The Biomass Productivity of Maize, Hemp and Faba Bean Multi-Crops

Abstract

Crop biomass is valuable not only from a nutritional and fodder point of view, but also from an energetic point of view. The main task is to increase biomass production while maintaining low nutrient and pesticide requirements and low ecological footprint. A stationary three-year field experiment was performed at the Experimental Station of Vytautas Magnus University, Lithuania, in 2020–2022. Single crops and mixtures of maize, hemp, and faba bean were investigated in terms of their biomass productivity. Results showed that as the crop diversification increases, the productivity of maize and hemp grown in the mixture decreases, while faba bean ensures high dried biomass productivity. During the three years of the experimentation, the highest total dried biomass was found in the ternary crop: on average, 1495.4 g m⁻² per year. The highest dried biomass of the ternary crop was established in the first year of experimentation: 2081.1 g m⁻². A decrease in biomass yields in the second and third years of the experiment was observed in all treatment plots. The results suggest that the low fertilization level (totally N 45, P 45, K 45 kg ha⁻¹) in the experiment should be increased if multi-crop cultivations are to be regrown during vegetative seasons.

1. Introduction

At present, humanity is facing increasing challenges; one of them is to prevent the climate change and to find ways to protect the nature from the damage caused to it. Solid fuel reserves are running out, so it is necessary to find energy alternatives to provide energy without depleting nature [1]. Plant biomass is a renewable energy resource that can be efficiently used for biofuel production [2]. Research is being conducted to naturally grow more plant (crop) biomass in the most natural ways and at the lowest cost to extract the most energy.

In order to achieve sustainable farming and increase crop productivity per unit area, growing multi-crops is increasingly proposed [3]. Inter-crop competition, when several plant species are grown simultaneously in the same area, has had positive results [4]. Studies have shown that multi-cropping systems in agriculture have higher yield potential than single cultures [5]. Furthermore, multi-cropping has been shown to reduce diseases and pests and improve soil properties [6,7]. Crop diversity increases the plant’s root network in the soil, improving food and water availability. In multi-crops, plant roots are located at different soil levels, which allows crops to capture soil moisture at different depths, thus increasing the water use efficiency of agrocenoses [8]. Multi-cropping affects the entire soil biota, increasing the abundance, diversity, and activity of soil microorganisms [9]. In multi-crop systems, plants compete with each other to use solar energy more efficiently [10]. Inter-crops protect the main crop against dormancy [11].

Faba beans are often found in multi-crops. This crop is highly nutritious due to their high protein content. Faba bean is a good source of mineral nutrients, vitamins, and many biologically active compounds. Equally important is the contribution of faba beans in maintaining the sustainability of agricultural systems, as they very efficiently fix atmospheric nitrogen [12]. Incorporating faba beans into cropping systems improves soil fertility. The faba bean root system establishes a symbiosis with Rhizobium bacteria, and the simultaneous biological nitrogen fixation is associated with a lower need for fertilizers in arable soil and increased soil biological activity. Therefore, growing faba beans in crop rotations with non-nitrogen-fixers, such as cereals or horticultural crops, is beneficial [13]. The amount of nitrogen that faba beans can fix depends mainly on the species used, nitrogen and phosphorus fertilization rates, soil properties, and symbiotic rhizomes in the soil [14,15]. Silberg et al. [16] reported that combining legumes in maize systems reduces the number of parasitic weeds such as striga (Striga asiatica L.).

Maize is an important cereal crop used for food and feed in many developed countries. In recent years, maize cultivation has increased primarily due to its use for biofuel production [17]. No other grain crop uses sunlight as efficiently as maize or has the same high yield per hectare. According to the results of several studies, maize is listed as the most important component in many intercrops [18].

Legumes fix atmospheric nitrogen and do not compete with maize for nitrogen reserves in the soil, thus producing higher maize biomass than when mono-cultivations [19]. Such a double crop is also popular because it yields two different grains [20]. Furthermore, Bilalis et al. [21] show that growing maize and faba beans together creates a dense crop that suppresses weeds by blocking sunlight.

Industrial hemp (Cannabis sativa L.) was one of the first species used by humans for economic purposes. In many regions of the world, they were grown as fiber for textiles. Hemp seeds are a source of protein and oil and are rich in saturated fatty acids. Their inflorescences have been used medicinally because of their high cannabinoid content. Due to its unique properties, hemp biomass is also useful for biofuel production [22]. In the colder climate regions of Northern Europe, fiber hemp is grown as an energy crop with high biomass [23]. Fiber hemp is well known for its high biomass productivity (24 t ha⁻¹). Studies conducted at the Institute of Natural Fibers in Poland have shown that hemp is a valuable raw material for energy production (heat of combustion ~18 MJ kg⁻¹) [24]. Moreover, Kolarikova et al. [25] state that hemp has a high calorific value, which is comparable to wood. Furthermore, hemp forms a dense crop, which is why it is excellent at inhibiting the growth of weeds. Hemp also acts as a natural insect repellent, limits the growth of nematodes in the soil, and can reduce greenhouse gas emissions by sequestering ~2.5 tons of CO₂. Hemp has been observed to improve soil quality through its ability by phytoremediating heavy metals [26]. Due to its beneficial agronomic properties, hemp is increasingly found in multi-crop systems.

To achieve the maximum result of multi-cropping, it is necessary to take of the advantages of each plant grown in the multi-crop. For this, appropriate combinations, arrangement and proportions of cultivated plant species are selected [27]. So far, scarce data are available on the cultivation of maize, hemp, and faba bean in the multi-crops, the amount of biomass produced, and its use for energy purposes [28]. The objective of our research was to assess the impact of crop diversification on the biomass of individual crop species and the total biomass of crop mixtures (multi-crops). We hypothesize that crop diversification up to a three-crop mixture will increase biomass productivity of per unit area.

2. Materials and Methods

2.1. Experimental Site

A short-term stationary field experiment was performed in 2020–2022 at the Vytautas Magnus University, Agriculture Academy, Experimental station (54°52′ N, 23°49′ E). The annual average precipitation rate at the experimental site is close to 600 mm, and the length of the vegetative season lasts 150–160 days with between 300 and 500 mm of precipitation. Based on the amount of precipitation, this Lithuanian territory is a zone of surplus moisture. Usually, the coldest month is January, and the warmest and the most humid is July.

The soil of the experimental field is a deeper gleyic saturated loam (45.6% sand, 41.7% silt, 12.7% clay) Planosol (Endohypogleyic-Eutric, Ple-gln-w [29]. Soil pH_HCl varies from 7.3–7.8; total nitrogen content, 0.08–0.13%; humus, 1.5–1.7%; available phosphorus, 189 to 280 mg·kg⁻¹; available potassium, 97–118 mg·kg⁻¹; available sulfur, 1.2–2.6 mg·kg⁻¹; and exchangeable magnesium, 436–790 mg·kg⁻¹.

In Lithuania, the variation of meteorological conditions is very high between the growing seasons and growth stages because of the uneven boreal intermediate maritime-continental climate. Thus, during the 2020–2022 vegetative seasons, air temperatures and rainfall were uneven. April mainly was colder and dryer than the long-term average. May was also colder, but with surplus humidity (

Table 1. Differences (±) of air temperatures and precipitation rates from the long-term average during the 2020–2022 vegetative seasons. Kaunas Meteorological Station.

Months	2020	2021	2022	Long-Term Average (Since 1974)
Air temperatures °C
April	0.0	−0.7	−0.7	6.9
May	−2.7	−1.8	−2.2	13.2
June	+2.9	+3.4	−1.6	16.1
July	−1.3	+3.9	−0.8	18.7
August	+1.4	−0.8	+3.6	17.3
Precipitations rates mm
April	−37.3	−7.6	−2.7	41.3
May	+32.7	+59.9	+22.3	61.7
June	+22.4	−36.6	+0.7	76.9
July	−36.2	−48.2	+3.9	96.6
August	+3.9	+33.3	−50.2	88.9

). June was warmer (except in 2022) than usual conditions, but precipitation was distributed differently in each experimental year.

July is the warmest month in Lithuania, but in 2020 and 2022, the air temperature was less than average, while in 2020 and 2021, it was arid. In 2022, the precipitation conditions were similar to the long-term average. During the experimentation, August was uneven: in 2020, it was close to average; in 2021, wetter; in 2022, much dyer than long-term conditions. Air temperatures in August also varied between experimental years.

2.2. Treatments and Agronomic Practice

Maize (Zea mays L., Pioneer selection), hemp (Cannabis sativa L., cultivar Austa SK), and faba bean (Vicia faba L., cultivar Vertigo) were grown in the experiment. Three types of cultivations were sown: mono, binary, and ternary. A total of seven treatments were tested (

Table 2. Crop bio-diversity [30].

Treatments	Cultivation	Abbreviation
single crop	maize	MA
	hemp	HE
	faba bean	FB
binary crop	maize + hemp	MA + HE
	maize + faba bean	MA + FB
	hemp + faba bean	HE + FB
ternary crop	maize + hemp + faba bean	MA + HE + FB

).

The plots were randomly distributed with three replications. The size of the plot was 8 m². In total, there were 21 plots in the experiment. The pre-crop was oat. In 2021–2022, all experimental plots were regrown in the same places as continued crops (monoculture) according to the experimental treatments to highlight the cumulative effects.

The experimental soil was plowed in autumn and cultivated in spring before sowing. In April, before sowing, mineral fertilizer NPK 15:15:15 (300 kg·ha⁻¹) was applied each year of experimentation. We used a low fertilization level to establish the effect of multi-cropping on soil fertility. Plots were sown by hand according to specially created schemes (Figure 1).

The crops inter-rows were loosened 1–2 times. No pesticides were used. Crop biomass production was harvested by hand after a short 103–105-days vegetative season, before the faba bean biomass had broken down. At that time, the biomass of maize and hemp had not yet reached its maximum, but faba beans produce abundant and valuable biomass that we did not want to lose. As research results have shown, our strategy works. In addition, there is a lack of plant biomass for fuel production in Lithuania in August.

2.3. Methods and Analysis

To evaluate crop productivity, plants were cut in a row of 1 m length with at least in 5 spots per plot, and a compound sample was formed. Samples for each species of plant were taken separately, so a total of 36 samples were collected for detailed evaluation (Figure 2).

Biomass was dried under laboratory conditions to the lowest air humidity, and at a temperature of 105 °C to absolutely dry humidity. The results of biomass drying are presented in the article.

Experimental data were processed using one-factor analysis of variance (ANOVA) from the statistical software SELEKCIJA (vers. 5.00, author dr. Pavelas Tarakanovas. Lithuanian Institute of Agriculture, Akademija, Kedainiu distr., Lithuania). The significance of differences between the control and treatments in Figure 2, Figure 3 and Figure 4 was estimated by the least significant difference (LSD) test. A significant difference was indicated as follows: * when p ≤ 0.05 (significant at 95% probability level), ** when p ≤ 0.01 (significant at 99% probability level). Lower-case letters were used to mark a significant difference between treatments at the 95% probability level. The interaction of meteorological conditions during three vegetative seasons with crop biomass productivity indicators was determined. Therefore, the averages of the indicators are not presented in the article, and individual experimental years were analyzed.

3. Results and Discussion

3.1. Impact of Crop Diversification Level on Maize Productivity

Using crop biomass for biofuel production has attracted great interest in many scientific and engineering disciplines. Maize biomass is a primary resource for biofuel production [31]. In all three years of the experimentation, the lowest dried biomass was found in the maize multi-crop grown together with hemp and faba bean. Here, the dried biomass of maize differed by an average of 3.2 times and the maize was highest (Figure 3).

In 2020, the highest dried biomass (446.1 g m⁻²) was of the single maize crop, i.e., 3 times higher than in the multi-crop (135.8 g m⁻²). In 2021, the difference in the dried biomass of maize between the single crop (384.7 g m⁻²) and the ternary crop (109.9 g m⁻²) increased to 3.5 times. The year 2022 was distinguished by the fact that the significantly highest dry biomass of maize was determined not in a single crop but in a binary crop of maize and faba bean (197.2 g m⁻²). Legumes fix atmospheric nitrogen and thus do not compete with maize for it. By not using nitrogen fertilizers and growing maize together with Fabaceae crops, maize yields were significantly increased [32]. Similarly, Ofori et al. [33] found that soil organic carbon increased from 0.37 to 0.82 when forage pea was intercropped with maize compared to a mono-cultivation of maize. This led to better growth of maize in a binary crop with peas. However, Tsubo et al. [34] found that growing faba bean with maize did not affect their yield. Inter-cropping cereals with legumes is a long-standing practice in tropical climates [18]. Dordas et al. [35] found that the dry biomass of the binary crop of faba bean and triticale was significantly higher than mono-cultivations of these plants.

3.2. Impact of Crop Diversification Level on the Hemp Productivity

For several decades, hemp has been considered a valuable crop due to its high productivity and calorific value comparable to wood [23]. According to Prade et al. [36], solid biofuel produced from hemp is not inferior in quality to willow wood.

Our experiment found no significant differences in dried hemp biomass between 2020 and 2021. The highest dried hemp biomass was in the binary hemp and maize crop (982.4 g m⁻² and 282.1 g m⁻², respectively); the lowest was in the ternary crop (723.9 g m⁻² and 159.8 g m⁻²) (Figure 4). Conversely, Geetha et al. [37] research showed that in a binary crop of sugarcane and hemp, sugarcane produced fresher biomass than in mono-cultivation.

In 2022, significant differences between treatments were identified. The significantly highest dried biomass of hemp was in the single crop (337.9 g m⁻²). The significantly lowest dried biomass of hemp was obtained from hemp growing in a ternary crop (76.7 g m⁻²), which, however, did not differ from the biomass of (MA+)HE and HE(+FB).

3.3. Impact of Crop Diversification Level on Faba Bean Productivity

Unlike maize and hemp, during all three years of the experiment, faba bean grown in a ternary crop showed the best productivity results. Here, the dry biomass of faba bean was the highest on average by 1070.2 g m⁻² per year (Figure 5).

Faba bean grown together with maize and with hemp in binary-crops showed less faba bean productivity. Similarly, Salama et al. [38] found that maize inter-cropped with forage cowpea reduced cowpea dry biomass because maize is a taller plant and grows faster, reducing the light available for cowpea. This is in agreement with the data of other authors, as Ewansiha et al. [39] suggest that reducing the shade produced by maize would increase productivity in maize–cowpea cropping system. This can be conducted by choosing appropriate spacing between plants. In our experiment, in conditions of Boreal climate conditions and long-day vegetative season, the development of faba bean in the early development stages was quicker than maize and a little slower than hemp (Figure 6). Besides, rows of cultivations were oriented to the South.

Köpke and Nemecek [40] pointed out that faba bean increases soil macronutrients and can maintain companion plant productivity. We expected that too, however as the crop diversification increases, the productivity of maize and hemp grown in the mixture decreases, while faba bean ensures high dried biomass productivity. Similarly, Stoltz et al. [41] found that the dry biomass of a single maize crop was 24–38% higher than in a maize and faba bean mixture. In our experiment, this difference reached 56% in 2020. This may have happened because the companion plants grown together with the faba bean did not have time to take advantage of the nutrients stored in the faba bean roots because the crops were harvested early and at the same time. Furthermore, all the biomass of the faba bean canopy was removed (taken) from the field. Thus, under conditions of low fertilization, faba beans provided more of their own nutrition than other plant species growing nearby.

3.4. Total Dried Biomass of Multi-Crops

During all three years of the experimentation, the highest total dried biomass was determined in the ternary crop: on average, 1495.4 g m⁻² per year. The highest dried biomass of the ternary crop was, in the first year of experimentation, 2081.1 g m⁻² and the lowest one, in the second year, 825.8 g m⁻², i.e., 2.5 times less (Figure 7). Bybee-Finley et al. [42] tested millet, sorghum, and hemp as single-crops and the same crops in mixtures for 6 years. In all years except the first one, the biomass of crops grown in mixtures was higher than that of mono-cultivations. Das et al. [43] stated that industrial hemp stalk yield per hectare (5437 kg) was similar to seed grass and sorghum, the difference being that hemp requires less input to grow. In our experiment, hemp produced 7000–9000 kg ha⁻¹ of dried biomass, in which stalks dominated. Aubin et al. [44] concluded that higher N rates maximally improved hemp growth, plant height, and biomass. Thus, growing hemp with legumes that help with nitrogen fixation can be expected to produce positive results.

The faba bean single crop had, on average, 647.6 g m⁻² dry biomass per year, which was the highest single crop. In all years of experimentation, the most similar dry biomass was found in maize and faba bean binary crops (806.9; 495.0; 558.3 g m⁻²).

Brooker et al. [45] found that multi-cropping systems with beans or peas could save synthetic fertilizers due to lower nutrient needs. In our experiment, decreased biomass yields in the second and third years of experiment were observed in all plots of treatments due to a decrease in available soil nutrients, especially after two vegetative seasons [46]. These results suggest growing multi-crop cultivations for one vegetative season, if the low fertilization level will be used. Franko et al. [47] concluded similarly. In our earlier investigations, living mulches in the low-fertilized maize cultivation competed with the main crop and impeded its development. Negative correlations were found between the biomass of mulch and maize canopy height (r₂₀₀₉ = −0.795, r₂₀₁₀ = −0.844; p ≤ 0.05), as well dry biomass of maize stems and leaves (r₂₀₀₉ = −0.74, r₂₀₁₀ = −0.689; p ≥ 0.05) [48].

4. Conclusions

In conditions of low fertilizer, as the crop diversification increases, the productivity of maize and hemp grown in the mixture decreases, while faba bean ensures high dried biomass productivity. As expected, during three years of experimentation, the ternary crop provided the highest total dried biomass. The highest dried ternary crop biomass was established in the first year of experimentation and was around 60% higher than that in 2021 and 24% higher than that in 2022.