1. Introduction
Sustainable farming systems are based on diverse production strategies to produce high-quality food under the premise of minimizing resource consumption. While the goal of modern agriculture to increase productivity in monocultures often contrasts with the concept of sustainability [
1,
2], the long-term benefits of diverse cropping systems have been demonstrated repeatedly [
3,
4,
5,
6,
7]. Maintaining soil fertility, enhancing weed suppression and decreasing the risk of yield losses may be achieved by increasing in-field crop diversity [
3,
8]. Relying on a broader range of crops can further promote flexibility towards environmental disturbances which is particularly important in a changing climate [
8].
One strategy of diversification in agriculture is intercropping [
1,
5], i.e., the simultaneous growing of two or more crops on the same field at the same time [
1,
2,
4,
9]. Potential benefits of intercropping include weed suppression and ecological pest and disease control, both of which are of particular importance in organic farming [
1,
5,
8]. Intercropping may also lead to more efficient resource use, mostly because of temporal or spatial differences in resource use by the individual crops. This resource partitioning among intercrops, along with other mechanisms, can increase the overall yield from a given area [
1,
2,
3,
4,
8,
9,
10,
11]. Thus, a functioning intercrop is based on partners that occupy complementary niches [
2,
11,
12]. However, competitiveness of the combined crops is strongly influenced by the environment and cultivation practices like time of planting, fertilization and pest control [
7]. As crop–crop interactions may change from one cropping environment to another, choosing the right partners and managing an intercrop is difficult [
1]. This is particularly true for intercropping poppy, due to a lack of experimental studies.
Apart from possible yield advantages, intercrops can provide additional benefits like enhanced weed suppression compared to single crops [
8], especially in additive intercrops where overall crop density is higher than in the respective monocultures. There are several studies proving a weed suppressing effect of crops grown as intercrops, most of them combining cereals with legumes [
8,
13,
14,
15,
16,
17]. While the exact mechanisms of weed regulation by intercrops are still debated, competition for resources such as light is altered when another crop is introduced to a system [
3,
4,
8]. Further, competition for nitrogen influences the intercrop performance as crops differ in nitrogen requirements and their dependence on soil nitrogen. This also applies to competition for nitrogen with weeds. For example legumes leave more soil N to be used by intercrops or weeds than non-nitrogen-fixating plants like barley [
18]. That is why total nitrogen uptake of an intercrop can be higher than in the respective sole crops and nitrogen acquisition of weeds will be limited [
19].
Resource partitioning and facilitation are the two main principles that usually contribute to enhanced weed suppression in intercrops [
4,
11]. While not all trials confirmed an advantage of intercropping in terms of weed regulation [
17,
20], Liebmann and Dyck (1993) considered most intercropping designs successful compared to the weed suppressing effect of at least one of the sole crops [
3].
Weed management is a crucial point in poppy cultivation. Due to slow initial growth resulting in a long phase with slow biomass build up, the suppression of poppy plants by weeds is frequently observed [
21,
22,
23,
24]. Secondly, late weed infestation can become problematic when green parts of tall weeds like
Chenopodium album L. are harvested together with the poppy capsules [
23]. It can make post-harvest drying necessary and a separation of
C. album seeds from poppy seeds is difficult due to similar color and size [
23]. Suppression by intercropping might not always be complete or more effective than herbicide use but could also be of economic interest as costs for herbicides, mechanical or manual weeding are reduced [
3]. Therefore, an intercrop design may be chosen to increase soil cover during the whole cropping period. An increased soil cover by intercropping further minimizes soil erosion, evaporation and allows for allelopathic effects between plants [
3,
25]. Intercropping can also be a measure to reduce total crop failure [
5,
26]. In the case of poppy, cultivation is often prone to failure due to its high requirements for seedbed preparation and crop management (weed regulation) in early development, making an intercropping experiment particularly worth looking into.
However, to our knowledge, there are no recently published studies on intercropping including poppy. Poppy belongs to a plant family that is otherwise not represented in European farming systems, which creates opportunities to integrate poppy into many crop rotations [
22]. Therefore, facilitating poppy cultivation by implementing it in an intercrop can promote its cultivation and contribute to general diversification of cropping systems using minor crop species.
Liebmann and Dyck (1993) [
3] define two categories of intercrops, one of them focusing on the main crop where the other crop is added for insurance against crop failure, minor economic uses, erosion control, soil fertility improvement and weed control. This contrasts with a system where both crops are of equal interest to the farmer. The first design applies better to our approach where poppy is the focus crop and spring barley and white clover were chosen as supporting crops. Both companion crops should primarily increase soil cover while not reducing poppy growth and yields. While barley may achieve an extra yield, thus reducing the risk of total crop failure, clover provides an external nitrogen source through nitrogen fixation, thereby limiting competition for soil N [
16,
27] and improving soil conditions for the following crop. A weed suppressing effect was presumed for all intercropping treatments due to increased plant density and ground cover.
Here we aimed to test if intercropping of poppy with both barley and white clover improves poppy cultivation by decreasing the risk of total yield losses and adding ecological benefits at the same time. Secondly, we aimed to discover key factors determining poppy intercrop performance and competitiveness by testing different seeding densities of barley and two sowing dates of white clover regarding plant growth, yield formation and weed suppression.
2. Materials and Methods
2.1. Site Description
Field trials were carried out at the site of Campus Klein-Altendorf (CKA), University of Bonn, for three consecutive years (2018–2020). The CKA is conventionally farmed but the field trial was predominantly managed under organic principles. This mostly means that the use of pesticides was avoided in these trials, but it must be acknowledged that weed occurrence would probably have been different on long-term organically managed soils. The experimental farm is located on the main terrace area of the Lower Rhine valley. Average annual precipitation and temperature are 603 mm and 9.4 °C, respectively, and the vegetation period lasts 165–170 days. Precipitation and temperature during the trial periods of 2018–2020 were measured at the CKA weather station (
Table 1). The main difference between years was that precipitation in 2018 and 2020 was almost always below the long term average while rainfall in 2019 was high, especially in the months of March, May and July.
The soil type at CKA is a highly fertile luvisol. Soil nutrient levels in 0–30 cm depth were determined at the beginning of each growing period (
Table 2). Samples were collected at five points per site and merged into a mixed sample. In 2019 and 2020 organic nitrogen fertilizer as fine grained horn meal (0–1 mm, 13% organically bound N) at 30 kg N ha
−1 was manually applied on all plots a few days after sowing. Preceding crops were winter wheat (2017 and 2018) and spring barley (2019).
2.2. Experimental Design
The trial was a complete randomized block design with four blocks arranged next to each other, each block comprising the eleven treatments so that each treatment was replicated four times. The plot size of each treatment was 3 m × 10 m. The main crop was the spring-sown poppy variety “Viola” (Zeno Projekte, Austria) that was tested in additive intercropping with white clover cultivar (cv.) “SW Hebe” (Camena Samen, Lauenau, Germany) and spring barley cv. “Laureate” (Syngenta, Maintal, Germany).
To explore if a delayed sowing of clover increases the compatibility of both crops, two different seeding dates of clover were tested in combination with poppy. Concerning barley, intercropping effects on poppy and barley yield formation were of primary importance resulting in the testing of three different seeding densities of barley for assessing competition effects. The seeding density of poppy remained the same in all treatments and throughout all years. Each crop was also grown as a sole crop for comparison. In total, 11 treatments were tested (
Table 3).
Seeding and harvest dates are summarized in
Table 4. Only the grains of barley and poppy seeds were harvested while clover was grown as green manure. Poppy seeds were sown with a precision air seeder for small seeds at 1.1 kg ha
−1 in both the intercrops and the sole crop. This resulted in poppy plant densities after seed emergence of 202 (±33; 2018), 210 (±28; 2019) and 69 (±44; 2020) plants m
−2 respectively. Row spacing between poppy rows was 50 cm (2018) and 45 cm (2019, 2020) so that every plot contained 6 poppy rows. Row spacing was changed to improve compatibility of the sowing machine and tractor to avoid effects on plant growth at the side of the plots. Clover and barley seeds were spread between poppy rows with a mechanical seed drilling machine with 11 cm row spacing. The seeding shares that would interfere with poppy rows were lifted. Therefore, intercropped plots contained 6 poppy rows and 16 rows of the respective intercrop (
Figure S1). Clover was sown at 5.3 kg ha
−1 (145 ± 25 plants m
−2 after emergence) and barley at 135, 270 and 450 germinable seeds m
−2 (
Table 3). Sole crops consisted of the same row pattern but with the intercropping partner left out.
The aim was an early sowing of poppy, preferably in March, but seedbed preparation is essential for its germination and wet soils in March of 2018 and 2019 prevented such an early sowing in those years. Sowing dates of barley and late white clover were adapted to soil conditions as well, resulting in different seeding dates between years.
For the purpose of observing weed suppression ability of the intercrops, no mechanical weed regulation or herbicides were applied after sowing of the second crop. In 2019 a single application of a grass herbicide (Fusilade, 1 L ha−1) was executed before crop emergence. This was done because of massive regrowth of previously grown cereals that would not have allowed any poppy growth at that time. In 2018 and 2020 mechanical weeding was done before seeding barley and late white clover (C2). Due to very low weed infestation after crop emergence in 2019, mechanical weeding was not necessary that year. The sites were fenced from seed emergence until harvest to prevent damage from hares.
2.3. Measurements
Growth parameters were analyzed depending on poppy developmental stage (BBCH) including leaf area index (LAI), plant biomass, weed coverage, weed number and weed species. The correspondence between BBCH stages and day of year are listed in
Table S1. Furthermore, the yield parameters capsule number or ear number, capsule weight, seed number and seed weight as well as straw weight were assessed at harvest. Data was generated for each crop separately on a single row from the center of each plot to reduce boundary effects.
For determining crop phenology, BBCH growth stages were used, with the extended cereal BBCH scale for barley and the general extended BBCH scale for clover [
28]. This latter scale was also used for identifying poppy growth stages but slightly modified. Main adaption was the exclusion of stage 2 and 4 due to overlapping with stages 3 and 5. Accordingly, the following principal growth stages occurred: 10–19,leaf development; 30–39, stem elongation; 50–59, inflorescence development; 60–69, flowering; 70–79, fruit development; 80–89, ripening. The BBCH stages were determined for every plot.
For fresh and dry matter determination, crop samples were collected on each plot from 0.1 m2. An area was chosen in the center of each plot that contained one poppy row and adjacent intercrops when present. The areas were chosen randomly but with distance to plot boarders and previously sampled zones. The plant material was sorted by crop species and its fresh weight was determined. For assessing dry matter, the plant samples were placed in a drying oven at 60 °C for 48 h and weighed again. As an intermediate step, before drying, leaves of the crop samples from 0.1 m² were separated from stems for leaf area measurement. Leaf area was determined by use of a Li-Cor 3100C leaf area meter (Li-Cor, Lincoln, NE, USA). Leaf area index (LAI) was obtained by dividing leaf area (m2) by the sampling area. Leaf area was first determined in 2019 and also analyzed in 2020.
For each of the four replicates, the dried material was further processed to analyze nitrogen content in the aboveground biomass. First the dry biomass was shredded with a SM 300 cutting mill (Retsch, Haan, Germany) at 3000 rpm and the use of a 0.25 mm sieve. For samples that were too small in weight for the SM 300 mill, material was pulverized with a MM 400 ball mill (Retsch, Haan, Germany) at 3000 rpm for 30 s. Then 6 mg of each ground sample were weighed into tin capsules for combustion and gas chromatographic analysis in the EA 3000 Elemental Analyser (HEKAtech, Wegberg, Germany).
Weeds were documented by weed number counting and weed species identification on three evenly distributed areas on each plot, each comprising 0.1 m², and weed ground coverage as well as crop ground coverage were estimated in percent.
At harvest, poppy and barley plants were collected manually from one row of each plot on three evenly distributed lengths of 1 m, respectively. Moisture content at harvest was 7–9% for poppy and 12–16% for barley. Number of plants and capsules or ears were counted. Poppy capsules and barley ears were cut off and counted and the straw was dried for 24 h at 104 °C and weighed afterwards. The dry capsules were weighed and cut with a small hack saw to separate seeds from capsules. Seeds of the whole sample were weighed to calculate total seed yield. Barley ears were processed with a Wintersteiger LD 180 laboratory thresher (Wintersteiger, Ried, Austria) and then seeds were weighed. One thousand seed masses of poppy and of barley were determined by weighing 1000 seeds that were counted with a Contador 2 Seed Counter (Pfeuffer, Kitzingen, Germany).
2.4. Data Analysis
For yield comparison the land equivalent ratio (LER) was computed for poppy and barley yields by the formula according to Mead and Willey (1980):
with
Ya = Yield of crop a as intercrop,
Yb = Yield of crop b as intercrops,
Sa = Yield of crop a as sole crop and
Sb = Yield of crop b as sole crop [
29]. The LER defines the relative land area that would be needed by a sole crop to produce the same yield as the intercropping system [
29]. An LER > 1 means for a substitutive design that more land would be needed if both crops were sown as sole crops thus indicating a benefit by intercropping. In additive intercropping, LERs between 1 and 2 could be expected due to increased plant density. pLERa and pLERb are partial LERs of each cultivar, describing their individual competitive ability by comparing yields in the intercrop to yields in monoculture.
The trial was analyzed as a one-factorial experiment and overall comparisons were done with a linear model including treatment and year as fixed factors. Due to consistent interactions between year and treatment, analyses of variance (ANOVA) were conducted for the individual years with treatment as a fixed factor for every measured parameter. Normal distribution (Shapiro–Wilk test) and homogeneity of variance (Levene’s test) was given for all data except weeds. Tukey HSD tests were chosen for all post-hoc comparisons except for weed data where the Scheffé test was applied. To stabilize estimates for LERs and pLERs, the ratios were calculated with individual plot values for the numerators (intercrops) and averages over all replicates for the denominators (sole crops). Analysis of variance for the LER values was performed with R software (version 1.2.1335) and all other statistical calculations were performed with SPSS 26 software (IBM Ehningen, Germany). Visualizations were made with Microsoft Excel 2019.
5. Conclusions
Poppy intercropped with clover was not severely impacted by competition as indicated by similar poppy yield, biomass, LAI and nitrogen content compared to poppy sole crops. This was mainly attributed to limited competition for water, light and soil nitrogen and a smaller demand of clover for exploiting these resources due to less biomass growth and its nitrogen fixation abilities. Seeding dates of clover did not have a major impact on yield and plant morphology but an earlier sowing date secured sufficient biomass build up to exploit the benefits of an increased ground cover. Therefore, implementing low growing intercrops like white clover into a poppy intercrop should be realized simultaneously to the poppy. Barley and poppy intercrops produced acceptable poppy yields compared to sole crops in two of three years. Early seeding dates and increased water supply enhanced the competitive abilities of barley, lowering poppy yields, LER, LAI, biomass and nitrogen content in intercropping with barley. Competition for nitrogen was strong particularly in the early phases of development. Higher seeding densities of barley tended to increase competitive effects, but the influence of climatic and management factors was stronger. Therefore, when choosing tall growing, biomass rich crops like barley, the poppy crop must be supported by earlier sowing or lower seeding densities of the second crop to ensure competitive equality in the intercrop. The effect of intercropping on weeds was small due to high variability in weed distribution and years with overall low weed infestation. However, a tendency towards lower weed ground cover in intercropping treatments was observed, in particular with barley. Higher plant densities in intercropping may be responsible for why poppy–clover intercrops were also less weedy than both sole crops.
In summary, intercropping poppy with barley requires precise management and earlier sowing of the poppy crop to limit the competitive abilities of barley. Clover appears to be a promising intercrop for poppy as it added ecological benefits while not affecting poppy performance. Due to great flexibility in choosing intercropping cultivars, seeding dates and densities, intercropping poppy can a promising cultivation strategy for poppy in the future.