**1. Introduction**

In Europe, cereal-legume mixtures have long been considered minor crops. However, interest in their cultivation has been growing in recent years, as they are considered an important element of agricultural diversification [1]. For example, in Poland, in 2019, the cultivation area of cereal-legume mixtures was 0.27% (29,300 ha) of the total arable land, of which the majority were spring mixtures [2]. The mixtures are cultivated in organic and sustainable agricultural systems [3,4]; they are cultivated mainly for high-protein fodder, green fodder, hay, or green manure [5–8].

Crop mixtures are essential for crop rotations in organic farming [9–11], contributing to several ecosystem services [12]; they are responsible for the maintenance of greater species diversity in crop-rotation [13,14], an increase in biologically bound nitrogen in soil [15,16], and a decrease in disease and pest outcomes [17]. Moreover, cereal-legume mixtures with varying rooting depth improve soil structure, i.e., by loosening deeper layers of soil [18,19], making mechanical operations easier. Contrarily, in conventional farming, which is cash-oriented, the role of cereal-legume mixtures is marginal. That is because

139

**Citation:** Puzy ´ ˙ nska, K.; Synowiec, A.; Puzy ´ ˙ nski, S.; Bocianowski, J.; Klima, K.; Lepiarczyk, A. The Performance of Oat-Vetch Mixtures in Organic and Conventional Farming Systems. *Agriculture* **2021**, *11*, 332. https:// doi.org/10.3390/agriculture11040332

Academic Editors: Urs Feller and Les Copeland

Received: 19 January 2021 Accepted: 3 April 2021 Published: 8 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

mineral fertilizers and pesticides replace the mixtures' nutritional and pesticide properties. There has been a trend in agriculture in recent years to shift from traditional conventional farming to sustainable, more environmentally friendly farming, increasing the inclusion of these mixtures in crop rotation [20].

One of the spring cereal-legume mixtures, relatively popular in cultivation in temperate climate, is oats with common vetch [21]. Both components of this mixture differ in soil and climatic requirements and agrotechnology. They offer a premise for an appropriate selection of species and cultivars, and proportions of mixture, for sowing [22]. According to many authors, crop mixtures' yielding depends on the proper selection of cultivars [13,23–25]. Common vetch is a valuable component of these mixtures due to the high protein content of its seeds. However, vetch grown in a mixture with oats is characterized by little competitive potential, especially for light [26]. This translates into a lower growth of vetch that develops smaller seeds of lower nutrient content.

On the other hand, even though a highly competitive species in mixtures [27], oats support the companion crop from lodging [28]. The maximum demand for water and nutrients of both mixture components elapses during the vegetation. For that reason, interspecific competition in mixtures is lower than in the case of intraspecific competition in pure sowing [20].

Several indices measure the condition of the crop canopy, i.e., the leaf area index (LAI) and the leaf chlorophyll content (SPAD). The LAI informs about the leaves' area, which is equal to the assimilation area [29]. On the other hand, the SPAD shows the relative content of chlorophyll in the leaves, translating into their nitrogen nutrition [30]. As a result, there is a relationship between the LAI and SPAD values and the seed yield [13,31,32]. Klima et al. [13] correlate higher values of LAI of spring cereals mixtures with higher mixtures' yields than their pure sowings. However, the LAI of the oat-vetch mixture has not been studied so far.

The main aim of the study was to compare the yield and yield components of mixtures of oats (*Avena sativa* L.) with common vetch (*Vicia sativa* L.) in two farming systems differing in fertilization and plant protection means. The selection of oat cultivars on the yield of mixtures, including temperature and rainfall during 2012–2014, was also analyzed. Additionally, the leaf area index (LAI) of mixtures and the relative chlorophyll content (SPAD) in oats and vetch leaves were measured in two phases of plants' growth.

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

#### *2.1. Field Site and Experiment Descriptions*

The four-field crop rotation: potato—winter wheat—oats and common vetch mixture— winter spelt, in a randomized split-split-plot design, has been carried out since 2009 in the Experimental Station Mydlniki-Kraków, Poland (50◦04 N, 19◦51 E, 280 m a.s.l., Figure 1), on Stagnic Luvisol (SL) soil [33]. All crops were present each year, which means that the mixture of oats and vetch was sown every year following the winter wheat.

The investigations for this paper were carried out in the years 2012–2014. The examined soil texture was loam developed from loess; pH (KCl) 6.04; Ntot 0.858 g kg−1; P 423.2 mg kg−<sup>1</sup> soil; K 148.2 mg kg−<sup>1</sup> soil; and Corg 7.34%.

The first factor of the experiment was the farming system: (i) organic—without any artificial mean; and (ii) conventional with synthetic pesticides and mineral fertilizers. The second factor was selecting the oat cultivars: 'Celer' or 'Grajcar' for the mixture with common vetch cv. 'Hanka'. The course of temperature and precipitation in 2012–2014 was considered the third factor.

The oat and vetch mixtures were sown at the optimal agrotechnical dates for southern Poland, 23 March 2012; 16 April 2013; and 20 March 2014, at a planned density of plants per m<sup>−</sup><sup>2</sup> 500 and 75 for the oats and vetch, respectively. The mixtures were sown on plots of 24 m<sup>2</sup> area (3 × 8 m), using a plot drill (Hege 80) at a row space of 13.0 cm. A total of 16 plots were present each year (four replications for every mixture in both systems). Soil tillage

was similar in organic and conventional plots. It consisted of a deep pre-winter plowing (October) and shallow seedbed tillage using an active harrow and a string roller (April).

**Figure 1.** Location of the study site. Source: https://www.google.com/maps/ (Accessed on 17 January 2021).

Every four years, 30 tons of composted manure per hectare was used under potato in the conventional and organic system. A mineral fertilization (kg ha−1) of 80 N, 65 P, and 100 K was applied only in the conventional plots. The doses of fertilizers followed good agricultural practices and generally accepted principles of spring cereal cultivation. Nitrogen was applied as ammonium nitrate (34% N); one-third of the dose administered before sowing, and two-thirds as a top dressing. The potassium salt (60% K2O) and triple superphosphate (40% P2O5) were used in full doses before pre-winter plowing in October.

Additionally, in the conventional plots only, three-stage fungicide protection combined with pest control program was applied. Treatments were performed when the economic threshold of pests was exceeded, with a ca. one-month intervals between them. The following pesticides were used: fungicides—prochloraz + tebuconazole or thiophanate-methyl + conazole; insecticides—deltamethrin; beta-cyfluthrin or chlorpyrifos.

In the organic plots, only a mechanic weed control was performed each year by a Weeder harrow at the end of oats' tillering and a manual weed removal before the mixture's harvest.

#### *2.2. Description of Cultivars*

According to the breeders' recommendations, the crop cultivars selected for this study are intended to cultivate mountainous areas of temperate climate, where they yield well.

The yellow-grained oat cv. 'Celer' has a 120 days to ripening phase BBCH 85 (German "Biologische Bundesanstalt, Bundessortenamt, und Chemische Industrie") from sowing. The mass of a thousand grains is 41.0 g. The grains have a relatively high proportion of husk (28.2%). The protein and fat content of the grains are 6 (on a 9-point scale, where 9 means most favorable, 5—average 1—least favorable content). Plants are resistant to coronary and stem rust and of good resistance to other diseases. The cv. Celer is relatively short (90 cm), with high lodging resistance. The advised sowing rate of seeds is 550–600 m<sup>−</sup>2. Breeder: Małopolska Hodowla Ro´slin (HR), Sp. z o. o., Poland.

The oat cv.'Grajcar' is an early sown cultivar of medium-early ripening, equal to 120 days to the ripening phase (BBCH 85) from sowing. It is a yellow-grained oat, with an average thousand-seed mass of (35.3 g). The grains have a relatively high proportion of husk (29.5%). The protein and fat content are 6 and 7, respectively. The plants are highly resistant to coronary and stem rust. It has average soil requirements. The plants are relatively short (89 cm). The advised sowing rate of seeds is 550–600 seeds m<sup>−</sup>2. Breeder: Małopolska Hodowla Ro´slin (HR), Sp. z o. o., Poland.

'Hanka' is common vetch (*Vicia sativa* L.) cultivar of a traditional type of growth, i.e., not self-ending. Plants are lush, 50–160 cm high, rich in leaves ending with sticking tendrils; seeds are brown. The cultivar is very fertile, with seeds of high protein (32%) and low tannins (0.05%). Seeds are ready for harvest 120 days after sowing. The thousand-grain mass is 52 g. It can be grown for seeds, green fodder, or green manure. The cultivar is appropriate for mixing with cereals. Breeder: Firma Nasienna Granum, Poland.

#### *2.3. Leaf Area Index and Leaf Greenness Index*

Two indexes of a canopy condition were measured. First, the leaf area index (LAI), characterizing the leaf assimilation area capable of absorbing photosynthetically active radiation (400–700 nm), using a SunScan Canopy Analysis System—SS1-COM Complete System (SunScan Canopy Analysis System, Delta-T Devices Ltd., Burwell, Cambridge, UK). Second, the leaf relative chlorophyll content in soil plant analysis development values (SPAD), using a 502DL chlorophyll meter (Minolta SPAD-502DL, Spectrum Technologies Inc., Plainfield, IL, USA).

The following formulas were used for the calculation of the LAI index (Equations (1) and (2)): 

$$K(\mathbf{x}, \theta) := \frac{\sqrt{\mathbf{x}^2 + \tan \left(\theta\right)^2}}{\mathbf{x} + 1.702 \left(\mathbf{x} + 1.12\right)^{-0.708}} \tag{1}$$

where:

*x* is the ellipsoidal leaf angle distribution parameter (ELADP), *θ* is the zenith angle of the direct beam,

$$\tau(\mathbf{x}, \theta) := \exp(-K(\mathbf{x}, \theta)L) \tag{2}$$

where:

*τ* is the gap fraction,

*L* is the leaf area index,

*K*(*<sup>x</sup>*,*θ*) is the extinction coefficient.

The measurement of relative chlorophyll content by the chlorophyll meter was according to the formula (Equation (3)):

$$\mathbf{M} = \mathbf{k} \, \log\_{10} \frac{\mathbf{I}\_{0(650)} \mathbf{I}\_{(940)}}{\mathbf{I}\_{(650)} \mathbf{I}\_{0(940)}} \tag{3}$$

where:

*k*is a confidential proportionality coefficient = 40;

I0(650) is the intensity of incident monochromatic light at 650 nm wavelength;

I(940) is the intensity of transmitted light at 940 nm wavelength;

I(650) is the intensity of transmitted light at 650 nm wavelength;

I0(940) is the intensity of incident monochromatic light at 940 nm wavelength.

The LAI and the SPAD measurements were performed each year on two dates, i.e., LAI1 and SPAD1 in the oats' tillering phase (BBCH 29), and LAI2 SPAD2 in the grain watery ripe phase (BBCH 71). The SPAD measurements were performed separately for oats and vetch plants, while the LAI were measured for the mixtures' canopy at four random spots per plot. The SPAD was measured on leaves of 25 plants of oats and vetch per plot. For the measurement, only fully developed leaves were chosen. The oat's SPAD readings were taken from the middle part of the leaf blade; for vetch this area was the middle leaflet on the pinnate leaf.

#### *2.4. Yield Measurements*

Before harvesting, the oat and vetch plants were sampled to determine the number of oat panicles per m<sup>−</sup><sup>2</sup> and grains per panicle, and the number of vetch pods and seeds per pod. The plants were sampled from four random spots of 0.125 m<sup>−</sup><sup>2</sup> each (0.25 m × 0.5 m) across each plot, but three edge rows on both plot sides were omitted. All sampled plants were analyzed, and the results were recalculated toa1m<sup>2</sup> area.

The harvest was carried out with a plot harvester (Seedmaster, Wintersteiger) at the oats' fully ripe growth stage (BBCH 97). After the harvest, the oats' grain and vetch seeds from each plot (24 m2) were weighed. Additional samples of grains and seeds (ca. 20–40 g) were taken to determine their dry mass at 105 ◦C for 24 h. The yield (t ha−1) was then calculated at 15% seed moisture. The thousand-grain mass of the oats and seeds of vetch were also determined.

The spatial arrangemen<sup>t</sup> of the experiment, with genotypes (cultivars) and farming systems including replications, is in Supplementary Figure S1. A flowchart of the methods is in Supplementary Figure S2.

#### *2.5. Statistical Analysis of Results*

The normality of the distribution of the observed traits was tested with Shapiro–Wilk's normality test to check whether the analysis of variance (ANOVA) met the assumption that the ANOVA model's residuals follow a normal distribution. Next, the effects of the main factors of the experiment: (i) farming system, (ii) oat cultivars, and (iii) years, and all the interactions between them, were estimated with a linear model for threeway ANOVA. The relationships between the traits were assessed based on Pearson's correlation coefficients and tested with the *t*-test. Tukey's test at *p* ≤ 0.05 tested the significance of mean differences.

The results were also analyzed with multivariate methods. The canonical variate analysis (CVA) was applied to present a multi-trait assessment of the similarity of the investigated treatments in a lower number of dimensions with the least possible loss of information. This enabled the graphic illustration of the variation in the traits of all treatments under analysis. The Mahalanobis distance was suggested as a measure of similarity of multi-trait treatments, whose significance was verified employing critical value *Dcr* known as the least significant distance. Pearson's simple correlation coefficients were estimated between values of the first two canonical variates and values of the original individual traits to determine the relative share of each original trait in the multivariate variation of the treatments [34]. The GenStat v. 18 statistical software package was used for all the analyses. The GenStat v. 18 codes that have been implemented for the analyses are in Appendix A.

#### *2.6. Weather Conditions*

The weather data were collected from a meteorological station located in the Experimental Station Mydlniki-Kraków, Poland.

The sums of precipitation and the average daily air temperature in 2012–2014 differed from the standard multiyear period (1951–2000).

The humidity conditions (Figure 2) are based on the monthly precipitation for each study year. The distribution of precipitation in individual months is important for grainlegume mixture development. According to [35], the total rainfall during the vegetation period of oats in a temperate climate should range from 270 to 400 mm. The water demands of oats increase during their growth, reaching their highest values in June and July. The common vetch also has a high water demand, especially during flowering.

**Figure 2.** Sum of precipitation (mm) during the study.

The amount of precipitation in individual months and years was characterized according to the criterion of [36] for southern Poland, which classifies each month and year as "regular", or as one of three levels of "dryness", or as one of three levels of "excessive rainfall". The April–August of 2012 were dry (86% of the norm). During this year, the months of April and July were regular, May was very dry, August was dry, and June was very humid. The April–August of 2013 were classified as regular (99% of the norm). However, during this year, a large variation in precipitation was found, e.g., the months of April, July, and August were defined as very dry, May was humid, and June was extremely humid. The April–August of 2014 was regular (100.1% of the norm), with May classified as "wet" and June as "very dry" (Figure 3).

**Figure 3.** The sum of precipitation (mm) in the April–August period in the years of study 2012–2014 compared to the multiyear (1951–2000). Descriptors dry and regular correspond to April–August periods of the 1951–2000 multiyear.

Large fluctuations in the air temperature were observed in individual months and years of the study (Figure 4). In all study years, the average temperature (◦C) was higher than the standard multiyear period (1951–2000). The air temperature was classified based on deviations in individual months of the April–August period from the norm for Krakow (Poland), according to [37]. April and June 2012 were warm, while May, July, and August were very warm. In 2013, April and August were regular, May and June were warm, and July was extremely warm. In 2014, April was warm, and May, June, and August were regular. July 2014 was an extremely warm month.

**Figure 4.** Mean temperatures (◦C) during the study and in the 1951–2000 multiyear. Descriptors very warm and warm correspond to the 1951–2000 multiyear.
