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Photosynthetically active radiation determining yields for an intercrop of maize with cabbage
Europ. J. Agronomy 69 (2015) 32–40
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European Journal of Agronomy journal homepage: www.elsevier.com/locate/eja
Photosynthetically active radiation determining yields for an intercrop of maize with cabbage Qingsuo Wang a,∗ , Dongbao Sun a,∗,1 , Hong Hao a , Xuejiao Zhao a , Weiping Hao a , Qiong Liu b a b
Institute of Environment and Sustainable Development in Agriculture, CAAS/ Key Laboratory of Dryland Agriculture, MOA, Beijing 100081, China Meteorological Bureau of Shouyang, Shanxi Province, Shanxi 045400, China
a r t i c l e
i n f o
Article history: Received 19 January 2015 Received in revised form 16 May 2015 Accepted 28 May 2015 Keywords: Maize Cabbage Intercropping Yield PAR Dryland
a b s t r a c t In order to explicate influential mechanism for yields of an intercrop of maize with cabbage, seven treatments for different intercropping rows of two crops were designed and conducted in the central Shanxi Province of China, and yield and PAR for each component crop of the intercropping treatments were measured. PAR of intercropped maize was higher than its monocropping while PAR of intercropped cabbage was lower than its monocropping. PAR highlighted a decline trend for intercropped maize and an increase trend for intercropped cabbage with increased rows of two crops in intercropping, respectively. Relationship between PAR and the number of intercropping rows of two crops was a negative linear function for intercropped maize and a positive logarithmic function for intercropped cabbage, respectively. Grain yield of intercropped maize declined and fresh yield of intercropped cabbage increased with intercropping rows of two crops, respectively. Relationship between yield and the number of intercropping rows of two crops was a logarithmic function, negative for intercropped maize and positive for intercropped cabbage, respectively. Generally, a positive linear function between yield and PAR in the intercropping system was developed for both intercropped maize and cabbage. M4 C6 is recommended for an optimum combination of this intercropping system. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Growing two or more crops proximately on a given piece of land is called intercropping which has been identified some significant types including row intercropping, strip intercropping, relay intercropping, within-row intercropping and mixed intercropping. Agroforestry, a mixture of crops and trees or shrubs, is thought as one type of intercropping. Intercropping has been experienced as a common practice of traditional cropping systems for thousands of years. The more agricultural intensification in terms of crop breed-
Abbreviations: PAR, photosynthetically active radiation; Mm, maize monoculture; Cm, cabbage monoculture; M1 C1 , one row of maize intercropped with one row of cabbage; M1 C2 , one row of maize intercropped with two rows of cabbage; M2 C3 , two rows of maize intercropped with three rows of cabbage; M3 C4 , three rows of maize intercropped with four rows of cabbage; M3 C5 , three rows of maize intercropped with five rows of cabbage; M4 C6 , four rows of maize intercropped with six rows of cabbage; M6 C8 , six rows of maize intercropped with eight rows of cabbage. ∗ Corresponding authors. Tel.: +86 10 82109756; fax: +86 10 82109560. 1 These authors contributed equally to this study and share first authorship. http://dx.doi.org/10.1016/j.eja.2015.05.004 1161-0301/© 2015 Elsevier B.V. All rights reserved.
ing, mechanization, fertilizer and pesticide use has been limiting expansion of intercropping so that it is far less common in Europe, North America, and some parts of Asia. In many areas of the world, however, intercropping still is a very important cropping system, especially for small farms or organic farms. Intercropping has been paid more and more attention as a very significant alternative way of sustainable agriculture in view of protection of environment, efficient utilization of resources, reduction of pest and disease incidence, and resistance to drought and frost risks (Lithourgidis et al., 2011). The most remarkable advantage of intercropping is to produce a greater yield or income than any crop monocropping. This yield advantage of intercropping mainly depends on the mutual benefit or different ecological niches (spatial, temporal, and nutritious) between component crops so that they can make efficient use of resources such as light, water and nutrients that would otherwise not be utilized by each sole crop. The especially important thing of intercropping is to efficiently utilize solar radiation resource together for the component crops, but different intercropping systems show different ways of obtaining solar radiation energy. Relay intercropping can make use of overlap time between first and
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second intercropped crops and prolong solar radiation utilization time comparing with each crop monocropping (Corlett et al., 1992; Connolly et al., 2001). For intercropping an early-maturing crop with a late- maturing crop, the early-maturing crop can efficiently use solar radiation which is not fully used by the late-maturing crop in the early growing period so that the intercropping system can sufficiently capture sunlight energy during the whole growing season (Keating and Carberry, 1993). Intercropping between cereal and legume (climbing) crops can obtain more solar radiation by increasing total leaf area index of the component crops comparing with each crop sole cropping so that the whole intercropping system can increase light use efficiency, e.g., a maize-cowpea intercrop (Ghanbari et al., 2010). Intercropping a tall-statured crop with a short-statured crop can also improve light use efficiency due to decreasing sunlight reflectance by intercepting three-dimensional light instead of planar light each sole crop obtains. For the tallstatured crop of an intercrop, its edge rows get more solar radiation and produce higher yields called the edge yield advantage comparing with the inner rows. For the short-statured crop, its edge rows can be a certain degree of shade from the tall-statured crop and get less solar radiation which is favorable to shade-tolerant crops such as edible mushrooms, some vegetables and medicinal plants. However, intercrops of the tall- and short-statured crops are unfavorable to many short-statured crops intolerant to shade and often make their yields decline, such as intercropping maize with soybean (Wang et al., 2012), groundnut (Awal et al., 2006), tomato (Dong, 2012), chili pepper (Gong et al., 2010). Intercropping crops with trees or shrubs always gives less crop yields comparing with the sole crop growing (Pei et al., 2000; Yuan et al., 2001; Shi et al., 2009). The intercropped short-statured crops often get a gradual increase of yields with their rows (Hayder et al., 2003; Liu et al., 2012; Wang et al., 2012; Gong et al., 2010) due to being less shaded and more sunlight intercepting. However, quantitative relationships between yield and solar radiation or the number of intercropping rows of maize and cabbage remain unclear. Shouyang County, located in the middle of Shanxi Province, is dominated by dryland farming due to less precipitation. Maize is one of main cereal crops and cabbage is one of main vegetables in this region. Its uniquely natural conditions such as high elevation, cool climate, high difference of daily temperature, less incidence of plant disease and insect pests make cabbage good quality with tight shape, crisp texture, refreshing taste, high calcium and vitamin C. Cabbage has been traditionally grown in summer and harvested in autumn in order to escape spring drought and make good use of high daily temperature difference in autumn, and used as fresh vegetable and pickled or fermented vegetable for winter and spring seasons. In recent years, however, some cabbage is planted in spring and harvested in the middle summer which is supplied for hot season markets in the eastern China. Most of cabbage is grown in monoculture while some is intercropped with other crops such as maize and millet as strip intercropping favorable to the mechanized farming. The objectives of this work is to explore the quantitative relationships between yield and photosynthetically active radiation (PAR) or the number of intercropping rows of two crops in a maize–cabbage intercrop system, and to determine reasonable strip structure of the maize–cabbage intercrop.
2. Methods 2.1. Study site This study was conducted at the Dryland Agricultural Experiment Station in Shouyang County, Shanxi Province (37◦ 51 N and 113◦ 05 E). The landscape is Loess Plateau in China. The altitude
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is 1035 m above sea level. It belongs to a temperate continental semi-humid climate with annual average temperature of 7.6 ◦ C and annual average precipitation of 489.5 mm from 1981 to 2010 which means cold weather and less precipitation. Drought has frequently happened and dryland farming is dominated in this region. In 2011, average temperature was 7.1 ◦ C with a maximum of 32.9 ◦ C and a minimum of −24.1 ◦ C, and annual precipitation was 635.6 mm. In 2012, average temperature was 6.9 ◦ C with a maximum of 33.0 ◦ C and a minimum of −23.2 ◦ C, and annual precipitation was 574.5 mm. Soil type is Cinnamon with the parent material of loess. The top soil at 0–20 cm depth contained 22.99 g/kg of organic matter, 1.15 g/kg of total nitrogen, 16.02 mg/kg of nitrate, 118.44 mg/kg of available phosphorous and 11.82 mg/kg of available potassium, and the pH was 8.2 in April, 2011. One crop a year is the current cropping system. The major crops are maize, millet, buckwheat, and the main vegetables are cabbage and potato.
2.2. Experiment design This study was conducted in 2011 and 2012. Maize cultivar Jindan 48 and cabbage cultivar Wanfeng adapted to the local conditions were selected. Seven treatments of a maize–cabbage strip intercrop were designed with the number of intercropping rows of maize and cabbage, respectively, i.e., one row of maize versus one row of cabbage (M1 C1 ), one row of maize versus two rows of cabbage (M1 C2 ), two rows of maize versus three rows of cabbage (M2 C3 ), three rows of maize versus four rows of cabbage (M3 C4 ), three rows of maize versus five rows of cabbage (M3 C5 ), four rows of maize versus six rows of cabbage (M4 C6 ) and six rows of maize versus eight rows of cabbage (M6 C8 ). Monoculture of maize (Mm ) and cabbage (Cm ) was as a control, respectively. Two crops were planted in a north–south row direction. Row spacing was 60 cm for both crops. Plant spacing was 30 cm for maize and 60 cm for cabbage, respectively. Each treatment was duplicated three times which were located according to a spatially balanced randomized plot design. Each plot at least had one complete intercropping strip for two intercropped crops. Length of the plots was 10 m. Maize was grown on April 30, and cabbage was transplanted on June 20 as seeding. Both crops were harvested in early October each year. Chemical fertilizers such as urea and superphosphate were applied into soil before maize was planted each year. The fertilizer application was 150 kg N hm−2 and 84 kg P2 O5 hm−2 , respectively. Weeds and insects were effectively controlled.
2.3. Measurement of PAR Incident PAR was measured with Li-250A Light Meter among which was installed Li-190SA Line Quantum Sensor. Sunny days were selected to observe for this work. The measurement was operated at 2 h intervals from 6:00 to 18:00 each day selected. PAR of intercropped maize each treatment was measured at different heights in each row of the intercropping strip. The height measured was the bottom, 25 cm, 50 cm, 100 cm, 150 cm and the top of maize. PAR was serially measured from east to west in the horizontal direction and from bottom to top in the vertical direction. PAR of intercropped cabbage each treatment was measured at the top (about 30 cm high) in each row of the intercropping strip from east to west. PAR of sole maize and cabbage was measured for 4 rows in the middle of plots as two intercropped crops were done respectively. The measurement of PAR each treatment was duplicated two times for M1 C1 , M1 C2 , M2 C3 , M3 C4 and M3 C5 , and one time for M4 C6 , M6 C8 , Mm and Cm due to too many points for measuring and lack of equipments and labors. If PAR was measured three replicate times for each treatment, it would take long time at
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2 h intervals so that the difference of PAR measured from beginning to end might be large than that between the treatments. 2.4. Measurement of yields Yields of intercropped and sole crops were harvested and recorded according to each row in the middle of plots each treatment. Ears of maize were air-dried for 2–3 months, grains each ear of maize were striped artificially, and grain sample selected randomly was 500 g for each row. All samples of maize were dried in an oven (105 ◦ C, 8 h) to test the water content and calculate grain yield of maize. External leaves of cabbage were removed artificially, then the fresh weight each cabbage was weighed. Yields of intercropped maize and cabbage were calculated according to the actual harvested area each plot while those of sole maize and cabbage were done according to 4 rows in the middle of plots. 3. Results 3.1. Effect of a maize–cabbage intercrop on PAR The most important feature of a maize–cabbage intercrop was change of PAR spatial distribution that was to alter the planar light respectively obtained by monocropping of two crops to three-dimensional light intercepted by mixed cropping patterns. In particular, intercropped maize as a tall-statured crop also obtained an additional part of lateral incident light apart from the top light, made reflecting light a decline to the sky and improved PAR of its inner canopy so that PAR in the intercropping strips was stronger than that of its monoculture (Fig. 1). In contrast, intercropped cabbage as a short-statured crop got PAR less than its monoculture because of shade from maize, the tallstatured crop (Fig. 2). On July 29, 2012, intercropped maize got PAR of 422.5–505.3 mol m−2 which was 20.3–43.9% higher than that of its sole cropping while intercropped cabbage obtained 398.1–709.7 mol m−2 which was 17.8–53.9% less than that of its monocropping. 3.2. Relationship between PAR and the number of intercropping rows of maize and cabbage Intercropped maize showed a decline trend of PAR with increase of its rows because of incident solar radiation being dramatically intercepted by the leaves and stems from outside to inside of the intercropping strips (Fig. 1). On August 6, 2011, maize of M1 C2 obtained PAR of 517.7 mol m−2 s−1 which was 44.1 mol m−2 s−1 and 53.1 mol m−2 s−1 more than that of M2 C3 and M3 C4 , respectively. However, one-row intercrop treatment of maize did not always get higher PAR than two and three-row intercropping patterns because intercropped maize got more lateral light with increase of intercropping rows of cabbage. If the number of intercropping rows of maize was the same, PAR of intercropped maize increased with intercropping rows of cabbage in the different intercropping patterns because increased rows of intercropped cabbage resulted in an increase in incident solar radiation on the side of intercropped maize. On August 6, 2011, maize of M1 C2 got 10.9% of PAR more than that of M1 C1 , and maize of M3 C5 did 2.9% of PAR more than that of M3 C4 . Intercropped cabbage showed an increase trend of PAR with increase of its rows in the intercropping strips because of less shade from maize, but the increased value of PAR was a successive decline among the different intercropping patterns (Fig. 2). On August 6, 2011, PAR of one row, two rows, three rows, four rows, five rows, six rows and eight rows of intercropped cabbage was 201.4 mol m−2 s−1 for
M1 C1 , 357.6 mol m−2 s−1 for M1 C2 , 449.6 mol m−2 s−1 for M2 C3 , 593.6 mol m−2 s−1 for M3 C4 , 616.8 mol m−2 s−1 for M3 C5 , 659.0 mol m−2 s−1 for M4 C6 and 671.7 mol m−2 s−1 for M6 C8 . In general, PAR of intercropped maize in different growing stages and the number of its intercropping rows presented a negative linear function with R2 of 0.47–0.93 which was significant for most of the observation dates (Fig. 3). PAR of intercropped cabbage in different growing stages and the number of its intercropping rows gave a positive logarithmic function with R2 of 0.80–0.99 which was extremely significant for all of the observation dates (Fig. 4). Once intercropping rows of cabbage were ≥6, the difference of PAR was much less between the treatments. 3.3. Relationship between yield and the number of intercropping rows of maize and cabbage For the maize–cabbage intercrop, edge rows of intercropped maize got more solar radiation and produced higher yields called the edge yield advantage comparing with the inner rows. The edge yield advantage each row decreased sharply from outside to inside of the intercropping strips. Intercropped maize demonstrated the highest yield for one-row intercrop treatment, a sharp decline of yield for two-row intercrop treatment and three-row intercrop treatment. According to statistical analysis, the yield difference of intercropped maize was significant between one-row and three-row intercrop treatments, and not significant between the treatments once intercropping rows of maize were ≥3 (data not shown). Relationship between yield and the number of intercropping rows of maize was a negative logarithmic function with R2 of 0.82 and 0.93 for 2011 and 2012, respectively (p < 0.01) (Fig. 5). Because intercropped cabbage as the short-statured crop could be certainly shaded from maize as the tall-statured crop, its edge rows got less solar radiation and showed less yield called the edge yield disadvantage comparing with the inner rows. The edge yield disadvantage also decreased sharply from outside to inside of the intercropping strips. Intercropped cabbage highlighted the lowest yield for one-row intercrop treatment, and a sharp increase of yield for two-row intercrop treatment. If intercropping rows of cabbage continued to grow, the increased value of yield declined successively. According to statistical analysis, the yield difference of intercropped cabbage was significant between one-row and three-row intercrop treatments and between three-row and fiverow intercrop treatments, and was not significant between the treatments once intercropping rows of cabbage were ≥5 (data not shown). Relationship between yield and the number of intercropping rows of cabbage was a positive logarithmic function with R2 of above 0.9 for 2011 and 2012, respectively (p < 0.01) (Fig. 6). 3.4. Relationship between yield and PAR of intercropped maize and cabbage For the different treatments of the maize–cabbage intercrop, both intercropped maize gotten more solar radiation and cabbage shaded from maize highlighted a positive linear correlation between yield and PAR (p < 0.01) (Fig. 7). This further explicates that PAR is extremely important for yields of the intercropped crops. 4. Discussions For maize–cabbage intercrop, intercropped maize as the tallstatured crop could obtain more PAR than its sole cropping while intercropped cabbage as the short-statured crop got less PAR due to more shade from maize comparing with that sole
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Fig. 1. PAR of intercropped maize in different growing periods in 2011 and 2012*. *PAR is the daily average measured at different heights in each row of intercropped and sole maize. M, C, m, and Arabic numerals stand for maize, cabbage, monoculture, intercropping and the number of intercropping rows, respectively.
cropping, so the intercrop system made intercropped maize produce a higher yield comparing with its sole crop and intercropped cabbage get less yield comparing with its sole crop. This conclusion was similar to many previous studies such as intercrop systems of maize–soybean (Wang et al., 2012), maize–chili pepper (Gong et al., 2010), maize–groundnut
(Awal et al., 2006), maize–tomato (Dong, 2012). In fact, PAR of the component crops was related with the number of their rows in the intercropping strips. Relationship between PAR and the number of intercropping rows of two crops was a negative linear function for intercropped maize and a positively logarithmic function for intercropped cabbage,
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Fig. 2. PAR of intercropped cabbage in different growing periods in 2011 and 2012*. *PAR was the daily average measured at the top in each row of intercropped and sole cabbage. M, C, m, and Arabic numerals stand for maize, cabbage, monoculture, intercropping and the number of intercropping rows, respectively.
respectively. In addition, change of PAR with the number of intercropping rows of cabbage influenced at least one of its phenological events, i.e., the folding period in 2011 was postponed at least 10 days for M1r C1r comparing with the other treatments including sole cabbage, but the major phenological events of maize occurred
simultaneously on sole crops and intercrops according to our work (data not shown). The intercrop patterns showed a logarithmic function between yield and the number of intercropping rows of two crops. Such a logarithmic function was negative for intercropped maize and
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Fig. 3. Relationship between PAR and the number of intercropping rows of maize in 2011 and 2012P*. *PAR is the daily average measured at different heights in each row of intercropped maize.
positive for intercropped cabbage, respectively. Some research works demonstrated that yields of intercropped tall-statured crops had a decline trend with their rows (Marshall and Willey, 1983; Chen et al., 2004; Wang et al., 2012) which was more or less the
same to this study. Yields of intercropped short-statured crops had a significantly positive correlation with their rows (Yuan et al., 2001) which was not consistent with the results of this study.
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Fig. 4. Relationship between PAR and the number of intercropping rows of cabbage in 2011 and 2012. *PAR is the daily average measured at the top in each row of intercropped cabbage.
Yield of intercropped maize and cabbage for the different intercropping patterns was a positive linear correlation with their PAR (p < 0.01). Such a result was also proved by some previous works on intercrops of maize–soybean (Ma et al., 1994), corn-
mungbean in coconut based farming system (Dauzat and Eroy, 1997), paulownia–wheat (Jiang et al., 1994), corn–soybean–oat (Jurik and Van, 2004). This further explains that PAR is extremely important for yields of the intercropped crops.
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Fig. 5. Relationship between yield of and the number of intercropping rows of maize in 2011 and 2012.
Fig. 6. Relationship between yield of and the number of intercropping rows of cabbage in 2011 and 2012.
Maize and cabbage are two shade-intolerant crops. For maize–cabbage intercrop, intercropped maize as the tall-statured crop formed the edge yield advantage by obtaining more solar radiation comparing with the inner rows while intercropped cabbage as the short-statured crop developed the edge yield disadvantage by intercepting less solar radiation due to more shade from maize comparing with the inner rows. The edge yield advantage of intercropped maize was at the expensive of the edge yield of intercropped cabbage (Iragavarapu and Randall, 1996). The yield or income advantage of the intercropping systems is revealed comparing with each sole crop once the increase degree of edge yield of the intercropped tall-statured crops is more than the decrease degree of that of the intercropped short-statured crops. In order to get maximum benefit from the maize–cabbage intercropping system, the key is to efficiently utilize the edge yield advantage of the tall-statured crop and restrain the edge yield disadvantage of the short-statured crop. Therefore, the number of intercropping rows of maize must be decreased, on one hand, so that every
intercropping row has the edge advantage more or less, and the number of intercropping rows of cabbage must be increased, on the other hand, so that the shade from maize falls and the yield is a little decrease or equal comparing with its monoculture. In view of mechanization of intercrops, 4 rows of intercropped maize are suitable, but more than 6 rows are not recommended; at least 4 rows of intercropped cabbage are recommended and its 6 rows are appreciate. Therefore, M4 C6 is recommended for an optimum combination of this intercropping system. The edge row effect is different from different crops, e.g., 2 or 3 rows for maize, 4 rows for cotton and 3 rows for wheat (Liu, 1994). Therefore, the first thing is that the type of crops must be considered for intercrops when parameter of intercropping rows of the component crops is designed. The main reason for high benefit of the intercropping systems is to form proper canopy structure of intercrops so that they utilize differently temporal and spatial environment resources which the phenomenon is called the complementary effect.
Fig. 7. Relationship between yield of and PAR of intercropped maize and cabbage in 2011 and 2012*. *PAR is the total mean of daily average for the dates observed for every row at different heights of maize and at the top of cabbage.
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5. Conclusions The PAR difference resulted from change of intercropping rows of maize and cabbage was a main factor influencing their respective yields of the different intercrop patterns. The yield, either increase of intercropped maize due to more PAR being obtained or decrease of intercropped cabbage due to more shade from maize, was a positive linear function of PAR which was related to the number of intercropping rows of two crops with a negative linear function for maize and a positive logarithmic function for cabbage, respectively. The yield was also a logarithmic function of the number of intercropping rows of two crops, negative for maize and positive for cabbage, respectively. Our finds indicated that M4 C6 is an optimum combination of this intercropping system in the regions similar to this study site. Acknowledgements This project was funded by Ministry of Science and Technology of Chian (2012BAD09B01) and Chinese National Natural Science Foundation (31401344). We thank Prof. H. P. Zhou at the Institute of Agricultural Environment and Resources, Shanxi Academy of Agricultural Sciences, for providing the information on soil in the station. We are extremely grateful to two anonymous reviewers for giving many helpful suggestions. References Awal, M.A., Koshi, H., Ikeda, T., 2006. Radiation interception and use by maize/peanut intercrop canopy. Agric. For. Meteorol. 139, 74–83. Chen, C., Westcott, M., Neill, K., Wichman, D., Knox, M., 2004. Row configuration and nitrogen application for barley–pea intercropping in Montana. Agron. J. 96, 1730–1738. Connolly, J., Goma, H.C., Rahim, K., 2001. The information content of indicators in intercropping research. Agric. Ecosyst. Environ. 87, 191–207. Corlett, J.E., Black, C.R., Ong, C.K., Monteith, J.L., 1992. Above-and-below-ground interactions in a leucaena/millet alley cropping system. II. Light interception and dry matter production. Agric. For. Meteorol. 60, 73–91.
Dauzat, J., Eroy, M.N., 1997. Simulating light regime and intercrop yields in coconut based farming systems. Eur. J. Agron. 7, 63–74. Dong, H.F., 2012. Influences of tomato/maize intercropping on photosynthetic characteristics of tomato leave. J. Anhui Agri. Sci. 40, 7696–7698. Ghanbari, A., Dahmardeh, M., Siahsar, B.A., Ramroudi, M., 2010. Effect of maize (zea mays L.)-cowpea (vigna unguiculata L.) intercropping on light distribution, soil temperature and soil moisture in arid environment. J. Food Agric. Environ. 8, 102–108. Gong, X.J., Teng, Y.F., Qian, C.R., Yu, Y., Ma, J.T., 2010. Physiological effect of maize/chili pepper intercropping I. The influence of different intercropping patterns on photosynthetic rate and yield of maize/chili pepper. Chin. Agric. Sci. Bull. 26, 111–114. Hayder, G., Mumtaz, S.S., Khan, A., Khan, S., 2003. Maize and soybean intercropping under various levels of soybean seed rates. Asian J. Plant. Sci. 2, 339–341. Iragavarapu, T., Randall, G., 1996. Border effects on yields in a strip-intercropped soybean, corn, and wheat production system. J. Prod. Agric. 9, 101–107. Jiang, J.P., Zhu, J.J., Liu, T.Z., He, S.M., Zhou, Z.M., Su, F.J., 1994. Related changes of wheat yield and photosynthetically active radiation in paulownia/wheat intercropping system. Acta Agric. Boreali Sinica 9, 133–137. Jurik, T.W., Van, K., 2004. Microenvironment of a corn–soybean–oat strip intercrops system. Field Crops Res. 90, 335–349. Keating, B.A., Carberry, P.S., 1993. Resource capture and use in intercropping: solar radiation. Field Crops Res. 34, 273–301. Lithourgidis, A.S., Dordas, C.A., Damalas, C.A., Vlachostergios, D.N., 2011. Annual intercrops: an alternative pathway for sustainable agriculture. Aust. J. Crop Sci. 5, 396–410. Liu, J.L., Tang, Y.J., Liang, Y.Y., Chen, J.S., Chen, J., 2012. The influence of varieties and rows of intercropped corn and soybean on the yield and quality of soybean. Jiangsu Agric. Sci. 40, 84–85. Liu, X.H., 1994. Science of Cropping System. China Agriculture Press, Beijing. Ma, J., Ma, S.Y., Cheng, Y.S., Mao, J.C., Bai, S.B., 1994. An analysis of effect of intercropping of maize with soybean. J. Northwest Sci. tech. Univ. Agric. For. 22, 80–84. Marshall, B., Willey, R.W., 1983. Field Crops Res. 7, 141–160. Pei, B.H., Yuan, Y.X., Jia, Y.B., Wang, W.Q., 2000. A study on light utilization of poplar-crop intercropping system. Scientia Silvae Sinicae 36, 13–18. Shi, X.L., Guo, X.P., Bi, H.X., Li, Y.H., 2009. Light competition and yield of fruit-crop intercropping system in western shanxi province. J. Beijing For. Univ. 31, 115–118. Wang, X.L., Yan, X.D., Xu, Y.P., Liu, Z.K., 2012. Study on the photosynthetic characteristics of intercropping system of maize and soybean, Journal of Hebei Agricultural Science, 16, 33–35, 59. Yuan, Y.X., Wang, Y., Li, J.Q., Feng, C.H., Zhang, S.L., 2001. The effect of poplar-crop intercropping space on wheat growth and yield. Chin. J. Eco-Agric. 9, 88–91.
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European Journal of Agronomy journal homepage: www.elsevier.com/locate/eja
Photosynthetically active radiation determining yields for an intercrop of maize with cabbage Qingsuo Wang a,∗ , Dongbao Sun a,∗,1 , Hong Hao a , Xuejiao Zhao a , Weiping Hao a , Qiong Liu b a b
Institute of Environment and Sustainable Development in Agriculture, CAAS/ Key Laboratory of Dryland Agriculture, MOA, Beijing 100081, China Meteorological Bureau of Shouyang, Shanxi Province, Shanxi 045400, China
a r t i c l e
i n f o
Article history: Received 19 January 2015 Received in revised form 16 May 2015 Accepted 28 May 2015 Keywords: Maize Cabbage Intercropping Yield PAR Dryland
a b s t r a c t In order to explicate influential mechanism for yields of an intercrop of maize with cabbage, seven treatments for different intercropping rows of two crops were designed and conducted in the central Shanxi Province of China, and yield and PAR for each component crop of the intercropping treatments were measured. PAR of intercropped maize was higher than its monocropping while PAR of intercropped cabbage was lower than its monocropping. PAR highlighted a decline trend for intercropped maize and an increase trend for intercropped cabbage with increased rows of two crops in intercropping, respectively. Relationship between PAR and the number of intercropping rows of two crops was a negative linear function for intercropped maize and a positive logarithmic function for intercropped cabbage, respectively. Grain yield of intercropped maize declined and fresh yield of intercropped cabbage increased with intercropping rows of two crops, respectively. Relationship between yield and the number of intercropping rows of two crops was a logarithmic function, negative for intercropped maize and positive for intercropped cabbage, respectively. Generally, a positive linear function between yield and PAR in the intercropping system was developed for both intercropped maize and cabbage. M4 C6 is recommended for an optimum combination of this intercropping system. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Growing two or more crops proximately on a given piece of land is called intercropping which has been identified some significant types including row intercropping, strip intercropping, relay intercropping, within-row intercropping and mixed intercropping. Agroforestry, a mixture of crops and trees or shrubs, is thought as one type of intercropping. Intercropping has been experienced as a common practice of traditional cropping systems for thousands of years. The more agricultural intensification in terms of crop breed-
Abbreviations: PAR, photosynthetically active radiation; Mm, maize monoculture; Cm, cabbage monoculture; M1 C1 , one row of maize intercropped with one row of cabbage; M1 C2 , one row of maize intercropped with two rows of cabbage; M2 C3 , two rows of maize intercropped with three rows of cabbage; M3 C4 , three rows of maize intercropped with four rows of cabbage; M3 C5 , three rows of maize intercropped with five rows of cabbage; M4 C6 , four rows of maize intercropped with six rows of cabbage; M6 C8 , six rows of maize intercropped with eight rows of cabbage. ∗ Corresponding authors. Tel.: +86 10 82109756; fax: +86 10 82109560. 1 These authors contributed equally to this study and share first authorship. http://dx.doi.org/10.1016/j.eja.2015.05.004 1161-0301/© 2015 Elsevier B.V. All rights reserved.
ing, mechanization, fertilizer and pesticide use has been limiting expansion of intercropping so that it is far less common in Europe, North America, and some parts of Asia. In many areas of the world, however, intercropping still is a very important cropping system, especially for small farms or organic farms. Intercropping has been paid more and more attention as a very significant alternative way of sustainable agriculture in view of protection of environment, efficient utilization of resources, reduction of pest and disease incidence, and resistance to drought and frost risks (Lithourgidis et al., 2011). The most remarkable advantage of intercropping is to produce a greater yield or income than any crop monocropping. This yield advantage of intercropping mainly depends on the mutual benefit or different ecological niches (spatial, temporal, and nutritious) between component crops so that they can make efficient use of resources such as light, water and nutrients that would otherwise not be utilized by each sole crop. The especially important thing of intercropping is to efficiently utilize solar radiation resource together for the component crops, but different intercropping systems show different ways of obtaining solar radiation energy. Relay intercropping can make use of overlap time between first and
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second intercropped crops and prolong solar radiation utilization time comparing with each crop monocropping (Corlett et al., 1992; Connolly et al., 2001). For intercropping an early-maturing crop with a late- maturing crop, the early-maturing crop can efficiently use solar radiation which is not fully used by the late-maturing crop in the early growing period so that the intercropping system can sufficiently capture sunlight energy during the whole growing season (Keating and Carberry, 1993). Intercropping between cereal and legume (climbing) crops can obtain more solar radiation by increasing total leaf area index of the component crops comparing with each crop sole cropping so that the whole intercropping system can increase light use efficiency, e.g., a maize-cowpea intercrop (Ghanbari et al., 2010). Intercropping a tall-statured crop with a short-statured crop can also improve light use efficiency due to decreasing sunlight reflectance by intercepting three-dimensional light instead of planar light each sole crop obtains. For the tallstatured crop of an intercrop, its edge rows get more solar radiation and produce higher yields called the edge yield advantage comparing with the inner rows. For the short-statured crop, its edge rows can be a certain degree of shade from the tall-statured crop and get less solar radiation which is favorable to shade-tolerant crops such as edible mushrooms, some vegetables and medicinal plants. However, intercrops of the tall- and short-statured crops are unfavorable to many short-statured crops intolerant to shade and often make their yields decline, such as intercropping maize with soybean (Wang et al., 2012), groundnut (Awal et al., 2006), tomato (Dong, 2012), chili pepper (Gong et al., 2010). Intercropping crops with trees or shrubs always gives less crop yields comparing with the sole crop growing (Pei et al., 2000; Yuan et al., 2001; Shi et al., 2009). The intercropped short-statured crops often get a gradual increase of yields with their rows (Hayder et al., 2003; Liu et al., 2012; Wang et al., 2012; Gong et al., 2010) due to being less shaded and more sunlight intercepting. However, quantitative relationships between yield and solar radiation or the number of intercropping rows of maize and cabbage remain unclear. Shouyang County, located in the middle of Shanxi Province, is dominated by dryland farming due to less precipitation. Maize is one of main cereal crops and cabbage is one of main vegetables in this region. Its uniquely natural conditions such as high elevation, cool climate, high difference of daily temperature, less incidence of plant disease and insect pests make cabbage good quality with tight shape, crisp texture, refreshing taste, high calcium and vitamin C. Cabbage has been traditionally grown in summer and harvested in autumn in order to escape spring drought and make good use of high daily temperature difference in autumn, and used as fresh vegetable and pickled or fermented vegetable for winter and spring seasons. In recent years, however, some cabbage is planted in spring and harvested in the middle summer which is supplied for hot season markets in the eastern China. Most of cabbage is grown in monoculture while some is intercropped with other crops such as maize and millet as strip intercropping favorable to the mechanized farming. The objectives of this work is to explore the quantitative relationships between yield and photosynthetically active radiation (PAR) or the number of intercropping rows of two crops in a maize–cabbage intercrop system, and to determine reasonable strip structure of the maize–cabbage intercrop.
2. Methods 2.1. Study site This study was conducted at the Dryland Agricultural Experiment Station in Shouyang County, Shanxi Province (37◦ 51 N and 113◦ 05 E). The landscape is Loess Plateau in China. The altitude
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is 1035 m above sea level. It belongs to a temperate continental semi-humid climate with annual average temperature of 7.6 ◦ C and annual average precipitation of 489.5 mm from 1981 to 2010 which means cold weather and less precipitation. Drought has frequently happened and dryland farming is dominated in this region. In 2011, average temperature was 7.1 ◦ C with a maximum of 32.9 ◦ C and a minimum of −24.1 ◦ C, and annual precipitation was 635.6 mm. In 2012, average temperature was 6.9 ◦ C with a maximum of 33.0 ◦ C and a minimum of −23.2 ◦ C, and annual precipitation was 574.5 mm. Soil type is Cinnamon with the parent material of loess. The top soil at 0–20 cm depth contained 22.99 g/kg of organic matter, 1.15 g/kg of total nitrogen, 16.02 mg/kg of nitrate, 118.44 mg/kg of available phosphorous and 11.82 mg/kg of available potassium, and the pH was 8.2 in April, 2011. One crop a year is the current cropping system. The major crops are maize, millet, buckwheat, and the main vegetables are cabbage and potato.
2.2. Experiment design This study was conducted in 2011 and 2012. Maize cultivar Jindan 48 and cabbage cultivar Wanfeng adapted to the local conditions were selected. Seven treatments of a maize–cabbage strip intercrop were designed with the number of intercropping rows of maize and cabbage, respectively, i.e., one row of maize versus one row of cabbage (M1 C1 ), one row of maize versus two rows of cabbage (M1 C2 ), two rows of maize versus three rows of cabbage (M2 C3 ), three rows of maize versus four rows of cabbage (M3 C4 ), three rows of maize versus five rows of cabbage (M3 C5 ), four rows of maize versus six rows of cabbage (M4 C6 ) and six rows of maize versus eight rows of cabbage (M6 C8 ). Monoculture of maize (Mm ) and cabbage (Cm ) was as a control, respectively. Two crops were planted in a north–south row direction. Row spacing was 60 cm for both crops. Plant spacing was 30 cm for maize and 60 cm for cabbage, respectively. Each treatment was duplicated three times which were located according to a spatially balanced randomized plot design. Each plot at least had one complete intercropping strip for two intercropped crops. Length of the plots was 10 m. Maize was grown on April 30, and cabbage was transplanted on June 20 as seeding. Both crops were harvested in early October each year. Chemical fertilizers such as urea and superphosphate were applied into soil before maize was planted each year. The fertilizer application was 150 kg N hm−2 and 84 kg P2 O5 hm−2 , respectively. Weeds and insects were effectively controlled.
2.3. Measurement of PAR Incident PAR was measured with Li-250A Light Meter among which was installed Li-190SA Line Quantum Sensor. Sunny days were selected to observe for this work. The measurement was operated at 2 h intervals from 6:00 to 18:00 each day selected. PAR of intercropped maize each treatment was measured at different heights in each row of the intercropping strip. The height measured was the bottom, 25 cm, 50 cm, 100 cm, 150 cm and the top of maize. PAR was serially measured from east to west in the horizontal direction and from bottom to top in the vertical direction. PAR of intercropped cabbage each treatment was measured at the top (about 30 cm high) in each row of the intercropping strip from east to west. PAR of sole maize and cabbage was measured for 4 rows in the middle of plots as two intercropped crops were done respectively. The measurement of PAR each treatment was duplicated two times for M1 C1 , M1 C2 , M2 C3 , M3 C4 and M3 C5 , and one time for M4 C6 , M6 C8 , Mm and Cm due to too many points for measuring and lack of equipments and labors. If PAR was measured three replicate times for each treatment, it would take long time at
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2 h intervals so that the difference of PAR measured from beginning to end might be large than that between the treatments. 2.4. Measurement of yields Yields of intercropped and sole crops were harvested and recorded according to each row in the middle of plots each treatment. Ears of maize were air-dried for 2–3 months, grains each ear of maize were striped artificially, and grain sample selected randomly was 500 g for each row. All samples of maize were dried in an oven (105 ◦ C, 8 h) to test the water content and calculate grain yield of maize. External leaves of cabbage were removed artificially, then the fresh weight each cabbage was weighed. Yields of intercropped maize and cabbage were calculated according to the actual harvested area each plot while those of sole maize and cabbage were done according to 4 rows in the middle of plots. 3. Results 3.1. Effect of a maize–cabbage intercrop on PAR The most important feature of a maize–cabbage intercrop was change of PAR spatial distribution that was to alter the planar light respectively obtained by monocropping of two crops to three-dimensional light intercepted by mixed cropping patterns. In particular, intercropped maize as a tall-statured crop also obtained an additional part of lateral incident light apart from the top light, made reflecting light a decline to the sky and improved PAR of its inner canopy so that PAR in the intercropping strips was stronger than that of its monoculture (Fig. 1). In contrast, intercropped cabbage as a short-statured crop got PAR less than its monoculture because of shade from maize, the tallstatured crop (Fig. 2). On July 29, 2012, intercropped maize got PAR of 422.5–505.3 mol m−2 which was 20.3–43.9% higher than that of its sole cropping while intercropped cabbage obtained 398.1–709.7 mol m−2 which was 17.8–53.9% less than that of its monocropping. 3.2. Relationship between PAR and the number of intercropping rows of maize and cabbage Intercropped maize showed a decline trend of PAR with increase of its rows because of incident solar radiation being dramatically intercepted by the leaves and stems from outside to inside of the intercropping strips (Fig. 1). On August 6, 2011, maize of M1 C2 obtained PAR of 517.7 mol m−2 s−1 which was 44.1 mol m−2 s−1 and 53.1 mol m−2 s−1 more than that of M2 C3 and M3 C4 , respectively. However, one-row intercrop treatment of maize did not always get higher PAR than two and three-row intercropping patterns because intercropped maize got more lateral light with increase of intercropping rows of cabbage. If the number of intercropping rows of maize was the same, PAR of intercropped maize increased with intercropping rows of cabbage in the different intercropping patterns because increased rows of intercropped cabbage resulted in an increase in incident solar radiation on the side of intercropped maize. On August 6, 2011, maize of M1 C2 got 10.9% of PAR more than that of M1 C1 , and maize of M3 C5 did 2.9% of PAR more than that of M3 C4 . Intercropped cabbage showed an increase trend of PAR with increase of its rows in the intercropping strips because of less shade from maize, but the increased value of PAR was a successive decline among the different intercropping patterns (Fig. 2). On August 6, 2011, PAR of one row, two rows, three rows, four rows, five rows, six rows and eight rows of intercropped cabbage was 201.4 mol m−2 s−1 for
M1 C1 , 357.6 mol m−2 s−1 for M1 C2 , 449.6 mol m−2 s−1 for M2 C3 , 593.6 mol m−2 s−1 for M3 C4 , 616.8 mol m−2 s−1 for M3 C5 , 659.0 mol m−2 s−1 for M4 C6 and 671.7 mol m−2 s−1 for M6 C8 . In general, PAR of intercropped maize in different growing stages and the number of its intercropping rows presented a negative linear function with R2 of 0.47–0.93 which was significant for most of the observation dates (Fig. 3). PAR of intercropped cabbage in different growing stages and the number of its intercropping rows gave a positive logarithmic function with R2 of 0.80–0.99 which was extremely significant for all of the observation dates (Fig. 4). Once intercropping rows of cabbage were ≥6, the difference of PAR was much less between the treatments. 3.3. Relationship between yield and the number of intercropping rows of maize and cabbage For the maize–cabbage intercrop, edge rows of intercropped maize got more solar radiation and produced higher yields called the edge yield advantage comparing with the inner rows. The edge yield advantage each row decreased sharply from outside to inside of the intercropping strips. Intercropped maize demonstrated the highest yield for one-row intercrop treatment, a sharp decline of yield for two-row intercrop treatment and three-row intercrop treatment. According to statistical analysis, the yield difference of intercropped maize was significant between one-row and three-row intercrop treatments, and not significant between the treatments once intercropping rows of maize were ≥3 (data not shown). Relationship between yield and the number of intercropping rows of maize was a negative logarithmic function with R2 of 0.82 and 0.93 for 2011 and 2012, respectively (p < 0.01) (Fig. 5). Because intercropped cabbage as the short-statured crop could be certainly shaded from maize as the tall-statured crop, its edge rows got less solar radiation and showed less yield called the edge yield disadvantage comparing with the inner rows. The edge yield disadvantage also decreased sharply from outside to inside of the intercropping strips. Intercropped cabbage highlighted the lowest yield for one-row intercrop treatment, and a sharp increase of yield for two-row intercrop treatment. If intercropping rows of cabbage continued to grow, the increased value of yield declined successively. According to statistical analysis, the yield difference of intercropped cabbage was significant between one-row and three-row intercrop treatments and between three-row and fiverow intercrop treatments, and was not significant between the treatments once intercropping rows of cabbage were ≥5 (data not shown). Relationship between yield and the number of intercropping rows of cabbage was a positive logarithmic function with R2 of above 0.9 for 2011 and 2012, respectively (p < 0.01) (Fig. 6). 3.4. Relationship between yield and PAR of intercropped maize and cabbage For the different treatments of the maize–cabbage intercrop, both intercropped maize gotten more solar radiation and cabbage shaded from maize highlighted a positive linear correlation between yield and PAR (p < 0.01) (Fig. 7). This further explicates that PAR is extremely important for yields of the intercropped crops. 4. Discussions For maize–cabbage intercrop, intercropped maize as the tallstatured crop could obtain more PAR than its sole cropping while intercropped cabbage as the short-statured crop got less PAR due to more shade from maize comparing with that sole
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Fig. 1. PAR of intercropped maize in different growing periods in 2011 and 2012*. *PAR is the daily average measured at different heights in each row of intercropped and sole maize. M, C, m, and Arabic numerals stand for maize, cabbage, monoculture, intercropping and the number of intercropping rows, respectively.
cropping, so the intercrop system made intercropped maize produce a higher yield comparing with its sole crop and intercropped cabbage get less yield comparing with its sole crop. This conclusion was similar to many previous studies such as intercrop systems of maize–soybean (Wang et al., 2012), maize–chili pepper (Gong et al., 2010), maize–groundnut
(Awal et al., 2006), maize–tomato (Dong, 2012). In fact, PAR of the component crops was related with the number of their rows in the intercropping strips. Relationship between PAR and the number of intercropping rows of two crops was a negative linear function for intercropped maize and a positively logarithmic function for intercropped cabbage,
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Fig. 2. PAR of intercropped cabbage in different growing periods in 2011 and 2012*. *PAR was the daily average measured at the top in each row of intercropped and sole cabbage. M, C, m, and Arabic numerals stand for maize, cabbage, monoculture, intercropping and the number of intercropping rows, respectively.
respectively. In addition, change of PAR with the number of intercropping rows of cabbage influenced at least one of its phenological events, i.e., the folding period in 2011 was postponed at least 10 days for M1r C1r comparing with the other treatments including sole cabbage, but the major phenological events of maize occurred
simultaneously on sole crops and intercrops according to our work (data not shown). The intercrop patterns showed a logarithmic function between yield and the number of intercropping rows of two crops. Such a logarithmic function was negative for intercropped maize and
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Fig. 3. Relationship between PAR and the number of intercropping rows of maize in 2011 and 2012P*. *PAR is the daily average measured at different heights in each row of intercropped maize.
positive for intercropped cabbage, respectively. Some research works demonstrated that yields of intercropped tall-statured crops had a decline trend with their rows (Marshall and Willey, 1983; Chen et al., 2004; Wang et al., 2012) which was more or less the
same to this study. Yields of intercropped short-statured crops had a significantly positive correlation with their rows (Yuan et al., 2001) which was not consistent with the results of this study.
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Fig. 4. Relationship between PAR and the number of intercropping rows of cabbage in 2011 and 2012. *PAR is the daily average measured at the top in each row of intercropped cabbage.
Yield of intercropped maize and cabbage for the different intercropping patterns was a positive linear correlation with their PAR (p < 0.01). Such a result was also proved by some previous works on intercrops of maize–soybean (Ma et al., 1994), corn-
mungbean in coconut based farming system (Dauzat and Eroy, 1997), paulownia–wheat (Jiang et al., 1994), corn–soybean–oat (Jurik and Van, 2004). This further explains that PAR is extremely important for yields of the intercropped crops.
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Fig. 5. Relationship between yield of and the number of intercropping rows of maize in 2011 and 2012.
Fig. 6. Relationship between yield of and the number of intercropping rows of cabbage in 2011 and 2012.
Maize and cabbage are two shade-intolerant crops. For maize–cabbage intercrop, intercropped maize as the tall-statured crop formed the edge yield advantage by obtaining more solar radiation comparing with the inner rows while intercropped cabbage as the short-statured crop developed the edge yield disadvantage by intercepting less solar radiation due to more shade from maize comparing with the inner rows. The edge yield advantage of intercropped maize was at the expensive of the edge yield of intercropped cabbage (Iragavarapu and Randall, 1996). The yield or income advantage of the intercropping systems is revealed comparing with each sole crop once the increase degree of edge yield of the intercropped tall-statured crops is more than the decrease degree of that of the intercropped short-statured crops. In order to get maximum benefit from the maize–cabbage intercropping system, the key is to efficiently utilize the edge yield advantage of the tall-statured crop and restrain the edge yield disadvantage of the short-statured crop. Therefore, the number of intercropping rows of maize must be decreased, on one hand, so that every
intercropping row has the edge advantage more or less, and the number of intercropping rows of cabbage must be increased, on the other hand, so that the shade from maize falls and the yield is a little decrease or equal comparing with its monoculture. In view of mechanization of intercrops, 4 rows of intercropped maize are suitable, but more than 6 rows are not recommended; at least 4 rows of intercropped cabbage are recommended and its 6 rows are appreciate. Therefore, M4 C6 is recommended for an optimum combination of this intercropping system. The edge row effect is different from different crops, e.g., 2 or 3 rows for maize, 4 rows for cotton and 3 rows for wheat (Liu, 1994). Therefore, the first thing is that the type of crops must be considered for intercrops when parameter of intercropping rows of the component crops is designed. The main reason for high benefit of the intercropping systems is to form proper canopy structure of intercrops so that they utilize differently temporal and spatial environment resources which the phenomenon is called the complementary effect.
Fig. 7. Relationship between yield of and PAR of intercropped maize and cabbage in 2011 and 2012*. *PAR is the total mean of daily average for the dates observed for every row at different heights of maize and at the top of cabbage.
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5. Conclusions The PAR difference resulted from change of intercropping rows of maize and cabbage was a main factor influencing their respective yields of the different intercrop patterns. The yield, either increase of intercropped maize due to more PAR being obtained or decrease of intercropped cabbage due to more shade from maize, was a positive linear function of PAR which was related to the number of intercropping rows of two crops with a negative linear function for maize and a positive logarithmic function for cabbage, respectively. The yield was also a logarithmic function of the number of intercropping rows of two crops, negative for maize and positive for cabbage, respectively. Our finds indicated that M4 C6 is an optimum combination of this intercropping system in the regions similar to this study site. Acknowledgements This project was funded by Ministry of Science and Technology of Chian (2012BAD09B01) and Chinese National Natural Science Foundation (31401344). We thank Prof. H. P. Zhou at the Institute of Agricultural Environment and Resources, Shanxi Academy of Agricultural Sciences, for providing the information on soil in the station. We are extremely grateful to two anonymous reviewers for giving many helpful suggestions. References Awal, M.A., Koshi, H., Ikeda, T., 2006. Radiation interception and use by maize/peanut intercrop canopy. Agric. For. Meteorol. 139, 74–83. Chen, C., Westcott, M., Neill, K., Wichman, D., Knox, M., 2004. Row configuration and nitrogen application for barley–pea intercropping in Montana. Agron. J. 96, 1730–1738. Connolly, J., Goma, H.C., Rahim, K., 2001. The information content of indicators in intercropping research. Agric. Ecosyst. Environ. 87, 191–207. Corlett, J.E., Black, C.R., Ong, C.K., Monteith, J.L., 1992. Above-and-below-ground interactions in a leucaena/millet alley cropping system. II. Light interception and dry matter production. Agric. For. Meteorol. 60, 73–91.
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