Evaluating pea and barley cultivars for complementarity in intercropping at different levels of soil N availability

Field Crops Research 72 (2001) 185±196

Evaluating pea and barley cultivars for complementarity in intercropping at different levels of soil N availability H. Hauggaard-Nielsena,*, E.S. Jensena,b a

b

Plant Biology and Biochemistry Department, Risù National Laboratory, DK-4000 Roskilde, Denmark Department of Agricultural Sciences, Section of Agroecology, The Royal Veterinary and Agricultural University, Agrovej 10, DK-2630 Taastrup, Denmark Accepted 9 July 2001

Abstract Two ®eld experiments were carried out on a temperate sandy loam using six pea (Pisum sativum L.) and ®ve spring barley (Hordeum vulgare L.) cultivars to determine cultivar complementarity in the intercrop for grain yield, dry matter production and nitrogen (N) acquisition. Crops were grown with or without the supply of 40 or 50 kg N ha 1 in the two experiments. Cultivars were grown as sole crops (SC) and as mixed intercrops (IC) using a replacement design (50:50). The land equivalent ratio (LER), which is de®ned as the relative land area under SC that is required to produce the yields achieved in intercropping, were used to compare cultivar performance in intercropping relative to sole cropping. Barley was the stronger competitor in the intercrops and as a result barley grain yield and nitrogen uptake in IC were similar to SC. The per plant pea grain production and aboveground N accumulation in IC were reduced to less than half compared to SC pea plants due to competitive interactions. Application of N caused a dynamic change in the intercrop composition. Competition from barley increased with N application and the pea contribution to the combined intercrop grain yield decreased. The LER values showed that in the intercrop plant growth resources were used on average 20% more ef®cient without N application and 5±10% more ef®cient with N application. The choice of pea cultivar in the intercrop in¯uenced the intercrop performance to a larger degree than the choice of barley cultivar. Furthermore, pea cultivar  cropping systems interactions was observed, indicating that cultivars performed differently in sole and intercrops. An indeterminate pea cultivar competed strongly with barley causing a greater proportion of peas in the intercrop yield, but caused a reduced N uptake and yield of barley. Determinate peas with normal leaves caused the highest degree of complementary use of N sources by allowing barley to exploit the soil N sources ef®ciently, while they contribute with ®xed N2. However, difference in performance among cultivars was observed. Using the indeterminate pea cultivar combined IC grain yield was in general lower than the greatest sole crop yield and vice versa for the determinate pea cultivars. Up to 22% …LER ˆ 1:22† greater combined IC grain yield was observed in several mixtures using determinate pea cultivars. From the present study, it is was concluded that there is a need for breeding suitable pea cultivars for intercropping purposes, since cultivars bred for sole cropping may not be the types, which are the most suitable for intercropping. For optimized N-use in pea±barley intercrops it is concluded that important traits for the intercropped pea are: (1) determinate growth, (2) a medium competitive root system for soil inorganic N and other nutrients during early growth, (3) high light Abbreviations: SC, sole cropping; IC, mixed intercropping; N, nitrogen; LER, land equivalent ratio; SNF, symbiotic N2 ®xation * Corresponding author. Present address: Department of Agricultural Sciences, Section of Agroecology, The Royal Veterinary and Agricultural University, Agrovej 10, DK-2630 Taastrup, Denmark. Tel.: ‡45-3528-3454; fax: ‡45-3528-2175. E-mail address: [email protected] (H. Hauggaard-Nielsen). 0378-4290/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 3 7 8 - 4 2 9 0 ( 0 1 ) 0 0 1 7 6 - 9

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absorption capacity by peas growing underneath the canopy of the higher barley component and (4) early establishment of symbiotic N2 ®xation to support a high growth rate during early growth stages. Fertilized pea±barley intercrops gave a 15% higher net income than fertilized barley sole cropping and is regarded as a better safeguard for the farmer's earnings compared to pea sole cropping known for variable yields and poor competitive ability towards weeds. # 2001 Published by Elsevier Science B.V. Keywords: Pea; Pisum sativum L.; Barley; Hordeum vulgare L.; Intercropping; Cultivars; Grain yield; N accumulation; Land equivalent ratio

1. Introduction Intercropping (IC) is known as a practice, which can improve the utilization of available resources and cause yield advantages and increased yield stability compared to sole cropping (SC) (Ofori and Stern, 1987; Trenbath, 1976; Vandermeer, 1989; Willey, 1979). Yield advantages occur when intercrop components compete only partly for the same plant growth resources. Thus, when the interspeci®c competition is less than intraspeci®c competition (Vandermeer, 1989; Willey, 1979). Ideally, cultivars suitable for IC should enhance the complementary effects between species (Davis and Woolley, 1993). Symbiotic N2 ®xation (SNF) is important for the supply of nitrogen in low-input agriculture (Peoples et al., 1995a; van Kessel and Hartley, 2000). Grain legume and cereals grown as mixed intercrops (IC) offers an opportunity to increase the input of SNF into temperate agro-ecosystems, while simultaneously utilizing the inorganic N sources ef®ciently, and maintaining the yield level and stability (Jensen, 1996; Anil et al., 1998). Jensen (1996) found that pea only acquired 8% of the total soil N taken up in a pea± barley IC, underlining the stronger competitive ability of barley for soil inorganic N. The main explanation for the intercropping advantage in cereal and grain legume intercrops relate to the complimentary in N use (Chalk, 1998; Willey, 1979). In 1997, about 4.5 Mt of grain legumes (Bourdillon, 1999) were produced in the European Union, of which 80% was peas. However, the grain legume production constituted only about 5% of the total protein production required in European animal production (Gatel and Champ, 1999). The main protein source is imported GMO soybean meal from USA and Latin America. Consumers in Europe have reservations toward GMOs. Furthermore, the European ban on meat and bonemeal in animal feedstuffs has resulted

in a greater demand for protein sources. Therefore a greater production of grain legume proteins in an environmental-friendly way seems to be pertinent for the future European animal production. A high concentration of soil inorganic N inhibits SNF (Peoples et al., 1995b; Sprent and Minchin, 1985), and reduces N input from SNF to the cropping system. We assume that cereal±legume IC results in an improved use of N resources in the intercropping system: the cereal ef®ciently exploits soil inorganic N, eventually alleviating the inhibitory effect of elevated soil N levels in fertile soils on SNF. Beside their SNF ability and break crop effects in cereal rich temperate crop rotations, grain legumes are rich in protein compared to other starchy plant products. However, grain legume sole cropping in systems with limited use of herbicide is questionable, and grain legume±cereal intercrops is recommended for such systems (Hauggaard-Nielsen et al., 2001). In temperate regions there is a long tradition for intercropping of clover-grass pastures for grazing and silage, and pea±barley mixtures for silage. However, grain legume±cereals intercrops for harvest as mature seeds is still limited. Most plant breeding programs for crop improvement have tended to develop cultivars for SC systems (Davis and Woolley, 1993; Nelson and Robichaux, 1997; O'Leary and Smith, 1999). These cultivars are normally used in IC systems. Carr et al. (1998) showed that the forage yield of pea±barley and pea±oat intercrops were greater with cereal cultivars developed for intercropping than when using cultivars developed for grain production. Finding the best traits for use in intercropping is a challenging task, due to the many cultivar  cropping system interactions. Assuming that the main advantage of intercropping cereals and grain legumes relates to the improved N use, we hypothesize that important traits for the cereal in intercropping systems are: (1)

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root systems with high competitive ability for soil N sources, (2) aerial parts allowing good support and reasonable amount of light to penetrate to the shorter pea component, and (3) thus allowing good growth of the associated pea. Suitable pea cultivars for intercropping should ideally have: (1) a medium competitive root system able to obtain suf®cient soil inorganic N and nutrients during early growth, and (2) early establishment of SNF, to support a high growth rate during early growth stages. The aim of the present study was to evaluate the effect of pea and barley cultivars on grain yield, dry matter production and N acquisition in pea±barley intercrops at different levels of soil N availability. The study involved six pea and ®ve barley cultivars, which were originally bred for sole cropping. 2. Materials and methods 2.1. Site description and experimental design The experiments were carried out on experimental ®elds at Risù National Laboratory, Denmark (558410 N, 128050 E) in 1982 (Experiment I) and

187

1984 (Experiment II). The 25-year mean annual rainfall at Risù is 550 mm, mean annual air temperature 88C with maximum and minimum daily air temperature of 168C (July) and 18C (February). The rainfall in the two growing seasons (April±August) were 252 mm (Experiment I) and 211 mm (Experiment II) compared to a 25-year mean of 217 mm. The greater rainfall in Experiment I was due to a rather wet May. The soil was a sandy loam with 115 g kg 1 clay, 145 g kg 1 silt, 490 g kg 1 ®ne sand and 250 g kg 1 coarse sand (Typic Hapludalf) representative for the eastern part of Denmark. The soil pH in water was 7.0. According to routine soil sampling every ®ve years pH is very stable on this soil showing a high percent base saturation. No liming has been conducted on the experimental ®elds for the last 30 years. The soil contained 10 g kg 1 total C and 1 g kg 1 total N in the topsoil (0±25 cm). White mustard (Sinapis alba L.) was grown in the preceding year to each experiment. Before sowing, soil inorganic N concentrations in the topsoil (0±25 cm) were 45 kg N ha 1 (Experiment I) and 30 kg N ha 1 (Experiment II). Field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) cultivars (Table 1) were grown

Table 1 Characteristics of barley and pea cultivars. Source: Department of Variety Testing, Danish Institute of Agricultural Sciences Species

Cultivar

Cultivar characteristics

Barley

Claudia (C) Golf (G) Jarl (J)

Medium early dwarf cultivar, widespread lateral growth, high stem strength, rather resistant against pests Medium early, medium-tall cultivar, medium lateral growth, medium stem strength and rather sensitive toward pests Very early, medium-tall cultivar with narrow leaves (minimum leaf biomass), medium lateral growth, high stem strength, rather sensitive toward pests Very early dwarf cultivar, poor lateral growth, high stem strength, rather resistant against pests Medium early, medium-tall cultivar, medium lateral growth, medium stem strength, rather resistant against pests

Mona (M) Nery (N) Pea

Allround (a) Bodil (b) Finale (f) Progreta (p) Salome (s) Stehgolt (st) a

Short and early flowering, early and uniform maturity, white-flowered, dwarf cultivar, normal leaves, medium-size blue- to light-green coloured grains, determinate growth, medium stem strength, root length, group 1a Short and early flowering, early and uniform maturity, white-flowered, dwarf cultivar, normal leaves, medium-size light-yellow to light-green coloured grains, determinate growth, medium stem strength, root length, group 1 Short and early flowering, early and uniform maturity, white-flowered, very dwarf cultivar, normal leaves, largesize green coloured grains, determinate growth, low stem strength, root length, group 1 Medium early flowering, early and uniform maturity, white-flowered, medium-tall cultivar, normal leaves with tendrils and characteristic leaflets, medium-size light-green coloured grains, determinate growth, medium stem strength, root length, group 2 Early flowering, early and uniform maturity, pink-flowered, medium-tall cultivar, normal leaves, large light-brown grains, indeterminate growth, low stem strength, root length, group 3 Medium early flowering, medium early and uniform maturity, white-flowered, medium to dwarf cultivar, normal leaves, small light-yellow grains, determinate growth, high stem strength, root length, group 1

Pea root length according to Jensen (1985). Group 1: 2.7±3.6 cm cm 3, group 2: 3.6±4.5 cm cm

3

and group 3: 4.5±5.4 cm cm 3.

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as sole crops (SC) and as a mixed intercrop (IC). Crops were grown without nitrogen (N) application ( N) or with the supply of 40 kg N ha 1 (Experiment I) or 50 kg N ha 1 (Experiment II) (‡N). Nitrogen was supplied once as Ca(NO3)2 just after sowing. Economical optimum nitrogen application rates for barley sole cropping would be 90±110 kg N ha 1, but to prevent too strong a competition from barley in the intercrop, medium fertilization levels were preferred to be able to detect cultivar  system  N fertilizer interaction in pea. The experimental layout was a randomized splitplot design with crops as main plot and N application as subplot in Experiment I and a lattice square design in Experiment II, with three replicates in both experiments. Each subplot (5.4 m2) consisted of 10 rows of length 4.4 m spaced 15 cm apart. 2.2. Site preparation Prior to sowing 30 kg P and 50 kg K ha 1 were broadcast on all plots. Fungicide treated (Thiram 80) seeds were sown on 7 April (Experiment I) and on 16 April (Experiment II) using a 10-row drill with 15 cm row distance. Barley and pea seeds were mixed before sowing. The number of seedlings in four rows of length 1 m was counted two weeks after seedling emergence. Weeds and leaf eating weevils (Sitona lineatus) were controlled with appropriate pesticides. 2.3. Cultivars and design The target SC plant density was 350 barley plants and 80 pea plants m 2. IC was sown in a 50:50 replacement design. Thus, the target intercrop density is 50% of the SC density of each crop. The principle of this design is that the interactions between IC components are not confounded by alterations in the plant density in IC compared to SC (De Wit and Van den Bergh, 1965; Trenbath, 1976). The cultivars used differed in time to ¯owering and maturity as illustrated by early (cv. Jarl and cv. Mona) compared to medium early barley cultivars (cv. Claudia, cv. Golf and cv. Nery). Barley cultivars had high (cv. Claudia, cv. Jarl and cv. Mona) and medium±high stem strength (cv. Nery and cv. Golf). Pea cultivars had either a determinate (cv. Allround, cv. Finale, cv. Progreta, cv. Bodil and cv. Stehgolt) or an indeterminate

(cv. Salome) growth form and had high (cv. Stehgolt), medium (cv. Allround, cv. Bodil, and cv. Progreta) and low (cv. Salome and cv. Finale) stem strength. Cv. Allround, cv. Bodil, cv. Finale, and cv. Stehgolt have the smallest root systems, followed by cv. Progreta with slightly longer roots and cv. Salome with the greatest root length (Table 1). In Experiment I, the composition of the established intercrops was close to those planned (i.e. 50%), but the cv. Finale pea plant population in IC was lower than expected. In Experiment II, barley constituted a greater proportion of the IC (ca. 65%) than planned. 2.4. Harvest and analysis Aboveground plant parts were harvested at maturity by cutting the plants 2 cm above the soil surface. The harvest area was 2.2 m2. IC was separated into the barley and pea components. Each component was weighed and subsamples were taken for determination of total dry matter content (DM) and composition of grain and straw after drying at 808C for 24 h. Total N was determined on ground samples using a semimicro Kjeldahl method which included nitrate (Bremner and Mulvaney, 1982). 2.5. Calculations and statistics The land equivalent ratio (LER) is de®ned as the relative land area (or growth resources) that is required when growing SC to produce the yield achieved in IC (Willey, 1979). LER for a barley±pea IC is the sum of the partial LER values for barley (LB) and pea (LP), in accordance with De Wit and Van den Bergh (1965): YB-IC YB-SC YP-IC LP ˆ YP-SC LB ˆ

LER ˆ LB ‡ LP

(1) (2) (3)

where YB-IC and YP-IC are the yields (total harvested DM) of barley and pea in IC, respectively, and YB-SC and YP-SC the yields of barley and pea in SC, respectively. LER > 1 indicates IC advantages in terms of improved use of environmental resources for plant growth. When LER < 1 resources are used more ef®ciently by SC than by IC.

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All measured variables were normally distributed and statistical analyses by ANOVA were performed using SAS software (SAS, 1990). The signi®cance of difference between treatments was estimated using the Tukeys Studentized range test with a ˆ 0:05, if a main effect or interaction was signi®cant. 3. Results 3.1. N accumulation in crops Total aboveground N accumulation was not significantly different between cultivars of barley in SC and between cultivars of pea in sole crop, except for cv. Salome with N supply (Figs. 1 and 2). Due to SNF, pea in SC accumulated 4 (without N application) to 2.5 (with N supply) times the amount of N accumulated in barley SC. Fig. 2. Accumulation of N in aboveground plant parts of barley and pea cultivars in sole and intercrops without ( N) and with (‡N) application of 50 kg N ha 1. Experiment II: all values are means of three replicates. Bars represent LSD0.05. For further details on cultivars, see Table 1.

Fig. 1. Accumulation of N in aboveground plant parts of barley and pea cultivars in sole and intercrops without ( N) and with (‡N) application of 40 kg N ha 1. Experiment I: all values are means of three replicates. Bars represent LSD0.05. For further details on cultivars, see Table 1.

Barley accumulated on average 85% of the SC N in IC, and no signi®cant differences in the ability to compete for soil N in intercrops were observed between the barley cultivars (Figs. 1 and 2). Due to competition from barley, the per plant N accumulation in pea was much lower in IC than in SC. The pea cultivars differed in their ability to acquire N, when intercropped with barley and signi®cant …p < 0:001† pea cultivar  cropping system interaction was observed in both experiments. Cv. Finale pea accumulated the lowest amount of N in intercrops, whereas the cv. Finale sole crop N accumulation did not differ from the other determinate cultivars. In Experiment I, the indeterminate cv. Salome pea accumulated signi®cantly greater amounts of N in intercrops than the other pea cultivars. Without N supply pea contributed with the largest proportion of N in the IC, but with N application barley was the dominant contributor (Figs. 1 and 2). The variation in total IC N accumulation was determined by the pea component N accumulation and the

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total IC N was not in¯uenced by N application. IC accumulated the same amount of N independent of N application (Figs. 1 and 2). 3.2. Grain yield In SC, the grain yields of barley cultivars were not signi®cantly different, but N application increased the yields (Figs. 3 and 4). The sole crop grain yields of pea cultivars were not in¯uenced by N application, except for cv. Salome and cv. Progreta peas for which grain yields were reduced when N was applied (Figs. 3 and 4). On average, barley IC produced grain yields around 85% of the SC yields, except in the mixtures with cv. Salome pea, where the barley yields were only 60%. Without N application, the per plant grain yields of determinate pea cultivars were on average 65% of the per plant yield in SC, and with N application only on average 36% of SC yields (Figs. 3 and 4). Fig. 4. Grain yields of barley and pea cultivars in sole and intercrops without ( N) and with (‡N) application of 50 kg N ha 1. Experiment II: all values are means of three replicates. Bars represent LSD0.05. For further details on cultivars, see Table 1.

Fig. 3. Grain yields of barley and pea cultivars in sole and intercrops without ( N) and with (‡N) application of 40 kg N ha 1. Experiment I: all values are means of three replicates. Bars represent LSD0.05. For further details on cultivars, see Table 1.

In Experiment I, no differences were observed between barley cultivars when intercropped with pea. In Experiment II, the grain yield of a barley cultivars was not signi®cantly different when intercropped with a speci®c pea cultivar, but cv. Golf performed slightly better than the other cultivars in IC (Fig. 4). Cv. Golf responded stronger to increased N availability than the other cultivars and stronger in IC than in SC (Fig. 4). The greatest barley yield in IC was found in associations with cv. Finale pea. In Experiment I, pea cultivar  cropping system interaction was observed (Fig. 3). The indeterminate cv. Salome pea was the superior yielding pea in IC, but the lowest yielding in SC. Cv. Finale had a relatively lower yield in IC compared to the other determinate cultivars and its yield performance in sole crop (Figs. 3 and 4). The barley cultivar did not in¯uence the grain yield of intercropped pea. In Experiment II, cv. Stehgolt pea had the greatest yield in the intercrops, but no pea cultivar  cropping system interaction was observed. The reduction in pea IC grain yield due to the increased N availability was found to be stronger

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in cv. Finale (55%) than in the other cultivars (on average 25%). Total intercrop grain yields were slightly lower or similar to the greater yielding SC, but ``over-yielding'' (the combined IC yield greater than the greatest sole crop yield) up to 22% was observed in several IC mixtures. The total IC yields were only slightly in¯uenced by N application, but the proportion of the pea in the IC was almost halved (Figs. 3 and 4). 3.3. Production ef®ciency of intercropping LER for total DM yields can be used to determine the ef®ciency of resource use in intercropping relative to sole cropping and the advantage from intercropping. Fig. 5 shows the partial LER values (LB and LP) and the combined LER value. In Experiment I, the average LER in treatments without N application was 1.22 compared to 1.06 with N application (Fig. 5). In the ICs with determinate pea cultivars the decline was due to reductions in both the partial pea and barley LER values. Cv. Claudia LB values were less reduced than those of cv. Nery in response to N fertilization. The ICs with the indeterminate cv. Salome peas behaved differently from the determinate pea ICs. The diagonal line in Fig. 5 is the border of species dominance. Symbols below the diagonal line indicate barley dominance and symbols above the diagonal line indicate pea dominance. The greater competitiveness of cv. Salome was indicated in the diagram. N application switched the dominance from barley to pea in the cv. Salome intercrops, as the only pea±barley mixture (Fig. 5). The average LER in Experiment II was 1.17 without N application and 1.05 with N application (Fig. 5b). Application of 50 kg N ha 1 decreased the LP from 0.35 to about 0.20, and LB were rather unaffected by N application. Interactions were found between pea and barley cultivars with respect to LER values (Fig. 5). In Experiment I, intercrops with cv. Claudia barley had the greatest LER (Fig. 5a). In Experiment II, the greatest LER values were obtained with cv. Stehgolt pea for three out of four barley cultivars (Fig. 5b). The overall grain yield advantage from intercropping was also evaluated by LER values (Table 2). The overall advantages were 14% in Experiment I and 11% in Experiment II (LER 1.14 and 1.11). The greater N

Fig. 5. Partial LER of barley (LB) and pea (LP) and the combined LER calculated from total aboveground biomass production without (open symbols) or with (closed symbols) application of 40 kg N ha 1 (Experiment I) and 50 kg N ha 1 (Experiment II). The obliquely lines show LER. The line crossing the xy coordinate 0.0 indicates a 50-to-50% balanced use of environmental sources for plant growth by pea and barley. All values are means of three replicates. For further details on cultivars, see Table 1.

availability reduced the advantage from intercropping and no advantage …LER  1† was found in cv. Nery, cv. Jarl and cv. Mona intercrops when averaged over pea cultivars (Table 2). Cv. Golf intercrops had the greatest LER value (1.19) averaged over pea cultivars and N levels. In Experiment I, the greatest LER values for grain yield were found with cv. Salome pea, and in Experiment II, the cv. Progreta intercrops had the greatest LER values (Table 2). The cv. Nery±Bodil and cv. Nery±Finale intercrops were present in both experiments and the average LER (over N levels) were similar in both experiments (1.13 in Experiment I and 1.09 in Experiment II).

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Table 2 LER values calculated from grain DM production in Experiments I and II Barleya

N treatmentb

Experiment I N N ‡N

Peaa a

b

f

s

Mean

1.23 1.05

1.23 1.02

1.27 0.99

1.15 1.26

1.22 1.08

C

N ‡N

1.16 1.03

1.15 1.12

1.08 0.99

1.17 1.38

1.14 1.13

Mean

N ‡N

1.20 1.04

1.19 1.07

1.18 0.99

1.16 1.32

1.18 1.11

p

b

f

st

Mean

1.33 1.05

1.15 1.04

1.18 1.00

1.16 0.97

1.21 1.02

Experiment II N N ‡N

expenses (Table 3). In general, pea SC were the most costly crops to grow followed by pea±barley intercrops and barley SC. N application in pea SC reduced the net income in contrast to barley SC increasing the net income by almost 250%. N application to the intercrop raised the net income by 100 Euro ha 1. Compared to pea sole cropping, the net income of pea±barley intercropping was 35 and 10% lower without and with N application, respectively. In contrast, the net income of pea±barley intercropping was 200 and 15% higher than barley cropping, without and with N application, respectively. Extra costs for separation of intercropped pea and barley grains after harvest were included in the calculations.

J

N ‡N

1.25 1.02

1.12 1.00

1.11 0.99

1.18 0.97

1.17 1.00

M

N ‡N

1.05 1.04

1.07 1.01

1.17 0.99

1.13 0.95

1.10 1.00

G

N ‡N

1.24 1.17

1.21 1.14

1.32 1.14

1.22 1.04

1.25 1.12

Mean

N ‡N

1.22 1.07

1.14 1.05

1.20 1.03

1.17 0.98

1.18 1.03

4. Discussion 4.1. Competitive interactions in pea±barley intercrops Barley was the dominant component in most of the intercrops, as also observed by Jensen (1996, 1998) and Hauggaard-Nielsen et al. (2001). The only exception was when barley was intercropped with the indeterminate cv. Salome pea cultivar. In these intercrops, pea was the dominant species possibly due to temporal growth pattern and a root system with greater density than the determinate cultivars (Jensen, 1985). Cereals take up nutrients, especially N, mainly during the vegetative growth stages (Jensen, 1996) and the associated vigorous growth may cause shading of the legume and thereby reduce its growth during later

a

For further details on cultivars, see Table 1. The N supply was 0 ( N) and 40 or 50 kg N ha Experiments I and II, respectively. b

1

(‡N) in

3.4. Economical aspects of cropping strategy Application of N increased the production cost due to the costs of fertilizer and increased grain-drying

Table 3 Economical aspects of pea and barley sole cropping compared to pea±barley intercropping using data from Experiment II. The economical calculations are based upon the newest 2001 estimates for the Danish market published by the Danish Agricultural Advisory Center (www.lr.dk) including EU subsidies Cropping

Crop

Fertilizer (kg N ha 1)

Grain yield (Mg ha 1)

Straw yield (Mg ha 1)

Production cost (Euro ha 1)

Income from sale (Euro ha 1)

Net income (Euro ha 1)

Sole cropping

Pea

0 50 0 50

6.0 5.9 3.9 5.9

3.6 3.8 2.5 4.6

742 773 667 716

1347 1331 853 1153

605 559 186 436

0 50 0 50

2.2 1.1 2.9 4.9

1.1 0.6 2.1 4.1

732 764

1127 1259

394 494

Barley Intercropping

Pea Pea Barley Barley

H. Hauggaard-Nielsen, E.S. Jensen / Field Crops Research 72 (2001) 185±196

growth stages (Fujita et al., 1992). At increased levels of soil N availability the competition for light between component crops is intensi®ed and suppresses the growth of the legume (Fujita et al., 1992). In the present study, this effect was observed in terms of the decreased partial pea and total LER values for the determinate pea cultivars when N was applied (Fig. 5 and Table 2). This is in agreement with Waterer et al. (1994) and Jensen (1996). Three of the barley cultivars exhibited a very strong competitive effect when N was applied resulting in no grain yield advantages in intercrops with these cultivars (Table 2). Without N application, however, the LER values were generally >1 indicating complementary resource use. 4.2. Effect of barley cultivar in intercropping The barley cultivars used in this study all showed a high degree of plasticity in terms of N uptake, since when grown in the intercrops at 51±66% of the sole crop plant population, barley still accumulated on average, 85% of N taken up by the SC. In terms of the effect of barley cultivar on the pea growth and N accumulation, no differences were observed between the cultivars. We conclude that there were no major differences among the barley cultivars used in this study. However, the most suitable cultivars for intercropping appeared to be those cultivars with medium early maturity and high stem strength. Cv. Golf barley has these characteristics and germinates a little later than the other cultivars, which may reduce the early interspeci®c competition. Cereal cultivar differences in shoot and root response to high and low nitrogen supply have been observed (e.g. Gorny, 1993, 2001) and such differences may be intensi®ed in intercrops using cultivars with deeper and more vigorous root growth under interspeci®c competitive growth conditions. Knowledge about cereal cultivars root response to intercropping growth conditions is a valuable adaptive characteristic for breeding suitable barley cultivars for intercropping purposes. 4.3. Effect of pea cultivar in intercrops Suitable pea cultivars for intercropping should ideally have: a low competitive ability for soil inorganic N during early growth, but early establishment of SNF

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to support a high growth rate during early growth stages. In the present study, it was not possible to differentiate uptake of soil N from SNF. It has earlier been shown by Jensen (1996) that pea uptake of soil N in intercropping may be less than 8% of the total soil N taken up. Thus, it was assumed that differences in accumulated N in the determinate pea cultivars mainly re¯ected differences in SNF. Pea cultivars with an indeterminate growth habit such as cv. Salome have a greater root system and a greater proportion of their vegetative growth after ¯owering as compared with determinate pea cultivars. Cv. Salome showed a much greater competitive ability towards barley compared with the other pea cultivars. This was probably due to increased competition for soil N, since barley N uptake was much lower in intercrops with cv. Salome than in the other intercrops. Nelson and Robichaux (1997) found that total shoot length of cowpea was positively correlated with cowpea yield when intercropped with millet. The tall cv. Salome pea can grow into upper levels of the barley canopy potentially gaining increased access to higher incident light levels, in contrast to shorter determinate cultivars. Less shading of the legume component in an IC system may increase photosynthesis and SNF (Fujita et al., 1992). The better performance of cv. Salome pea in intercrops was counterbalanced by a lower partial LER for barley and the total biomass LER for cv. Salome±barley intercrops were not greater than for other ICs (Fig. 5). Cv. Finale was the cultivar with the lowest competitive ability in intercrops, and cv. Progreta was slightly weaker than cv. Bodil, cv. Allround and cv. Stehgolt. Cv. Finale was the shortest cultivar and had soft stems (Table 1), which may have in¯uenced the ability of this cultivar to grow to the top of the canopy, and as a result there was more shading by barley. Analysis of factors in¯uencing the competitive ability of determinate and dwarf pea cultivars is required to make a selection for intercropping in breeding programs. In contrast to the present study, it is recommended for future studies to include cultivar root system morphology and distribution when evaluating suitability for growth under competitive intercropping conditions. Based on total IC N accumulation, including SNF, the pea cultivars could be ranked as follows: cv. Salome > cv: Stehgolt > cv: Bodil > cv: Allround >

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cv: Progreta > cv: Finale. If N accumulation by barley is included, the ranking is: cv: Stehgolt > cv: Bodil > cv: Allround, because these three pea cultivars showed the highest degree of interspeci®c competitive ability and induced a reasonable amount of N2 ®xation without detriment to the barley soil N uptake. Semi-lea¯ess or lea¯ess pea cultivars were not included in this study. It would be valuable to evaluate effects of pea leaf type under pea±barley intercrop growth conditions. Only those pea±barley combination intercrops containing cv. Salome were able to increase the advantage from intercropping when N was applied (Table 2). Thus under there conditions, the barley component may contribute to an improved standing ability of cv. Salome (Karpenstein-Machan and Stuelpnagel, 2000). Also, intercropped cv. Salome showed the greatest competitive ability among the pea cultivars. Thus if a high proportion of indeterminate and tannin rich pea grain is required (e.g., cattle concentrates), we conclude that indeterminate peas seems to be the best choice for intercropping. In contrast, if the focusing on advantages of intercropping cereals and grain legumes is to improve the use of N sources in the cropping system, the determinate cultivars cv. Allround, cv. Bodil or cv. Stehgolt with normal leaves are the best choice for intercropping. Compared to cv. Salome these cultivars appear to cause the highest degree of complementary use of N sources by allowing barley to exploit the soil N ef®ciently, while the pea contributes with symbiotically ®xed N2. Crop rotation, choice of cultivars, soil fertility, nitrate levels, quality requirements that crops may have to meet in animal fodder and/or human consumption, commodity price and the availability of a market, are all factors in¯uencing crop preference by farmers. For intercropping to be biologically advantageous, the intercrop component crops and cultivars need to be chosen with care. There is a need to address fundamental biological interactions and changing intercrop component proportions in order to be able to manage the desired intercrop output. 4.4. Socio-economic considerations Pea sole cropping gave the highest net income followed by pea±barley intercropping and barley sole cropping, respectively (Table 3). However, pea grain yields in the study year were beyond the average level

of about 4.5 Mg ha 1 for the eastern part of Denmark. Furthermore, peas are known for variable yields and to be weak competitors for weeds (e.g. Hauggaard-Nielsen et al., 2001). Pea sole cropping, however, are less favoured in cropping systems without use of herbicides, e.g. in organic farming systems, and Danish farmers are beginning intercrop pea with a cereal in ®elds with high weed pressure. The fertilized pea± barley intercrop performance resulted in an intermediate net income (Table 3) and is regarded as a better safeguard for the farmer's earnings compared to pea sole cropping. If the European Union succeeds in boosting the protein crop production in Europe through, e.g. higher prices, one might expect to see a higher degree of grain legume sole cropping or grain legume±cereal intercrops with a lower proportion of the cereal. It is important to remember, however, that an intercropped cereal is a valuable component to improve competitive ability towards weeds and also to provide physical support to reduce pea lodging. Intercropped barley ef®ciently exploited soil inorganic N sources while at the same time atmospheric N2 is ®xed by the pea component without compromising yield level and stability, as shown by HauggaardNielsen et al. (2001) and Jensen (1996). The economical calculations showed that such implementation of crop complementarity in typical cereal rich rotations can result in income bene®ts for the farmer (Table 3). 4.5. Breeding of pea and barley for intercropping The present study indicates that there is a need for developing suitable pea cultivars for intercropping purposes. The ®rst steps toward development of re®ned breeding programs should be the elaboration of the mechanisms of competition. The present study showed that breeding of suitable pea cultivars should be the highest priority. Important traits for the intercropped pea are: (1) determinate growth, (2) a medium competitive root system for soil inorganic N and nutrients during early growth, (3) high absorption capacity of light which penetrates through the cereal canopy, and (4) early establishment of SNF to support a high growth rate during early growth stages. Further improvement of the use of N by legume± cereal intercrops may require pea cultivars with different characteristics than those used in the present study. Early, dwarf and determinate pea cultivars

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resulted in the greatest intercropping advantages, while normal-leafed cultivars should be compared with semi-lea¯ess cultivars to evaluate light absorption, SNF and productivity in intercropping. In such comparison, however, it is important to realize that pea lodging, which led to the development of semi-lea¯ess cultivars, is only of minor importance when intercropped with barley, because barley contributes to an improved standing ability of pea (KarpensteinMachan and Stuelpnagel, 2000). The present work demonstrates that breeding programmes for pea and barley cultivars, as SC, are not suf®cient for adaptation to intercropping. Currently, conventional plant breeding may not be responding to farmer demand to a range of cultivars for speci®c needs. Finding the best pea traits for example to use in legume±cereal intercrops is complicated due to many cultivar-cropping system interactions and farmers may need to involved in breeding programs. 5. Conclusion Higher priority should be given to improve the IC composition selection of pea cultivars capable of improving the SNF input into the cropping system without depressing the yield of the barley component. However, it is dif®cult to select for such traits, due to the many cultivar  cropping system interactions in IC systems. Acknowledgements We thank Merete Brink Jensen for skilled technical assistance and Dr. Kristian Thorup-Kristensen for helpful comments on the draft. References Anil, L., Park, R.H.P., Miller, F.A., 1998. Temperate intercropping of cereals for forage: a review of the potential for growth and utilization with particular reference to the UK. Grass Forage Sci. 53, 301±317. Bourdillon, A., 1999. Advantages and constraints of grain legume for the feed market. In: AEP (Eds.), Proceedings of the Third European Conference on Grain Legumes, Valladolid, Spain, 1988, p. 5. Paris, France, p. 510.

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