Introduction
Developing countries like Ethiopia are particularly damaged by climate change due to their nature-dependent agrarian economy (Emerta, Reference Emerta2013). Agriculture is inextricably linked to weather and climate, which are key output variables (Roudier et al., Reference Roudier, Sultan, Quirion and Berg2011). Climate change affects crop yield and quality (Southworth et al., Reference Southworth, Randolph, Habeck, Doering, Pfeifer, Rao and Johnston2000), so appropriate adaptation options (Haverkort and Verhagen, Reference Haverkort and Verhagen2008; Yin, Reference Yin2013; Singh et al., Reference Singh, Nedumaran, Traore, Boote, Rattunde, Prasad, Singh, Srinivas and Bantilan2014) should be chosen to mitigate the negative effects of climate change while maximizing the benefits of positive impacts to feed the world's rapidly growing population (Daccache et al., Reference Daccache, Weatherhead, Stalham and Knox2011; Wart et al., Reference Wart, Kersebaum, Peng, Milner and Cassman2013). As a result, land-use intensification and agricultural diversification have increased (Alene et al., Reference Alene, Manyong and Gockowski2006). Planting cereal with legumes in an intercropping system is an excellent solution to this problem among sustainable farming methods (Agegnehu et al., Reference Agegnehu, Ghizaw and Sinebo2006). Panda (Reference Panda2010) defines intercropping as the simultaneous cultivation of two or more crop species in the same land, sharing resources during all or part of their growing season. It is a fascinating prospect, aiming for increased crop diversity within agricultural systems, and that is becoming increasingly widely recognized as a vital pillar of long-term development (Jensen et al., Reference Jensen, Bedoussac, Carlsson, Journet, Justes and Hauggaard-Nielsen2015; Laurent et al., Reference Laurent, Etienne Pascal, Hauggaard-Nielsen, Christophe, Corre-Hellou, Jensen, Loïc and Eric2015). As the availability of agricultural land decreases, intercropping becomes more significant and intense (Niguse and Reddy, Reference Niguse, Reddy, Woldeyesus, Zerihun and Nigussie1996). Intercropping is a commonly used land system in tropical regions where farming is limited to the number of inputs (Laurent et al., Reference Laurent, Etienne Pascal, Hauggaard-Nielsen, Christophe, Corre-Hellou, Jensen, Loïc and Eric2015). It is known to increase a farm's productivity, lower labor peaks, minimize crop failure and provide the highest possible net income (Bitew et al., Reference Bitew, Alemayehu, Adego and Assefa2019).
In Ethiopia, intercropping is prevalent, and it usually contains both cereal and legumes. Cereal is the most common food source, and its output is far higher than that of legumes (Willey, Reference Willey1979). Wheat is the most significant crop because it provides the majority of the country's calories. In the region, lupines are also employed as a versatile crop (Yeheyis et al., Reference Yeheyis, Kijora, van Santen and Peters2012). Subsistence farmers have used this legume as a supplementary crop in areas with poor soil fertility (Yeheyis et al., Reference Yeheyis, Kijora, van Santen and Peters2012; Bantie et al., Reference Bantie, Abera and Woldegiorgis2014).
The benefits of intercropping are known to be limited by a variety of factors such as component crop sowing timing, crop variety and crop selection. These variables can also limit the crop's productivity (Pal et al., Reference Pal, Kalu, Norman and Adedzwa1988). In intercropping system, the correct crop combination, component crop planting date and planting pattern that can best harness soil nutrients with less competition and strong facilitation between component crops are particularly critical (Munnu, Reference Munnu2003; Lithourgidis et al., Reference Lithourgidis, Dordas, Damalas and Vlachostergios2011; Bantie et al., Reference Bantie, Abera and Woldegiorgis2014). Additive (keep the density of one species constant while varying the density of other species) design may be used to combine species in intercropping systems, depending on the farmers' interests and the value of the crops (Connolly et al., Reference Connolly, Goma and Rahim2001). Farmers in northwest Ethiopia employ simple natural principles to manage bread wheat-lupines intercropped in additive design to maximize complementarity and avoid competition. There has been no systematic study effort to choose compatible lupine varieties with bread wheat under additive design intercropping with correct planting date for adopting the right system using diverse scenarios of sole crop production. Thus, the objective of this study was to evaluate different lupins and planting dates might affect the production of bread wheat-lupins under an additive design intercropping system in Northwest Ethiopia.
Materials and method
Descriptions of the study sites
During the rainy seasons of 2019 and 2020, a field experiment was conducted in two key wheat-producing districts in northwest Ethiopia (Adet and Debre Tabor districts). Adet is located at a latitude and longitude (11o16′N 37o29′E) with an altitude of 2240 meters above sea level (AARC, 2011). Debre Tabor is located latitude and longitude of 11o51′N 38o1′E with an elevation of 2630 meters above sea level. According to Bahir Dar Meteorology station (Reference Bahir Dar Meteorology station2020), the Adet district received a total rainfall of 1592.1 and 1228.1 mm during the 2019 and 2020 cropping seasons, respectively (Fig. 1), while the Debre Tabor district received 1926.1 and 1739 mm for the years 2019 and 2020, respectively (Fig. 2). The minimum and maximum temperatures at Adet were 5.4 and 30.2°C in the 2019 season, respectively and 6.8 and 30.7°C in the 2020 season. The minimum and highest temperatures at Debre Tabor for the cropping seasons of 2019 and 2020 were 7.5 and 25.3°C and 6.2 and 25.5°C, respectively.
Fig. 1. Average monthly rainfall and temperature distribution at Adet throughout the two-year experiment (Bahir Dar Meteorology station, 2020).
Fig. 2. Average monthly rainfall and temperature distribution during the two experimental years (2019–2020) at Debre Tabor (Bahir Dar Meteorology station, 2020).
Soil samples were gathered in the respective year at five spots diagonally at 0–20 cm soil depth and composited to characterize the overall soil parameters of both experimental sites before the start of the experiment. The Adet Agricultural Research Center's Soil Laboratory assessed the composite soil samples. To determine soil texture, total nitrogen, pH, available phosphorous, organic carbon and cation exchange capacity, the soil sample was air-dried, crushed and sieved using a 2 mm sieve. Table 1 shows the findings of the soil analysis as well as the methodologies used.
Table 1. Soil properties of the study sites before the experimenta
pH: potential of hydrogen, CEC: cation exchange capacity, OC: organic carbon, TN: total nitrogen, Ava.P: available phosphorous, USDA: the U.S. Department of Agriculture.
a Data were mean of 2 year (2019 and 2020).
b Panda (Reference Panda2010).
c Landon (Reference Landon1991).
Experimental treatments, design and procedures
Bread wheat was intercropped with two lupine varieties at varied planting dates at the Adet and Debre Tabor experimental sites (Table 2). The treatments included an additive series of lupine row intercropping with bread wheat at four planting dates and two varieties of lupine. Furthermore, the sole cultures of the three crops were considered as a control. The experiments were laid out in a 4 × 2 factorial randomized complete block design (RCBD) with three replications plus the sole crop of the wheat, local lupine and sweet lupine were also included for comparison purposes. The gross and net areas of the experimental plot were 3.6 m × 2 m (7.2 m2) and 3 m × 2 m (6 m2) with a distance of 0.5 and 1 m between adjacent plots and replications, respectively.
Table 2. Description of the treatment combinations for the bread wheat and sweet lupine intercropping experiment
The most dominant and adaptable variety of bread wheat (Taye), local lupine (white lupine) and sweet lupine (Sanabor) were used in the present study. Bread wheat was a primary crop, with local lupine and sweet lupine as secondary crops, all of which were planted at the same time, two weeks, four weeks and six weeks after wheat was planted. Wheat was sown on the 24th and 25th of June in 2019 and 2020, respectively. Bread wheat seeds were manually drilled in rows at the specified 20 cm inter-row spacing, while lupine seeds were planted every two rows at 40 cm × 10 cm inter and intra-row spacing, respectively, in an additive design (2:1) intercropping system. Fertilizer rates of 74/46 N/P2O5 kg ha−1 and 120/46 N/P2O5 kg ha−1 were used at Adet and Debre Tabor respectively. The whole amount of phosphorous in each experimental site was applied in the band at the time of sowing in the form of Di-ammonium phosphate (DAP). On the other hand, nitrogen in the form of urea was applied in two splits where one-third was applied at the time of sowing and the remaining two-thirds were applied at tillering stages in the band with a depth of 2 cm. Lupine in all cropping systems was supplied with 18/46 N/P2O5 kg ha−1 in the form of Diammonium phosphate (DAP) at the time of planting. The lupine benefited from self-fixed nitrogen, but it required a starter fertilizer (diammonium phosphate) to get started. Weeds were uniformly managed by hand weeding. For planted crops, the fertilizer application method was in the band at a depth of 2 cm. Following the growth of the crops, weeding and other management actions were carried out.
Data collection and measurements
The number of branches per plant (NBPPL), number of pods per branch (NPOPB) and number of seeds per pod (NSEPPO) were measured at physiological maturity from ten randomly picked lupine plants in the net plot area using the row planting method. Treatment 1, treatment 2, treatment 3, treatment 4, treatment 5, sole sweet lupine and sole local lupine at the Adet experimental site had 183, 140, 170, 127, 179, 140 and 170 days to physiological maturity from sowing to harvesting, respectively. Treatment 1, treatment 2, treatment 3, treatment 4, treatment 5, sole sweet lupine and sole local lupine at the Debre Tabor experimental site had 204, 164, 188, 156, 176, 156 and 188 days to physiological maturity from sowing to harvesting, respectively.
The component crops' total above-ground biomass (BY) from the net plot area was collected and sun-dried for two weeks at an average air temperature of 25–27°C until complete drying was achieved. The grain yields (GY) of bread wheat and lupines were determined from the total biomass yield (BY). The GY of bread wheat and lupines were dried, threshed, cleaned and adjusted to a moisture content of 12 and14%, respectively.
Data and analysis
Data analysis for the NBPPL, NPOPB, NSEPPO, the component crop GY and total BY was conducted using the general linear model (GLM) procedure of SAS version 9.2 (SAS Institute, 2008) for each site and year. The data were combined over years since values for the error mean square of the two years were homogenous (Gomez and Gomez, Reference Gomez and Gomez1984). In the combined analysis, location and treatment were considered fixed effects and year as a random effect. Crop characteristics that showed significant differences (P < 0.05) were further tested for mean separation. Temporal niche differentiation (TND), land equivalent ratio (LER), competition ratio and production efficiency are indices, and their formulas were used to produce values for each treatment, which were then interpreted using the indices' values. GY obtained on plot bases were converted to hectare bases and expressed in tons per hectare for statistical analysis.
Temporal niche differentiation (TND)
TND is a metric that measures how much of the intercrop's entire growing period differs from that of the two-component species when produced as a sole crop. The formula for calculating it was devised by Yu et al. (Reference Yu, Stomph, Makowski and van der Werf2015):
where P overlap represents the period of overlap of the growth period of the intercropped species, while the P system represents the duration of the whole intercrop. In theory, TND is zero when both species are sown and harvested at the same time; TND would be one in the case of double cropping and TND would be found between zero and one when both species are sown and harvested at the same or different date. High TND means the duration of overlap is short, the reduction of growth resource competition and approaches to one. Low TND means the duration of overlap is long, high growth resource competition and approaches to zero.
Land-use efficiency
The LER is a measure of land utilization efficiency in an intercropping system. When compared to mono-cropping, it demonstrates the efficiency of intercropping for the exploitation of environmental resources (Zhang et al., Reference Zhang, Yang and Dong2011). Willey and Reddy (Reference Willey and Reddy1981) devised the formula for calculating the LER.
where Yi and Ym are yields of component crops in intercrop and sole cropping, respectively, and n is the number of crops involved. The value of unity is the critical value. When LER is equal to one, component crops are complementary. When the LER is more than one, intercropping benefits the species' development and yield. When the LER is less than one, however, intercropping harms the growth and production of plants farmed in combinations (Willey and Reddy, Reference Willey and Reddy1981).
Competitive ratio (CR)
To quantify competitiveness among component crops, the competitive ratio was used (Zhang et al., Reference Zhang, Yang and Dong2011). The following formula was used to calculate the competitive ratio of the component crops:
where CRW and CRL denote bread wheat and lupine CR, respectively. PLERW and PLERL are the partial LERs of wheat and lupines, respectively, and ZWL and ZLW are the seed proportion of wheat intercropped with lupine varieties and the seed proportion of lupine varieties intercropped with wheat, respectively. The yields of wheat and lupine varieties in intercropping are YWIC and YLIC, respectively, while the yields of wheat and lupine varieties in sole cropping are YWSC and YLSC, respectively. Wheat was a competitor if CRW was greater than one, while lupine suppressed wheat output if CRW was less than one.
Production efficiency
The area time equivalent ratio (ATER) is a relatively new measure for determining the effectiveness of cropping systems (Samant, Reference Samant2015). Hiebsch and McCollum, (Reference Hiebsch and McCollum1987) calculated the ATER as follows:
where PLERW and PLERL are a partial LER of wheat and lupines, respectively; TW denotes the duration of wheat maturity, TL denotes the duration of lupine maturity, and T denotes the period of the intercropping system. The null hypothesis was that ATER equaled one, implying that two crops are complementary. When the ATER was more than one, intercropping favored component crop growth and yield. Intercropping, on the other hand, had a detrimental impact on the component crops' development and yield when ATER was less than one. According to Jolliffe and Wanjau, (Reference Jolliffe and Wanjau1999), intercropping productivity was calculated in terms of TLO yields as the sum of the component GY.
Results and discussion
Growth responses of lupines in bread wheat-lupine intercropping
Growth responses of the lupines were significantly (P < 0.01) influenced by intercropping treatments (Table 3). The greatest NBPPL was recorded for local lupine planted two weeks (S2V1) after the bread wheat, followed by simultaneous planting of local lupine (S1V1) and bread wheat, planting of local lupine four weeks (S3V1) after bread wheat, and simultaneous planting of sweet lupine (S1V2) and bread wheat intercropping at the Adet experimental site (Table 3). While the greatest NBPPL was recorded by simultaneous planting of local lupine (S1V1) and bread wheat intercropping system at the Debre Tabor experimental site (Table 3). In both experimental sites, the lowest NBPPL was recorded by the planting of sweet lupine four weeks (S3V2) after bread wheat, planting of local lupine six weeks (S4V1) after bread wheat, and planting of sweet lupine six weeks (S4V2) after bread wheat (Table 3). The greatest NPOPB was recorded by the planting of local lupine two weeks (S2V1) after bread wheat, simultaneous planting of sweet lupine (S1V2) and local lupine (S1V1) with bread wheat intercropping system at the Adet experimental site. The highest NPOPB was recorded with simultaneous planting of local lupine (S1V1) and bread wheat, and planting of local lupine two weeks (S2V1) after bread wheat at the Debre Tabor experimental site (Table 3). While the lowest NPOPB was recorded by the planting of sweet lupine four weeks (S3V2) after bread wheat, planting of local lupine six weeks (S4V1) after bread wheat, and planting of sweet lupine six weeks (S4V2) after bread wheat at both experimental sites (Table 3). At Adet, the simultaneous planting of local lupine (S1V1) and bread wheat and planting of local lupine two weeks (S2V1) after bread wheat had the highest NSEPPO. While at Debre Tabor, the simultaneous planting of local lupine (S1V1) and bread wheat, planting of local lupine two weeks (S2V1) after bread wheat, and planting of local lupine four weeks (S3V1) after bread wheat ranked among the highest (Table 3). Significantly greater local lupine yield attributes in bread wheat-local lupine intercropping compared to bread wheat-sweet lupine intercropping in all treatments could be attributed to (i) Long TND indicates that the overlap is brief, reducing growth resource competition and (ii) Long TND resulting from efficient resource usage. This result was in line with Bitew et al. (Reference Bitew, Alemayehu, Adgo and Assefa2020), who reported that greater GY in some cereal-legume intercropping compared with other cereal-legume intercropping, possibly due to long TND. In a bread wheat-lupine intercropping system, the lowest NSEPPO was reported for the planting of sweet lupine four weeks (S3V2) after bread wheat, planting of local lupine six weeks (S4V1) after bread wheat, and planting of sweet lupine six weeks (S4V2) after bread wheat. Planting of sweet lupine four weeks (S3V2) after bread wheat, planting of local lupine six weeks (S4V1) after bread wheat, and planting of sweet lupine six weeks (S4V2) after bread wheat scored the lowest in all growth parameters, which could be due to: (i) sowing sweet lupine seeds four weeks after the main crop, sweet lupine did not have a conducive environment to germinate and grow in the bread wheat-sweet lupine intercropping system (ii) sowing both local and sweet lupine six weeks after the main crop, both sweet lupine, and local lupine did not sprout due to a lack of seed germination conditions, shadowing effects of the component crop, and low soil temperature due to persistent rain.
Table 3. Yield and yield attributes of legumes in bread wheat-legumes intercropping system in Northwest Ethiopia
Temporal niche differentiation (TND); the number of branches per plant (NBPPL); the number of pods per branch (NPOPB); the number of seeds per pod (NSEPPO); grain yield (GY); total biomass yield (BY).
GY & BY expressed in ton per hectare (t ha−1).
Data were combined over years (2019 and 2020).
*,**, *** are the significant difference at a probability level of, 0.05, 0.01 and 0.001, respectively. Means with the same letter are not significantly different within columns.
Yield responses of lupine under bread wheat-lupine intercropping
The highest GY were recorded for the simultaneous planting of local lupine (S1V1) and bread wheat and planting of local lupine two weeks (S2V1) after bread wheat followed by the treatment combinations S3V1, S1V2 and S2V2 after bread wheat at the Adet experimental site (Table 3). While at Debre Tabor, GY was only greater for the simultaneous planting of local lupine (S1V1) and bread wheat relative to the simultaneous planting of sweet lupine (S1V2) and bread wheat (Table 3). The highest total BY was recorded by the simultaneous planting of local lupine (S1V1) and bread wheat, followed by planting of local lupine two weeks (S2V1) and four weeks (S3V1) after bread wheat at Adet (Table 3). While at Debre Tabor, the highest total BYs were recorded by the simultaneous planting of local lupine (S1V1) and bread wheat, planting of local lupine for two weeks (S2V1), and four weeks (S3V1) after bread wheat. However, the simultaneous planting of local lupine (S1V1) and bread wheat had a greater BY than the simultaneous planting of sweet lupine (S1V2) and bread wheat and planting of sweet lupine two weeks (S2V2) after bread wheat (Table 3). When compared to short TND intercropping treatments, long TND intercropping treatments produced the maximum GY and total biomass. This could be due to (i) intercropping treatments with high TND and lower inter-specific competition; or intercropping treatments with low TND and higher inter-specific competition. (ii) Local lupine, as opposed to sweet lupine, may lessen the shadowing effect of the component crop, allowing for more efficient photosynthesis. This result was similar to Bitew et al. (Reference Bitew, Alemayehu, Adgo and Assefa2020), who suggested that the significantly greater GY intercropping treatments could be attributable to efficient resource usage due to the wide TND. Similarly, intercrops of different varieties are adopted widely to exploit the length of the growing period (Lithourgidis et al., Reference Lithourgidis, Dordas, Damalas and Vlachostergios2011) and increase light interception over time (Keating and Carberry, Reference Keating and Carberry1993; Zhang et al., Reference Zhang, van der Werf, Bastiaans, Zhang, Li and Spiertz2008). The lupines with the lowest GY and total biomass were planted four weeks later with sweet lupine and six weeks later with both types of lupine in the bread wheat-lupine intercropping system. This could be attributed to minimal inter-specific competition and intercropping treatments with high TND.
Land use efficiency in bread wheat-lupine intercropping
The partial land equivalent ratio (PLER) of bread wheat in all treatments was greater than 0.5 at both experimental sites, according to the findings. While the PLER of lupines is less than 0.5 at Adet, it is larger than 0.5 at DebreTabor (Table 4). PLER values greater than 0.5 for cereal and legume suggested that both component crops had an advantage in terms of growth resource utilization during their co-growing period in these cereal-based intercropping systems (Bitew et al., Reference Bitew, Alemayehu, Adego and Assefa2019). PLER of lupine was less than 0.5, indicating that bread wheat had an advantage and lupine had a disadvantage in this intercropping system. The PLER of bread wheat, on the other hand, was higher than that of legumes, which explains why bread wheat has a stronger competitive ability than legumes. Thus, determine that bread wheat contributed the most to the combination yield and LER, confirming farmers' argument for cultivating legumes as an intercrop. This result is consistent with Wang et al. (Reference Wang, Li, Christie, Zhang, Zhang and Bever2017) in wheat-faba bean, and winter wheat-legumes (Guiducci et al., Reference Guiducci, Tosti, Falcinelli and Benincasa2018). Similarly, in pearl millet-cluster bean, yield advantage has also been seen (Yadav and Yadav, Reference Yadav and Yadav2001).
Table 4. Effects of intercropping treatments on land use efficiency in Northwest, Ethiopia
PLERBW, partial land equivalent ratio of bread wheat; PLERLL, partial land equivalent ratio of local lupine; PLERSL, partial land equivalent ratio of sweet lupine; LER, land equivalent ratio in Adet and Debre Tabor experimental sites.
aData were combined over years (2019 and 2020).
The LER was greater or equal to one almost in all intercrops at both experimental sites, however, planting of sweet lupine four weeks (S3V2) after bread wheat, planting of local lupine six weeks (S4V1) after bread wheat, and planting of sweet lupine six weeks (S4V2) after bread wheat was lower than the sole crop's in Debre Tabor because the legumes sown with bread wheat did not germinate, grow or contribute to the overall land output yield in the intercropping system (Table 4). In Adet, all of the LER was greater than or equal to one. At the Adet experimental site, the highest LER was reported from S1V2 (1.3), followed by S2V2 (1.23) in the bread wheat–lupine intercropping system. While at Debre Tabor experimental site, the highest LER was reported from S2V2 (1.67), followed by S1V2 (1.6), S2V1 (1.6) and S3V1 (1.6) in the bread wheat-lupine intercropping system, as shown in Table 4. Except for S1V2 and S2V2 at Adet, the higher LER obtained in all scenarios of bread wheat-legume intercropping could be attributed to wider TND between intercropped crops, resulting in lower inter-specific competition among different treatments of the same legume intercropped with the main crop. Bitew et al. (Reference Bitew, Alemayehu, Adego and Assefa2019) found similar outcomes in cereal-legume intercropping, which they attribute to larger TND resulting in less inter-specific competition. The second reason could be that intercropping increases the availability of applied nutrients and improves the efficiency of nutrient consumption by linked cereals, boosting overall land output yield (Kiwia et al., Reference Kiwia, Kimani, Harawa, Jama and Sileshi2019). Sweet lupine intercropping at the same time and two weeks later in bread wheat-sweet lupine intercropping produced higher LER than local lupine in bread wheat-local lupine intercropping. In many intercropping systems, both competition and facilitation occur, according to Vandermeer, (Reference Vandermeer1989), and it is conceivable to achieve the net result of LER>one, in which complementary facilitation contributes more to the interaction than competitive interference. According to the findings, sole cropping requires 23 to 30% more land in Adet, whereas sole cropping requires 60 to 67% more land in Debre Tabor, implying that an intercrop yield advantage of 60 to 67% can be acquired over sole cropping. Intercrops' increased yield and land-use efficiency suggest that farmers could produce more without placing more land under cultivation. The rise in yields is intended to diversify the household's food and revenue sources. This result was consistent with other research findings, such as finger millet-white lupine intercropping systems (Bantie et al., Reference Bantie, Abera and Woldegiorgis2014), pearl millet-cluster bean intercropping systems (Yadav and Yadav, Reference Yadav and Yadav2001), and pea–barley intercropping systems (Chen et al., Reference Chen, Westcott, Neill, Wichman and Knox2004). In Debre Tabor, however, S3V2, S4V1 and S4V2 resulted in yield reductions of 7, 5 and 7%, respectively, as compared to sole cropping. This is primarily owing to the component crop's reduced yield. In Debre Tabor, the LER values derived from S3V2, S4V1 and S4V2 were all less than one, indicating that these blends were inferior to their respective solo crops. In wheat-pea (Naudin et al., Reference Naudin, Corre-Hellou, Pineau, Crozat and Jeuffroy2010) and barley-lupine (Bantie et al., Reference Bantie, Abera and Woldegiorgis2014), LER was shown to be less than one. This suggested that for this intercropping to boost production, the appropriate component crop and planting dates were required.
Competition between component crops
At both study sites, bread wheat was more competitive (CRBW ranging from 1.08 to 4.8) than sweet lupine (CRSL ranging from 0.24 to 0.88) and local lupine (CRLL ranging from 0.21 to 0.92) in bread wheat-lupines intercropping (Table 5). According to Zhang et al. (Reference Zhang, Yang and Dong2011), if the CR of cereal was larger than one, cereal was a competitor, however, if the CR of cereal was less than one, the legume crop suppressed cereal production. In all intercropping experiments, the CR of bread wheat had only been more competitive than the CR of sweet lupine and local lupine. The fast-growing capacity and the high number of tillers of bread wheat in mixtures likely result in higher CR of bread wheat than CR of sweet lupine and CR of local lupine. This result contradicts the findings by Bantie et al. (Reference Bantie, Abera and Woldegiorgis2014) for cereal-legume intercropping. In bread wheat-lupine intercropping treatments, there was less competition in lupine (CRLL of 0.21 and 0.72 in Adet and Debre Tabor in Table 5, respectively) due to the wide TND at Adet (0.48) and Debre Tabor (0.36) (Table 3). According to Yu et al. (Reference Yu, Stomph, Makowski and van der Werf2015), cereal–legume intercropping systems with a large TND have low competition for growth resources. There was less competition of local lupine ranging from 0.31 to 0.21 and 0.92 to 0.72 at Adet and Debre Tabor in bread wheat-local lupine intercropping treatments while TND widens from 0.27 to 0.48 and 0.23 to 0.36 (Table 3), respectively, in bread wheat-local lupine intercropping treatments. Similarly, while TND increased from 0.05 to 0.14 and 0.08 to 0.17 in Adet and Debre Tabor, respectively, in bread wheat-sweet lupine intercropping treatments, sweet lupine competition decreased from 0.38 to 0.24 at Adet but not in Debre Tabor (Table 5).
Table 5. Competitive ratio of bread wheat, local lupine and sweet lupine intercropped at Adet and Debre Tabor Experimental sites
CRBW, competitive ratio of Bread wheat in intercropping; CRSL, competitive ratio of sweet lupin in intercropping; CRLL, competitive ratio of local lupine in intercropping.
Data were combined over years (2019 and 2020) at both locations.
Production efficiency
At both study sites, the ATER was higher when bread wheat was intercropped with sweet lupine than with local lupine (Table 6). By raising soil nitrogen (N) through rhizobacteria's N-fixing activity, legume intercropping can improve soil fertility by allowing more fixed N to remain in the upper soil layers and be available to plants (Hauggaard-Nielsen et al., Reference Hauggaard-Nielsen, Gooding, Ambus, Corre-Hellou, Crozat, Dahlmann, Dibet, von Fragstein, Pristeri, Monti and Jensen2009; Chapagain and Riseman, Reference Chapagain and Riseman2014).
Table 6. Production efficiency of bread wheat-legume intercropping in Northwest, Ethiopia
ATER, area time equivalent ratio; Data were combined over years (2019 and 2020) at both locations.
Greater ATER was reported in the simultaneous planting of sweet lupine (S1V2) and bread wheat (1.29), followed by the planting of sweet lupine (S2V2) after bread wheat (1.22) at the Adet experimental location. At the Debre Tabor location, the treatment combination of planting sweet lupine two weeks after bread wheat (S2V2) had the highest ATER (1.53), followed by simultaneous planting of sweet lupine (S1V2) after bread wheat (1.39) intercropping (Table 6). This revealed that sole cropping would necessitate 29 and 53% more area, respectively, to attain the same yield as intercropping. This difference was due to the climatic difference in Adet and Debre Tabor as shown in Figures 1 and 2. The additional crop output obtained by the intercropping treatments when compared to sole cropping was also linked to the maximum land-use efficiency for the intercrop (Table 4). In support of these findings, interspecific interactions between legume and cereal caused cereal to acquire more soil N, which pushed the legume to fix more N, boosting the system's land-use efficiency, according to Li et al. (Reference Li, Zhang and Zhang2013). This is consistent with the findings of Bantie et al. (Reference Bantie, Abera and Woldegiorgis2014) for cereal-legume intercropping. All bread wheat-lupines intercrops showed ATER values greater than one at both locations, except for simultaneous planting of local lupine (S1V1) and bread wheat, planting of local lupine two weeks (S2V1) after bread wheat and planting of local lupine four weeks (S3V1) after bread wheat at the Adet experimental site, showing that these combinations are superior to sole crops. Similarly, yield advantages have been shown in various non-legume-legume intercropping systems, such as bean-wheat (Hauggaard-Nielsen et al., Reference Hauggaard-Nielsen, Ambus and Jensen2001), ground nut-cereal fodders (Ghosh, Reference Ghosh2004), barley-pea (Chen et al., Reference Chen, Westcott, Neill, Wichman and Knox2004) and faba bean-barley (Trydemanknudsen et al., Reference Trydemanknudsen, Hauggaard-Nielsen, Jornsgard and Steenjensen2004), as compared to corresponding sole crops. An ATER of less than one was recorded in wheat-pea intercropping (Naudin et al., Reference Naudin, Corre-Hellou, Pineau, Crozat and Jeuffroy2010), which contradicted this result.
Conclusion
The simultaneous planting of sweet lupine (S1V2) and bread wheat (1.3) had the greatest LER at the Adet experimental site. While in Debre Tabor experimental site, planting of sweet lupine two weeks (S2V2) after bread wheat planting (1.67) had the highest LER.
Bread wheat outcompeted sweet lupine and local lupine in bread wheat-lupine intercropping at both study sites. When bread wheat was intercropped with sweet lupine, the ATER was higher than when intercropped with local lupine. The simultaneous planting of sweet lupine (S1V2) and bread wheat (1.29) had the greatest ATER at the Adet experimental site. While at Debre Tabor experimental site, planting of sweet lupine two weeks (S2V2) after bread wheat planting (1.53) had the greatest ATER. Based on the findings of this study, In Adet, the simultaneous planting of sweet lupine (S1V2) and bread wheat intercropping could be recommended for improvement of productivity and production efficiency. While in Debre Tabor, planting of sweet lupine two weeks (S2V2) after bread wheat intercropping could be recommended for improvement of productivity and production efficiency, as well as a realistic option for improving family food security in the study areas and areas with similar agroecology. Further research should be focused on (i) evaluating the quality of bread wheat in the intercropping of bread wheat and lupine under additive series intercropping and (ii) the impact of intercropping systems of bread wheat and legumes on soil physical, chemical and biological properties, as well as water usage efficiency.
Data
On reasonable request, the corresponding author, Birhanu Bayeh, will provide the data that support the findings of this study.
Acknowledgements
The authors want to thank everyone who assisted with data collection at the experimental sites, data processing, writing and publishing this paper. We are also appreciative for the financial support provided by the Ministry of Science and Higher Education and Debre Tabor University for this experiment.
Conflicts of interests
There are no conflicts of interest declared by the authors.
