2.1. Experimental Research Site and Design
Experimental studies were carried out in 2020–2022 on a farm in Panevėžys district, Lithuania (55°40′27.7″ N 24°08′43.9″ E). The experimental research was carried out in an area with an average annual precipitation of 600–650 mm. During the research years, the average annual air temperature was about 7.5 °C. It was more than one degree higher than the long-term annual temperature (about 6.3 °C). Winter wheat was sown for two consecutive years in the same field (22.4 ha) using the no-tillage method according to precision farming technologies. In the first year of the study (2020–2021), three technologies were applied with four replications: URS—uniform rate seeding (as control), VRS—variable rate seeding, and VRS + VRF—variable rate seeding with variable rate fertilization. In the second year of the study (2021–2022), four technologies with three replications were applied: URS—uniform rate seeding (as control), VRS—variable rate seeding, VRSD—variable rate and depth seeding, and VRSD + VRF—variable rate and depth seeding with variable rate fertilization.
To enable the application of precision farming technologies, a full-field scan of the apparent electrical conductivity (ECa) was carried out before the start of the experimental studies. An EM38-MK2 electromagnetic induction soil scanner (Geonics Ltd., Mississauga, ON, Canada) and a Trimble EZ-Guide 250 global positioning system (GPS) coordinate locating device (Trimble Navigation Ltd., Alpharetta, GA, USA) with a GPS antenna were used for this scan. Based on the soil ECa data and using QGIS (Open-Source Geographic Information Systems) software (version 3.16, Hannover, Germany), the experimental field was divided into 5 soil management zones (MZs): MZ1—>28.6 mS m−1, MZ2—27.3–28.6 mS m−1, MZ3—25.7–27.3 mS m−1, MZ4—24.2–25.7 mS m−1, and MZ5—22.6–24.2 mS m−1. Soil samples were taken from each MZ, and soil texture was determined in the Agrochemical Research Laboratory. The soil texture was sandy loam in the first four zones (MZ1–MZ4) and loamy sand in the last zone (MZ5). In the first year of the study, a regional average rate of 180 kg ha−1 of winter wheat (Skagen variety) was applied using the URS and, in the middle zone (MZ3), using the VRS and VRS + VRF variants. In the variable rate variants, the seeding rate was reduced by 10% and 20% in the higher ECa zones MZ2 and MZ1, respectively, while, contrarily, in the lower ECa zones, the seeding rate was increased by 10% and 20% in zones MZ4 and MZ5, respectively. In the second year of the study, the average seeding rate was 162 kg ha−1, while in the other years, the MZ rate was reduced or increased in the same way as in the first year. Since variable seeding depth was also included in the second year, the same principle as that for seeding rate was applied to this technological parameter. Using the URS variant and in zone MZ3, the seeding depth was 3.0 cm; in MZ2 and MZ1, it was 10% and 20% lower; and in MZ4 and MZ5 it was 10% and 20% higher, respectively. Seeding at variable rates and variable depths was carried out using a Horsch Avatar 6.16 SD direct drill.
Using the VRS + VRF and VRSD + VRF precision seeding technologies, precision variable rate fertilization was applied simultaneously. Rates of phosphorus, potassium, and nitrogen fertilizers by year can be seen in the tables in the following subsection. Approximately 60% of the phosphorus fertilizer rate was applied during the seeding process, and the remainder of the phosphorus fertilizer and the total potassium fertilizer rate were applied immediately after seeding, according to the phosphorus and potassium maps obtained from the chemical analysis of soil samples. Fertilization was carried out with a Rauch Axis H50.2 centrifugal mineral fertilizer spreader with a working width of 36 m. Nitrogen fertilizer (180 kg N ha−1) was applied 3 times. The first time, a uniform rate of 60 kg N ha−1 was applied on all technologies; the second time, a variable rate of 70 kg N ha−1 was applied, and the third time, the remainder of the rate was applied. For the variable rate application, N-uptake and nitrogen fertilizer requirement maps were prepared using a Yara N-Sensor ALS optical nitrogen sensor (Yara International ASA, Norway). Spraying with growth regulators and fungicides was carried out with a Horsch Leeb PT 270 self-propelled sprayer with a working width of 36 m.
Winter wheat yield was determined by randomly sampling 5 plant samples from each technology and each replication. Depending on the variant and management zone, each sample consisted of 69 to 147 productive stems with ears. The samples were manually cut from a 1.0 m long row, threshed in the laboratory, and weighed, and the grain yield per hectare was calculated.
2.2. Life Cycle Assessment
The utilization of the life cycle assessment (LCA) was involved in assessing the environmental impact of using variable rates of seeding, plant protection products, and fertilizers in the cultivation of winter wheat [
14,
15]. This assessment did not include the technological operations and materials used, the rate of which did not change across technologies (herbicides, fungicides, diesel, machinery, etc.). The boundary of this LCA system (
Figure 1) includes only those materials that were used at a variable rate. To allow for a qualitative comparison between LCA data, a specific functional unit (FU) was selected. In this study, the FU was defined as one kilogram of winter wheat grain.
LCA serves as a widely employed instrument for evaluating the potential environmental consequences and resource utilization associated with a product or service system throughout its entire life cycle. This life cycle encompasses activities ranging from the extraction of raw materials to the production and utilization phases, as well as waste management and transportation [
16]. The LCA methodology adheres to the standards set by the International Commission of Standardization, specifically ISO 14040 [
17] and ISO 14044 [
18]. To conduct the LCA analysis in this study, SimaPro 9 software was employed. Data related to materials, as shown in
Table 1 and
Table 2, were obtained from the Ecoinvent V3 database [
19].
The seed rate in kilograms in the tables differed for each year, but the number of seeds was the same (about 4.2 million seeds per ha). In the second year, the seeds were smaller and lighter. Therefore, the seeding rate seems to be lower at first glance, but in reality, the number of seeds and plants was the same. For the same number of seeds and plants, the same amount of chemical fertilizers was used in both years.
The assessment of the environmental impact related to the cultivation of winter wheat was specifically focused on midpoint impacts, utilizing the CML-IA baseline V3.06/EU25 methodology. For evaluation purposes, 11 distinct impact categories were employed, as presented in
Table 3. These categories are standard in LCA and help to better understand the impact of activities in different categories. It is also possible to evaluate the differences between different variable rate technologies for different impact categories.