**1. Introduction**

Modern agriculture has to face several challenges in the next decades. It has to secure an adequate supply of food, feed, fuel, and fiber for a growing world population under the current situation that 24 billion Mg of fertile soil are annually lost due to erosion, and an increasing land desertification [1,2]. Additionally, there is a loss of biodiversity, which is partly caused by expansion and intensification of modern agriculture [3]. In landscapes dominated by agriculture, a loss of pollinators and wildflowers as their food resources among other things can be observed [4]. While 90% of the plants are dependent on animal pollination, 58 out of 130 crop pollinating bee species in the EU are threatened. Pollinators contribute up to 35% to the global crop yields and if their numbers continue to decline, global food

supply can no longer be guaranteed [5]. For their survival, pollinators need habitat diversity [6]. In most agricultural landscapes, the structural diversity declines or no longer exists [7].

To counteract these current problems, future cropping systems should be productive and sustainable in ecological, economic, and social ways. Therefore, intercropping could make a valuable contribution.

Intercropping is the practice of cultivating more than one crop simultaneously on the same area of land. There are some different forms of intercropping, like strip-, row-, mixed-, or relay-intercropping, just to mention the most popular ones [8]. The advantages of intercropping range from ecological and economic to social benefits. Gebru [9] listed the most common advantages of intercropping, which can be found in literature; it prevents soil from erosion and desiccation by more ground coverage, yields can be more stable in unsteady seasons and higher under normal growing conditions, and there is an income diversification by different crops and different working peaks.

Intercropping has always been the most widespread form of cropping system in (sub-) tropical and developing countries. Maize (*Zea mays* L.) is one of the most cultivated plants in these regions and ensures the food supply of the population. With 48.5 million Mg harvested in 2018 in the least developed countries, maize (grain and silage) ranked in second place after paddy rice, before wheat [10].

For these developing countries intercropping maize is the common practice. Maize is mostly intercropped with legumes (*Phaseolus vulgaris* L., *Glycine max* (L.) MERR., *Arachis hypogaea* L.), vegetables (*Solanum tuberosum* L., *Raphanus sativus* var. *sativus* L., *Spinacia oleracea* L., *Cucurbita* spp. L.), or cereals like wheat (*Triticum aestivum* L.) [10–18]. Mostly, legume crops were intercropped due to their ability for biological nitrogen fixation [19].

In the industrialized countries maize-intercropping is generally not used. Knörzer et al. [20] showed that in Africa and Asia the small farm structure and the land scarcity make intercropping common.

Especially the cropping of maize related to some negative environmental effects due to its growth habits and cropping system. Maize has a slow initial development and late canopy closure, which encourage erosion and nitrate leaching into groundwater bodies [20–24].

Maize cultivation covers large areas of land. In Germany, 2.6 million ha out of 11.7 million ha of arable land were cropped with maize in 2018, 84% for silage maize production [25]. In the federal state of Baden-Württemberg, 17% of the 814,600 ha arable land were cultivated with silage maize and 7% with maize for grain use in the same year [26].

Therefore, intercropping could be an option to increase the biodiversity in agricultural ecosystems [27]. The additional cropped plants will elongate the flowering period which could offer a food basis and create habitats for small vertebrates and some arthropods (hymenoptera, coleoptera, lepidoptera, and diptera). Additional ground coverage by the partner can reduce nitrate leaching and erosion. A study from Denmark with silage maize showed that intercropping with *Festuca rubra* (L.) could reduce nitrate leaching by 15–37% (depending on soil type and crop rotation) [28]. Yields comparable to those in monocropping are achievable, as shown in Iran. Javanmard et al. [29] showed that intercropping of maize with vetch (*Vicia villosa* ROTH), bitter vetch (*Vicia ervilia* (L.) WILLD.), berseem clover (*Trifolium alexandrinum* L.), and common bean (*Phaseolus vulgaris* L.) resulted in higher biomass yields due to the use of different soil layers by the root systems of the different intercropping partners By increasing the crude protein of the harvest material, the purchase of protein feed might be reduced. Especially for the regions in Baden-Württemberg, such as the Markgräfler Land, Bruchsal-Mannheim-Heidelberg, Kraichgau, Stuttgart-Heilbronn, Main-Tauber-Kreis, and Upper Swabia with a high biogas plant and/or livestock density and maize cultivation in general and also high nitrate loads in the groundwater, maize intercropping could be an interesting alternative [30].

Therefore, if maize cultivation uses large areas of land, and the current cultivation system has negative effects on the environment (monoculture, late canopy closing, nitrate leaching, limited habitats, and food resources), and small vertebrates and arthropods, then intercropping partners that provide an additional flowering aspect and ground coverage can be beneficial for biodiversity. To the best of our knowledge, there are only few approaches to create a biodiversity aspect in silage maize with *Phaseolus* beans or flowering mixtures [31–35]. Therefore, the objectives of this study were to: (i) test different legume and non-legume plants for their suitability for intercropping systems with maize, and to enhance the flowering aspect in silage maize stands; (ii) determine the effects of these partners on maize growth and yield; and (iii) determine the effects on composition of the harvested biomass.

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

#### *2.1. Site Conditions and Climate*

The field experiments were carried out from 2018 to 2019 on two experimental sites at the Centre for Agricultural Technology Augustenberg (Ettlingen and Forchheim am Kaiserstuhl) and at the experimental station Tachenhausen of the Nuertingen-Geislingen University in Southwest Germany (Table 1).


**Table 1.** Site characteristics of the three experimental sites Ettlingen (ET), Tachenhausen (TH), and Forchheim am Kaiserstuhl (FAK).

The weather in 2018 was dry and hot, with only 261 mm precipitation during the main growing season (March to October) at ET (deficit of 270 mm, compared to the mean long-term annual precipitation), 388 mm at TH (deficit of 153 mm) and 290 mm at FAK (deficit 371 mm). Spring 2019 was more favorable than in 2018. April was rather warm, while May was cold. Summer was warm again. During the main growing season in 2019 482 mm precipitation occurred in ET (deficit 49 mm), 597 mm in TH (deficit 76 mm), while with 585 mm in FAK, a precipitation plus of 56 mm was documented. Climate charts can be found in the Appendix A (Figures A1–A3).

## *2.2. Experimental Design*

At ET, the previous crops were sweet corn (*Zea mays* L.) in summer 2017 and a mixture of *Sinapsis alba* (L.) and *Raphanus sativus* var. *oleiformis* (Pers.) as green manure over winter. In TH 2017, winter wheat (*Triticum aestivum* L.) and in 2018 spring barley (*Hordeum vulgare* L.) were grown as previous crops followed by a fallow over both winter times. In FAK in both years, the previous crop was potato (*Solanum tuberosum* L.), also followed by fallow over winter.

ET was only rated in 2018. Therefore, a split-split-plot design with three replicates was used. The three factors tested were; the amount of nitrogen fertilizer (N-Level, main plot), the placement of the IFP seeds (seed placement, subplot 1) and nine different intercropped flowering partners (2018) (IFP, subplot 2). The three levels of nitrogen fertilization consist of 0%, 50%, and 100% of the required nitrogen demand of a sole silage maize crop. The seed placement of the IFP was either between the maize rows (BR) or in the maize rows (IR). Therefore, the IFP was sown close to the maize rows, which should simulate a simultaneous sowing of maize and IFP. Sowing rates of the IFP were set according to the amount used to establish a sole crop stand (Table 2). The flowering partners for intercropping were chosen according to their flowering properties and the attractiveness/food supply for insects. Nitrogen fertilization took place on 25 April 2018 and 19 April 2019. The required fertilizer nitrogen amount in 2018 was 172 kg ha−<sup>1</sup> under 100% and 86 kg ha−<sup>1</sup> under 50%. The used fertilizer was ALZON 46 (46% total-nitrogen as urea with 2-cyanoguanidine and 1,2,4-triazole as nitrification inhibitor). Fertilizer amount was based on accepted local fertilization recommendations, taking residual soil nutrient from soil tests into account.


