**1. Introduction**

A growing demand for food in developed and developing countries as well as natural disasters such as drought, disease, and pests are becoming a major challenge for agricultural production in the 21st century. Today, purely species-specific crops dominate the cultivation of cereals. The opposite option to pure sowing of cereal species may be cereal mixtures, mainly interspecies, which are currently estimated to account for 1% of this group of agricultural crops [1]. Cereals and cereal-and-legume mixtures are an essential link in the transition of sustainable agriculture and organic farming [2]. In Central Europe, a disturbing trend is the high percentage of cereals in the sowing structure, which results in a succession of cereal crops for several years. Moreover, each simplification in the tillage system increases the disturbance of biological balance in the agricultural environment. The dominance of one cereal species, or of one cultivar within a species in a given area, promotes the development of pathogens, which causes a decrease in yields. Cereal mixtures maintain better plant health by increasing the biodiversity of the canopy [3,4].

The increase of plant productivity based on biodiversity is conditioned by more e ffective use of the interrelationships among plants in a mixed crop [5,6]. One of the concepts for increasing plant productivity in this cultivation system is to optimize plant species selection in mixed sowings to make complementary use of available space, water, and nutrients [7,8]. This concept was presented by Li et al. [9] who claimed that the complementary e ffect is associated with a better use of space by one of the components of the mixture, which has not been fully utilized by other components less tolerant to the given habitat conditions. It can be done by using plant architecture as a strategy to allow one member of a mix to capture sunlight that would otherwise be unused. According to Li et al. [9], this concept is considered a spatial complementarity, and the phenomenon of competitiveness in that concept is defined as the properties of the species that are characterized by faster development and better control of the space, which limits the development of other components in mixed sowing [10–12]. In mixed sowing, lower weed infestation, poorer pest infestation, and better resistance to lodging are observed due to the production of lower and more flexible stems [13,14]. Di fferent species in mixtures better penetrate the soil thanks to their di fferent root systems and enable a more e fficient use of fertilization, which can be applied in smaller doses [3]. A two-species mixed sowing, consisting of species of di fferent crop groups, is not a common practice in mechanised systems due to higher labour input, mainly during sowing and harvesting, as well as due to the instability in yields due to weather conditions [3,13,15]. An alternative agrotechnical solution is to compose mixtures or mixes within one group of plants, e.g., cereals. Cereal species mixtures can increase the intra-species diversity of the cropping system diversity by increasing genetic diversity in the canopy. Such use of intra-species diversity is well suited to mechanised systems that are designed to manage a single species, as it can provide benefits from reduced disease, weed, and insect pressure as well as improve yield level and quality per hectare [16,17]. In large farms, where the share of cereals in the farming systems often reaches 75% and where genetic uniformity lead to a biological imbalance in the fields, the use of multispecies cereal mixtures becomes a desirable solution [18].

In central Europe, there is a large variation in soil quality, which is a major problem in terms of increasing the productivity of cereal crops. To minimize the impact of soil variability on yield, mixed sowing is promoted. Hong et al. [6] have shown that, on small and large farms, the yield is determined by the sowing method. The larger the field area, the greater the overall variability of soil quality, which favours yields of mixtures compared to sole crop/pure stand, due to the dominance in the canopy of this cereal species for which the soil characteristics are appropriate. Cultivating naked cultivars of spring barley and oats in pure sowings, due to lower yields, is less economical than pure sowing of the covered forms of these cereals. It is recommended to cultivate naked cultivars of these cereals in mixtures intended as fodder on one's own farm due to better quality of grains of naked forms [19–21]. Mixed sowing increases biodiversity in the fields and contributes to the sustainability of crop production [3,7]. On large farms, the percentage of cereals in the monoculture is high, and crop simplifications often lead to an imbalance in biological diversity. Growing cereals in mixtures contributes to improving crop health. In addition, mixtures are more tolerant of unfavourable weather conditions and varied habitat conditions across the field than the tolerance of pure crop sowings of cereals. Due to lower susceptibility to climatic factors of limiting nature (shortage of precipitation, large temperature fluctuations), barley exhibits higher yield reliability than other spring cereals. The advantage of barley (as a component of the mixture) is the greatest resistance to drought among spring cereals due to a lower transpiration coe fficient and high root suction power. Barley as a component of the mixture, in comparison with other cereal species, is very sensitive to soil acidification [22].

The inclusion of naked barley and naked oats into a mixture increases the protein and fat contents in the grains of the mixture, which contributes to their better forage value. It is advantageous to cultivate naked cereal cultivars in mixtures that are intended for fodder because of better quality of naked grain forms [23]. In the absence of the husk, the metabolised energy of the naked oat kernel can be comparable to or higher than that of wheat [24]. Naked oat kernels have also been shown to have a higher content of metabolised energy, lipids, linoleic acid, protein, essential amino acids, and starch than husked oat cultivars [25,26]. These characteristics make naked oats potentially more suitable as a feed source than other cereals particularly for poultry (MacLeod et al., 2008). Oat grain has a number of nutritional benefits compared to other cereals [27]. It has a high lipid content compared to wheat and barley, which comprises principally unsaturated oleic and linoleic fatty acids as well as high concentrations of the amino acids' lysine, methionine, and cysteine [23]. For human nutrition, they are a source of soluble fibre and β-glucans, which both can have positive e ffects on health [23,28].

The oats are characterized by high phytosanitary properties and, thus, they can reduce the infestation of the mixture canopy by fungal diseases. A novelty of the study lies in the comparison of di fferent spring species' composition of mixtures when taking into account new type cultivars (naked vs. covered) of barley and oats. This creates new possibilities to increase the yield and quality of grains without increasing the expenditure on chemical means of grain production under conditions of sustainable agriculture. In the light of unpredictable environmental variation factors, the grea<sup>t</sup> impediment is choosing the right cultivar or cultivar mixture. Loˇcmele et al. [29] highlighted that it is unclear how many cultivars and which type of cultivar should be used to compose the mixture.

The aim of the study was to compare the yield, protein yield, and net metabolic energy of di fferent variants of spring cereal mixtures with the share of naked cultivars of spring barley and oats at di fferent sowing densities, depending on soil quality. The scientific hypothesis assumed that grain yield di fferentiation within cereal mixture variants would be di fferent than protein and metabolic energy yield diversification due to the lower grain yield, but a higher content of protein and metabolic energy in the grains of naked forms of barley and oats as well as wheat. A higher grain yield is expected from a mixture of hulled forms of spring barley and oats. However, the yield of protein and metabolic energy of mixtures with the share of naked forms of barley, oats, and wheat, should be similar to that of covered forms.

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

The field experiment with di fferent combinations of spring cereal mixtures was conducted as part of field experimentation of Podlaskie Agricultural Advisory Centre in Szepietowo (AAC), Poland (52◦52, 22◦32), in the years 2013–2015. Two, two-factorial field experiments (with the same treatments) on di fferent types of soils were performed. The experiments were conducted on better-quality soil: Albic Luvisols (developed in loamy sand on loam), and on poorer quality soil: Haplic Arenosols (developed in loamy sand on sand), (Table 1). The first (random) factor was study years while the second factor was 10 sowing combinations. Mixture variants di ffered in the species composition—hulled covered barley (*Hordeum vulgare* L. cv. Skarb), naked barley (cv. Gawrosz), covered oats (*Avena sativa* L. cv. Krezus), naked oats (*Avena nuda* L. cv. Nagus), and wheat (*Triticum aestivum* L. cv. Nawra), as well as in sowing density (Table 2). The field experiment was carried out in four replications and the size of a single plot was 15 m<sup>2</sup> (length 10 m, width 1.5 m). Each plot consisted of 12 rows with a row-spacing of 12.5 cm. Grains were treated with thiram (37.5%) and carboxin (37.5%) (Oxafun T) and sowed using an Oyjord plot drill to a depth of 4 cm. After that, the sowing plots were harrowed, using a light harrow. The cultivation of barley, wheat, and oats in pure sowing was well recognized in earlier studies by the authors [22,30]. Leszczy ´nska and Noworolnik [22] proved that productivity of covered oat and barley is related to soil quality. In rich soil (clay soil), oat yields at 4.9 t ha−<sup>1</sup> and barley on 5.36 t ha−1, while in pure (sandy soil) oat yields at 4.46 t ha−<sup>1</sup> and barley on 3.87 t ha−1. Szmigiel and Oleksy [30] indicated that cultivation of covered cultivars of oat or barley in pure sowing was more beneficial than cultivation of naked cultivars of this species. The covered oat cv. 'Chwat' yielded 58% higher (5.85 t ha−1) compared to naked cv. 'Akt' (3.71 t ha−1) while covered barley cv. 'Rodos' yielded 11% higher (4.41 t ha−1) compared to naked cv. 'Rastik' (3.97 t ha−1). Zaj ˛ac et al. [18] indicated that wheat in pure sowing was yielded at 8 t ha−1.


**Table 1.** Nutrient content and soil pH of the experimental field in AAC Szepietowo in 2013–2015.


**Table 2.** Experimental scheme with spring cereal mixtures.

\* The sowing of each component in the mixture results from the recommended quantity for each species in pure sowing in accordance with the agricultural practice.

The tillage included pre-winter ploughing. In spring, a combined implement for soil tillage was used. The seeds before sowing were treated with Scenic 080 FS (100 mL + 500 L water/100 kg grain). The row spacing was 12 cm. In the autumn, phosphorus and potassium fertilization at a dose of 20 kg ha−<sup>1</sup> P2O5 (triple superphosphate) and 80 kg ha−<sup>1</sup> K2O (potassium salt 60%) were applied. Nitrogen fertilization in the dose of 60 kg ha−<sup>1</sup> N (ammonium nitrate 34%) was applied before sowing. Sowing was performed between 5–15 April. At harvest, the grain yield of mixtures from each plot was weighed, and grain samples were taken to determine the yield sharing of individual partners in the mixture, 1000 grain weight, and total protein content. The harvest of cereal mixtures was carried out at the stage of full maturity of cereals (Biologische Bundesanstalt, Bundessortenamt i Chemical industry-BBCH 89) in the period from 6 to 15 August.

Herbicides were used during the years of research, including: 1. Puma Uniwersal 069 EW (content of the active substance: phenoxaprop-P-ethyl- 69 g L−<sup>1</sup> ethylester of 2-(4-(6-chloro-1,3- benzoxazole-2-yloxy) phenoxy)propanoic acid. 2. Secateurs 125 OD (iodosulfuron-methyl-sodium 25 g <sup>L</sup>−1, amidosulfuron 100 g <sup>L</sup>−1), or 3. Weedlock Trio 540 SL (mecoprop (compound of the phenoxy acid group—as potassium salt)–300 g L−<sup>1</sup> (24.31%) M C PA (compound of the phenoxy acid group—as potassium salt)—200 g L−<sup>1</sup> (16.20%) dicamba (a compound from a group of benzoic acid derivatives—in the form of potassium salt) 40 g L−<sup>1</sup> (3.24%), which effectively destroyed dicotyledonous weeds. The problem (in a few treatments) was the occurrence of wild oats on mixed plots with oats (these weeds were removed manually).

The fungicide Soligor 425 EC (active ingredient: prothioconazole 53 g L−<sup>1</sup> (5.4%), spiroxamine 224 g L−<sup>1</sup> (22.9%), and tebuconazole 148 g L−<sup>1</sup> (15.1%), were used during the growing season. In the years of the study, there was a low level of cereal leaf beetle infestation of cereals below the economic harmfulness threshold.

The grain yield was determined at 15% humidity. Protein content was determined by the Kiejdahl method in the Main Laboratory of Chemical Analyses of IUNG-PIB in Pulawy. The grain energy value of cereal mixture was determined (taking into account the share of components) by converting the grain yield into net energy (MJ), when assuming the values calculated for pigs based on animal nutrition standards [31].

The yield suppression ratio (YSR) of the individual components of the mixture was calculated according to the methodology that Weigelt and Jolliffe presented [32]. The values of the yield suppression ratio were calculated from the ratio of the percentage share (weight) of grains of individual species in the yield to their percentage in the sowing material.

Analysis of variance (ANOVA) for randomized complete block design was performed for most data using the Statistica® software computer program package. Treatment means were compared using Tukey test at *p* = 0.05. Subsequently, six orthogonal contrast for selected treatment were performed using Statistica® software.

The following data were used to calculate the protein yield and energy value of the mixture grain yield: mixture grain yield, percentage share of components in grain yield, protein content in grain of individual components, and the value of metabolic energy of 1 kg of grain.

Methods of analyses were conducted according to methodology. Analysis of *p*-available was conducted based on the colorimetric assay and the Egner-Riehm DL method (PN-R-04023, 1996). Analysis of K-availability was conducted based on the photometric method (PN-R-04022, 1996).
