**1. Introduction**

Soybean seed is the world's primary source of plant protein. In temperate climate conditions soybean cultivation remains relatively inefficient [1]. Therefore, yellow lupine seeds [2–4] have become a promising alternative protein source. Cultivation can be carried out by pure sowing or by intercropping with other spring crop species.

Due to intercropping of lupine with other plants, the seed yield decreases; however, the protein yield is noticeably higher [5]. For coexisting species, we distinguish different intercropping systems depending on the time of sowing and spatial placement [6]. Cereals (wheat, barley, and oats) and legumes (yellow lupine, narrow leaf lupine, and peas) are grown mainly in mixed intercropping (MI), i.e., they are planted in the same rows.

As a consequence of MI, the optimization of fertilization techniques is considerably limited, and the implementation of herbicide control is not possible. Corn, along with other species, is grown by utilizing strip intercropping (SI), i.e., alternating strips of various species [7–10]. If strips of a single species are wide enough and adjusted to the technical capabilities of cultivating tools, it also becomes possible to optimize the cultivation practice for individual species.

In integrated conditions, particularly in organic production systems, the cultivation of cereals with legumes in mixtures is considered to be a suitable source of concentrated feed [11,12]. Intercropping also serves ecological functions: it increases biodiversity, positively affects the soil condition, and suppresses

weed infestation [13]. Therefore, the inability to optimize cultivation practice and di fficulties regarding herbicide control should not constitute an obstacle or become a deterrent in MI implementation. Because of enhanced utilization of habitat capacity, MI crops are generally more stable in subsequent years as compared to pure crop yields of species used in MI [14–19]. Unfortunately, the co-occurrence of individual species may also contribute to unfavorable e ffects. The adverse e ffects vary considerably, and they are strictly dependent on weather conditions. As a consequence, varying qualities of yields are obtained in subsequent growing seasons regardless of the fact that the same agrotechnical assumptions are being implemented [20], which, in turn, leads to di fficulty in balancing feed resources [21].

In the scientific literature, the subject of SI primarily relates to the cultivation of soybeans and corn [22]. Available resources pertaining to SI of yellow lupine with other plant species are rather limited [23]. It is known, however, that the yield of yellow lupine seeds in MI with oats is largely dependent on environmental factors. MI in low moisture soil conditions leads to competition between lupine and oats, and it shows to be asymmetric to the detriment of lupine. Consequently, MI results in a considerably smaller yield of yellow lupine [24,25]. Since the interaction between species occurs exclusively at the strips' border, it has been ascertained that SI cultivation of yellow lupine with spring cereals can be justified. In SI, yellow lupine adversely responds to close proximity with oats and triticale [23]. However, the response of yellow lupine in the proximity of other plant species (potential components for SI) remains undetermined. While taking into consideration the asymmetry of competition between various species, row separation with a technological path presents a viable option. Therein lies the advantage of SI over MI. Separating the species tends to diminish the competition effect and utilize the positive phenomenon of the border e ffect: namely, an increase in the yield of plants cultivated adjacent to an area devoid of vegetation [26,27].

The aim of our study was to determine the proximity e ffect of spring wheat, triticale, barley, and peas on yellow lupine cultivation and to estimate its yield in strip intercropping with the abovementioned plant species.

#### **2. Materials and Methods**

## *2.1. Experiment Site*

The field trial was carried out between 2008 and 2010. The experiment was conducted at the Research Station of the Faculty of Agriculture and Biotechnology in Mochełek (53◦13 N; 17◦51 E) (Figure 1). The results presented in this manuscript are part of previously published studies related to the proximity e ffect (PE) on other species [28–31]. Accordingly, the methodology presented in this experiment coincides with the cited studies.

**Figure 1.** Site of field experiment at Mochełek, Kuyavian-Pomeranian voivodeship, Poland [32,33].

The experiment was conducted on loam sand texture luvisol soil (LV) [34]; the pre-crop was winter oil seed rape. Depending on the research year, the Corg content was 6.2–6.6 <sup>g</sup>·kg−<sup>1</sup> d.m. of soil, and the content of absorbable forms P and K was 63–69 and 94–172 mg·kg−<sup>1</sup> respectively, soil pH (1M KCl) was between 5.2–6.6.

During the growing season, the temperature amplitude was similar for all three years of the conducted research (Figure 2). In 2009, however, April and the first two decades of June were characterized by warmer temperatures as compared to the rest of the year. The year 2010 was marked by a much warmer July. Distribution of rainfall also varied significantly. Modest precipitation was observed from the third decade of April to the second decade of June of 2008. During this time period, in any of the decades, the rainfall did not exceed 10 mm. In 2010, rainfall not exceeding 10 mm per decade was reported between the first decade of June and the second decade of July.

**Figure 2.** Precipitation and air temperature (2008–2010) at the site of the field experiment.

## *2.2. Experiment Design*

The source data come from a multiple 3-year field experiment. The layout of the experiment is demonstrated in Figure 3. The plot was 150 cm wide and consisted of 12 rows of plants separated by 12.5 cm. Figure 3A represents one of four replications (randomized complete blocks) with all the neighboring species of yellow lupine and their paths. The experimental treatment consisted of yellow lupine's row layout (Figure 3B), four rows of separation termed PE (proximity effect) in relation to the neighboring species (wheat, triticale, barley, and oat) or separated from an unplanted path referred to as BE (border effect) (Figure 3C). The first adjacent row was located 12.5 cm from the first row of the neighboring species/path. The experimental plot consisted of successive plant rows each measuring four meters long. The mean result of each treatment of adjacent plants (from right and left sides of the plot) was considered as a single replication. Based on the results of previous studies [23], the fourth plant row was no longer subjected to the influence of neighboring plants, representing the internal canopy (control). The orientation of the plots' longer side was north-south.

**Figure 3.** Experiment design: single block (**A**), PE single plot (**B**), and border effect (BE) single plot design (**C**).

#### *2.3. Elements of Agrotechnical Practices*

Each plant species was sown simultaneously between 25 March and 5 April. In order to ensure even spacing between each plant, the cereal seeds were precisely placed on a seeding belt made of blotting paper. Plant density was 45 pcs·m<sup>−</sup><sup>1</sup> (360 pcs·m<sup>−</sup>2). The seeding strips were placed in the soil at a depth of 4 cm. Seeds of lupine and peas were sown manually; planting density was 10 pcs·m<sup>−</sup><sup>1</sup> (80 pcs·m<sup>−</sup>2).

The following cultivars were planted: yellow lupine 'Lidar', spring wheat 'Bombona', spring triticale 'Doublet', spring barley 'Antek', and pea 'Ramrod'.

Macro-nutrients were applied during the spring months: 30 kg <sup>P</sup>·ha−1, 66 kg K·ha−<sup>1</sup> and 34 kg N·ha−1. In the phenological phase BBCH 22–25 (tillering stage), N fertilization (34 kg N·ha−<sup>1</sup> dose) was used with cereals only. Herbicide active substance-linuron (Alfalon 450SC), at a dose of 1 dm3·ha−<sup>1</sup> was applied to each crop.

#### *2.4. Samples and Measurements*

Harvest sampling from each row was conducted manually. The measurements of yellow lupine plants included:


Weight was recalculated for 1 m of the row.

## *2.5. Data Analysis*

Single year data concerning all characteristics of yellow lupine in strip intercropping were calculated using one-way ANOVA in a four reps (block) model. The three-year synthesis of variance, based on statistic F (Fisher) in a mixed model, tested the null hypotheses regarding year as random effect and treatments (yellow lupine's row neighboring to one of four species) as fixed effect (Table 1). The post-hoc calculation according to HSD Tukey's test (*p* = 0.05) was used for the separation of means of yellow lupine traits. For data verification, the R core team software package was used.


**Table 1.** Significance of factor and significance of interaction factor and years in ANOVA.

 significant *p* < 0.05; \*\* significant *p* < 0.01; - not significant.

\*

Index of the proximity effect (IPE) was based on the results acquired from three rows closest to the neighboring species; IPE reflects the quotient of trait values for the given order and the fourth order.

$$\text{PE} = \frac{\text{R}\_{(1,2,3)}}{\text{R}\_{(4)}} \tag{1}$$

where R(1,2,3) is the seed weight of plants from 1st or 2nd or 3th row; and R(4) is the seed weight of plants from the 4th row.

IPE = 1 implies neutrality of the tested species. IPE < 1 indicates a negative impact of the neighboring species on yellow lupine. IPE > 1 indicates positive influence of the neighboring species on yellow lupine. Index of the border effect (IBE) was calculated as well. In this instance, the yellow lupine plants were adjacent to a vegetation-free area and separated by a technological path or a path dividing the plots. The interpretations of the IBE and IPE values are the same.

The proposed predictive analysis is to adopt the results of the yield from this study to the practical utilization of yellow lupine in SI with various species. As the sowing is practiced by a 3-m-wide seed driller, we applied, in reference to yield estimation for each linear meter, 3-m-wide strips (24 rows), with a row spacing of 12.5 cm. Estimated yield (Figures 4 and 5) was calculated based on the following formulas:

$$
\chi\_{no\ proximity} = 24 \times r\_4 \tag{2}
$$

$$Y\_{ome\ side\ provinity} = r\_1 + r\_2 + r\_3 + 21 \times r\_4 \tag{3}$$

$$\chi\_{\text{two side proximity}} = 2 \times r\_1 + 2 \times r\_2 + 2 \times r\_3 + 18 \times r\_4 \tag{4}$$

where *r*1–4 represents the yield in the next row from the neighboring species.

**Figure 4.** Estimated yellow lupine yield (g) for each linear meter of 3-m-wide strips, depending on the type of proximity.

**Figure 5.** Estimated yellow lupine yield difference (%), depending on neighboring species/path.

The total yield and yield structure of SI for two species in an area of one hectare were also estimated (Figure 6). These crops were estimated considering the immediate vicinity of strips, and strips separated by a 50-cm-wide path. For estimates, 17 rows, each 3-m-wide were adopted for both species (34 rows in total). The above setup resulted in arable fields of 102 × 98 m for SI without paths and 114.75 × 87.1 m for SI with paths. Estimated cereal yields were based on the results from the same experiment that was already published in other articles [28–31].

**Figure 6.** Estimated strip intercropping (SI) yield, and yield structure, depending on neighboring species/path.
