Maize-bean intercropping yields in Northern Germany are comparable to those of pure silage maize

European Journal of Agronomy 112 (2020) 125947

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Maize-bean intercropping yields in Northern Germany are comparable to those of pure silage maize

T

Jenny Fischera, , Herwart Böhma, Jürgen Heβb ⁎

a b

Thünen Institute of Organic Farming, Trenthorst 32, 23847, Westerau, Germany University of Kassel, Department of Organic Farming and Cropping Systems, Nordbahnhofstr. 1a, 37213, Witzenhausen, Germany

ARTICLE INFO

ABSTRACT

Keywords: Organic farming Zea mays Phaseolus vulgaris Phaseolus coccineus Vicia faba Total yield Bean leaf mass Crude protein yield

Maize-bean intercropping is currently gaining popularity among farmers in Germany, although there is hardly any data available on the yield potential under moderate maritime climate conditions in comparison to pure maize cultivation as well as on the choice of the bean cultivar. To assess the agronomic potential of maize-bean intercropping for organic farming, field-experiments over two growing seasons were conducted in Northern Germany (53°46′N, 10°30′E). A split-plot design with two sowing times of the beans as main-plot-factor and different bean species and cultivars (Phaseolus vulgaris, cv.: Cobra, Eva, Tarbais, P. coccineus, cv.: Preisgewinner, Weiße Riesen, Vicia faba cv.: Isabell) as subplot-factor was applied. In both years, the average intercropping yield (11 t ha−1 in 2011 and 14 t ha−1 in 2012) of the two sowing dates was comparable to that of maize-control, except for Maize-Eva in 2011. The highest intercropping yields in 2011 were shown by cv. Tarbais and Preisgewinner with bean shares ranging from 34% to 39%. By contrast, the late-sown beans in 2012 had a share of 6–12% only, reflecting the overall higher maize yield level. Compared to maize control, the intercropping with Tarbais and Preisgewinner, which achieved the highest leaf mass scores, led to a significant improvement in crude protein yield by 36% and 18% in 2011, respectively. Earlier ripening bean cultivars (e.g. cv. Eva and Cobra) tend to lose their leaves before harvest. So cultivars with a longer growth period (e.g. cv. Tarbais), similar to maize, should be used to achieve high bean and crude protein yields. Our results demonstrate the importance of cultivar choice for achieving high bean yields that have the potential to improve feed quality. Tarbais and Preisgewinner were the most suitable cultivars for maize-bean intercropping, as they achieved the highest bean yields and improved the crude protein yield in 2011. Even though further optimization is indispensable, we have shown that maize-bean intercropping can fit into organic farming systems under moderate maritime climate.

1. Introduction Maize-bean intercropping is mainly used in the subtropics, where Phaseolus beans are one of the most important protein sources for human consumption (Broughton et al., 2003; Caldas and Blair, 2009). The knowledge about synergistic effects of maize-bean intercropping has a long tradition in small-scale farming in South America (Morgado and Willey, 2008) and parts of Africa (Alemayehu, 2014), even though maize-bean intercropping is also practiced in other countries like Turkey (Çiftçi et al., 2006), Thailand (Punyalue et al., 2015), Austria (BMLFUW, 2014) and France (Tarbais Haricot Cooperative, 2019). The main reasons for maize-bean intercropping are the increased crop performance as a result of an improved resource use efficiency (AlbinoGarduño et al., 2015; Molatudi and Mariga, 2012), the better weed ⁎

suppression (Workayehu and Wortmann, 2011; Bilalis et al., 2010) as well as a reduced pest (Altieri et al., 1978) and disease incidence (Fininsa, 2003). Because most research deals predominantly with improving the lowinput systems for human consumption, very little is known about the suitability of maize-bean intercropping for organic farming systems that aim to enhance the energy- and protein supply for animal husbandry from roughage. However, the supply with on-farm grown forage of high quality is of particular importance, especially in organic farming, because the use of concentrates is restricted by the Council Regulation (Commission Regulation (EC) No 889/2008, 2008) on organic production. As maize silage can be applied as energy fodder and roughage at once it is a superior feed in comparison to other roughage components like grass or grass-clover, especially in organic dairy production.

Corresponding author. E-mail addresses: [email protected] (J. Fischer), [email protected] (H. Böhm), [email protected] (J. Heβ).

https://doi.org/10.1016/j.eja.2019.125947 Received 6 February 2019; Received in revised form 27 August 2019; Accepted 29 August 2019 1161-0301/ © 2019 Elsevier B.V. All rights reserved.

European Journal of Agronomy 112 (2020) 125947

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Thus, the organic maize production in Germany has increased from 16,900 in 2011 to 20,000 ha in 2016 (Schaack et al., 2017) of which 40% is used as silage and corn cob mix. However, the use of maize silage is restricted by its low crude protein content (Armstrong et al., 2008; Mlakar et al., 2011) that reduces microbial growth and milk production (Clark et al., 1992). Therefore, it is necessary to add protein rich fodder components to fulfill the high protein demands in intensive organic husbandry. According to Früh et al. (2015), the self-sufficiency rate of Germany for crude protein is about 63%. Consequently, 37% of the crude protein demand for animal husbandry has to be covered by import. By enhancing the protein content of the silage, the import and costs for protein supplements for livestock rations can be reduced (Armstrong et al., 2008), while the nutritive value is enhanced (Riday and Albrecht, 2008). A feasible measure to increase the protein concentration under the restrictions of organic farming is the intercropping with legumes. Especially the climbing Phaseolus beans have a high potential for intercropping, as the maize plants provide a structure for the beans to entwine around them. Also, faba bean as the most common bean species grown in organic agriculture in Germany (Schaack et al., 2017) is used by some farmers for intercropping with maize. Thereby it is possible to include an additional legume crop in the rotation which enhances soil fertility by its biological nitrogen fixation. To evaluate the growth potential of maize-bean intercropping for organic farming systems in Northern Germany field-experiments over two growing seasons were conducted at the Thünen Institute of Organic Farming. The aim of the experiment was to examine:

to 6 cm depth at the beginning of May (Table 2) with a row distance of 75 cm. The main-plot factor ‘sowing time’ comprised two sowing times: (T1) beans were sown soon after the maize; (T2) beans were sown at the four-leaf development stage (BBCH 14, Meier (2001)) of the maize. The later sowing time (T2) enabled a mechanical weed control with a share hoe prior to bean seeding. Note that ‘sowing time’ is associated with weed control, but for brevity’s sake, we call this factor ‘sowing time’ because differences in weed control are a consequence of the different sowing times. The specific sowing and management dates of the experimental years are presented in Table 2. In order to reduce the interspecific competition between maize and bean plants the common seed density of 11 viable seeds m−2 was reduced to 8 viable seeds m−2 in the intercropping plots. In both main factor plots (T1, T2), pure maize stands of the common seed density (11 viable seeds m−2, M11) were used as control variant. Moreover, pure maize stands with reduced seed density (8 viable seeds m−2, M8) were used in order to assess the yield potential of maize at lower plant densities. In the subplot-factor ‘crop stand’ different bean species and cultivars (Table 3) in intercropping with maize were evaluated in comparison to maize in pure stand. For the intercropping with maize three cultivars of common runner beans (Phaseolus vulgaris [Pv], cv. Cobra, Eva, Tarbais) and two cultivars of scarlet runner beans (Phaseolus coccineus [Pc], cv. Preisgewinner, Weiße Riesen) and additionally, the intercropping potential of a faba bean (Vicia faba [Vf], cv. Isabell) were tested. The beans were sown in alternating rows in a depth of 3 cm and a distance of 15 cm to the maize. For the climbing beans, the seed density was 6 viable seeds m−2 whereas the faba bean was sown with 19 viable seeds m-2 owing to their differing growth habit.

a) The yield potential of maize-bean intercropping in comparison to maize in pure stand under moderate maritime climate conditions b) The influence of sowing time of the beans on yield and yield composition of the intercrop c) The effect of different bean species and cultivars on yield and yield composition of the intercrop

2.3. Agronomic evaluation The potential of the different intercropping variants was evaluated by agronomic parameters assessed during the vegetation period and at harvesting. The rate of field emergence was assessed by counting the number of plants in two rows on a row length of 3 m in the plot centers. The number of plants per m2 was then calculated by taking into account the row distance of 75 cm. At the time of maize flowering (BBCH 65, Table 2) the plant height, the presence of weeds and bean leaf mass scores were recorded. Plant height of maize and beans was measured on three plants per plot for each crop. The presence of weeds per plot was estimated using a score from one to nine, following the basic instruction of the Bundessortenamt (2000). The leaf mass of the beans was scored according to a self-developed scale from one to five. At score 1 the beans have grown upwards very quickly and close to the maize with very low bean leaf mass and almost uncovered interspaces. In contrast, beans with the highest score 5 were entwined into the top of the maize plants with a very high leaf mass in the lower section, so that the interspace was completely covered by beans up to 40 cm height. According to the different, not climbing, growth habit the faba bean could reach a maximum leaf mass score of 3. Prior to harvesting, the separate yield of maize and beans per plot was assessed by cutting, manually separating and weighing four maize plants with their appending beans. Each crop component was dried at 105 °C for 48 h to calculate the bean and maize percentage on the basis of their dry matter (DM) content. The dry matter yield was calculated by the fresh matter yield, weighed within the self-propelled maize harvester of Haldrup® (Ilshofen, Germany) and the dry matter content of samples dried at 105 °C for 48 h. The dry matter yield of each crop component was then calculated on the basis of the share of bean and maize on total yield. Thus, the total dry matter yield [t DM ha−1] is the sum of the intercropped maize with bean as well as the yield of maize in pure stand. The crude protein yield was calculated by the total dry matter yield and the crude protein content, which was analyzed according to the Kjeldahl-method (VDLUFA, 1976-2012).

2. Material and methods 2.1. Site characteristics The field experiments were carried out in 2011 and 2012 in Northern Germany (53°46′N, 10°30′E). The experimental farm was managed according to the EU-regulation of organic farming (Commission Regulation (EC) No 889/2008, 2008). The climate is moderate maritime with a mean annual precipitation of 693 mm and a mean temperature of 9.0 °C in the long-term average (1986–2016). In both experimental years, the spring was about 2–3 °C warmer than the long-term average, while precipitation was about 80–90 mm lower until the end of May. During the main growth phase (June-End of September) there was about 99 mm more rainfall in 2011 whereas it remained drier (-43 mm) in 2012. The deviations of precipitation and temperature in the experimental years 2011 and 2012 from the long-term average are shown in Table 1. According to the World Reference Base for soil resources the main soil type of the experimental farm is classified as stagnic luvisol with a loamy texture (44.1% sand, 37.5% silt, 18.4% clay) in the topsoil (0–30 cm) and a high bulk density (1.8) in the subsoil (30–90 cm). 2.2. Field experiment The maize-bean intercropping experiments were conducted in a randomized split-plot design with four field replications and a plot size of 45 m2 (3 m x 15 m). The maize-bean intercropping experiment was integrated into the maize field of the experimental farm. Therefore, the cultivation took place after biennial grass-clover according to the crop rotation. Silage maize (Zea mays [Zm], cv. Fabregas, FAO 210) was drilled with a pneumatic precision seed drill (Planter 3, Kuhn®, France) 2

European Journal of Agronomy 112 (2020) 125947

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Table 1 Monthly precipitation [mm] and temperature [°C] in the long-term average (1986-2016) and their deviations in the experimental years 2011 and 2012.

Year

Precipitation [mm] Long term average 1986-2016

Deviation 2011

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year

59.17 48.20 48.96 39.76 50.32 67.15 71.69 77.60 55.66 52.86 56.67 65.15 693.19

−19.67 11.20 −33.86 −30.36 −24.72 17.75 37.21 51.10 −7.16 −26.06 −49.07 56.55 −17.09

2012

Temperature [°C] Long term average 1986-2016

Deviation 2011

2012

28.13 −29.90 −41.86 −12.56 −25.32 8.95 −5.39 −31.60 −14.76 −6.56 −28.87 3.25 −156.49

1.19 1.64 4.11 8.15 12.52 15.35 17.86 17.30 13.69 9.21 5.02 2.23 9.02

0.31 −0.94 −0.01 3.05 0.88 1.15 −1.16 −0.20 1.21 0.79 −0.12 2.17 0.58

0.91 −2.34 2.69 −0.85 0.38 −0.95 −0.86 0.30 −0.09 −0.01 0.58 −1.53 −0.12

the early sowing block in 2012, no evaluable data of the plant height and the separate yield of maize and beans could be recorded. Therefore a one-factorial randomized block design for the factor ‘crop stand’ was applied for the analysis of these data. Score values of bean leaf mass and weeds did not satisfy the assumptions of normally distributed data, so they were analyzed by a nonparametric ANOVA (proc npar1way, SAS 9.3) to calculate MonteCarlo estimates of the exact p-values. In the first step, the KruskalWallis test was used to identify significant differences between the population medians of the variants. To check which variants differ, a pairwise comparison was carried out afterward, using the Wilcoxonrank-sum test (α = 0.05).

Table 2 Crop management and dates of crop emergence, maize flowering and harvesting in the main factor plots (T1, T2) in the experimental years 2011 and 2012. Harvesting Year

2011

2012

T1 Seeding

Maize Beans Harrowing Howing Maize Beans

Weeding Emergence Maize flowering Harvesting

T2

2. May 6. May – – 17. May 07. Jun 04. Aug 19. Oct

14. Jun – 14. Jun 26. Jun

T1 3. May 8. May 8. May – 16. May 6. Jun 16. Aug 26. Oct

T2 30. May 29. May 14. Jun

3. Results 3.1. Agronomic evaluation

2.4. Statistical analysis

In 2011, the rate of field emergence (Table 4) of maize showed no significant differences for the factors 'crop stand’, ‘sowing time’ as well as for the interaction of both, whereas in 2012 a significant interaction of ‘crop stand x sowing time’ (p = 0.007) was found. In the first year, the maize control (M11) had a low emergence rate of 64% that was comparable to the intercropping variants. In contrast to this, in 2012 it obtained an emergence rate of 84%, within the early sowing strip (T1), which was significantly lower than those of maize in intercropping with early-sown beans. The rate of field emergence of the beans was significant for the factor ‘crop stand’ in 2011 (p < 0.001), while a significant interaction of ‘crop stand x sowing time’ (p < 0.001) was found in 2012. In 2011, the lowest emergence rate of 64% was observed in the faba bean, which was significantly lower than the Phaseolus beans (Table 4).

According to the differing weather conditions between the years and the lower rate of field emergence of the early-sown beans in 2012, the data analysis was done separately for each year. The data evaluation was done with an analysis of variance, using a mixed model procedure (proc mixed, SAS 9.3). Normality of residues was checked with the Shapiro-Wilk-Test and QQ-plots. If significant differences were found (α = 0.05), least-squared-means were compared using the TukeyKramer posthoc test for adjustment of p-values and Kenward-Roger adjustment for correcting the degrees of freedom. Letter display was generated with the %mult-macro (Piepho, 2012). For the agronomic evaluation in 2011 a split-plot design was used with ‘sowing time’ as main-plot-factor, ‘crop stand’ as subplot-factor and ‘replication x sowing time’ as random factor. Due to the failure of bean plants within

Table 3 Overview of the variants and the main characteristics of the tested bean species and cultivars in intercropping. The seed density is given in viable seeds per square meter (vsm−2) and the thousand grain weight (TGW) in gram. The maturity groups are estimated on the basis of the ripening time within the field experiments. Type

Sole cropping

Intercropping maize with:

Species

Zea mays (Zm)

Vicia faba (Vf)

Phaseolus vulgaris (Pv)

Fabregas

Isabell

Cobra

Eva

Tarbais

Preisgewinner

Weiße Riesen

19 652 coloured brown early Maize-Isabell MI

6 350 purple black medium Maize-Cobra MC

6 407 white white early Maize-Eva ME

6 619 white white medium-late Maize-Tarbais MT

6 1178 red red-black late Maize-Preisgewinner MP

6 1200 white white late Maize-Weiße Riesen MW

Cultivar −2

Seed density [vsm TGW [g] Flower colour Seed coat colour Maturity Crop stand abbreviation

]

11 310 . . FAO 210 Maize, control M11

8

Maize, reduced M8

3

Phaseolus coccineus (Pc)

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Table 4 Rate of field emergence of maize and bean in the experimental years 2011 and 2012 (M = maize, 11 = control with standard seed density of 11 seeds m−2, 8 = reduced seed density of 8 seeds m−2, I = Isabell, C = Cobra, E = Eva, T = Tarbais, P = Preisgewinner, W = Weiße Riesen). All values are the Least Square Means with the Standard Error (LSMeans ± SE) of the field replicates. LSMeans with no letter in common indicates significant differences between the crop stands (within one column), n.s. = not significant. Significant differences between the two sowing times within the same crop stand are indicated by an asterisk (Tukey-Kramer Test: p < 0.05). Since the sowing density was calculated based on the expected germination capacity, emergence rates can exceed 100% if the germination capacity was underestimated. Year

2011

2012

Unit

[%]

[%]

Crop

Maize

Bean

Maize

Crop stand

Sowing time

Ø T1 + T2

Ø T1 + T2

T1

T2

within

T1

T2

within

Z. mays

M11 M8 MI MC ME MT MP MW

64.0 74.5 67.8 70.4 63.4 62.4 67.1 63.5

. . 64.0 99.0 90.5 90.4 84.0 81.8

84.4 a ± 4.1 103.2 bc ± 4.1 97.6 b ± 4.1 104.0 bc ± 4.1 108.7 bc ± 4.1 102.4 bc ± 4.1 104.0 bc ± 4.1 112.2 c ± 4.8

95.6 ab ± 4.1 107.1 bcd ± 4.1 110.3 d ± 4.1 107.9 cd ± 4.1 97.6 ac ± 4.1 95.2 a ± 4.1 94.4 a ± 4.1 96.8 ac ± 4.1

n.s. n.s. * n.s. n.s. n.s. n.s. *

. . 64.0 c ± 6.2 16.5 a ± 6.2 3.8 a ± 6.2 9.3 a ± 6.2 43.5 b ± 6.2 40.8 b ± 6.2

. . 68.5 a ± 6.2 96.3 b ± 6.2 99.0 b ± 6.2 91.8 b ± 6.2 107.5 b ± 6.2 97.5 b ± 6.2

. . n.s. * * * * *

Z. mays + V. faba Z. mays + P. vulgaris Z. mays + P. coccineus

n.s. ± 4.7 n.s. ± 4.7 n.s. ± 4.7 n.s. ± 4.7 n.s. ± 4.7 n.s. ± 4.7 n.s. ± 4.7 n.s. ± 5.0

a ± 5.4 c ± 5.4 bc ± 5.4 bc ± 5.4 b ± 5.4 b ± 5.8

Bean

Looking at the differences within the same crop stand, the cultivars Tarbais and Preisgewinner achieved significantly higher leaf mass scores at the early sowing time than at the late sowing time in 2011. In contrast, the bean leaf mass scores of the early sowing time in 2012 were inferior to the late sowing time for all cultivars except Isabell and Weiße Riesen. The comparison between the years showed that the leaf mass scores of all early-sown cultivars, except the faba bean, were significantly higher in the first experimental year than in the second. Significant differences between the weed scores were found in 2011 for the early-sown crop stands (Pr ≥ Chi-Square = 0.002), and the latesown crop stands (Pr ≥ Chi-Square = 0.002) as well. In contrast, no significant differences were found between the weed scores of the early and late-sown crop stands in 2012. The lowest weed scores were found in the intercropped cultivar Tarbais at both sowing times, which was significantly lower than the maize variants (M11, M8) and Maize-Eva at both sowing times, as well as the early-sown intercrops with Isabell, Cobra and Weiße Riesen. Furthermore, the early-sown Maize-Preisgewinner had significantly lower weed scores as the early-sown intercropped cultivars Weiße Riesen, Cobra and Eva. In contrast, the highest weed scores were detected for M8 at both sowing times. This crop stand differed significantly from the intercropped cultivars Isabell, Tarbais and Preisgewinner at both sowing times, as well as Weiße Riesen at the early sowing time in 2011. Likewise, M11 had a significantly higher weed score than the earlysown Maize-Tarbais and all late-sown intercropping variants, except Maize-Weiße Riesen. In 2012, all crop stands within the early sowing strip had weed scores in the range of 8.0, whereas those of the late sowing time were in the range of 2.8. Significant differences between the weed scores within the same crop stand were detected, except for the pure maize stands (M11, M8). In 2011, Maize-Tarbais and Maize-Preisgewinner had significantly lower weed scores at the early sowing time than at the late sowing time.

Therein the cultivar Cobra had the highest emergence rate of 99%, which was significantly higher than those of the P. coccineus cultivars (MP, MW). In contrast, the rate of field emergence of the early-sown beans in the second year was very low. The P. vulgaris cultivars had the significantly lowest emergence rates (MC: 16%, ME: 4%, MT: 9%), which differed significantly from the P. coccineus cultivars (MP: 43%, MW: 41%) and the faba bean. Comparable to 2011, the late-sown Phaseolus beans achieved emergence rates above 90%, while the latesown faba bean had a significantly lower emergence rate of 68%. Hence, in comparison of the sowing dates in the second year, all bean cultivars, except the faba bean, had a higher rate of field emergence when sown late. The plant height of maize showed no significant differences between the variants in both years. In 2011 the maize was 112 cm high, whereas it reached a height of 280 cm in the second year. Likewise, no significant differences were found between the plant heights of the beans in the first year that was 67 cm on average. In contrast to this, the plant height of the beans was significantly affected by the factor ‘crop stand’ at the late sowing time in 2012 (p < 0.001). Preisgewinner (P. coccineus) was the significantly tallest bean with a height of 231 cm. The cultivars Tarbais and Weiße Riesen were 200 cm high and differed significantly from the early ripening bean cultivars (Isabell, Eva, Cobra). Of these, Cobra was the smallest bean with a height of 148 cm, whereas the faba bean, that reached a height of 160 cm in intercropping, was significantly taller than Cobra. 3.1.1. Weeds and bean leaf mass scores The bean leaf mass scores of the early-sown beans in 2011 and 2012 differed significantly (Pr ≥ Chi-Square = 0.001), whereas no significant differences could be detected between the bean leaf mass scores of the late sowing times in both years. The highest bean leaf mass with a score of 4.3 and 4.0 (Fig. 1) was found in the early-sown Tarbais and Preisgewinner in 2011, respectively. In contrast, Weiße Riesen and the early ripening cultivars Cobra, Eva and Isabell had significantly lower bean leaf mass scores than Tarbais. While in 2011 and 2012 no significant differences were observed between the bean leaf mass scores of the late-sown crop stands, the early-sown beans of the second year also differed significantly. The faba bean had the highest bean leaf mass score of 2, while all other cultivars except Weiße Riesen had significantly lower bean leaf mass scores. While the P. coccineus cultivars still achieved a leaf mass score of 1, almost no beans were found in the two early-ripening P. vulgaris cultivars Cobra and Eva. Hence, the cultivar Eva had a significantly lower bean leaf mass score compared to both P. coccineus cultivars.

3.1.2. Dry matter content, yield and crude protein yield The tested crop stands achieved an average dry matter content of 33% at harvest. The highest contents of 34–36% DM were found in the pure maize stands (M11, M8), as well as in the intercrops with the early maturing bean cultivars (MI, MC, ME). In contrast to the early-ripening cultivars (MI, ME), which achieved higher dry matter contents at the early sowing time, the late-ripening cultivars (MT, MP, MW) showed an opposite trend. Hence the lowest content of 27% DM was found in the early-sown Maize-Weiße Riesen, while the faba bean intercrop had the highest content of 40% DM at the early sowing time. 4

European Journal of Agronomy 112 (2020) 125947

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Fig. 1. Comparison of the median scores of bean leaf mass (grey bars) and weeds (black bars) in pure maize and maize-bean intercropping in the years 2011 and 2012 (M = maize, 11 = control with standard seed density of 11 seeds m−2, 8 = reduced seed density of 8 seeds m−2, I = Isabell, C = Cobra, E = Eva, T = Tarbais, P = Preisgewinner, W = Weiße Riesen). Medians with no upper case letter in common are significantly different for the bean leaf mass scores, whereas medians with no lower case letter in common indicate significant differences between the weed scores (within the same sowing time), n.s. = not significant (Wilcoxon Rank Sum Test: p < 0.05).

DM ha−1, while the yield within the late sowing strip was about 5 t DM ha−1 higher. Likewise, the relative yield in 2011 showed significant differences between the crop stands (p = 0.007). The highest relative yield of 111.8% was achieved by the intercropped cultivar Preisgewinner, which differed significantly from Maize-Eva. This cultivar also differed significantly from M11; hence the relative yield was about 14.1% lower. Although the other crop stands varied in a range of 0.70–1.06%, no significant difference to M11 could be identified. Correspondingly, no significant differences were detected for the relative yield in the second year, which varied in a range of 91.7–100.8%. The highest maize dry matter yield was achieved by M11 in both years, which was significantly higher than the maize yields of the intercropping variants. Likewise, M8 was significantly higher than all intercropping variants, except the crop stand Maize-Isabell and Maize-

Referring to dry matter yield (Table 5) significant effects between the crop stands were found for total yield in 2011 (absolute: p = 0.009, relative: p = 0.007) and the maize yields in both years (p < 0.001). In contrast to this, the total dry matter yields in 2012, as well as the dry matter yields of the beans in both years, showed no significant differences between the crop stands. In 2011, Maize-Preisgewinner had the highest total yield of 12.3 t DM ha−1, which was significantly higher than those of all other crop stands except M11 and Maize-Tarbais. In contrast, Maize-Eva had the lowest total dry matter yield, which differed significantly from M11 as well as Maize-Tarbais and Maize-Preisgewinner. However, no significant differences were found between the crop stands for total yield in 2012, which averaged 14.1 t DM ha−1, whereas significant differences (p < 0.001) were detected between the sowing times. The variants of the early sowing strip achieved a yield of 11.6 t 5

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Table 5 Absolute and relative total dry matter yield and absolute dry matter yield differentiated between maize and bean in the experimental years 2011 and 2012 (M = maize, 11 = control with standard seed density of 11 seeds m−2, 8 = reduced seed density of 8 seeds m−2, I = Isabell, C = Cobra, E = Eva, T = Tarbais, P = Preisgewinner, W = Weiße Riesen). All values are the LSMeans ± SE of the field replicates. LSMeans with no letter in common indicates significant differences between the crop stands (within one column), n.s. = not significant (Tukey-Kramer Test: p < 0.05). Total Dry Matter Yield

Crop stand Z. mays Z. mays + V. faba Z. mays + P. vulgaris Z. mays + P. coccineus

2012

Maize Dry Matter Yield

Bean Dry Matter Yield

2011

2012

2011

2012

Year

2011

Type Sowing time Unit

absolute Ø T1 + T2 [t ha−1]

relative Ø T1 + T2 [%]

absolute Ø T1 + T2 [t ha−1]

relative Ø T1 + T2 [%]

absolute Ø T1 + T2 [t ha−1]

absolute T2 [t ha−1]

absolute Ø T1 + T2 [t ha−1]

absolute T2 [t ha−1]

M11 M8 MI MC ME MT MP MW

11.3 bc ± 0.6 10.5 ab ± 0.6 10.4 ab ± 0.6 10.4 ab ± 0.6 9.6 a ± 0.6 11.2 bc ± 0.6 12.3 c ± 0.6 10.7 ab ± 0.6

100.0 bc ± 5.3 93.7 ab ± 5.3 93.8 ab ± 5.3 92.3 ab ± 5.3 85.9 a ± 5.3 101.2 bc ± 5.3 111.8 c ± 5.3 96.5 ab ± 5.3

14.6 14.2 13.5 14.4 14.4 14.2 13.6 13.9

100.0 n.s. ± 5.4 97.4 n.s. ± 5.4 92.3 n.s. ± 5.4 100.3 n.s. ± 5.4 100.8 n.s. ± 5.4 99.4 n.s. ± 5.4 94.2 n.s. ± 5.4 91.7 n.s. ± 5.7

11.3 e ± 0.5 10.5 e ± 0.5 6.5 ab ± 0.5 6.6 ab ± 0.5 5.4 a ± 0.5 6.8 bc ± 0.5 8.2 d ± 0.5 7.8 cd ± 0.5

17.6 17.0 15.8 16.0 15.7 14.7 14.8 14.6

. . 4.0 3.9 4.2 4.4 4.2 3.0

. . 0.9 0.7 0.7 1.4 1.3 1.0

n.s. ± 0.6 n.s. ± 0.6 n.s. ± 0.6 n.s. ± 0.6 n.s. ± 0.6 n.s. ± 0.6 n.s. ± 0.6 n.s. ± 0.6

Cobra in 2012. In comparison to the other intercropping variants, Maize-Preisgewinner had the highest maize yield of 8.2 t DM ha−1 in 2011, which differed significantly from those of the intercropped V. faba and P. vulgaris cultivars. In 2012, the intercrops with the late maturing cultivars (MT, MP, MW) had significant lower maize yields than the intercrop with the earlier maturing cultivar Cobra, which was about 1.2–1.4 t DM ha-1 higher. However, no significant differences could be found for bean dry matter yield between the intercropping stands in both years. In the first year, the beans had an average yield of 4.0 t DM ha−1, whereas the beans yielded 1.0 t DM ha−1 in the second year. For the crude protein yield (Fig. 2), a significant influence of the crop stands was found in 2011, whereas no significant effect of the sowing times could be determined. The highest crude protein yield of 1.06 t CP ha−1 was found for Maize-Tarbais in 2011, which differed significantly from all other crop stands. In comparison to the pure maize stands (M11, M8), this cultivar exceeded the crude protein yield of M11 by 0.28 t ha−1, while M8 yielded 0.36 t ha−1 less crude protein.

d ± 0.5 cd ± 0.5 ac ± 0.6 bc ± 0.5 ab ± 0.5 a ± 0.5 a ± 0.5 a ± 0.5

n.s. ± 0.3 n.s. ± 0.3 n.s. ± 0.3 n.s. ± 0.3 n.s. ± 0.3 n.s. ± 0.3

n.s. ± 0.2 n.s. ± 0.2 n.s. ± 0.2 n.s. ± 0.2 n.s. ± 0.2 n.s. ± 0.2

Similarly, Maize-Preisgewinner had a significantly higher crude protein yield of 0.92 t CP ha−1 than the pure maize stands and Maize-Eva. Hence, the lowest crude protein yield of 0.70 t CP ha−1 was detected in M8, which differed significantly from all intercropping variants, except Maize-Eva. In contrast to the first year, no significant differences between the crop stands could be proven for the crude protein yield in 2012, which was in the range of 1 t ha−1. 4. Discussion 4.1. Agronomic evaluation The dry conditions during seedbed preparation in 2011 resulted in a coarse soil structure, which negatively affected the field emergence rate of maize (Table 4). In the second year, the rate of field emergence was influenced by the interaction of the sowing time and the crop stand, but all variants, except the maize control (M11) within the early sowing strip, had sufficient maize germination rates (> 90%).

Fig. 2. Crude protein yield [t CP ha−1] of pure maize and maize in intercropping with different bean species and cultivars in the years 2011 and 2012 (M = maize, 11 = control with standard seed density of 11 seeds m-2, 8 = reduced seed density of 8 seeds m-2, I = Isabell, C = Cobra, E = Eva, T = Tarbais, P = Preisgewinner, W = Weiße Riesen). The crude protein yield was calculated by the total dry matter yield and the crude protein contents. All values are the LSMeans ± SE of the field replicates. LSMeans with no letter in common indicates significant differences between the crop stands in the same year, n.s. = not significant (Tukey-Kramer Test: p < 0.05). 6

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As a result of the dry spring and the low precipitations in May (Table 1), the field emergence of the early-sown beans was delayed in 2011 and 2012. While the late-sown beans emerged within two weeks, the early ones needed more than four weeks, in both experimental years. The coarse soil structure in 2011 also inhibited the rate of field emergence of some bean cultivars. Hence, only 64% of the faba bean plants emerged (Table 4), whereas the climbing beans achieved sufficient emergence rates above 80% after the rainfalls in June 2011. Owing to the larger seeds and the associated higher water requirement, the P. coccineus beans had a rate of field emergence that was about 10% lower than that of the P. vulgaris beans, which achieved emergence rates above 90%. The field emergence rate (Table 4) of early-sown beans in 2012 was very low due to drought in May and the beginning of June and severe weed coverage. Especially the climbing P. vulgaris beans had very low emergence rates, while the P. coccineus beans and the faba bean achieved higher emergence rates. Nonetheless, the few existing beans were strongly suppressed by weeds, leaving almost no beans when harvesting this main factor plot. Conversely, the late-sown climbing beans achieved satisfactory emergence rates of almost 100% due to the successful mechanical weed control prior to seeding. Only the faba bean had a lower emergence rate of 68.5%, which was comparable to the early sowing time. These results demonstrate that, due to their slow youth development, the beans have low competitiveness against weeds during their initial growth. Therefore, they place high demands on weed management until tendril formation, especially when weather conditions are unfavorable at the time of field emergence. This is confirmed by Stagnari and Pisante (2011) and Burnside et al. (1998), who identified a critical phase of weed control from four to five weeks after emergence in P. vulgaris beans. For the plant height of maize and beans, that were extremely low in 2011, no significant differences were detected neither between the crop stands nor the sowing times. The reduced growth rate was probably the result of the dry conditions until the beginning of June as well as the inhibited field emergence due to the coarse seedbed. Nonetheless, intercropping had no negative effects on the plant height of maize in both experimental years. This is in line with the experiments of Hirpa (2014) in Ethiopia and Davis and Garcia (1983) in Columbia that did not reveal significant effects of intercropping on maize plant height even though a higher ratio of beans to maize was tested here than in our experiment. In contrast, the organic farming experiments conducted in Slovenia by Bavec et al. (2005) resulted in an increased plant height of maize in intercropping, which might be the result of light competition and the different climate. In contrast to the first year, the growth rate of the early-maturing climbing bean cultivars (Eva, Cobra) was negatively affected in 2012. This indicates that these cultivars have lower competitiveness than the late-maturing cultivars (Tarbais, Preisgewinner, Weiße Riesen). The increased growth rate of the faba bean, which has grown much taller in intercropping than it is usual in sole cropping, was probably the result of the increased competition for light. Same is reported from experiments in southern Sweden; where the faba beans in intercropping with forage maize were significantly taller than those from sole cropping, too (Stoltz and Nadeau, 2014). These results indicate that the interspecific competition strongly depends on the choice of maize and bean cultivar, the planting pattern and the planting density of the intercropping partners.

associated with the effect of the weed control, as in the main-plot of the late-sown bean variants (T2) an additional weed control with a hoe was possible prior seeding the beans (Table 2). While the differences between the weed scores of T1 and T2 were not distinctive in 2011, they were more pronounced in 2012. Nevertheless, the low weed scores of Tarbais at both sowing times in 2011 (Fig. 1) were the result of improved competitiveness against weeds associated with the highest bean leaf mass scores. Likewise, weeds were also reduced by Preisgewinner and the faba bean that had high leaf mass scores, too. In contrast, the lower leaf mass scores of Eva and Cobra indicate that the bean growth of these early-ripening cultivars was inhibited by maize that is a stronger competitor (Bavec et al., 2005). Therefore they could not obtain significant lower weed scores than M11 in the early sowing strip. Moreover, it should be considered that, beside the lower competitiveness of these cultivars, some P. vulgaris cultivars tend to lose their leaves at the time of ripening as a result of breeding operations. Therefore, the rapid senescence of the cultivar Eva caused severe biomass losses due to the falling of leaves prior harvesting when the vegetation period was extended, while the faba bean matured so early that grain loss occurred. As a result of the potential biomass-losses and the risk of weeds, these cultivars are not suitable for intercropping with maize of the maturity group 210 or higher. The lower leaf mass scores at the late sowing time (Fig. 1) are in line with the experiments of Dawo et al. (2007), where the highest bean biomass was achieved when beans were sown simultaneously with the maize. This shows that the growth advantage of maize increases the competitive effects on beans when they are sown later. Furthermore, later ripening cultivars like Tarbais and Preisgewinner may also benefit from the prolonged vegetation period due to the early sowing time. However, no significant differences could be observed between the latesown crop stands owing to the smaller variation between the variants. Nonetheless, the lowest weed scores were observed in the faba bean and the cultivar Tarbais, which had a slightly higher leaf mass. Contrary to this, the highest weed scores (Fig. 1) were found in the pure maize stands with uncovered inter-row spaces. Hence, the potential weed suppression was lower in these crop stands, whereas it increased by the narrower inter-row spaces and the better soil coverage in intercropping with beans. This is in line with the results of Bilalis et al. (2010), who examined the effects of intercropping on weeds in organic farming in Greece. Comparable to our results, a higher soil coverage in intercropping was observed here than in pure maize, whereby weeds were significantly reduced by the limited light availability. The very low bean leaf mass scores (Fig. 1) of the early-sown climbing Phaseolus beans in 2012 were attributed to the low rate of field emergence (Table 4). Moreover, the slow juvenile development and the drought weather until the beginning of June (Table 1) negatively affected bean growth. Thus, all crop stands had very high weed scores (Fig. 1) in the range of 8.0, causing strong suppression of the beans by weeds. Only the faba bean was able to achieve satisfactory leaf mass scores, but it could not achieve a significant reduction in weeds (Fig. 1). In contrast, the reduced weed pressure by the effective mechanical weed control in combination with the increased rainfall after sowing the late beans resulted in higher bean leaf mass scores that were comparable to those of the previous year. Hence, the weed scores in the late sowing strip were below those of the early sowing time (Fig. 1). Likewise, positive effects of mechanical weeding in maize-bean intercropping were reported by Nurk et al. (2017). Examining the differences within the same crop stand, it became apparent that in most cases the bean leaf mass scores were opposite to the weed scores. This indicates that the high leaf mass scores of the early-sown Tarbais and Preisgewinner were associated with a considerable weed reduction in 2011. In contrast, the significantly higher weed population in 2012 was favored by the very low bean leaf mass scores in the early-sown variants. However, no significant differences could be detected between the late-sown crop stands in both years as the beans had lower leaf mass scores, while the weed pressure was

4.1.1. Weed and bean leaf mass scores As the inter-rows were completely covered by bean leaves up to a height of more than 30 cm, the cultivars Tarbais and Preisgewinner achieved the highest bean leaf mass scores (Fig. 1) when sown early in 2011. The significantly higher leaf mass score of Tarbais indicates that this cultivar is well adapted to intercropping; hence it is traditionally cultivated with maize (de Sainte Marie and Bérard, 2009; Tarbais Haricot Cooperative, 2019). The effect of the factor ‘sowing time’ is 7

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already reduced by the mechanical weed control prior seeding.

The intercropped cultivars Cobra, Isabell and Weiße Riesen significantly improved the crude protein yield (+ 24–26%) compared to M8, but not to M11. Nonetheless, all intercropping variants enhanced the crude protein yield by 0.1 t CP ha−1 (15%) on average. This is also reflected in the results of Geren et al. (2008) and Dawo et al. (2007), who reported an increase of the crude protein yield by 14%–31% in intercropping with cowpea, dwarf bean and Phaseolus beans. While the mixture of 100% maize + 50% beans (Dawo et al., 2007) achieved a relative increase of 18.3% crude protein yield, a more substantial improvement of 24.8% and 26.0% was achieved with a reduced maize density of 75% and 50% respectively. This is in accordance with the results of the first year of our study, whereas no significant differences between the variants could be detected in the second year. This might be explained by the fact that beans accounted for 26.5–38.9% of the total yield in the first year, whereas beans had a considerably lower yield of 6.2–12.4% in the second year. Owing to the low rate of field emergence of maize in 2011, the share of bean dry matter yield increased up to 38% at reduced maize densities, whereas the drought conditions in 2012 resulted in stronger competition and lowered the share of bean yield to 6%. Correspondingly, Nurk et al. (2017) states that the bean development is generally favored by low seed densities of maize as well as high seed densities of the beans. Therefore, maize seed densities should be reduced by 25–30% in intercropping in order to minimize competition for the beans. In contrast, stronger reductions might result in higher yield differences in comparison to sole-cropped maize at standard seed densities, like it was observed by Armstrong et al. (2008). Also, the results of Nurk et al. (2017) showed, at one organic and one of two conventional sites in the first year, that the strongly reduced sowing density of 5 maize plants less in intercropping led to a significant reduction of the total yield by 4-5 t ha−1. As our results demonstrate, beans are generally able to compensate yield losses of maize, resulting from the lower seed density as well as interspecific competition, which is in line with the experiments of Bavec et al. (2005) and Dawo et al. (2007). After all, our experiments have shown that maize-bean intercropping can be adapted to the conditions of organic farming systems in moderate maritime climate. Nonetheless, further investigations are necessary in order to optimize this cultivation system with regard to the composition of the crop stands and their yield potential under this management and site characteristics. In particular, the early sowing time, which has a greater potential for high bean yields (Nurk et al., 2017), requires solutions to minimize the risk of weed suppression. Moreover, further experiments are indispensable to check whether the results obtained can be validated by a larger data set from a series of representative experiments.

4.1.2. Dry matter content, yield and crude protein yield On average, all intercropping variants achieved dry matter (DM) contents of 30–37%, which is required for ensiling (Spiekers, 2011). As the early-sown faba bean came to senescence some weeks before harvesting, this crop stand achieved the highest content of 40% DM. Likewise, the early-ripening P. vulgaris cultivars Eva and Cobra achieved higher dry matter contents in intercropping when sown simultaneously with the maize than with later sowing owing to the prolonged vegetation period and their advanced maturity. As a result of the higher bean leaf mass (Fig. 1), the later ripening bean cultivars (Tarbais, Preisgewinner, Weiße Riesen) had lower dry matter contents in intercropping when sown early instead of late. In 2011 the total dry matter yields (Table 5) were very low, due to the low rate of field emergence of the maize as well as the delayed bean emergence. Nonetheless, total dry matter yields in intercropping were comparable to the maize control (M11) in both years. Only the yield of Maize-Eva was significantly reduced by 14.1% in 2011, which is probably the result of a stronger interspecific competition between maize and bean or higher weed coverage due to the low bean leaf mass. Despite a range of 9.1% in relative yields in 2012 (Table 5), no significant differences could be identified between the crop stands. Due to the higher maize seed density in M11, maize dry matter yield was significantly higher than in the intercropping variants in both years. Comparable to our results Dawo et al. (2007) found the highest dry matter yields in pure maize stands at standard seed rate (10 plants m−2). Even though the yield of M11 was about 0.8 and 0.6 t DM ha-1 higher than that of M8, the two maize variants did not differ significantly from each other. This shows that the maize benefits from better resource availability at reduced plant densities. By contrast, the significant difference between M8 and the maize yield in intercropping (Table 5) shows that the higher overall plant density in intercropping has led to competitive effects on maize. Within the intercropping variants, the significantly highest maize yield in 2011 was realized by Maize-Preisgewinner, although this cultivar achieved high bean leaf mass scores (Fig. 1). Since the maize yield of the variants with intercropped V. faba and P. vulgaris cultivars was significantly lower than that of Maize-Preisgewinner, it can be concluded that the interspecific competition was stronger for these cultivars. The strongest reduction, which is due to higher weed coverage and stronger interspecific competitive effects, was observed for the crop stand Maize-Eva. In contrast to our results, where maize yield was reduced by 0.6–5.9 t DM ha-1 due to the reduced maize seed density in intercropping and interspecific competitive effects, Dawo et al. (2007) observed higher maize yields that were attributed to a better N-supply in intercropping compared to pure maize with reduced seed densities. Referring to Dusa (2009), the decrease in maize yield can also be the result of smaller cobs in intercropping. Opposing is reported by Albino-Garduño et al. (2015) who observed reduced maize grain yields in sole cropping as a result of a higher uptake of photosynthetic active radiation in the entire intercropping canopy. Although the bean yield (Table 5) of the cultivars Tarbais and Preisgewinner was slightly higher than those of the early maturing cultivars, it was not possible to identify significant differences. Nonetheless, the experiments of Dawo et al. (2007) confirm that the highest bean dry matter yields were achieved when they were planted simultaneously with the maize. Similar results were also achieved by Fininsa (1997) for bean seed yield. The evaluation of the crude protein yield (Fig. 2) reveals that the bean cultivars can have a significant effect if sufficiently high bean yields are achieved in intercropping like it was observed in 2011. As it had the highest bean leaf mass scores (Fig. 1) and therefore the highest bean yield (Table 5), the cultivar Tarbais significantly enhanced the crude protein yield by 35.4% relative to the maize control. Likewise, a relative increase of 17.2% was achieved by the cultivar Preisgewinner.

5. Conclusion Our experiments demonstrate that maize-bean intercropping can be adapted to moderate maritime climate, while the total yield in intercropping is comparable with that of pure maize stands at standard seed densities. As legumes have high protein contents, maize-bean intercropping has the potential to improve the energy and protein supply from on-farm grown roughage. Nonetheless, the choice of the appropriate bean cultivar is crucial for the success of this cropping system, as bean species and cultivars differ according to their competitive ability and maturity. Especially some early-ripening bean cultivars like Eva tend to lose their leaves at maturity, while the faba bean Isabell ripened much earlier than the maize. So the later sowing time (T2) is more suitable for these cultivars whereas the later maturing cultivars can benefit from the prolonged vegetation period at the early sowing time (T1). Therefore bean cultivars, with a longer growth period, similar to the maize should be used in order to achieve high bean and crude protein yields. Due to their high leaf mass and the associated advantages in weed suppression as well as the possible increase in protein content, the cultivars Tarbais and Preisgewinner appear to be 8

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particularly suitable for maize-bean intercropping. Nonetheless, it has to be considered that harvesting might be delayed by high bean leaf masses at harvesting, as a result of the lower dry matter contents of the beans. However, intercropping of maize with high yielding bean cultivars can have the potential to reduce the amount of protein-rich concentrates in animal feeding. Thus it is a promising system that can contribute to close the existing protein gap in organic farming. For this purpose, however, it is necessary to further optimize the cultivation system with regard to high proportions of bean yields in intercropping by the appropriate choice of cultivar and sowing time.

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