planting usually has a favorable envi- ronment for germination. The area between the rows receives throughout the day, while the broccoli transplants reduce evaporative demand in the surface due to reduction of wind currents. As the germi- nates and begins to grow, the broccoli foliage enlarges and gradually shades the soil between the broccoli rows. Eventually, the broccoli suppresses the legume’s growth through shading and dominates resource (nutrients, , sunlight, etc.) use in the field. Following broccoli harvest, the previously suppressed legume enters the steep upward portion of its growth Increasing Sustainability by curve and dominates the field through- Intercropping out the remainder of the summer. The result is a complete stand of capable of fixing N and retaining in the R.M. Coolman and G.D. Hoyt soil additional nutrients for fu- ture . Relay intercropping thus results in a mutually beneficial rela- atural ecosystems exhibit the tionship between the broccoli and le- Additional index words. legumes, relay intercropping, cover crops, capacity for both competi- gume components. competition, cooperation, vegetables N tion and cooperation (Odum, The effects of intercropping 1971). The ability of different plant Summary. Plant interactions are both species to cooperate to mutual advan- on plant interactions competitive and cooperative. Farmers tage is often the result of physical Intercropping has been an agri- use intercropping to the mutual changes in plant structures or manipu- cultural practice for thousands of years advantage of both main and secondary lation of the surrounding environment. (Kass, 1978). Intercropping was used crops in a multiple--production This principle of mutualism can be commonly in the United States until system. A vegetable crop has a applied to plant interactions in an ag- the 1940s, when energy-intensive, competitive advantage over a younger ricultural system. Intercropping is the modern developed in re- secondary cover crop interseeded before vegetable maturity. Non- growing of two or more crops simulta- sponse to the development of power legume intercropped cover crops can neously on the same field (Andrews and machinery, , and chemical use soil N, while a legume intercrop Kassam, 1976; Sanchez, 1976). The (Horwith, 1985). In less- can increase N in agricultural systems practice of relay intercropping involves developed countries, the majority of by biological N fixation. Intercrop- planting a second crop after an initial farmers still practice intercropping be- ping also may be more efficient than crop has reached maturity, but before cause of a lack of access to either credit, monocropping in exploiting limited it is ready for harvest. One application inputs, or machinery. resources. Relay-planting main crop of relay intercropping is to divide crops Ofori and Stern (1987) suggest and intercrop components so that into two categories: the main compo- that growing two or more crops simul- resource demands (nutrients, water, nent is the crop of primary importance taneously is more efficient than mono- sunlight, etc.) occur during different periods of the growing season can be and has the desired yields; the second cropping for exploitation of limited an effective means of minimizing crop, or secondary component, pro- resources. One major concern regard- interspecific competition. Intercrop- vides added economic and/or envi- ing intercropping systems on infertile ping systems often exhibit less crop ronmental benefits (Willey, 1979a). is the accelerated depletion of damage associated with and Figure 1 shows a conceptual model of mineral nutrients when both crops are plant pathogen attacks, and they the crop components when summer harvested. Intercropping often results provide . annual legumes are intercropped in a in accelerated harvesting of soil re- strip-tilled broccoli (Brassica oleracea sources as a short-term gain for - var. italica) production system (Cool- ers with limited resources. This con- man, 1989). Such summer annual le- cern is reduced if nurse crops are not gumes may be relay-intercropped into harvested from the system, but instead Department of Soil Science, North Carolina State Uni- a stand ofmaturing broccoli to achieve their capacity to capture and recycle versity, Mountain Horticultural Crops Research & Ex- an economic yield in the broccoli com- nutrients for use by the following pri- tension Center, 2016 Fanning Bridge Rd., Fletcher, NC 28732. ponent. The legume provides benefits mary crop is used to the farmer’s ad- of biological N fixation, weed control, vantage. The advantages and disad- The cost of publishing this paper was defrayed in part by the payment of page charges. Under portal regulations, and soil conservation. vantages of intercropping systems, the this paper therefore must hereby be marked advertise- A summer annual legume sown competitive constraints, and the man- ment solely to indicate this fact. immediately following broccoli trans- agement techniques that can be used

HortTechnology • July/Sept. 1993 3(3) 309 vie for the same nutritional and water requirements in the same space- one of the plants usually receives less than it requires. Overlapping in space and time, the growth of two or more crops often results in decreased yields of both crops due to competition for limited essential resources (Dalal, 1977; Wahua and Miller, 1978; Wanki et al., 1982). Any development of intercrop- ping systems must evaluate the effects of competition on crop yields. Crop protection Intercropping systems often ex- hibit less crop damage associated with insect and plant pathogen attacks, al- though the results are quite variable. Previous research has shown a reduc- tion of insect damage in intercropped systems compared to (Kass, 1978; Nickel, 1973; Perrin, 1977). Francis and Sanders (1978) reported lower attacks of fall army- worm (Spodeptera frugiperda) in a corn-bean intercrop than in a corn (Zea mays) . Vandermeer (1989), however, conducted a similar experiment using identical cropping patterns in Nicaragua and failed to detect any differences in insect pests under both rain-fed and irrigated con- ditions. Evidence also exists that inter- to facilitate mutually beneficial plant in time. As plants begin to mature, the cropping reduces both nematode and interactions are reviewed below. uptake of both nutrients and water disease attack (Castillo et al., 1976; declines. A second crop planted dur- Egunjobi, 1984). Many mechanisms Below-ground interactions ing this period may be an effective way for reduced insect, nematode, and dis- Little is known about inter- of avoiding interspecific root competi- ease problems associated with inter- action in intercropping systems. Tren- tion (Sanchez, 1976). cropping have been proposed (Aiyer, bath (1974) considered yield advan- Competition for water and nutri- 1949; Root, 1973; Trenbath, 1976). tages in mixtures of varieties to be ents under relay intercropping affects Unfortunately, while each suggest a the result of differential root growth. the suppressed crop component in two mechanism of control for a particular Another view is that differences in ways (Baldwin and Tinker, 1972). First, situation, no single explanation is uni- rooting patterns are the result of mu- the of suppressed plants are less- versal. tual avoidance of root systems (Baldwin developed at the interface with roots Intercropping with a cover crop et al., 1972; Raper and Barber, 1970). of the dominant crop. Second, sup- often provides weed control in the Differences in root systems, particu- pressed crops are likely to show an cropping system. One proposed larly rooting depth, lateral root spread, increased root : shoot ratio. Shallow- mechanism of weed control is compe- and root density, have been found to rooted primary crops of an intercrop- tition of the secondary crop with weed affect competition for water (Babalola, ping system may force companion crops species. Vandermeer (1989) reported 1980; Haynes, 1980). Haynes con- to develop deeper rooting systems that that a recent review by Liebman ( 1986) cluded that proper management of explore the soil matrix more exten- found that 20 of 23 intercropping root systems can minimize root com- sively than their monoculture coun- systems involving cover crops sup- petition for water. terparts (Fisher, 1976; IRRI, 1972). pressed weed growth significantly. Willey (1979a) counters that root Such root avoidance may assist in de- Individual intercropping systems avoidance also should occur in mo- veloping root systems that use soil to a show more variable weed control ef- nocultural cropping systems. He con- greater depth (Trenbath, 1974). fects. For example, a maize- cludes that increased exploration of intercrop in Nigeria had no suppres- the soil matrix by roots in intercrop- Above-ground interactions sive effect on weeds early in the season, ping systems does not have a direct Light, water, mineral nutrients, but controlled weed growth effectively advantage over monocultures. How- oxygen, and carbon dioxide are all later in the cropping season (Ayeni et ever, relay intercropping has the ad- required for plant growth (Trenbath, al., 1984). On the other hand, a maize- vantage of separating rooting systems 1976). Competition occurs when two cassava system at the same location did

310 HortTechnology • July/Sept. 1993 3(3) not suppress weed growth (Unamma tically. Ofori and Stern (1987) sug- way to develop beneficial plant inter- and Ene, 1983). Again, while reviews gested that intercropping non-legumes actions in intercropping systems. ofresearch on intercropping and weed with legumes increased the N fixation control arc impressive, individual crop- of the legume component. Their re- Conclusions ping systems respond differently and, sults suggested that non-legume com- The adoption of intercropping therefore, must be evaluated on their ponents had a competitive advantage systems has a number of potential ad- own merit based on site-specific con- for soil N that forced legume compo- vantages and disadvantages that are ditions and the crops used. nents to maximize their biological N- often site- and crop-specific. Inter- fixing capabilities. They contend that cropping can be a more-efficient means Nutrients legumes generally take a subordinate of exploiting the resources required Intercropping with legumes may role in legume-cereal intercroppings. for plant growth. Relay-planting crop have either beneficial or detrimental components so that resource demands effects because interspecific competi- Relay-intercropping systems occur during different periods of the tion, the nitrogen-fixing capacity of One way to reduce competition growing season can be an effective the legumes, planting times, and soil for limited water and mineral nutrients means of minimizing interspecific com- fertility levels can affect nutrient ac- is to plant crops at different times of petition. Yet, little is known about cruement (Sanchez, 1976). Numer- the growing season in a relay-inter- root interactions in individual inter- ous experiments have demonstrated a cropping system (Andrews and Kassam, cropping systems, which makes it dif- yield advantage of legume and non- 1976). Vegetable crop components ficult to predict yields, use ofresources, legume mixtures over monocultural planted first must have a competitive or biological interactions for a given counterparts (Agboola and Fayemi, advantage over younger secondary system. The effect ofintercropping on 1972; Kurtz et al., 1952; Trenbath, cover crop components. This results in insect, nematode, disease, and weed 1976; Willey, 1979b). One interpreta- a reduced growth pattern for the crop problems is site-specific and difficult tion of this advantage is that the non planted later in the season. Willey to predict and, therefore, must be evalu- legume component uses soil N, while (1979a) suggested that the main way ated for each new intercropping com- the legume component uses N from to achieve complementarity, or coop- bination. the atmosphere (Hall, 1974; Snaydon eration, between crop components is and Harris, 1979). Sanchez (1976) to develop crop growth patterns so suggested that legume-cereal inter- that each crop exerts its major de- Literature Cited croppings can be beneficial when the mands on resource use at different Agboola, A.A. and A.A. Fayemi. 1972. Fixa- cropping system is managed to mini- times in the growing season. Compe- tion and excretion of nitrogen by tropical mize interspecific competition. Relay tition, therefore, can be controlled legumes. Agron. J. 64:409-412. intercropping of shade-tolerant legume through adjusting the time when each Aiyer, A.K.Y.N. 1949. Mixed cropping in species between rows of a main crop crop has reached its maximum de- . Indian J. Agr. Sci. 19:439-543. may be one method of minimizing mand for water and nutrients. Baker interspecific competition. Legumes and Yusuf (1976) compared yield in- Andrew, D.J. and A.H. Kassam. 1976. The importance of in must be planted early enough in the creases in cereal mixtures to differ- increasing world food supplies, p. l-11. growing season to accumulate appre- ences in date of maturity, and esti- In: R.I. Papendick, P.A. Sanchez, and G.B. ciable amounts of N from the atmo- mated that no advantage could be Triplett (eds.). Multiple cropping. Amer. sphere to be effective N contributors gained unless the temporal difference Soc. Agron., Madison, Wis. Spec. Publ. (Coolman, 1988; Eaglesham et al., in cereal maturity exceeded 30 to 40 27. 1982). Nitrogen contributions in re- days. Ayeni, A.O., W.B. Duke, and I.O. Akobun- lay-cropping systems tend to be site- In addition, dominant and sup- du. 1984. Weed interference in maize, cow- specific, being affected by crop com- pressed crops can develop under dif- pea and maize/cowpea intercrop in a sub- ponents, planting dates, and timing of ferentially planted intercropping sys- humid tropical environment. I. Influence N release from legume residues tems (Huxley and Maingu, 1978). This of cropping season. Weed Res. 24:264- (Sanchez, 1976). relationship actually may be used to an 279. Most intercropping systems ex- advantage in intercropping systems that Babalola, O. 1980. Water relations of three hibit competition for N between crop use legumes as a nurse crop for N fixa- cowpea cultivars ( Vigna unguiculata L.). components (Ofori and Stern, 1987). tion (Gliessman, 1985). Interseeding Plant & Soil 56:59-69. Legume-cereal intercropping systems legumes with indeterminate growth have less N competition when legumes patterns into a main crop may suppress Baker, E.F.I. and Y. Yusuf. 1976. Research are fixing N actively (Trenbath, 1976). legume growth initially, but also may with mixed crops at the Institute for Agri- If N fixation is inhibited, the legume exhibit a greater ability to contribute cultural Research, Samaru, Nigeria. Symp. on intercropping in semi-arid areas. 10-12 behaves like other crop species and N to the system. Indeterminate le- May 1976. Morogora, Tanzania. p. 127- competes for soil N actively (Ofori and gume species have a longer growing 133. Stern, 1987). Wahua (1983) found season and tend to partition less total that management of crop growth stages N to reproductive activities than le- Baldwin, J.P. and P.B. Tinker. 1972. A played an important role in N compe- gumes with determinate growth pat- method ofestimating the lengths and spa- tition. He concluded that, when corn terns (Eagelsham et al., 1982). These tial patterns of two interpenetrating root systems. Plant & Soil 37:209-213. and sorghum (Sorghum spp.) had si- results suggest that proper sowing time multaneous maximum N demands, of indeterminate legumes into trans- Baldwin, J.P., P.R. Tinker, and P.H. Nye. yields of both crops were reduced dras- planted vegetable crops may be one 1972. Uptake of solutes by multiple root

HortTechnology • July/Sept. 1993 3(3) 311 systems from soil. II. The theoretical ef- Haynes, R.J 1980. Competitive aspects of Snaydon, R. W. and P.M. Harris. 1979. fects of rooting density and pattern on the grass-legume association. Adv. Agron. Interactions below ground: The use of uptake of nutrients from soil. Plant & Soil 33:227-261. nutrients and water, p. 188-201. In: R.W. 36:693-708. Willey (ed.). Proc. Intl. Wkshp. on Inter- Horwith, B. 1985. A role for intercropping cropping. ICRISAT, Hyderabad, India. Castillo. M.B., M.S. Alejar, and R.R. in modern agriculture. Bioscience 35:286- Harwood. 1976. Nematodes in cropping 291. Trenbath, B.R. 1974. Biomass productivity patterns. II. Control of Meloidogyne incog- of mixtures. Adv. Agron. 26:177-210. nita through cropping patterns and cul- Huxley, P.A. and Z. Maingu. 1978. Use of Trenbath, B.R. 1976. Plant interactions in tural practices. Philippine Agr. 59:295- a systematic spacing design as an aid to the 312. study of inter-cropping: Some general con- mixed crop communities, p. 129-170. In: siderations. Expt. Agr. 14:49-56. R.I. Papendick, P.A. Sanchez, and G.B. Coolman, R.M. 1989. Legume relay inter- Triplett (eds.). Multiple cropping. ASA, International Rice Research Institute. 1972. cropping and broccoli production in west- CSSA, and SSSA, Madison, Wis. ASA Spec. ern North Carolina. MS Thesis. North Multiple cropping. In IRRI Annu. Rpt. for Publ. 27. 1972, Los Banos, Philippines. p. 21-34. Carolina State Univ., Raleigh. Unamma, R.P.A. and L.S.O. Ene. 1983. Dalal, R.C. 1977. Effect of intercropping Kass, D.C.L. 1978. cropping Weed interference in cassava-maize inter- on maize with soya bean on grain yield. systems: A review and analysis. Cornell crop in the rainforest of Nigeria. Tropical Trop. Agr. (Trinidad) 54:189-191. Univ., Ithaca, N.Y. Cornell Intl. Agr. Bul. root crop production and uses in Africa. 32. Proc. 2nd Triennial Symp. Intl. Soc. Trop. Eagelsham, A.R.J., A. Ayanaba, V.R. Rao, Kurtz, T., S.W. Melsted, and R.H. Bray. Root Crops. Africa Branch IDRC. p. 59- and D.L. Eskew. 1982. Mineral N effects on 62. cowpea and soybean crops in Nigerian soil. 1952. The importance of nitrogen and Plant & Soil 68:183-192. water in reducing competition between Vandermeer, J.H. 1989. The of intercrops and corn. Agron. J. 44:13-17. intercropping. Cambridge Univ. Press. Egunjobi, O.A. 1984. Effects of intercrop- Cambridge, U.K. ping maize and grain legumes and Liebman, M.Z. 1986. Ecological suppres- treatments on populations of Pratylenchus sion of weeds in intercropping systems: Wahua, T.A.T. and D.A. Miller. 1978. brachyurus Todfrey (Nematoda) on the Experiments with barley, pea, and mus- Relative yield totals and yield components yield of Maize (Zea mays L). Protection tard. PhD Diss. Univ. of California, Berke- of intercropped sorghum and soybeans. Ecol. 6:153-167. ley. Agron. J. 70:287-291. 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