1. Introduction
The cultivation of spring cereal mixtures (SCMs) is an element of crop rotation typical for Polish agriculture [
1]. In 2015, cereals accounted for 73.3% of the total crop area of Poland, including 10.7% of SCMs. In 2017 and 2018, cereals accounted for 70.1% and 72.1%, respectively, and in the same years, the SCMs accounted for 11.6% and 12.7%, respectively [
2].
The spring cereal mixtures are applied both in organic and sustainable agricultural systems [
3,
4,
5], mainly as a source of feed (grains) for livestock [
6,
7,
8]. This method of crop cultivation involves the simultaneous sowing of usually two different species of spring cereals, in different proportions [
9,
10]; their grains are mixed before sowing.
Cultivation of SCMs has many advantages and is desirable in sustainable agriculture owing to current ecological trends related to reducing the amount of mineral fertilizers and pesticides, as a part of integrated pest management [
11,
12,
13]. It results in less environmental pollution and lower outlays on agronomic practices [
14]. This is because of the fact that SCMs are less infested by pathogens and insect pests than pure sowings [
15,
16], which results from a reduced number of plants of a given species susceptible to a particular pest [
17]. The components of mixtures are characterized by a different growth pattern [
10], and as a result, they better cover soil, protecting it from water loss. This also results in a reduced weed infestation, as species in the mixture compete more effectively with weeds [
18] and promote biodiversity in a canopy [
19].
Many authors [
20,
21,
22] confirmed the relationship between grain yield and leaf area index (LAI). LAI is referred to as the ratio of the surface of assimilation organs of a crop, mainly leaves, to the surface of soil. The LAI value depends on genetic characteristics and habitat factors [
23]. For various crops, the LAI is several times larger than the surface of the soil, for example, for small-seeded legumes, 4–5 times, and for other crops, 2–4 times [
24].
The SCMs are characterized by greater yield stability than pure sowing [
25]. This is because of, among others, the fact that, in the adverse weather and soil conditions (e.g., drought) for the first species (component), the second component of the SCM finds more favorable conditions and increases the yield, compensating for the lower yield of the first one [
26,
27]. This compensation is also associated with the complementary use of soil resources, that is, nutrients and water, which results from the diverse architecture of the root systems of SCMs. Mixtures are an important element in increasing species diversity in crop rotation [
28,
29], which is supportive to stabilizing the yield of following crops [
30].
A common component of SCMs is oats, called a phytosanitary plant [
31]. Oats are a cereal species with a very well-developed root system, capable of taking water and nutrients from deeper layers of soil. Moreover, as an allelopathic crop, they influence, by root exudates, the abundance and composition of soil-pests, as well as the composition of soil microorganisms, which together contribute to the soil biological activity [
32]. In this way, oats also stimulate the growth and development of the other component of SCM [
33].
The available literature lacks current studies on the yield, competitiveness of components, and economic aspects of the cultivation of SCMs in different cropping systems, especially in the long term and in the extensive conditions of mountainous agriculture. Decisions related to the selection of the production structure are made based on both the production/quality characteristics and economic results [
34], also including the system of agricultural subsidies [
35]. Moreover, subsidies in the mountainous areas of southern Poland are one of the most important and motivating factors for a farmer to produce organically [
3]. All of this became a reason for undertaking our research. The profitability of cultivation is determined by the relation between the value of the obtained crop and the incurred production costs, which include all elements throughout the production process. For this reason, our research also included economic analysis of standard gross margin of SCMs’ cultivation.
The aims of our study were to (i) analyze the yield, competitiveness, and leaf area index (LAI); and (ii) assess the economic indicators of spring cereals in pure or mixed sowings in integrated or organic crop rotations, over nine years (three rotations of crops) in the mountainous area of southern Poland.
2. Materials and Methods
A field experiment was carried out in the years 2011–2019 in the Mountainous Experimental Station in Czyrna near Krynica Górska, southern Poland (545 m a.s.l.; 49°25’, N 20°58’ E). The soil was acid (pH
KCl = 5.1), brown soil—Cambisol [
36], formed from weathered flysch material, composed of loam with a medium skeleton content. The chemical composition of soil was as follows: 0.22% N
tot.; 46.2 mg kg
−1 soil P; 203.3 mg kg
−1 soil K; 1.84% C
org. The experiment was set up in a two-factorial split-block design, with four replications. The total area of a single plot was of 30.8 m
2, with 22 m
2 of a harvested area. Three full rotations of the crops (nine years) were included in the results.
There were two systems (first factor) of the experiment: (1) integrated, with mineral fertilization and chemical pesticides; and (2) organic, without any synthetic additives. Each of the systems was composed of six three-field crop rotations (second factor): (1) potato fertilized with manure (33 t ha
−1); (2) spring cereal pure sowing or a spring cereals mixture—six variants in total (
Table 1); and (3) spring vetch. The density of cereals in the mixture was reduced by 50%, in relation to the pure sowings. All of the systems and crops were present each year, which means that each crop was grown nine times throughout the whole study period. Three full rotations of the crops (nine years) were included in the results.
In the integrated crop rotation, a mineral fertilization for cereals in pure sowings and their mixtures was balanced, based on the content of nutrients in soil, quality of the preceding crop, and forecasted yield. In autumn, 34 kg ha−1 P and 55.6 kg ha−1 K were applied before a deep ploughing in October. For cereals in spring, a total dose of 72.0 kg ha−1 N was divided into two equal doses, one applied before sowing and a second in the shoot formation. Grains were coated with karboxine + thiram (60 g + 60 g per 100 kg of grains). Weeds in the pure sowings and mixtures were controlled by tribenuron methyl (12 g ha−1).
In the organic crop rotation, no chemical fertilizers nor pesticides were applied. Weeds in the cereals were mechanically controlled by a Weeder harrow, run two times in by the end of tillering/beginning of shooting (BBCH 29–30 [
37]).
In the stage of grains development (BBCH 70–71), samples of cereals were collected from each plot from 1 m
2. An area of leaves was measured from 20 shoots per sample, using an LI-COR 3100 Area Meter (LI-COR Biosciences GmbH, Bad Homburg vor der Höhe, Germany). Next, the average area of leaves per shoot was multiplied by the total number of shoots per 1m
2 [
24]. On this basis, a leaf area index (LAI) was calculated:
At harvest, the grain was collected from each plot (22 m2). The yield was expressed as per ha at 15% of seed moisture content.
To assess the competition between the components of the mixtures, two competition indices were calculated: land equivalent ratio (LER) (2) [
38] and competitive ratio (CR) (3) [
39].
where Y
ii—yield of species
i in a pure sowing, Y
jj—yield of species
j in a pure sowing, Y
ij—yield of species
i in a mixed sowing with a species
j, and Y
ji—yield of species
j in a mixed sowing with a species
i.
If LER value is greater than one (LER > 1), it means that the mixture is more effective than the pure sowing [
40,
41].
where
Zij—proportion of species
i in the mixture with species
j and
Zji—proportion of species
j in the mixture with species
i.
If CR = 1, it means that there are equal competitive abilities of species
i and
j. If CR
i > 1, it means that species
i is more competitive than species
j. If CR
i < 1, it means that species
j is more competitive than species
i [
40,
41,
42].
The economic indicators were calculated. The amount of cash outlay on the means of production was taken as the basis for the agricultural techniques used in the experiment, as well as the consumption of pesticides, fertilizers, and seeds. The values were calculated per area of 1 hectare. The commodity value of the harvested crops and the prices of the means of production were according to data contained in the market analyses developed at the Department of Market Research IERiGZ-PIB, Warsaw, Poland [
43]. An additional source of data was the farm production calculations compiled by the Department of Economics and Agricultural Management MODR, Karniowice, Poland [
44]. All calculations took into account the prices of the last, that is, the year 2019. The average yield of crops during the 2011–2019 study period was included in the calculations. The amount of human labor expenditure was adopted [
45]. The costs of agrotechnical operations were determined using the method [
46]. The standard gross margin was calculated from the difference between the value of products obtained and the direct costs incurred. The direct profitability index, which characterizes the relation of production value to direct costs, was determined [
47]. The labor consumption in the cultivation of cereals and cereal mixtures was 9.5 working hours per ha
−1 and was based on the workload involved in the experiment.
The mean value of nine years was used in the statistical analysis. Before the examination, the results of the experiment were tested for normality of distribution as well as homogeneity of variance by Shapiro–Wilk and Brown–Forsythe tests (Statistica PL ver. 13.1, StatSoft, Krakow, Poland). Both tests turned insignificant values (p = 0.1 for both tests), a basis for performing the analysis of variance (ANOVA). The results were subjected to a two-factor ANOVA for a split-block design with four replications, using FR-ANALWAR-4.3 Microsoft Excel-based package (author: Prof. F. Rudnicki; UTP University of Science and Technology, Bydgoszcz, Poland). The significance of differences between means was tested using the Tukey test (p ≤ 0.05).
Weather Conditions
The weather data were collected from the meteorological station located in the Mountainous Experimental Station in Czyrna near Krynica Górska. The average precipitation in January, April, July, and August for the years 2011–2019 was similar for the multi-year (1962–1990) period. On average, the precipitation in May 2011–2019 was ca. 17 mm more, and in June 2011–2019 was ca. 24 mm less, than for the multi-year period (
Table 2).
The average temperature for the vegetative period (April–August) of the 2011–2019 years was 2.4 °C higher as compared with a similar period of a multi-year (
Table 3). On average, April and August of the 2011–2019 years were particularly warmer than for a multi-year, by 2.8 and 3.4 °C, respectively. Moreover, the average temperatures in January and May–July of 2011–2019 were ca. 2 °C higher from those of the multi-year period (
Table 3).
4. Discussion
Our study presents the results of a nine-year-long field experiment with six different crop rotations, each in two systems—integrated and organic. The results indicated that the average yield of spring cereal grain obtained during the next nine growing seasons in the organic crop rotation was about 18% lower than in the integrated one. This mainly results from the lack of use of easily absorbable fertilizers, as well as pesticides in the organic system. As shown by Kumar et al. [
48], at low nutrient availability, especially during early growth of cereals, higher investments in root system development can significantly trade off with aboveground productivity, and strong competition can further strengthen such effects.
It was also found that, in both systems, the yielding of two-species spring cereal mixtures (SCMs) was higher than the cereals in pure sowing, which is consistent with the results of other authors [
26,
27]. This phenomenon consists of a number of factors, including the complementary use of habitat resources [
49] or mutually stimulating allelopathic effect of cereals [
50].
Of the three different SCMs, oats with spring barley, oats with spring triticale, and spring triticale with spring barley, higher yields were observed for the mixtures with barley as a component. A mixture of oats and barley was characterized by a particularly high grain yield. Barley was the dominant component in this mixture, which posed a strong competitive effect on oats, as indicated by its high competitiveness ratio (CR = 1.06). Moreover, the barley yield in the mixture with oats was high, and even higher than in pure sowing, taking into account that the plant density reduced by half in mixture compared with pure sowing. Despite the dominance of barley in the mixture with oats, both components of this mixture act complementarily. Spring barley is a low cereal, but with a fast growth rate, high tillering, and a short ripening period [
51]. On the contrary, oats are a high cereal, ripening relatively late. As pointed by Shaaf et al. [
52], faster initial growth of spring barley favors its stronger tillering. This results in a competitive advantage of spring barley over oats in the early stages of growth [
53,
54]. Sobkowicz [
55] points out that, in the phase of emergence, a competition of root systems for soil resources is more important than those of the aboveground parts for light. This applies especially to spring barley, which, in the early phases of growth, produces a large root system [
55]. According to Cousens [
56], competition for light begins in the tillering phase. The competitive advantage of oats over spring barley begins in the flowering phase. From this phase, plants of oats are higher than spring barley plants. As pointed by Hecht et al. [
57], in later growth phases, when barley density is higher, the stem mass fraction increases, while the root mass fraction decreases. In the later growth period (watery ripe, BBCH 71), higher plants of oats develop greater panicles and grains [
53]. So-called height convergence may be observed for the mixtures; specifically, a shortening of the long-culmed cereal species and an increase in the length of the short-culmed species. As a result, the mixtures have a decreased lodging and a higher yield [
58]. This phenomenon can also be observed for the mixture of oats and barley [
59].
In the present study, a high value of the land equivalent ratio (LER), which is an indicator of crop productivity [
60], was also found for the SCMs. This is consistent with the results of other authors [
61]. Interestingly, the average sum of LERs for the SCMs in the organic system was equal to 1.11 and was significantly higher than for those SCMs in the integrated system. This may indicate a complementary and more effective use by the components of SCMs of limited habitat resources, especially in the organic system. Rudnicki [
62] showed that, as soil conditions deteriorate, the SCMs are more effective compared with pure sowing. However, at better and fertilized crop stands, the yields of mixtures are similar to the yields of pure sowing. Among the examined SCMs, the highest LER value was recorded for a mixture of oats with spring barley (LER = 1.14). This result further confirms the complementarity of the components of this mixture.
In the scientific literature so far, there are no detailed results of studies on the leaf area index (LAI) of SCMs. Available studies on LAI of pure cereal sowing show that there is a directly proportional relationship between this trait and grain yield [
63]. Our results may partly explain the tendency to obtain higher yields of SCMs in comparison with pure sowing. The results of this study showed that, in both SCMs containing barley, the LAI value was the highest, and the presence of barley affected this result. This likely resulted from the difference in the height of the components of the mixtures. Barley, as a low cereal, develops leaves in the lower layers of the canopy, and effectively uses space for assimilation of photosynthetically active radiation.
Organic farming is perceived as an agricultural system that balances multiple sustainability goals by promoting global food and ecosystem security. Whether organic agriculture can expand is determined by its economic competitiveness with the other agricultural systems [
64,
65]. In our study, we found that the average value of standard gross margin without subsidies was almost twofold higher in the organic crop rotation than in the integrated one. This result is perspective for an organic system and is in accordance with research by Crowder and Reganold [
64]. They examined the financial performance of organic and conventional agriculture by conducting a meta-analysis of a global dataset spanning 55 crops grown on five continents. They found out that, without organic premiums, benefit/cost ratios and net present values of organic agriculture were significantly lower than those for conventional agriculture. However, when actual premiums were applied, organic agriculture was significantly more profitable (22–35%) and had higher benefit/cost ratios (20–24%) than conventional agriculture. The authors conclude that organic agriculture can continue to expand even if premiums decline [
64]. On the contrary, Rosa-Schleich et al. [
66] underline that the ecological benefits for the farmer were partly insufficient to outbalance economic costs in the short term, even though these practices have the potential to lead to higher and more stable yields, increase profitability, and reduce risks in the long term. Still, ecological-economic performance of organic practices is highly context-dependent [
66]. One of the factors that can increase profits and reduce the risks of the organic cropping system is a proper selection of crops [
67,
68]. In our research, this condition was met by including pure sowing of spring barley in the crop rotation. As shown by Omokanye et al. [
8], the profits from cultivating mixtures are variable, and not always higher than those of selected crops in pure sowings. Moreover, a significant role in our research was played by subsidies, which are of the highest importance in both the organic system as well as the mountainous areas of southern Poland, where the less favored conditions occur [
35].
5. Conclusions
The results of our long-term research carried out in the mountainous areas of southern Poland revealed that yield of SCMs and leaf area index in pure and mixed sowing in the organic crop rotation was lower than in the integrated one, by 18% and 16%, respectively. At the same time, the average yield of spring cereal mixtures with barley was higher than that of pure cereal sowing. Under the conditions studied, the highest yield was obtained for a mixture of oats and spring barley, which could be partially explained by the higher leaf area index value for this mixture (LAI = 2.19). The yields of the other mixtures (oats with spring triticale and spring triticale with spring barley) were higher than the yield of the one of the components of the mixture in pure sowing. Moreover, the average sum of LERs for mixtures in the organic system was 1.11 and was significantly higher than for mixtures in the integrated system. In mixtures, barley displayed the highest competitiveness ratio. The analysis of the land equivalent ratio (LER) also showed that, under the examined conditions, spring cereal mixtures are a more effective form of cultivation than pure sowings. Each time, LER values for the mixtures exceeded 1. At the same time, despite lower yields of spring cereals in the organic crop rotation, the average value of standard gross margin without subsidies in the organic crop rotation was almost twice as high as in the integrated one. Among pure and mixed sowings, the highest value of standard gross margin without subsidies was found for spring barley cultivation. Summing up, ecological cultivation of spring cereal mixtures, having many pro-environmental values and showing a standard gross margin, should be recommended especially in the mountainous areas of southern Poland.