were weighed. To bring the yield to the standard moisture, two achene samples, 50 g each, were taken to determine their moisture content. Achene moisture was determined according to “Oil seeds. Method for humidity determination” [27]. This method is for determining the weight loss of a sample of oil seeds, expressed as a percentage, dried in an air-heated oven for a fixed set of conditions: temperature (130 °C) for a drying duration of 40 min.

2.6. Statistical Analysis

The data that were collected during the two-year multi-factor experiments for two locations were subjected to an analysis of variance (ANOVA) to determine the significance of factors and their interactions. The differences among the treatment means were estimated using the Fisher’s least significant difference (LSD) test at a 5% level of significance (α = 0.05), as well as the relative experimental error (m%)—a statistical measure calculated based on experimental data. In addition, a correlation analysis was conducted on the yield and yield component relationships, and average values and standard errors (SE) were calculated [28].

2.7. The Ecological Plasticity Indicator

Ecological plasticity is understood as the degree of adaptability of a variety or hybrid to environmental conditions, expressed in producing a high and high-quality yield under a range of soil and weather conditions [29]. For the practical use of quantitative estimates of plasticity, Eberhart and Russell [30] introduced the linear regression coefficient (b_i) [31].

For our calculations, we used data on the average yield Y_ij by treatments i and by years j. Each treatment is a combination of the fertilizer application rate, sowing date, and seeding rate. The average yield for the experiment (Y) was determined as follows:

$$Y=\frac{1}{vn}\sum _{j=1}^n\sum _{i=1}^v{Y}_{ij}$$

(2)

where v is the number of treatments and n is the number of years.

For the calculation of the linear regression coefficient (b_i) or ecological plasticity, first, the indices of environmental conditions (I_j) were estimated:

$$I_j=\frac{1}{v}\sum _{i=1}^v{Y}_{ij}-\frac{1}{vn}\sum _{j=1}^n\sum _{i=1}^v{Y}_{ij}$$

(3)

where the first term on the right side of the equation is the average yield for all treatments for a given year and the second term is the average yield for all treatments and for all years. This index reflects how favorable the growing conditions are for each year.

Finally, b_i, which characterizes the ecological plasticity, was calculated for each treatment:

$$b_i=\sum _{j=1}^n{Y}_{ij}{I}_j/\sum _{j=1}^n{I}_j^2$$

(4)

where the nominator is the sum of the products of the yield for a given treatment for a given year by the corresponding value of the environmental condition index, and the denominator is the sum of the squared indices of the environmental conditions.

The ecological plasticity indicator b_i shows how the hybrid used in the treatments reacts to changes in the growing conditions. If b_i > 1, then the hybrid is more responsive to changes in conditions. In this case, changing the indicator allows for increasing the yield. When b_i < 1, the hybrid reacts less to changes in the conditions of the treatment. When b_i =1, the hybrid optimally matches the conditions of the treatment and the environment. When b_i = 0, the changes in conditions do not affect the yield [29].

3. Results and Discussion

3.1. Assessment of Meteorological Conditions

During the past two decades, an instability in oil crop yield has been observed in Kazakhstan. Although sunflower is a crop that is better adapted to water stress than other crops, the main factor that affects the yield of sunflower under rainfed conditions is still irregular and insufficient rainfall during the growing season. An analysis of sunflower productivity shows that meteorological conditions, i.e., rainfall and the sum of the active temperatures during the growing season, have a significant effect on the final yield [32,33].

According to the agrometeorological data from the experimental fields, the parameters of the of environmental conditions during growing seasons were as follows:

LLP “Naydorovskoye” is in the dry steppe zone with a total precipitation less than 145 mm during April–October. The effective temperature regime of the growing season (the sum of daily average temperatures above 10 °C) is 2250 °Cd. The HTC values for the sunflower growing season ranged from 0.6 to 0.8, which represents slightly dry to insufficient moisture [34].

LLP “NK AES” is located in the steppe zone with a total precipitation ranging from 260 to 280 mm for the period of April–October. The sum of effective temperatures of the growing season is 2386 °Cd. The HTC value was 0.93, which represents insufficient moisture.

The dynamics of the HTC values in the dry steppe zone over the two study years were characterized by sharp jumps during the growing season (Figure 2). An analysis of the HTC values for the sunflower growing season in the dry steppe zone shows that the 2021 growing season was characterized by favorable moisture in June (slightly dry) and the second half of September (insufficient moisture), which corresponded to the germination and ripening phases (

Table 1. The Hydro-thermal Coefficient (HTC) values and corresponding agricultural drought classes for the 2021 and 2022 growing seasons in two study zones.

		HTC
		Dry Steppe		Steppe
Month	Decade	2021	2022	2021	2022
May	II	0.08 (I)	0.33 (I)	0.3 (II)	0.17 (I)
	III	0.02 (I)	0.77 (IV)	0.36 (II)	0.17 (I)
June	I	0.59 (III)	0.21 (I)	0.34 (II)	0.07 (I)
	II	0.26 (I)	0.71 (IV)	0.54 (III)	1.3 (VI)
	III	0.73 (IV)	0.21 (I(I)	0.06 (I)	1.4 (VI)
July	I	0.01 (I)	1.29 (VI)	0 (I)	1.3 (VI)
	II	0.24 (I)	0.23 (I)	1.75 (VII)	0.71 (IV)
	III	0.03 (I)	0.66 (III)	0.15 (I)	1.8 (VII)
August	I	0.09 (I)	0.81 (IV)	0.61 (III)	0.33 (II)
	II	0.08 (I)	0.26 (I)	0.05 (I)	1.41 (VI)
	III	0.13 (I)	0 (I)	0.53 (III)	1.96 (VII)
September	I	0 (I)	0.03 (I)	0.44 (II)	0.13 (I)
	II	0.78 (IV)	0.3 (II)	0.84 (IV)	0.17 (I)
	III	0.42 (II)	0.12 (I)	0.32 (II)	0.92 (IV)
For vegetation period		0.25 (I)	0.43 (II)	0.46 (II)	0.85 (IV)

Note: In brackets the agricultural drought classes are given.

). The critical growth period of “budding-flowering” was very dry (HTC < 0.2). The best year in terms of soil moisture level was 2022 during the first half of the growing season and for the period of “budding-flowering” with a HTC = 1.2–0.8, which corresponds to sufficient moisture for the development of the reproductive part of the crop. The average soil moisture level for the 2021 growing season was 0.25, i.e., very dry, and for 2022, it was 0.43, i.e., dry (Table 1).

In the steppe zone, the dynamics of changes in the HTC were also characterized by significant unevenness during the growing season (Figure 2). In 2021, on average, the growing season was dry (HTC = 0.46), while, in 2022, there was insufficient moisture (HTC = 0.85) (Table 1). The critical growth periods “budding-flowering” in 2021 occurred under dry conditions (HTC = 0.5) with a significant accumulation of moisture in the second part of July during the formation of the flower head (HTC = 1.8), which favorably affected the overall productivity of the crop. In 2022, the moisture level increased from the first phases of growth to maturation (from 0.65 to 1.0), which resulted in an increase in yield (Table 1). An analysis of the HTC values for the two experimental sites showed a higher level in soil moisture for the experimental site in the steppe zone.

3.2. Effective Temperature, Precipitation, and Phenology of Sunflower

According to Shpaar et al. [35], sunflower has high demands for both temperature and moisture. The optimal temperature for germination ranges between 12 and 15 °C [32], with the sum of the active temperature for the period from sowing to germination ranging from 140 to 160 °Cd. For early maturity varieties and hybrids, a sum of effective temperatures from 1600 to 1800 °Cd is required for ripening, while, for mid-maturity varieties, this is around 2000 °Cd [36].

An analysis of the temperature conditions during the growth and development of the sunflower hybrid “Baiterek 17” shows that there were no significant differences during the first growing season among the treatments, including fertilizer application and sowing dates (

Table 2. The effective temperature sum (°Cd) for the development phases of the sunflower “Baiterek 17” under different fertilizer application rates and sowing dates for the dry steppe and steppe zones.

Fertilizer Application	Sowing Date	Sowing -Emergence	Emergence–Budding	Budding–Flowering	Flowering–Full Ripeness	Emergence–Full Ripeness	Sowing -Emergence	Emergence–Budding	Budding–Flowering	Flowering–Full Ripeness	Emergence–Full Ripeness
The dry steppe zone
		2021					2022
P0 *	15 May	282	662	555	908	2124	132	844	343	966	2124
P90 **		282	662	555	905	2132	132	864	339	963	2165
P0	20 May	283	750	668	820	2239	126	922	344	911	2177
P90		283	750	668	830	2248	126	922	344	910	2186
The steppe zone
		2021					2022
P0	10 May	302	602	718	847	2167	198	657	541	1111	2309
P90		302	489	801	882	2172	198	657	558	1124	2339
P0	20 May	295	645	687	840	2172	227	560	642	942	2181
P90		295	622	656	885	2153	227	597	642	966	2204

Note: *—without fertilizer application, **—application of ammonium phosphate (90 kg/ha phosphate content).

). There was a faster emergence in 2022, with a decrease in the temperature sum for the sowing-emergence period from 132 to 126 °Cd in the dry steppe zone from 10 May to 20 May. The critical growth period for “budding-ripening” in the dry steppe zone took place at a temperature sum from 1360 to 1390 °C for the early planting date and from 1380 to 1423 °Cd for the late planting date.

In the steppe zone in 2022, the temperature sum for the first period was 198 °Cd for the 10 May sowing date and 227 °Cd for the 20 May sowing date. For the period of “budding-flowering”, a higher temperature sum from 1400 to 1600 °Cd was recorded for the early sowing dates and was somewhat similar for the late sowing date. There was a slight decrease in this indicator for the fertilized treatments. In general, the temperature sum to reach full maturity (emergence-full ripeness period) in the dry steppe zone was 2120–2160 °Cd and in the steppe zone was 2160–2340 °Cd. A longer growing season and, consequently, a higher total temperature for the growing season in the steppe zone is associated with an increase in the soil moisture for that region area and, thus, longer activity of the leaves with respect to photosynthesis and the production of carbohydrates.

With respect to precipitation, there was a significant difference between the steppe and dry steppe zones for both the 2021 and 2022 growing seasons. However, agricultural practices such as fertilizer application and sowing dates did not show a significant difference for the total precipitation (

Table 3. Total precipitation (mm) the during the different growth and development phases of the sunflower “Baiterek 17” under different fertilizer application rates and the sowing dates for the dry steppe and steppe zones.

Fertilizer Application	Sowing Date	Sowing–Emergence	Emergence–Budding	Budding–Flowering	Flowering–Full Ripeness	Emergence–Full Ripeness	Sowing Emergence	Emergence–Budding	Budding–Flowering	Flowering–Full Ripeness	Emergence–Full Ripeness
The dry steppe zone
		2021					2022
P0 *	15 May	0.8	23.4	9.8	33.2	67.2	3.6	21.7	11	30.2	66.5
P90 **		0	23.4	9.8	46.8	80	3.6	21.7	11	30.7	67.0
P0	20 May	0.5	22.4	9.8	44.3	77	2.3	52.6	18	21.1	94
P90		0.5	22.4	9.8	44.3	77	2.3	52.6	18	21.1	94
The steppe zone
		2021					2022
P0	10 May	2	24.7	66.9	44.9	138.5	4.9	29.8	41.3	83.8	159.8
P90		2	24.7	67.3	44.9	138.9	4.9	29.8	41.3	84.8	160.8
P0	20 May	2.4	22.6	74.0	45.3	144.3	3.3	52.7	67.5	49.4	172.9
P90		2.4	22.6	68.7	44.3	138	3.3	52.7	67.5	49.4	172.9

Note: *—without fertilizer application, **—application of ammonium phosphate (90 kg/ha phosphate content).

).

The “Baiterek 17” hybrid belongs to the early maturing group of sunflower varieties. However, the duration of the growing season is affected by both weather conditions and crop management. Depending on the weather conditions during the growing season, it took 106–112 days on average for the growth and development of this hybrid over 2 years in the dry steppe zone of Kazakhstan (

Table 4. Duration of the growth phases (days) of the sunflower “Baiterek 17” under different fertilizer application, sowing dates, and seeding rates for the dry steppe and steppe zones.

Treatment			Sowing–Emergence	Emergence–Budding	Budding–Flowering	Flowering–Full Ripeness	Emergence–Full Ripeness	Sowing–Emergence	Emergence–Budding	Budding–Flowering	Flowering–Full Ripeness	Emergence–Full Ripeness	Two years Average
Fertilizer Application	Sowing Date	Seeding Rate Thousand/ha
			2021					2022
The dry steppe zone
P0 *	15 May	32	14	39	24	60	123	8	18	17	60	95	109
		41	14	39	24	62	125	8	18	17	58	93	109
		57	14	39	24	58	121	8	18	17	55	90	106
P90 **		32	14	40	24	65	129	8	19	18	60	97	113
		41	14	40	24	64	128	8	19	17	60	96	112
		57	14	39	24	62	125	8	18	18	60	96	111
P0	20 May	32	13	36	28	56	120	7	17	17	60	94	107
		41	13	36	28	58	122	7	17	17	59	93	108
		57	13	36	28	54	118	7	17	17	57	91	105
P90		32	13	36	28	59	123	7	17	17	61	95	109
		41	13	36	28	61	125	7	17	17	60	94	110
		57	13	36	28	57	121	7	17	17	59	93	107
The steppe zone
P0	10 May	45	15	32	35	58	125	14	33	27	74	134	130
		55	15	32	33	57	122	14	33	27	72	132	127
		65	15	32	32	57	121	14	31	27	73	131	126
P90		45	15	34	38	63	135	14	33	29	76	138	137
		55	15	32	34	63	129	14	33	27	75	135	132
		65	15	32	33	60	125	14	31	27	75	133	129
P0	20 May	45	13	32	37	46	115	11	34	28	72	134	125
		55	13	32	33	45	110	11	34	28	72	134	122
		65	13	32	31	44	107	11	31	34	72	137	122
P90		45	13	34	34	59	127	11	34	31	71	136	132
		55	13	32	34	59	125	11	34	31	71	136	131
		65	13	32	31	56	119	11	31	31	71	133	126

Note: *—without fertilizer application, **—application of ammonium phosphate (90 kg/ha phosphate content).

). A slight reduction in the duration of the growing season (by 1–3 days) was noted with an increase in plant density, as well as with the change from early to late sowing dates. In the steppe zone of Kazakhstan, the duration of the growing season ranged from 122 to 137 days, including 126–137 days for the early sowing date and 122–132 days for late sowing date, and with an increase in duration for the fertilized treatments (Table 4). Consequently, an increase in the soil moisture content resulted in an increase in the duration of the growing season by 15–25 days. Our results on the increase in the duration of the growing season with increasing levels of moisture and nutrition are confirmed by Nasieva et al.’s [37] study, in that the duration of the interphase periods was largely determined by the temperature regime and moisture supply. With a decrease in temperature and humidity, the duration of the growing season is reduced, especially during the period of sowing—emergence and emergence—the flower head formation. Fertilization (N45P30K45) lengthens the growing season by from 5 to 10 days [38].

3.3. Agrotechnology and Plant Density

The plant density should ensure the highest possible yield per unit area for specific soil and weather conditions [35]. A high-density crop under specific conditions consumes a large amount of soil moisture and nutrients for the formation of the plant vegetative mass, potentially resulting in a limited supply of soil moisture and nutrients during achene filling. On the other hand, if the planting density is too low, the crop does not fully use the soil moisture and nutrition for yield formation. Additionally, the risk of weed appearance increases. Therefore, the optimum plant density varies depending on the soil and weather conditions. In western Europe, the recommended density of sunflower plants is from 50,000 to 60,000 plants/ha, while for more arid regions of eastern Europe, this number is less at from 35,000 to 40,000 plants/ha. With an increase in moisture for chernozem soils, the optimum density is from 45,000 to 60,000 at a germination rate of 80% [39].

Obtaining the optimal number of seedlings and the formation of the necessary stem density is the main task of emerging agricultural technologies for adapting varieties and hybrids to the environment. According to the recommendations of the A.I. Baraev Scientific and Production Center for Grain Farming for regions of northern Kazakhstan with insufficient moisture, the density of sunflower plants should be from 25,000 to 40,000 plants/ha when the soil is wet up to 1 m. When sowing, the seeding rate should be increased by 25% with adjustments to poor germination and damage by soil pests and diseases [40].

Under the conditions of 2021–2022 in the dry steppe zone, at a seeding rate from 32,000 to 57,000 seeds/ha, the germination rate was 89.9–91.9% for the 15 May sowing date and 86.2–90.6% for the 20 May sowing date. Thus, the seeding rates did not have any effect on emergence.

In the steppe zone, the germination of plants for the early sowing dates ranged from 86.6 to 90.5% and decreased with an increase in the seeding rate. Fertilizer application did not have a significant effect on this indicator. For the 20 May sowing date, germination was 86.6–88.7%, with a slight decrease for an increase in the seeding rate. For the steppe zone, considering the 10 May planting date, there were 4.1–5.7 plants/m² for the 45,000 to 65,000 seeds/ha and 3.9–5.6 plants/m² for the 20 May planting date (Figure 3). This represented a germination rate of 87–90% for the 10 May planting date and 86–89% for the 20 May planting date.

The plant survival to harvest in the dry steppe zone was at the level of 87–92.2% for the 15 May sowing date with an increase of 3–4% for the fertilized treatments. Within this sowing period, there was a tendency for a reduction in plant survival to harvest with an increase in plant density of 2–3%. Similar patterns were observed for the 20 May planting date.

Thus, in the dry steppe zone, sunflower had a high germination for the early sowing of 15 May. For the steppe zone, there were no differences in seedling density for the different sowing dates. Plant survival remained at a high level in the dry steppe zone at 87–92%, compared to the steppe zone at 85–90%. An increase in soil moisture during the growing season resulted in an increase of 7–13% in the number of plants during the two years of study, resulting in a more intensive development of morphological and physiological parameters such as plant height and the diameter of the flower head, etc.

Our study in the dry steppe zone confirms that 30 thousand plants/ha with an even distribution over the area is the lowest limit for the sunflower growing efficiency [41]. Vasiliev [42] states that the optimal plant density of sunflowers in steppe zones is 45–50 thousand plants/ha. Our study in the steppe zone shows a slightly higher value of 55 thousand plants/ha.

The statistical processing of the relationship between plant density and yield showed that, in the dry steppe zone, the indicator had a moderate positive correlation with yield (r = 0.46 in 2021 and r = 0.59 in 2022). In the steppe zone, this relationship was stronger; in 2021, plant density had a high correlation with yield (r = 0.66) and, in 2022, this correlation increased (r = 0.76).

3.4. Agrotechnology and Yield Structure

Among the yield components that determine the plant productivity of sunflower crop, the size of the flower head, achene filling, and individual achene weight play important roles. Studies have shown that various factors such as environmental conditions and crop management have a significant impact on the yield and yield components of different sunflower hybrids [33,43]. For instance, for conditions in southern Romania, an increase in the number of plants from 50,000 to 60,000 and further to 70,000 plants/ha led to a decrease in the total number of flower heads. At the same time, under favorable growing conditions, an increase in plant density increased the yield and, under less favorable growing conditions, reduced the yield [43].

In this study, for the conditions in the dry steppe zone, the size of the flower head varied between 11.7 and 13.3 cm for the early and between 10.5 and 12.6 cm for the late planting date for the P₀ treatments (

Table 5. The sunflower “Baiterek 17” yield and yield components for various fertilizer application rates, sowing dates, and seeding rates for the dry steppe and steppe zones.

Treatment			Flower Head Diameter, cm	Number Of Achenes Per Flower Head	Weight of Achenes Per Flower Head, g
Fertilizer Application	Sowing Date	Seeding Rate, Thousand/ha
The dry steppe zone
P0 *	15 May	32	12.2 ± 0.5	454 ± 39	24 ± 2
		41	12.8 ± 0.7	451 ± 43	22 ± 2
		57	12.9 ± 0.4	475 ± 49	21 ± 2
P90 **		32	14.6 ± 0.5	536 ± 55	29 ± 3
		41	14.9 ± 0.8	506 ± 53	26 ± 2
		57	14.1 ± 0.4	472 ± 43	23 ± 2
P0	20 May	32	11.9 ± 0.7	392 ± 33	19 ± 1
		41	12.2 ± 0.4	381 ± 38	16 ± 2
		57	11.2 ± 0.7	363 ± 34	15 ± 1
P90		32	14.7 ± 1.0	525 ± 85	24 ± 3
		41	14.6 ± 0.8	565 ± 104	19 ± 2
		57	13.2 ± 0.9	506 ± 92	15 ± 1
The steppe zone
P0	10 May	45	14.1 ± 0.6	1029 ± 20	67 ± 1
		55	14.9 ± 0.6	995 ± 21	61 ± 1
		65	14.5 ± 1.0	943 ± 34	58 ± 1
P90		45	17.0 ± 0.5	1225 ± 34	81 ± 3
		55	16.5 ± 0.7	1129 ± 55	70 ± 2
		65	16.8 ± 0.6	1205 ± 41	67 ± 2
P0	20 May	45	14.8 ± 0.3	866 ± 15	54 ± 4
		55	14.6 ± 0.4	990 ± 70	62 ± 2
		65	14.2 ± 0.5	984 ± 35	59 ± 3
P90		45	16.8 ± 0.9	1322 ± 22	70 ± 4
		55	18.3 ± 1.0	1215 ± 37	69 ± 4
		65	17.2 ± 0.6	1057 ± 27	66 ± 3

Note: *—without fertilizer application, **—application of ammonium phosphate (90 kg/ha phosphate content). Provided values are two years (2021–2022) averages with the standard error (SE).

). For the fertilized treatments, the size of the flower head increased up to 13.2–14.8 cm under a low plant density. There was a small difference between the sizes of the flower heads for the different sowing dates. For the steppe zone, there were significant differences between the sizes of the flower heads for the treatments with fertilizer applications. For example, the size of the flower head for the unfertilized treatments was 13.5–15.5 cm compared to 16.2–17.4 cm for the fertilized treatments with early sowing and the highest seeding rate (Table 5). However, there was no clear pattern based on either plant density or sowing date.

In our experiments, a delay in sowing time and increasing plant density led to a reduction in the size of the flower head, especially under drought conditions. Experiments conducted at the North Dakota State University confirm that increasing the density of sunflower crops leads to a decrease in indicators such as 1000 achenes weight and flower head diameter [44]. Data from Nasieva et al. [37] showed that, for an early sowing date, the size of the flower head was 14 cm and this decreased by 1.4 cm at a late sowing time. Research on dark chestnut soils showed that, when sowing in the first and second parts of May, the diameter of the sunflower head was between 12.8 and 14.5 cm, and, when sowing in the third part of May, the value of this indicator decreased to 9.4–11.7 cm [10].

In the dry steppe zone, there was a significant decrease in the number of achenes to 360–520 achenes per flower head or 18–20% less than the number of achenes per flower head found in the steppe zone (Table 5). For the 20 May sowing date, there was an increase in the number of achenes per flower up to 506–565 achenes for the treatments with fertilizer application, while, for the unfertilized treatments, there was a decrease in comparison to the 10 May sowing date.

In the steppe zone, the number of achenes per a flower head for the early sowing dates increased by 19–28% for the fertilized treatments (Table 5). For the late sowing date, there was a decrease in the total number of achenes per a flower head of 14% for the high seeding rate of 65,000 seeds/ha.

The weight of achenes per flower head in the dry steppe zone was higher (29 g) for the early sowing date (15 May) for the fertilized treatment at a low seeding rate (32,000 seeds/ha) (Table 5) and decreased by 23% for higher seeding rates. The same response was found for the unfertilized treatment. The maximum achene weight of 24 g per flower head was obtained at a seeding rate of 32 thousand seeds/ha. For the steppe zone, the maximum achene weight of a flower head was 81 g for the early sowing date (10 May) and a seeding rate of 45,000 seeds/ha (Table 5). The achene weight per flower head decreased from 14 to 18% when the seeding rate increased. For the late sowing date, there was reduction in the weight of 11% to 20% per flower head depending on the seeding rate. Nasieva et al. [37] found that, at an early sowing time, the number of achenes per flower head was 1097 with a weight of 38.1 g, and at a late sowing time, this was 1013 achenes weighing 34.1 g. In our experiments, the delay in sowing time did not lead to a significant decrease in this indicator in years with a good moisture supply during the flowering and seed filling period.

According to Peresadko [7], the weight of seeds per flower head decreased with an increasing seeding rate for the hybrid Oskil. At increased seeding rates of 60 and 70 thousands/ha, plant competition increased, which affected the formation of the number of seeds in the flower head. The sharpest decrease in this indicator was observed in the control group. In the fertilized treatment, the number of seeds in the flower head decreased with an increased plant density, but more gradually in comparison to the unfertilized treatment. Our study results also confirm this tendency.

The weight of 1000 achenes directly depended on the weather conditions during the ripening period and varied significantly for the two climate zones. In the dry steppe zone, the average weight by treatments varied between 34.5 and 54.7 g, while, in the steppe zone, it was varied between 51.3 and 66.2 g (Figure 4). The 1000 achenes average weight decreased by 14–28% for the late sowing dates for the fertilized treatment as a result of an increase in the number of achenes per flower head and by 8–12% for the unfertilized treatment. There was also a decrease in the 1000 achenes weight depending on the plant density. A similar pattern of decrease in the weight of 1000 achenes (from 56.6 to 46.0 g) and the diameter of the flower head (from 15.2 to 12.4 cm) with a higher plant density was noted by Thompson and Fenton [45].

In the dry steppe zone, the range of the 1000 achenes weight by treatments was small for most of them except for late sowing at a higher seeding rate for fertilized treatments (Figure 4). In the steppe zone, we could see bigger ranges of 1000 achenes weight for almost all treatments except three of them. The median and the average for the majority of the treatments in both zones were matching or close, meaning that the 1000 achenes weight for each treatment was more or less evenly distributed from the lowest to highest values.

The diameter of the flower head, the number of achenes per flower head, and the total weight of the achenes per flower head were highest for the early sowing dates, the fertilized treatment, and a low seeding rate (Table 4). This pattern was found for both the dry steppe and steppe zones in Kazakhstan. The relationship between the structural yield components showed that, in the dry steppe zone, it was affected by the weight of 1000 achenes (r = 0.37) in 2021, while, for 2022, the correlations between yield and the number of achenes, the weight of achenes per flower head, and the weight of 1000 achenes were 0.82, 0.79, and 0.54, respectively. In the steppe zone, this relationship was more significant; in 2021, the correlations between yield and the number of achenes per flower head and the achene weight per flower head were 0.37 and 0.44, respectively. In 2022, the correlations between yield and the number of achenes per flower head, the achene weight per flower head, and the weight of 1000 achenes were 0.88, 0.68, and 0.48 respectively.

3.5. Sunflower Yield

An analysis of the yield data showed that the yield variability was related to the soil and weather conditions and different crop management treatments. The yield in the dry steppe zone in 2021 for unfertilized treatments and the early sowing date (15 May) ranged between 760 and 1290 kg/ha, while, for the late sowing date (20 May), it ranged between 570 and 900 kg/ha. Depending on the seeding rate, in 2022, the yield decreased for the early sowing date to 530–670 kg/ha, and, for the late sowing date, to 340–490 kg/ha for unfertilized treatments (

Table 6. The sunflower “Baiterek 17” yield (kg/ha) under various fertilizer application rates, sowing dates, and seeding rates for the dry steppe and steppe zones.

Sowing Date	Seeding Rate, Thousand/ha	P0 *			P90 **
		2021	2022	Average	2021	2022	Average
the dry steppe zone
15 May	32	760	530	650	980	570	770
	41	990	500	750	1160	660	910
	57	1290	670	980	1370	800	1090
average by sowing date		1013	567	793	1170	677	923
20 May	32	570	340	460	860	390	630
	41	730	350	540	840	450	650
	57	900	490	700	860	540	700
average by sowing date		733	393	567	853	460	660
average for fertilizer application		873	480	680	1012	568	792
LSD05		280	200	-	280	200	-
m%		8.3	6.8	-	8.3	6.8	-
the steppe zone
10 May	45	1820	2380	2100	2030	3010	2520
	55	1980	2500	2240	2170	3080	2630
	65	2190	2790	2490	2350	3580	2970
average by sowing date		1997	2557	2277	2183	3223	2707
20 May	45	1850	1520	1690	2080	1980	2030
	55	2250	2320	2290	2550	2540	2550
	65	2350	2380	2370	2680	2820	2750
average by sowing date		2150	2073	2117	2437	2447	2443
average for fertilizer application		2073	2315	2197	2310	2835	2575
LSD05		560	260	-	560	260	-
m%		4.7	2.3	-	4.7	3.2	-

Note: *—without fertilizer application, **—application of ammonium phosphate (90 kg/ha phosphate content), LSD₀₅—the least significant difference at 5% significance level (LSD), m%—a relative error.

).

The highest yield of 1290 kg/ha for an unfertilized treatment for the conditions of the dry steppe zone was obtained for the 15 May sowing date at the highest seeding rate of 57,000 seeds/ha (Table 6). There was a decrease in yield by 26–30% for the 20 May sowing date in 2021. The LSD₀₅ test showed that differences between the yield values for 15 May and 57 thousand seeds/ha treatment and the other two seeding rate treatments were significant. There was a significant difference at a 95% confidence level between the early sowing date and the highest seeding rate, and the late sowing date and the highest seeding rate treatments as well. In 2022, there were no significant differences in yield between the sowing dates, as well as the seeding rates for the unfertilized treatments. For the fertilized treatments, there was a higher yield between 1160 and 1370 kg/ha for the 15 May sowing date at a seeding rate from 41,000 and 57,000 seeds/ha in 2021. The LSD₀₅ test confirmed this significant difference considering seeding rates. There was also significant difference at a 95% confidence level between yields for the early sowing date combined with a high seeding rate (1170 kg/ha) and late sowing date (860 kg/ha) as well. A similar tendency for an early sowing date and a high seeding rate to make a significant difference was true in 2022, but with a lower yield ranging from 660 to 800 kg/ha. For the 20 May sowing treatment, there was a decrease in yield by 30–32% in 2022 compared to 2021, with no significant differences in yield according to the LSD₀₅ test for the treatments with different seeding rates.

In the steppe zone in 2021, the yield slightly increased by 11.5–7.3% for the non-fertilized treatments for the 20 May sowing date with an increase in the seeding rate, and an increase of 11–13% for fertilized treatments. The LSD₀₅ test did not confirm significance of these results. In 2022, the 10 May sowing date with the highest seeding rate (65 thousand seed/ha) treatment resulted in the highest yield (2790 kg/ha) with significant differences compared to low seeding rates, according to the LSD₀₅ test. This treatment also showed significant differences with late sowing treatments with different seeding rates, but with low yields as well. In 2022, the highest yield of 3580 kg/ha was obtained for the early sowing dates at a seeding rate of 65,000 seeds/ha for the fertilized treatment (Table 6). The significance of the differences between this treatment and treatments with low seeding rates as well as the late sowing date treatment that all had lower yield values was confirmed by the LSD₀₅ test.

The experiment conducted by Allam et al. [14] with two sunflower hybrids, Vidok and Euroflora, at two sowing dates (1 May and 1 June) and three seeding rates (55,000, 83,000, and 166,000 plants/ha) showed similar results. The highest yield of 4470 kg/ha, based on a combination of sowing date and planting density, was produced in the case of a high plant density for the early sowing date. A study conducted in western Kazakhstan to determine the impact of sowing dates on sunflower productivity showed that the optimal time for sowing is the early period. The highest biological yield of 1720 kg/ha and oil content were obtained when sown around April 29 [15,37]. The optimal density of sunflower of 60,000 plants/ha in the steppe zone according to the study conducted by Pinkovskyi and Tanchyk [13] is quite close to what our results show at 65,000 plants/ha. According to some studies, the optimal seeding rate can be 70,000 seeds/ha [46], or 60,000 for chernozem soil [47].

A three-way ANOVA for “Baiterek 17” was conducted separately for two climatic zones in order to identify the relative share of the three factors, including sowing date (A), seeding rate (B), and fertilizer application rate (C), and their various combinations (

Table 7. The results of the analysis of variance (ANOVA) for yield for the three-factor (2 × 3 × 2) experiments of the sunflower “Baiterek 17”.

Source of Variation	SS	df	MS	F	Fcrit	t05	LSD05
	Dry Steppe Zone
Total	603	71	-	-	-
Replication	0.5	2	0.26	0.039	3.12		2.48
Factor A (sowing date)	109.7	1	109.65	16.489 *	3.97		108.7
Factor B (seeding rate)	20.4	2	10.2	1.533	3.12		15.9
Factor C (fertilizer)	22.9	1	22.93	3.448	3.97		49.6
Interaction AB	47.2	2	23.59	3.547 *	3.12		24.1
Interaction AC	0.9	1	0.87	0.131	3.97		9.84
Interaction BC	7.2	2	3.6	0.542	3.12		9.43
Interaction ABC	8.7	2	4.37	0.657	3.12
Residual (Error)	385.7	58	6.65	-	-	1.99
	Steppe zone
Total	1616	71	-	-
Replication	29.8	2	14.90	1.066	3.12		19.2
Factor A (sowing date)	96.5	1	96.48	6.903 *	3.97		101.9
Factor B (seeding rate)	379.1	2	189.55	13.56 *	3.12		68.4
Factor C (fertilizer)	235.1	1	235.15	16.83 *	3.97		159.1
Interaction AB	47.8	2	23.9	1.71	3.12		24.3
Interaction AC	7.9	1	7.93	0.567	3.97		29.2
Interaction BC	7.7	2	3.85	0.276	3.12		9.7
Interaction ABC	1.8	2	0.9	0.064	3.12
Residual (Error)	810.6	58	13.98	-	-	1.99

Note: SS—a sum of squares, df—a degree of freedom, MS—a mean of squares, F—an actual value of the F-ratio, F_crit—a critical value of the F-ratio at the 5% significance level, t_05—a value of the t-criteria at the 5% significance level, LSD_05—the least significant difference at 5% significance level (LSD), *—significant at a p-value of 5%.

).

In the dry steppe zone, sowing date (A) and its combination with seeding rate (AB) were significant at a 5% significance level, as confirmed by the Fisher F-ratio, which was greater than the critical value for the F-ratio (Table 7). The contribution to this variability by sowing date (A) was higher than the sowing date—seeding rate (AB) combination, as can be seen by the higher F-ratio value of 16.489 for A vs. 3.547 for AB. For the other two factors and the remainder of the first-order interactions, as well as the second-order interaction (ABC), the actual values of the Fisher criterion were less than the theoretical value, and thus the effects on yield variability were insignificant or not detected (Table 7).

In the steppe zone, the impact of each separate factor A, B, and C was significant at a 5% significance level (Table 7), and the fertilizer application (C) was the largest contributor to yield variance (F-ratio = 16.83), followed by seeding rate (B) with an F-ratio of 13.56 and sowing date (A) with an F-ratio of 6.903. However, none of the first- and second-order interactions between these three factors were significant.

3.6. Ecological Plasticity

Various studies have confirmed that a change in yield depends on favorable weather conditions for plant development, which, in turn, depend on the sowing date and, to some extent, the fertilizer application rate [48,49]. Global climate change due to an increase in the carbon dioxide concentration in the atmosphere causes changes in temperature and other weather conditions, resulting in changes in production practices. This, in turn, increases the risks for growers and producers. Varieties that combine high biological, economic, and technological properties and have sufficient ecological plasticity are of great importance for climate resilience. Varieties and hybrids that are of the “intensive” type compared to the “ordinary” type result in a significant increase in yield only when applying a high amount of fertilizer and the application of pesticides. However, this can lead to a decrease in plant resilience to environmental stresses. Therefore, the final yield always depends on resistance to adverse environmental factors.

Weather conditions have no repeatability and if the indicators of a variety/hybrid differ by years, then there is an interaction between the variety and environmental conditions of that year, which can be analyzed using the environmental condition index and the ecological plasticity indicator [30]. Under harsh soil and weather conditions, the plasticity of varieties and hybrids is in great demand.

The index Ij value for the conditions in 2021 in the dry steppe zone shows that it was more favorable for growth with an index value of 2.07, while 2022 was less favorable with an index value of −2.12 (

Table 8. The environmental condition index (I_j) and the ecological plasticity indicator (b_i) for the hybrid “Baiterek 17” for various fertilizer application rates, sowing dates, and seeding rates for the dry steppe zone.

Fertilizer Application	Sowing Date	Seeding Rate, Thousand/ha	2021–Yj	2022–Yj	∑Yi	Yi	bi
P0 *	15 May	31.7	7.62	5.28	12.9	6.5	0.5
		40.8	9.56	5.0	14.6	7.3	1
		57.1	12.88	6.7	19.6	9.8	1.4
P90 **		31.7	9.75	5.71	15.5	7.7	0.9
		40.8	11.64	6.6	18.2	9.1	1.2
		57.1	13.72	8.0	21.7	10.9	1.3
P0	20 May	31.7	5.74	3.44	9.2	4.6	0.5
		40.8	7.29	3.5	10.8	5.4	0.9
		57.1	8.95	4.9	13.9	6.9	0.9
P90		31.7	8.55	3.89	12.4	6.2	1.1
		40.8	8.40	4.54	12.9	6.5	0.9
		57.1	8.62	5.4	14.0	7.0	0.7
Yj—average for year i			9.4	5.2		Y = 7.3
∑Yj			112.7	62.96	∑∑Yj = 175.7
Ij			2.07	−2.12
∑Ij²			4.29	4.49		8.8

Note: *—without fertilizer application, **—application of ammonium phosphate (90 kg/ha phosphate content), yield values used for calculations are in c/ha.

). When analyzing the ecological plasticity indicator (b_i), it showed that the hybrid is more responsive to the planting date and fertilizer application. For the 15 May sowing date and the unfertilized treatment, the hybrid reacted strongly to a change in the seeding rate (according to b_i from 0.5 to 1.4), while, for the P₉₀ treatment, a smaller response to density was found (b_i = 0.9–1.3). For the 20 May sowing date, the hybrid reacted only slightly to the change in the seeding rate. The most optimal conditions for a given planting date were for a fertilized treatment at a seeding rate of 32,000 seeds/ha.

For the conditions of the steppe zone, 2022 was more favorable for cultivation with an environmental index (Ij) of 2.8, while 2021 was less favorable with an index of −1.9 (

Table 9. The environmental condition index (I_j) and the ecological plasticity indicator (b_i) for the hybrid “Baiterek 17” for various fertilizer application rates, sowing dates, and seeding rates for the steppe zone.

Fertilizer Application	Sowing Date	Seeding Rate, Thousand/ha	2021–Yj	2022-Yj	∑Yi	Yi	bi
P0 *	10 May	45	18.2	23.8	42.0	21.0	2.8
		55	19.8	25.0	44.8	22.4	2.8
		65	21.9	27.9	49.8	24.9	3.2
P90 **		45	20.3	30.1	50.4	25.2	4
		55	21.7	30.8	52.5	26.3	3.9
		65	23.5	35.8	59.3	29.7	4.9
P0	20 May	45	18.5	15.2	33.7	16.9	0.6
		55	22.5	23.2	45.7	22.9	1.9
		65	23.5	23.8	47.3	23.7	1.9
P90		45	20.8	19.8	40.6	20.3	1.4
		55	25.5	25.4	50.9	25.5	2
		65	26.8	28.2	55.0	27.5	2.4
Yj—average for year i			21.9	25.8		Y = 23.8
∑Yj			263.0	309.1	∑∑Yj = 572
Ij			−1.90	2.8
∑Ij²			3.61	7.84	11.45

Note: *—without fertilizer application, **—application of ammonium phosphate (90 kg/ha phosphate content), yield values used for calculations are in c/ha.

). The indicator of ecological plasticity (b_i) for the conditions of the steppe zone, on average over the study years, shows that the hybrid was more responsive to the sowing date and fertilizer application rate. The hybrid reacted most to the changes for the conditions of the 10 May sowing date. In this period, the hybrid strongly reacted to a change in the seeding rate; with an increase in plant density, the reaction intensified according to the bi values for the unfertilized treatments of 2.8–3.2, and for the fertilized treatments of 3.9–4.9. On the sowing date of 20 May, the hybrid reduced its response to changes in agricultural conditions, with values of 0.6–1.9 for the unfertilized treatment and values of 1.4–2.4 for the fertilized treatments, with an increase in the seeding rate from lower values up to the seeding rate of 65,000 seeds/ha.

Thus, it can be concluded that the hybrid “Baiterek 17” is plastic both under changing environmental conditions and agricultural technologies, and that it can be used for intensive agriculture. Improving plant nutrition and increasing the plant density to a seeding rate of 65,000 seeds/ha at an early sowing date of 10 May increases the plasticity in the steppe zone. For the dry steppe zone, with a sharper fluctuation in the soil moisture in the region, the plasticity of the hybrid decreases, but also, the maximum plasticity is found for an earlier planting date of 15 May and a higher seeding rate of 57,000 seeds/ha.

4. Conclusions

The growth and development of the sunflower hybrid “Baiterek 17”, on average over the two years, required 106–112 days in the dry steppe zone and 122–137 days in the steppe zone due to differences in weather conditions. There was a slight reduction in the duration of the growing season by 1–3 days at an increased seeding rate and for late sowing dates in both study areas. In the dry steppe zone, sunflower had a high germination for the early sowing date of 15 May. In the steppe zone, there were no differences in emergence for the various sowing dates. The reaction of crop to the seeding rate was more notable at both levels of nutrition, with and without a nitrogen-phosphorus fertilizer. An increase in soil moisture during the growing season in both regions and with higher seeding rates resulted in an increase in the number of plants per unit area (from 2.5 to 4.8 plants/m² in the dry steppe and from 3.5 to 4.9 plants/m² in the steppe zone). Higher values of achene numbers and achenes weights were produced for the early sowing dates for the fertilized treatments and with a lower seeding rate, which was found in both climate zones. On average, over the two years of study, the highest yield in the dry steppe zone was obtained with fertilization for the early sowing date and at a maximum seeding rate of 57 thousand seeds/ha. In the steppe zone, even in insufficient moisture conditions, significant yield differences were found for the early sowing (10 May), high seeding rate (65 thousand seeds/ha), and fertilized treatments. The study shows that the hybrid “Baiterek 17” has a high ecological plasticity under changing environmental conditions and, with an increase in moisture availability, it requires intensive agricultural practices (fertilization, increased seeding rate, and early sowing dates) to obtain a high yield.