1. Introduction
Legume crops are an essential component of sustainable agriculture, given the ecological services they provide, especially their ability to fix atmospheric nitrogen [
1]. However, legume cultivation is hampered by biotic and abiotic stresses, which can seriously affect yield [
2]. One of these is the competition of weeds, which is a serious challenge for managing these crops. Weeds may not only reduce crop yields, but they may also affect the quality of the grain [
3], serve as alternative hosts for diseases and pests [
4,
5], or even hinder harvesting. In general, weed control methods must be implemented for legume crops since they are inefficient competitors [
6]. Herbicides are the most widely used weed management method but present several drawbacks, such as high costs for the farmer, environmental damage, and the eventual appearance of weed resistance to the active ingredients. These resistances may be especially problematic, as no herbicides with new modes of action have appeared in recent years [
7]. Other strategies are mechanical control and management of the spatial arrangement of crops [
8,
9].
Faba bean (
Vicia faba L.) stands out as one of the most cultivated legumes worldwide for use in both animal feed and human consumption [
10]. Weeds are one of the major biological constraints for faba bean, reducing yields by up to 50%, although they appear more competitive with weeds than other legumes [
11].
Intercropping is the practice of growing two or more crop species simultaneously in one field. It may help in the control of weeds in many crops [
12], complementing herbicides if necessary [
13], thus becoming a very valuable tool for organic and sustainable agriculture [
14,
15,
16]. In the case of legumes, weed reduction has been reported when combined with diverse crops, such as corn, wheat, oat, barley, or rape [
17,
18,
19,
20,
21,
22,
23]. There are different types of intercropping methods depending on the arrangement and sowing densities of the combined crops. Arrangement types include mixed intercropping, when plants are completely mixed, and alternate intercropping, when rows of each crop alternate regularly [
24]. In addition intercropping, all crops are sown at their normal densities, while in replacement intercropping, the densities are adjusted for the presence of the other crop(s) [
25].
Different mechanisms explain the smothering of weeds in intercropping systems. The most common is the competition between crops and weeds for natural resources such as light, water, or nitrogen [
21]. Another one is allelopathy, which is a characteristic of plants that may be exploited for weed control [
26]. Allelopathy is defined as the capability of plants to exert positive or negative influence in the surrounding area by releasing chemicals [
27]. It has been known for a long time that several crops present allelopathic activity, such as oat, wheat, barley, and sorghum [
28].
The main objective of this work was to determine if it is possible to control weeds in faba bean by intercropping and, if possible, the best crop combination. To this effect, field trials were performed across different years. Additionally, experiments under controlled conditions were carried out to investigate the possible role of allelopathy in weed control in intercrops of faba bean. This work was included in the Ph.D. Thesis “Diversificación de cultivos para el control de estreses bióticos en leguminosas” by Ahmed Amarna [
29].
2. Materials and Methods
2.1. Field Trials
Five field trials (
Table 1) were carried out to assess the effect of different crop combinations on the control of weeds in faba bean from 2014 to 2018 in the experimental station at the Institute for Sustainable Agriculture (CSIC) in Córdoba (South of Spain). The soil type in all cases was silt loam, and the previous crops were monocrops of wheat or various legumes. Ploughing was made at a depth of 30 cm, with the help of a chisel, a harrow, and a cultivator. Detailed temperature and precipitation per month for each trial are shown in
Figure 1. The combinations tested were faba bean/wheat, faba bean/barley, and faba bean/pea as well as monocrops of the four crops and a monocrop of faba bean at 50% of the sowing density (doubling the distance between rows) to check the effect of density on weed infestation. The cultivars used were ‘Muchamiel’ (faba bean), ‘Califa’ (wheat), ‘Henley’ (barley), and ‘Messire’ (pea). Sowing densities were 80 seeds/m
2 for legumes and 200 seeds/m
2 for cereals. Sowing was performed by hand on furrows previously ploughed in the soil. Two different intercropping systems were evaluated including alternate with replacement at 50%, where rows of each crop were alternated, giving a final sowing rate of 50/50, i.e., half of the normal density of each crop, and alternate with addition, where rows were also alternated but to a final rate of 100/100, i.e., the normal density of each crop. Replacement intercropping was evaluated in trials Córdoba-15-r and Córdoba-16-r, and addition intercropping was evaluated in trials Córdoba-16-a, Córdoba-17-a, and Córdoba-18-a. The experimental plots (
Figure 2) had a length of 3 m and comprised eight rows with a 35 cm distance between them in the case of monocrops and replacement intercropping and 16 rows at 17.5 cm in the case of addition intercropping (making a total plot size of 8.4 m
2). The monocrop of faba bean at 50% had four rows at 70 cm per plot (
Figure 2). For each trial, the experiment was designed as a randomized complete block with four replications.
Two areas of 0.7 m2 were randomly evaluated in the central part of each plot at the faba bean maturity stage to assess weed infestation. The weeds were counted and classified by botanic families. Two diversification indices were calculated in trials Córdoba-16-a, Córdoba-17-a, and Córdoba-18-a as follows:
Richness (R) and
Relative density (D):
where R: richness; nF
i: number of weed families present in a given plot (i); and nF
T: total number of weed families found in the whole trial.
where D
ij: relative density of weed family j in plot i; N
ji: number of plants of weed family j present in plot i; and N
Ti: total number of weed plants in plot i.
Additionally, two evaluators visually estimated weed coverage as a percentage of the area covered by weeds. At the end of the experiment, in trials Córdoba-16-a and Córdoba-17-a, the weeds in a central area of 2 m2 in each plot were harvested, and their biomass was determined by drying them in an oven at 60 °C for three days and subsequently weighing them.
2.2. Controlled-Condition Experiments
Two experiments were carried out under controlled conditions to investigate the possible allelopathic effect of barley. Thus, four weeds that grew in the field trials were selected for this: Polypogon monspeliensis (L.) Desf., Matricaria camomilla L., Sinapis arvensis L., and Medicago truncatula Gaertm, which belong to the botanical families Poaceae, Asteraceae, Cruciferae, and Fabaceae, respectively. In the first experiment, 10 seeds of barley were sown in plastic pots (5 × 5 × 10 cm) filled with a mixture of sand and peat at a 1:3 rate (v:v) and grown in a growth chamber with a photoperiod of 12 h of visible light (150 μmol m−2 s−1 photon flux density) at 25 °C and 12 h of darkness at 20 °C. Two weeks later, the barley plants were removed, and 20 seeds of the same weed species were sown in the same pots maintaining the substrate. The weed plants were counted every two weeks for 42 days. Then, the weed plants were removed, dried in an oven at 60 °C for three days, and weighed to obtain their biomass. A treatment control was used, in which weeds were sown in pots where barley was not previously grown. All this was performed in separate pots for each of the four weeds. The second experiment was carried out in the same way as in the first one, but the barley plants remained in the pots when the weeds were sown, and they grew together until the end of the experiment. The design in the two cases was randomized with seven replications. Each of these experiments was performed twice.
2.3. Statistical Analysis
Considering the type of data, generalized linear models (GLMs) were performed to study the influence of the treatment on weed coverage and biomass in field trials, following the model:
where
Y is the dependent variable,
β0,
β1, and
β3 denote the estimated parameters (weights), and
ε is the residual. Model fits were evaluated through residual plots. Subsequently, we assessed the contribution of treatment, year, and treatment × year according to Cohen’s f factor [
30].
Given that the treatment × year interactions were significant (Cohen’s f > 0.4) for both coverage and biomass, and since this study’s main objective was to compare the intercropping treatments independently of the year × crop interaction, a Friedman nonparametric two-way analysis of variance was used. Subsequently, Dunn’s test was performed to compare the treatment mean ranks at p ≤ 0.05. Additional ANOVA analyses were carried out to study the effect of the treatment on weed richness and weed relative density in each experiment.
In the experiments under controlled conditions, the Poisson regression procedure was applied using the maximum likelihood estimation method to study how the plant counts depended on the presence or absence of barley plants. Thus, the data were fitted to the following equation:
where
Y is the number of plants,
β0 is the independent term, and
β1 and
β2 are the estimates associated with the treatment (presence or not of barley). The difference between treatments was evaluated according to the
p-value of the Poisson regression. Additionally, a t-test was performed to calculate the effect of barley presence on the biomass of weed species and compare the proportion of reduction of weed plants and weed biomass between the two types of experiments. Finally, the proportion of weed plants and biomass reduction among weed species was assessed by ANOVA analysis.
Data were analyzed using Statistix software (Version 10; Statistix, Tallahassee, FL, USA) and the R statistical software package (RStudio 2023.12.1 + 402).
4. Discussion
In this work, intercropping was evaluated as a tool to control weeds in faba bean in the South of Spain. The level of control attained was very high, reaching reductions of 92.7% in weed coverage when combined with barley and 76.6% and 46.1% in weed biomass when mixed with barley and wheat, respectively. The mixture of faba bean and barley, then, might be effective enough to dispense with the application of herbicides in this agroecosystem.
Previously, only one work had studied the effect of the combination of faba bean and barley on weeds. In that study, Dhima et al. [
31] reported that alternate replacement intercropping achieved levels of reduction of corn poppy (
Papaver rhoeas L.) of around 90%. As for other cereals, it was shown that a mixture of faba bean and wheat reduced weed biomass by 60% in alternate and mixed replacement intercropping [
32]. Similarly, Boutagayout et al. [
33] found that an alternate intercrop of faba bean with wheat and oat decreased weed biomass by around 50%. As far as we know, our work is the first to achieve such a level of weed suppression in faba bean validated across three field experiments. Equally, barley has proved to be successful in reducing broomrape (
Orobanche crenata Forsk.) infestation in faba bean [
34] and also to be effective in controlling rust disease in faba bean [
35]. In these cases, the intercropping systems were different from those assessed in this work, so it would be necessary to integrate them in the best way to maximize the benefits of faba bean/barley intercropping. Furthermore, it is remarkable that the combination of faba bean and wheat reduced weed biomass by 64% compared with the wheat monocrop, making this mixture an exciting option for farmers that can be combined with some herbicide applications. Pea, however, has proved to be a poor competitor against weeds, both as a monocrop and in a mixture with faba bean, which is in line with the fact that the combination of grain legumes alone is not a good strategy to control weeds. Actually, these combinations are not found in the literature on weed suppression by intercropping [
36].
No differences in weed diversity among all the tested treatments were found. No weeds belonging to a particular botanic family were more affected by any crop combination or monocrop than others. This is in contrast with what has been found in other intercropping systems [
37,
38], although there are also situations where weed diversity has not been influenced by intercropping [
38]. In our case, the weed community proved to be relatively stable regardless of the crop or crops present.
Of the two types of intercropping tested, alternate with replacement and alternate with addition, only the latter was adequate for controlling weeds. The relationship between crops and weeds is based on competition for available resources such as water, nutrients, and light [
39,
40]. Plant density is one key factor in the improvement of crop competitiveness against weeds [
41], and that is precisely the difference between both intercropping systems. In addition, intercropping plant density is doubled in comparison with alternate intercropping. The high weed pressure levels that we observed in the plots with faba bean sown at half density confirm the importance of plant density.
Plant density, however, is not the only mechanism that explains weed suppression in intercropping. If that were the case, similar results would have been observed with barley, wheat, or pea. Weed pressure levels for them as sole crops illustrate that not all crops have the same competitive ability against weeds: barley presents very low weed infestation compared with the other two, with pea ranking the highest. Other factors influence performance in the presence of weeds, such as plant architecture, vigor, and allelopathy [
42,
43].
Barley has been described as one of the most competitive crops against weeds by different authors [
44,
45]. The rapid biomass accumulation and high growth rates that barley shows at the beginning of its cycle are some of the reasons for this [
45]. Another reason is the efficiency of barley in taking nitrogen: it has been reported to be more competitive for nitrogen than pea in intercrops, thus depriving weeds of this nutrient. Beyond that, barley is considered a crop with high levels of allelopathy [
46], and as many as 44 potential allelochemicals have been identified so far [
47]. The two more important are the alkaloids Gramine and Hordeine, which appear in the leaves, roots, and root exudates of barley plants [
27,
47]. All this has made barley a common partner in crop diversification for weed control [
36].
The experiments under controlled conditions aimed at evaluating the role that barley allelopathy might have on our results. Different types of bioassays under controlled conditions may be used to assess the allelopathic ability of a plant species, such as testing extracts from the allelopathic plant [
48,
49], agar bioassays [
44], or pot screenings [
50], where plants are grown together in Petri dishes or pots with soil, respectively. We opted for pot screening because it may better reflect the conditions under which allelopathy operates. In addition, our design, in which removing barley plants is compared with the effect of not removing them, allows for discriminating allelopathy from competition effects.
The results from the pots where barley was removed before sowing the weeds showed a substantial allelopathic effect against them. All four weed species presented a decrease in plant emergence and biomass that can only be explained by the presence of chemical compounds in the soil that were previously released by the barley plants. These results also suggest that the main allelopathic effect is related to the first stages of seed germination and seedling development. Remarkably, there were no significant differences between the final number of weed plants in the pots where barley was removed and in those where it remained until the end. As for weed biomass, however, in the case of two species (
Sinapis arvensis and
Matricaria camomilla), the decrease was higher when barley remained than when it was removed. This is probably due to additional allelopathic effects, although it is more difficult to separate them from competition effects in this case. Barley allelopathy was previously tested on
Sinapis arvensis [
51,
52]. Still, as far as we know, this is the first time it has been assessed on the other three weeds.
The fact that there was no difference in the reduction in emerged plants and biomass between four weeds belonging to such different botanical families points to a global and non-discriminatory effect of barley in our case. These weeds are a sample of the ten families found in our area, and this global effect could explain the lack of differences in the composition of weed communities between the barley intercrops and the monocrops in our field experiments.