1987

Field Crops Research, 16 (1987) 43-52

43

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

The Combined Effects of Nitrogen Fertilizer and Density of the Legume Component on Production Efficiency in a Maize/Cowpea Intercrop System FRANCIS OFORI and W.R. STERN

Agronomy Group, School of Agriculture, University of Western Australia, Nedlands, W.A. 6009 (Australia) (Accepted 7 August 1986)

ABSTRACT Ofori, F. and Stern, W.R., 1987. The combined effects of nitrogen fertilizer and density of the legume component on production efficiency in a maize/cowpea intercrop system. Field Crops Res., 16: 43-52. The combined effects of applied nitrogen and of legume density on the yields and efficiency of cereal/legume intercropping were examined, using maize and cowpea. The levels of applied nitrogen were 0, 30, 60 and 120 kg N h a - 1, and intercrop cowpea densities were 80 000, 100 000, 120 000 and 150 000 plants h a - ~. The interaction of applied N and density of companion cowpea on yield of maize was negative. In maize, the yield losses due to intercropping were alleviated to some degree by N application, but in cowpea they were accentuated, and it appeared that maize was more competitive than cowpea. Maize was more efficient than cowpea in the utilization of N to produce grain; with each increment of N, efficiency declined in maize but was almost constant in cowpea. The Land Equivalent Ratio (LER), a measure of the efficiency of intercropping, declined with increasing levels of applied N but did not change significantly when intercrop cowpea density exceeded 100 000 plants h a - 1. The LER values followed trends in cowpea rather than maize yields, and this may be attributed to shading of cowpea by maize.

INTRODUCTION

While in a cereal/legume intercrop system the cereal has a high nitrogen requirement to attain its maximum yield, N2 fixing capacity and the yield of the associated legume may be seriously constrained by high levels of applied N (Ahmed et al., 1979; Rego, 1981; Baker and Blamey, 1985). It has been demonstrated that the efficiencies of cereal legume intercrop systems expressed in terms of the land equivalent ratio (LER) remain unchanged or decrease in response to increasing rates of nitrogen fertilizer (Ahmed et al., 1979; Searle et al., 1981; Baker and Blarney, 1985; Ofori and 0378-4290/87/$03.50

© 1987 Elsevier Science Publishers B.V.

44 Stern, 1986). From these reports, the trends in the LER appear to be determined by the yields of the legume component. In order to maximise the efficiency of cereal/legume intercrop systems relative to sole cropping, it has been suggested that N should be applied by banding slow-release fertilizers to the cereal component ( IAEA, 1980); this approach is thought to raise the efficiency of utilization of applied N and also to minimise the deleterious effects on N2 fixation of the companion legume. Component crop density is also a management variable that may influence the efficiency of cereal/legume intercrop systems. In a maize/groundnut combination, Spitters (1983) found that the efficiency of intercropping in terms of LER was related more to the legume yield than to the associated cereal yield. This observation has recently been confirmed with maize and cowpea by Ofori and Stern (1987). Although the efficiency of cereal/legume intercropping is reported to be high at low levels of N and to increase with the proportion of the legume in the mixture, it is not known to what extent, if any, high legume density can provide additional N to benefit the companion cereal. The aim of the present work was to examine whether combining moderate to high levels of N fertilizer with higher legume densities had any effects on the yields, the nitrogen utilization efficiency and the Land Equivalent Ratio (LER) of a maize/cowpea intercrop system. MATERIALSAND METHODS The experiment was conducted at an irrigated site in Waroona, West Australia, (32°51'S, 115°55'E) in summer, between December 1983 and April 1984. Details of the area and its climate were reported by Ofori and Stern (1986). The site had previously been cropped to maize for two seasons. The land was cultivated, flood-irrigated, and then fertilized with basal dressings of superphosphate (9.1% P) at 120 kg ha -1 and KC1 (49.8% K) at 100 kg ha -1. After seedbed preparation a pre-emergent herbicide, Dual, was applied to control weeds. The experiment was a factorial design replicated three times. The treatments were four levels of N at 0, 30, 60 and 120 kg N h a - 1, a sole crop of maize at each level of N, and intercropping mixtures of maize at about 53 000 plants ha-1 combined with four cowpea densities of about 80 000, 100 000, 120 000 and 150 000 plants h a - 1. Sole cowpea was planted at a single density of 100 000 plants ha-1 without applied N. Thus, in each experimental block, there were four sole maize, one sole cowpea and 16 intercropping mixtures of maize and cowpea. Each 6-m wide plot consisted of four raised beds (8.0 X 1.5 m wide) separated by irrigation furrows. In the sole maize, plant spacings were 0.75 X 0.25 m, and in the sole cowpea 0.75 X 0.125 m. In the intercrop mixtures, maize was sown centrally in each bed in rows 1.5 m apart and 0.125 m between plants.

45 The adjacent cowpea rows were 0.75 m apart; intercrop cowpea densities were achieved by adjusting within-row spacings at 9.0, 11.0, 13.0 and 16.0 cm. The maize cultivar XL66 (late maturing hybrid) and the indeterminate cowpea cultivar Banjo ( Cook, 1982 ) were used. Cowpea seeds were inoculated with a peat-based single strain of Rhizobium (CB 756 - 'Nitrogerm'). On 2 December 1983, seeds were sown by machine using appropriate sprockets and seed plates. After 15 days, plants were thinned to the desired densities and, where necessary, vacancies were filled with plants of similar age raised in jiffy pots. In the N plots, ammonium nitrate (34% N) was applied to maize rows at the appropriate rates at 21 and 42 days after sowing (split dressings). At physiological maturity (125 days after sowing), ten maize and ten cowpea plants were harvested for determinations of dry matter yield, N concentration and N uptake in straw and grain or seed. Plants were dried to constant weights in a forced-draught oven at 80 °C for dry matter yield determinations, and then ground in a Willey mill to pass through a 2-mm mesh. Total N content of the ground samples was determined by a semi-micro Kjeldahl technique (Bremner, 1965) which involved digestion in sulphuric acid using a 40-tube Tecator Digestion System 40 (1016 Digester). Nitrogen concentrations of the digests were determined by a Kjeltec 1030 Autoanalyzer, and N uptake was calculated as the product of yield and N concentration. Nitrogen utilization efficiency (NUE) was determined as: amount of yield/amount of N taken up and expressed as kg/kg N (Tanaka et al., 1984). Final grain and seed yields were determined from 20 maize and 40 cowpea plants harvested from the central beds in each plot. Cobs and pods were dried to constant weights at 70 °C before threshing. Intercropping efficiency relative to sole cropping of maize and cowpea was calculated by the Land Equivalent Ratio (LER) ( Mead and Willey, 1980 ). RESULTS There were significant negative interactions between the effects of N and component cowpea density, on dry matter and grain yields, and on the N parameters of maize; in cowpea, the interaction was only signficant with respect to dry matter yield.

Yields Dry matter yields of sole and intercrop maize increased progressively with applied N (Fig. la). In the sole maize, the yield response was linear up to 60 kg N ha-1, and substantially greater than in the intercrop maize. The relative yield depression of maize increased when level of N and density of the intercrop

46 a)

Maize

A T

b) ~800(

Cowpea

6O00

2

/

~ 16oo0

Sole crop

/

~

~

t~-

"~"

500o

12000

4000

'rooooi

3000

t 0

I 30

I 60

I 120

Applied nitrogen

l 0

I 30

I 60

I 120

(kg N ha -1)

Fig. 1. Effects of applied N and intererop cowpea density on dry matter yields of maize and cowpea. Vertical bars are LSD ( P < 0.05) to compare sole crops and intercrops. Figures in parentheses are intercrop cowpea density per ha.

cowpea were increased. At nil-N, there were no significant differences in dry matter yields between sole and intercrop maize. Intercropping maize with the recommended cowpea density ( i.e. 100 000 plants h a - 1) caused a maize yield reduction of between 16 and 21% as N application varied from 30 to 120 kg N ha -1. In cowpea, the response of dry matter yield to applied N was positive up to 60 kg N h a - 1at the lower intercrop cowpea densities, and there was no response at the higher densities (Fig. lb). Increasing the component cowpea density from 80 000 to 120 000 plants ha-1 resulted in increases in dry matter yield. In the intercropping mixtures with 100 000 plants ha-1, cowpea yield reductions of between 33 and 40% (relative to sole-crop yield) were noted as the N application was increased from 30 to 120 kg N ha-1. In maize, the trends in grain yield were similar to those of dry matter (Fig. 2a). At the nil-N, maize intercropped with cowpea at the lower densities (i.e. 80 000 and 100 000 plants h a - 1) produced marginally greater grain yields than the sole crop. Grain yield was reduced as the level of N and the density of the intercrop cowpea were raised. In maize intercropped with cowpea at 100 000 plants h a - 1, grain yield reductions of between 9 and 18% were noted, as the level of applied N was raised from 30 to 120 kg N h a - 1. Seed yield of cowpea responded to applied N up to 30 kg N h a - 1, and declined with higher levels of N, at all densities of cowpea (Fig. 2b). The seed yields of intercrop cowpea were similar in densities ranging between 100 000 and 150 000

47 a)

Maize

1ooo0

I

-[

b) Cowpea ~8oo

-- 90°°

~,500 == S°"c~°P

._

~

~

.(150,000}

";,

~

C

12o,ooo1 C1o0,ooo)

lo

500(

60C

l

n

I

I

I

I

I

0

30

60

120

0

30

60

120

Applied nitrogen (kg N ha-4)

Fig. 2. Effectsof applied N and intercropcowpeadensityon grain and seedyieldsof maize and cowpea.Verticalbars are LSD (P < 0.05) to comparesolecropsand intercrops.Figuresin parentheses are intercropcowpeadensityper ha. plants h a - 1, and differed only from those at 80 000 plants h a - 1 ( p < 0.05 ). At 100 000 cowpea plant h a - 1, intercrolSping caused cowpea seed yield reductions of between 35 and 49%.

Nitrogen parameters There were generally no significant differences in N concentrations, uptake, and NUE's of maize and cowpea between the intercrop treatments with cowpea densities of 80 000 and 100 000, and again between cowpea densities of 120 000 and 150 000 plant~ ha-1. Therefore only the data with intercrop cowpea densities at 100 000 (C1) and 150 000 (C2) plants ha -1 are reported. At 125 days in the nil-N treatment, N concentrations in the maize intercropped with cowpea were higher than in the sole-crop maize (Table 1 ). Application of N significantly ( P < 0.05 ) increased N concentrations and uptake of maize, but these were reduced by intercropping at the different levels of applied N. In intercrop cowpea, N concentration and uptake at 125 days decreased with applied N above 60 kg N h a - 1 ( Table 2 ). Nitrogen utilization efficiency (NUE), i.e. the capacity of N to produce yield, showed that maize was more efficient than cowpea in the utilization of N (Tables 1 and 2). For each kg of N taken up, maize produced 81 kg of grain and cowpea 31 kg of seed. NUE in sole maize decreased significantly ( P < 0.05) as applied N rose from 0 to 120 kg N ha-l; however, in the intercrop maize,

0.1 (0.1) 11.9 (7.7) 8.1 (4.7)

Nper cent N uptake NUE

C1 -- 100 000 plants ha- 1. C2 = 150 000 plants ha 1. Figures in parentheses are for grain.

N

1.1 64.9 94.1

Grain N per cent N uptake (kg ha- 1) NUE (kg kg N 1)

LSD (P<0.05)

0.8 101.5 128.8

1.2 65.3 86.3

1.0 112.3 102.8

C2

0.1 (0.1) 13.2 (8.6) 9.0 (5.3)

C

1.2 77.1 84.0

1.0 135.5 103.1

C1

0.2 (0.2) 26.6 (17.2) 18.0 (10.6)

NxC

1.2 93.1 83.2

0.9 138.9 118.3

Sole crop

Sole crop

Cowpea density

30

0

Applied nitrogen (kg N ha- 1)

At maturity N per cent N uptake (kgha -1) NUE (kg kg N - ' )

Parameter

1.2 84.1 82.5

0.9 128.8 107.7

C1

Cowpea density

1.2 73.9 85.6

0.9 112.4 113.4

C2

1.3 117.4 74.6

1.1 205.7 90.1

Sole crop

60

1.3 93.5 76.6

1.1 173.4 87.0

C1

Cowpea density

1.2 76.7 83.6

0.9 128.6 107.7

C2

1.6 155.7 62.9

1.3 270.6 75.2

Sole crop

120

1.4 105.9 72.7

1.1 172.1 94.2

C1

Cowpea density

1.1 71.8 82.8

0.9 136.3 107.2

C2

Effects of applied nitrogen (N) and intererop cowpea density (C) on nitrogen concentration (%), nitrogen uptake and nitrogen utilization efficiency (NUE) of maize tops and grain at 125 days

TABLE 1

oo

0.2 (0.2) 7.8 (4.8) 4.9 {2.1)

N per cent N uptake NUE

C1 = 100 000 plants ha- 1. C2 = 150 000 plants ha- 1. Figures in parentheses are for seed.

N

3.3 51.8 30.6

Seed N per cent N uptake (kg h a - 1) NUE kg kg N - 1)

LSD (P<0.05)

2.4 139.4 41.3

3.3 31.8 30.7

2.0 89.2 48.9

C2

0.2 {0.3) 8.7 (5.3) 5.4 (ns)

C

3.3 32.3 30.6

2.2 75.6 46.9

C1

ns (ns) ns (ns) ns (ns)

NxC

3.2 33.3 31.0

2.2 83.0 44.6

Sole crop

Sole crop

C,owpea density

30

0

Applied nitrogen (kg N ha 1)

At maturity N per cent N uptake (kg ha 1) NUE ( kg kg N - 1)

Parameter

3.2 35.6 31.4

2.1 90.2 48.1

C1

Cowpea density

3.2 31.3 31.7

2.3 86.3 44.7

C2

3.1 32.5 32.3

2.0 87.7 51.2

Sole crop

60

3.1 25.3 32.0

1.8 63.8 55.5

C1

Cowpea density

2.9 24.5 34.5

1.7 68.4 61.7

C2

Sole crop

120

C1

Cowpea density C2

Effects of applied nitrogen (N) and intercrop cowpea density (C) on nitrogen concentration (%), nitrogen uptake and nitrogen utilization efficiency (NUE) of cowpea tops and seed at 125 days

TABLE 2

5o

T

_ore

~ ~ . . o

~

~

\

/loo,ooo

w~.~

//120,000

~%--~

/ / .150,ooo

1.4

~' 1.2

1.0 I 0

I 30

I 60

Applied nitrogen

I 120 {kg N ha "l)

Fig. 3. Effectsof appliedN and intercrop cowpeadensity on the land equivlanetratio. Verticalbar is LSD (P<0.05) to compareN effects. the decreases were marginal at the different levels of N (Table 1 ). In cowpea, intercropping and applied N increased N U t for dry matter of tops; however, the efficiency of seed production remained almost constant in response to N ( Table 2).

Efficiency of intercropping The land equivalent ratios (LER) in response to applied N are shown in Fig. 3; yield advantages ranged from 21 to 69%. With rising level of applied N, there was a steady decline in the LER. However, with an increase in intercrop cowpea density from 80 000 to 100 000 plants ha -1, the increase in LER due to density, although the largest, was only marginal. There was no difference among LERs of intercrop cowpea density treatments above 100 000 plants h a - 1. DISCUSSION These results do not show synergistic effects of combining N fertilizer application with increasing intercrop legume density. Maize grain yields (both sole and intercrop) increased progressively with applied N and declined with increasing density of the associated cowpea; seed yields of intercrop cowpea declined with applied N and increased with increasing cowpea density in the mixture. The LER followed the yield trends of cowpea rather than of maize. These

51 observations support the evidence that LER declines at higher N-levels and appear to suggest that intercropping efficiency is greater under low than high fertility (Searle et al., 1981; Ahmed et al., 1981; Ofori and Stern, 1986), and further explains the lack of response to applied N of the LER in sorghum/pigeonpea and sorghum/soybean intercrop systems reported by Rego (1981) and Baker and Blamey (1985). The significant reductions in yields, N concentrations and uptake in response to applied N, of intercrop maize, and of cowpea at various densities, indicate that the component crops were competing for plant-available soil N and other growth-limiting factors, even at the lowest cowpea density (i.e. 80 000 plants h a - 1). The degree of suppression of cowpea was more than twice the effect of cowpea on maize, so that it may be argued that maize dominated cowpea in the competition for growth-limiting factors. The depressions of intercrop cowpea could be due to the shading by the companion maize. These results are consistent with similar observations from a sorghum/soybean intercrop system (Baker and Blamey, 1985 ). The yield loss of intercrop maize in response to applied N and associated cowpea density indicates a negative interaction of these factors on yield. It therefore suggests that cowpea did not benefit maize in terms of N; rather, it prevented maize from taking full advantage of the applied N. The efficiency of utilization of N fertilizer for the production of yield shows that maize was more efficient than cowpea and that this was not affected by intercropping. Nitrogen utilization efficiency for grain yield in maize decreased with rising N fertilizer application, but in cowpea seed yield, it remained constant. Similar results with maize and soybean as sole crops have been reported by Tanaka et al. (1984), and the almost constant response of the legume was attributed to possible compensation between N2 fixation and fertilizer-N absorption. ACKNOWLEDGEMENTS The senior author is grateful for an award under the Commonwealth Scholarship and Fellowship Plan (CSFP), and to the Ghana Cocoa Board Plantations Ltd. for granting study leave.

REFERENCES Ahmed, S., Gunasena, H.P.M. and Yang, Y.H., 1979. Proc. Final I N P U T S Review Meeting, 20-24 August 1979, at East-West Center, Honolulu, HI, 251 pp. Baker, C.M. and Blarney, F.P.C., 1985. Nitrogen fertilizereffectson yield and nitrogen uptake of sorghum and soybean, grown in solecropping and intercropping systems. Field Crops Res., 12: 233-240.

52 Bremner, J.M., 1965. Total nitrogen. In: C.A. Black (Editor). Methods of Soil Analysis, Part 2. Am. Soc. Agron., Madison, WI., pp. 1149-1178. Cook, K.J., 1982. Voluntary register of grain legume cultivars in Australia - cowpea cv. Banjo. J. Aust. Inst. Agric. Sci., 48: 127-128. IAEA, 1980. In: Proc. advisory group meeting on nuclear techniques in the development of fertilizer and water management for multiple cropping systems, FAO/IAEA, Ankara, Turkey, 8-12 October 1979, 154 pp. Mead, R. and Willey, R.W., 1980. The concept of a 'Land Equivalent Ratio' and advantages in yields from intercropping. Exp. Agric., 16: 217-228. Ofori, F. and Stern, W.R., 1986. Maize/cowpea intercrop system: effect of nitrogen fertilizer on productivity and efficiency. Field Crops Res., 14: 247-261. Ofori, F. and Stern, W.R., 1987. Relative sowing time and density of component crops in a maize/ cowpea intercrop system. Exp. Agric., 23: (in press). Rego, T.J., 1981. Nitrogen response studies of intercropped sorghum with pigeonpea. In: Proc. Int. Workshop on Intercropping, 10-13 January 1979, Hyderabad, India. ICRISAT, Hyderabad, pp. 210-216. Searle, P.G.E., Comudom, Y., Shedden, D.C. and Nance, R.A., 1981. Effect of maize/legume intercropping systems and fertilizer nitrogen on crop yields and residual nitrogen. Field Crops Res., 4: 133-145. Spitters, C.J.T., 1983. An alternative approach to the analysis of mixed cropping experiments. 2. Marketable yield. Neth. J. Agric. Sci., 31: 143-155. Tanaka, A., Yamaguchi, J., Miura, S. and Tamaru, H., 1984. Comparison of fertilizer nitrogen efficiency among field crops. Soil Sci. Plant Nutr., 30: 199-208.