1. Introduction
Intercropping is a management technique in which two or more species are grown on the same area at the same time [
1]. One of the most common intercropping practices is the cultivation of mixtures of legumes with cereals [
2]. Such a crop can be used for grain feed, green fodder, silage or green manure [
3]. Due to the global increase in demand for animal products [
4], the need for feeds with high nutritional value continues to grow [
5]. Cereals and legumes are important forage crops because of their nutritional value [
6]. However, forage obtained solely from cereals is characterized by a relatively low protein content; therefore, the inclusion of legumes that contain high levels of protein significantly improves the quality of the forage obtained [
7]. In addition to the higher quality of forage obtained from growing legumes with cereals, such cropping systems have higher yields and greater stability compared to monoculture, which is attributed to more efficient use of light, water and nutrients [
2]. Higher yields are also often linked to better suppression of weeds [
8] and diseases [
9] by intercropping. According to Nelson et al. [
10], sowing mixtures also mitigates the effects of water scarcity and drought on crop production. This is particularly important as climate change continues and periods of precipitation deficiency become more frequent [
11]. An important benefit of including legumes in crops with cereals is their symbiosis with nodule bacteria that guarantees the fixation of atmospheric nitrogen into a plant-available form [
12]. Biologically bound nitrogen can be utilized by both the legume and the crops grown with them in mixture [
13]. In addition, strong competition for soil nitrogen between legumes and cereals leads to an increase in atmospheric nitrogen fixation by legumes compared to single crops [
14]. In addition, a benefit of intercropping legumes with cereals is their positive impact on soil quality. The reason for this is primarily due to the benefits of the legume component, whose cultivation is known for restoring soil fertility [
15]. In addition to the mentioned increase in the pool of available nitrogen in the soil, legumes also have the ability to modify the pH of the rhizosphere and thus increase the availability of other nutrients. Also, the fact that legumes are deeply rooted can have a positive effect on soil structure [
16]. Intercropping systems are also characterized by a greater diversity of roots and residues, which improves the energy supply of soil microbial biomass through the release of secretions such as amino acids and organic acids [
17]. As a result, the effect of these residues on soil microbial biomass is more noticeable than in monoculture cultivation. The diversity of microbial community structure in the rhizosphere soil in an intercropping system is generally greater than in a monocropping system [
18]. Thus, intercropping legumes with cereals can halt the downward trend of productivity in a continuous cereal–cereal system [
19]. In order for the aforementioned benefits to be achieved to the highest possible degree, it is important to select the right plant species as well as the proportion of their sowing in order not to make the crop overly competitive [
3]. Lupine is one of the main legumes grown in Europe [
20] and can be a good alternative to soybeans due to its high protein content [
21]. Lupine species additionally have high tolerance to various environmental stresses, excess nitrates, lime or salinity and therefore can be grown in many areas [
22]. In contrast, lupine crops, like many legumes, are susceptible to weed infestation and yield variability due to water shortages [
23]. The limitation of using lupins for feed purposes for many years has been due to the presence of antinutritional factors, mainly quinolizidine alkaloids. Alkaloids give the plants a bitter taste, thereby reducing their nutritional value, intake and digestibility [
24]. The level of alkaloids is generally higher in the seeds of the plants than in the vegetative parts [
25]. Additionally, modern lupine varieties contain very low levels of alkaloids [
26]. Therefore, this plant can be successfully used as feed for livestock [
3]. Triticale, on the other hand, can produce large amounts of biomass, is stable in yield and grows well under arid and semi-arid conditions, and it is suitable for most soil types [
27]. In contrast, the quality of forage obtained from triticale is better than oats, but slightly worse than barley or corn [
28]. Barley, on the other hand, can grow in unfavorable agroclimatic conditions [
29] and marginal environments unsuitable for other cereals [
30]. Potentially, therefore, the simultaneous cultivation of narrowleaf lupine with spring triticale or spring barley can allow farms to increase self-sufficiency in the production of green fodder with a relatively high protein content.
Nitrogen and water deficit are considered to be the main factors limiting crop yields [
31]. Nitrogen, which is involved in physiological and metabolic activities of plants, is an important nutrient element in ecosystems and is an essential element for plant growth [
32]. Improper use of nitrogen fertilizers not only increases environmental costs and pollution from agricultural production but also reduces nitrogen use efficiency [
33]. Intercropping legumes with cereals can reduce the need for mineral fertilizers and thus lower production costs and reduce the environmental impact of agriculture [
34]. Rational management of mineral fertilizers is additionally an important part of agriculture due to the fact that they are the main component of cost intensity in cultivation [
35], and the global efficiency of nitrogen use from mineral fertilizers is only approximately 35% [
36].
The variability in yield and quality of yields obtained from intercropping legumes with cereals in relation to soil and climatic conditions and the share and species of each component in the sowing [
3] suggests the need for further field research in this research area. In addition, the high use of nitrogen fertilizers in agriculture and their negative impact on the environment highlights the need to develop field crop management techniques that reduce the use of mineral fertilizers without yield losses [
16]. Therefore, field research was carried out with the aim of evaluating how different mutual sowing ratios of narrowleaf lupine with spring triticale or spring barley and different levels of mineral nitrogen fertilization affect the obtained dry matter yield, total protein content and total protein yield. The research hypothesis assumed that the appropriate share of components in sowing and optimized doses of mineral nitrogen fertilization would produce optimal yields of satisfactory quality.
4. Discussion
Growing mixtures of legumes with cereals is considered an effective way to improve forage yields [
38]. Many scientific studies report higher yields compared to monoculture crops; however, the yields largely depend on the proportion of components in the sown mixtures [
39]. Research by Wang et al. [
40] demonstrated higher forage yields from 28.7 to 66.4% when the sowing ratio of oats to vetch was 50 + 50% compared to monocultures. On the other hand, Šarūnaitė et al. [
34] demonstrated 9% higher biomass yields in a crop with an equal share of pea and oat components, and 13% higher when the share was 60 + 40%, respectively, compared to a pea crop. On the other hand, Sohail et al. [
7], at a sowing rate of 70 + 30% for barley or oats + vetch, obtained higher biomass yields of 52 to 57% compared to the legume and comparable yields to the grain crop. The yield increases obtained for mixed crops compared to monocultures in our study are similar to those obtained by the cited authors. They amounted for mixtures of lupine with barley from 18 to 41% and mixtures with triticale from 40 to 66%. However, in contrast to the results presented by Baxevanos et al. [
41], the obtained yields of mixed crops were also higher than those of cereal monocultures. The increase in yield of mixtures with respect to monocultures is attributed to greater efficiency in the use of resources during plant growth, such as water, nutrients and light [
42]. This is supported by the results presented by Bouras et al. [
43], in which higher water use was demonstrated by the biomass of legume–legume mixtures, especially under conditions of limited water availability. On the other hand, research by Umesh et al. [
44] proved higher light interception and effective radiation utilization by mixtures of corn and sorghum with legumes compared to single crops. In turn, better nutrient utilization is linked to the root morphology of cereals and legumes [
45]. Cereals are characterized by roots that are much finer and generally occupy the top layers of the soil compared to lupins with a tap root [
46,
47]. Thus, cereals can explore larger volumes of topsoil [
48], forcing lupins to draw nutrients from deeper layers. Temporal resource requirements are also important. According to research by other authors [
23,
49,
50], barley and triticale show high biomass production in the early growth stages, much earlier than peas or lupins. However, from the flowering stage onward the growth rate of lupine increased significantly, while that of triticale decreased significantly [
23]. Thus, the significantly shifted period of maximum growth allows for complementarity of resource use over time.
Mineral fertilization with nitrogen in many conducted studies led to an increase in the yield of legume–cereal mixtures [
43,
51,
52,
53]. An analogous relationship was obtained in the studies conducted. According to Salinas-Roco et al. [
14], N fertilization in legume–cereal crops primarily benefits cereals by promoting increased photosynthesis and accumulation of photosynthetic pigments, resulting in better growth. Thus, cereals show high competition with legumes, especially in terms of soil nitrogen resources [
54]. Increased competition from cereals forces legumes, in turn, to rely heavily on atmospheric nitrogen fixation to meet their needs [
55]. The mutual competition for nitrogen between cereals and legumes has an additional agricultural benefit. Greater nitrogen uptake by cereals helps reduce nodule inhibition in legumes as a result of excess nitrogen in the soil [
14]. On the other hand, with high levels of biological nitrogen fixation, up to 70% of the nitrogen can be transferred to the soil [
56]. Therefore, this nitrogen can benefit crops grown in mixture with legumes [
57,
58]. However, field research demonstrated a lack of yield increase effect between nitrogen rates of 40 and 60 kg N ha
−1, while narrowleaf lupine had higher yields at lower nitrogen fertilization rates. Also, studies by Salinas-Roco et al. [
14] and Carton et al. [
23] found no significant improvement in the DM yields of legumes and mixtures with cereals with increasing nitrogen fertilization. Such crop response can be explained by the fact that legumes, as a result of the efficient fixation of atmospheric nitrogen, compensate for the reduced availability of nitrogen from mineral fertilizers [
59]. On the other hand, the higher DM yield of lupins when fertilized with 40 rather than 60 kg N ha
−1 can also be linked to the formation of nodules and thus the fixation of atmospheric nitrogen. Latati et al. [
60] indicated a higher biomass of chickpea nodules at reduced soil nitrogen content; thus, excess nitrogen causes inhibition of nodule formation [
61]. According to Tamiru et al. [
62], legumes can fix up to 111 kg N ha
−1 in a year. Thus, the effective fixation of atmospheric nitrogen can far exceed the amount of nitrogen supplied from mineral fertilizers, which can translate into the yield obtained. The availability of nutrients in the soil and the proper selection of crops to be grown in a mixture are linked to the quality of the resulting forage [
63,
64]. Legumes generally have a higher total protein content compared to cereals [
65], which was also confirmed in the presented field experiment. Thus, with an increase in the sowing rate of narrowleaf lupine, an increase in the concentration of total protein in the obtained green mass was recorded. Also, other authors recorded an increase in the total protein content of mixtures in legumes compared to cereal crops [
66,
67,
68]. The average increase in total protein content obtained in our study at 43% for mixtures with barley and 30% for mixtures with triticale was much higher than that reported by Soufan and Al-Suhaibani [
69] at 10% with barley–pea mixtures. The increase in protein content in the biomass obtained can also be linked to increased nitrogen availability, as higher levels of available nitrogen result in better protein biosynthesis [
70]. Similar to the studies conducted by Krga et al. [
71] and Tamta et al. [
72], an increase in the level of nitrogen fertilization resulted in an increase in the total protein content of the obtained yield. However, in the study conducted, the total protein content at 60 and 40 kg N ha
−1 showed no significant differences. This may be due to the aforementioned compensation of the amount of available nitrogen due to the fixation of atmospheric nitrogen by legumes. According to Stagnari et al. [
45], non-legumes can benefit from the additional nitrogen released into the soil by legumes. According to Li et al. [
58], legumes can provide up to 15% of the nitrogen in cereals grown in mixtures. Also, Clark et al. [
57] indicated that peas grown with barley contributed 19% of the nitrogen to the cereal plant after 70 days of cultivation. The total protein yield obtained depends on the total protein content of the biomass and the yields obtained. However, in both the authors’ earlier studies [
39] and the one presented here, the yield of the obtained green matter shows a greater influence on the yield of total protein. An analogous relationship was also presented by Faligowska [
12] in the case of lupine cultivation in monoculture. As a result, according to the findings of Wang et al. [
64], the increase in yield of mixtures compared to monoculture crops resulted in an increase in total protein yield in our study, except for mixtures with a predominance of cereals in sowing. Baxevanos et al. [
41], in their study of pea–oat mixtures, obtained a 27% increase in total protein yield compared to oat monoculture. On the other hand, Wang et al. [
64] reported an increase in total protein yield in mixtures of vetch with oats by 52% in relation to oats and by 150% in the case of vetch. The average increases in total protein yield obtained in their study were higher, at 72% for mixtures with barley and 54% for mixtures with triticale, compared to cereal monoculture. In contrast, the increase in relation to narrowleaf lupine was much lower (6–18%). Similarly to the studies conducted by other authors [
71,
73,
74], despite the significantly higher total protein content of narrowleaf lupine biomass, its low green matter yields resulted in low total protein yields.
Regarding LER, in many studies different authors showed values greater than 1 in the cultivation of different legume–cereal mixtures, indicating higher resource use efficiency of mixtures compared to monocultures [
75,
76,
77]. Also in our study, LER values for all tested mixtures were higher than unity. In addition, the LER value decreased as the seeding rate of barley or triticale in the mixture increased. Similar observations were also made by Dhima et al. [
78] in mixtures of faba bean with oats and Salinas-Roco et al. [
14] with mixtures of faba bean or pea with wheat. In our study, higher LER values were obtained with fertilizations of 40 and 60 kg N ha
−1 compared to those obtained with fertilizations of 0 and 20 kg N ha
−1. On the other hand, the results obtained by other authors [
51,
55,
79] demonstrated the opposite trend, where an increase in mineral nitrogen fertilization led to a decrease in LER value. However, the cited studies analyzed much higher nitrogen fertilization and compared it to no fertilization. According to Kamran et al. [
80], in legumes it is important to pre-fertilize with low doses of nitrogen in order to ensure optimal nitrogen availability at the early stages of growth when atmospheric nitrogen fixation does not occur. This may explain the increase in LER obtained in our study at the proposed nitrogen fertilization levels.
Due to the climate changes observed in recent years, manifested by increasing temperatures and water shortages [
11], it is important to analyze the yield of plants under varying weather conditions. In our own research, the lowest yields of mixtures of narrowleaf lupine with cereals were obtained in the year with the lowest precipitation during the growing season. Similar results were also obtained by Rad et al. [
81] in their study of other legume–cereal mixtures. However, it is crucial to consider the different stages of plant development and the availability of water during this time [
82]. According to research by Alghabari et al. [
83], water deficits during the flowering period of spring barley can lead to yield losses of approximately 40%, while according to Anwaar et al. [
84], water deficits during the reproductive stages of wheat lead to losses of 30%. On the other hand, in the case of legumes, various authors have observed higher yield losses occurring with water deficits at the flowering or pod beginning stages, for chickpea yield reductions of up to 90% [
85], soybeans by 45% [
86] or beans by 85% [
87]. This may explain the lower yields of mixtures obtained in our study in a year with a higher total precipitation but a much lower total during the flowering period of the crops grown. In addition, it can be concluded that cereals tolerate water shortages better compared to legumes. According to Ding et al. [
88], cereals maintain positive tissue turgor through osmotic regulation under even high water stresses, thereby maintaining stability in yield. Thus, cereals are more stable in yield under varying weather conditions [
66]. Therefore, cereals, when grown with legumes, can act as a yield-stabilizing component, even during growing seasons with rainfall deficiency. In the study conducted, the total protein content of the obtained green forage was found to decrease with increasing precipitation during the growing season. Lower total protein content in the biomass of millet with legumes in a year with higher precipitation was also found by Giannoulis et al. [
89] and Gill and Omokanye [
90] with many other legume–cereal mixtures. Crops under drought stress increase root length and thus root biomass as a result, and they are able to absorb greater amounts of water and nutrients from the soil [
91]. In addition, water shortages result in reduced plant height [
92], reduced leaf number [
93] and leaf area, which leads to lower total biomass [
94]. Based on these statements, it can be assumed that the reduction in the biomass of the vegetative parts of the plants as a result of water shortage and the possible greater availability of nutrients for plant uptake dilution of components will be lower, which will translate into higher nutrient content, including total protein. According to the specified correlation, lower yields in years with limited rainfall translated into lower total protein yields, despite higher total protein contents. In contrast, similar to the studies of Yang et al. [
51] and Lauriault et al. [
5], the LER value was higher in years with less precipitation during the growing season. The higher LER in years with rainfall deficiency may be due to the higher yield reduction of legumes compared to cereals. Thus, higher productivity of mixtures and better resource utilization became more apparent in dry years [
71].