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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 4, Num. 4, 1996, pp. 453-462
African Crop Science Journal,
Vol. 4. No. 4, pp. 453-462, 1996

Pearl millet/cowpea cropping system yields and soil nutrient levels

M. BAGAYOKO, S.C. MASON*, S. TRAORE and K.M. ESKRIDGE

University of Nebraska, Lincoln, NE 68583-0915 U. S .A
*Corresponding Author

Joint contribution of Institut d'Economie Rurale (IER), Barnako, Mali and Department of Agronomy, University of Nebraska, Lincoln, NE 68583-0915 U.S.A. Paper No. 11718 of the Journal Series of the Nebraska Agric. Res. Div.

(Received 18 November 1996; accepted 13 December 1996)


Code Number: CS96086
Sizes of Files:
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ABSTRACT

Pearl millet [Penisetum glaucum (L.) R.Br.] and cowpea [Vigna unguiculata (L.) Walp] are important crops in traditional bush-fallow production systems in West Africa. Human population growth is forcing producers to consider alternative cropping systems and fertilizer application to meet food needs, by continuous, intercrop and rotational cropping systems on pearl millet and cowpea grain and stover yield, and maintaining soft nutrient levels. A long-term cropping system study with nitrogen applications of zero 20 and 40 kg ha^-1 was initiated at the Cinzana Research Station near Segou, Mali in 1990. The field had a leached tropical ferruginous (Ustalf) soil. Nitrogen fertilizer application increased pearl millet and stover yield linearly per year, but had no effect on cowpea. Rotation with cowpea increased pearl millet grain yield by 17 to 31% each year between 1991 and 1995, but had little effect on cowpea yield. Intercropping reduced yield of both crops every year, but the Land Equivalent Ration indicated a 14 % average increase in land use efficiency. After four years, soil of plots with the different cropping systems had similar levels of nutrients, except for phosphorous which was higher in continuous cowpea plots. Soil of plots with all cropping systems had lower level of pH, K, Ca, Mg and cation exchange capacity than fallow plots,

indicating that all cropping systems were mining soil of nutrients. This research indicates that nitrogen fertilizer application up to 40 kg/ha and crop rotation with cowpea increase pearl millet grain and stover yield.

Key Words: Crop rotation, grain and stover yield, intercropping, nitrogen

RESUME

Le mil [Penisetum glaucum (L.) R. Br.] et le niebe [Vigna unguculata (L.) Walp] sont des cultures importantes dans les systemes traditionnels en Afrique de l'Ouest. La croissance demographique pousse les Producteurs a considerer d'autres systemes alternatifs de culture et l'utilisation des engrais chimiques pour satisfaire les besoins alimentaires et maintenir le statut des elements nutritifs du sol. L'objectif de cette etude etait de determiner les effects des systemes de monoculture et de rotation sur la production de grains et de biomasse du mil et du niebe et le niveau des elements nutritifs du sol. Une etude du stem de culture de longue duree avec l'apport de zero, 20 et 40 kg/hectare a ete initiee a la Station de Recherche Agronomique de Cinzana non loin de Segou au Mali en 1990. La parcelle experimentale etait un sol du type "Sols ferrugimeux tropicaux lessives". L'application de l'engrai azote a augmente le rendement grain et de biomasse du mil de facon lineaire chaque annee, mais n'a eu aucun effet sur la production du niebe. L'association du niebe au mil a reduit le rendement des deux cultures chaque annee, mais le rapport de la surface equivalente a indique une augmentation moyenne de 14% de l'efficacite d'utilisation de la terre. Apres quatre annees de culture, les sols des parcelles portant les differents systemes de culture avaient des niveaux similaires d'elements nutritifs, a l'exception du phosphore qui etait plus eleve dans les parcelles de monoculture du niebe. Les sols des parcelles de tous les systemes de culture avaient des niveaux de pH, K, Ca, Mg et la capacite d'echange cationique plus bas que ceux de la jachere, indiquant ainsi que tous les systemes de culture etudies minaient les sol de ses elements nutritifs. Cette etude montre que l'appliation de l'azote a la dose de 40 kg/hectare et la rotation avec le niebe augmente le rendement grain et de bimasse du mil.

Mots Cles: Rotation de la culture, rendement de grain, culture mixted, nitrogene

INTRODUCTION

Pearl millet [Penisetum glaucum (L.) R. Br.] and cowpea [Vigna unguiculata (L.) Walp] are important crops grown in water- and nutrient-limiting production zones in West Africa. In the past, low human population density allowed use of bush-fallow production systems to meet both food needs and maintain soil nutrient levels. Today this practice is declining due to population growth, forcing producers to consider the use of alternate cropping systems and/or fertilizer application to meet food needs. Fertilizer use in West Africa is limited due to the high importation cost and producers lack of capital (Kadi et al., 1990). Thus, low fertilizer input systems with moderate yield levels are attractive alternatives for small farmers.

Cropping systems research indicates that rotation of cereal and legume crops will enhance grain yields (Nicon, 1978; Roder et al., 1988; Clegg and Mason, 1994; Reddy et al. 1994, Bationo et al., 1996). Crop rotation can improve soil nutrient levels through biological nitrogen fixation (Hoshikawa, 1990) and through solubilisation of unavailable P through root exudation (Gardner et al., 1981 ). However, some studies show that yield enhancement due to crop rotation may actually increase nutrient removal from the soil (Bationo et al., 1996) partially mining the soil nutrients. This is especially true with removal of both stover and grain from fields, a common practice due to the use of stover for livestock feed and fuel. Crop rotation has also been reported to enhance yield by altering soil physical properties (Kadi et al., 1990), soil microbiological properties (Stoop and Van Staverery 19 81; Ellis et al., 1992), and by reducing pest problems (Clegg and Mason, 1994).

Intercropping is widely practiced in West Africa (Roder et al., 19 88) as a mean to increase efficiency of land use through more complete utilization of solar radiation (Keating and Carbeny, 1993), water (Morris and Garrity, 1993a) and nutrients (Morris and Garrity, 1993b). In addition, intercropping can spread labour needs and reduce pest problems (Van der Pol, 1992). Intercropping effects on soil nutrient levels are largely dependent on the quantity of nutrients removed in harvested grain and stover (Dalai, 1974), and competition or shading effects on biological nitrogen fixation by legumes (Reddy et al., 1994).

Few studies have compared the effects of pearl millet and cowpea cropping systems on yield and soil nutrient levels in semi-arid West African conditions. Bationo et al. (1996) found that pearl millet-cowpea crop rotation increased yield and soil mineral nitrogen compared to continuous pearl millet, but their study did not include intercrop treatments. In their study, crop rotation had no effect on soil organic matter, and lowered pH, cation exchange capacity and base saturation as compared to continuous pearl millet. The objective of this study was to determine the effects of continuous, intercrop and rotational cropping systems on pearl millet and cowpea grain and stover yield, and on soil nutrient levels in the Segou region of Mali.

MATERIALS AND METHODS

A long-term pearl millet and cowpea cropping study was initiated in 1990 at the Cincana Agricultural Research Station near Segou, Mali. The area is characterised by 650 mm average annual rainfall, and has low organic matter, low nutrient level, acidic, sandy leached ferriginous (Ustalf) soil. The experimental site had been in native fallow for approximately 10 years, and the soils possessed great microvariability as in this region.

The experiment was conducted in a split plot design with the whole plots in randomized complete blocks with four replications. Whole plots were eight cropping systems (Table 1) and sub-plots were nitrogen application rates of zero, 20 and 40 kg ha^-1 sidedress applied to pearl millet hills at the early tillering growth stage. Sub-plot size was 6.4 x 8 m.

TABLE 1. Cropping systems sequences for pearl millet and cowpea in 1990 through 1995

---------------------------------------------------------------------------
Cropping systems                 1990          1991          1992  
--------------------------------------------------------------------------
Continuous Pearl Millet          Pearl Millet  Pearl Millet  Pearl Millet   
Continuous Cowpea                Cowpea        Cowpea        Cowpea   
Continuous Intercrop             Intercrop     Intercrop     Intercrop   
Intercrop-Pearl Millet Rotation  Pearl Millet  Intercrop     Pearl Millet   
Pearl Millet-Intercrop Rotation  Intercrop     Pearl Millet  Intercrop   
Millet-Cowpea Rotation           Cowpea        Pearl Millet  Cowpea   
Cowpea-Millet Rotation           Pearl Millet  Cowpea        Pearl Millet   
Fallow                           Fallow        Fallow        Fallow    
--------------------------------------------------------------------------
Cropping systems                 1993          1994          1995 
--------------------------------------------------------------------------
Continuous Pearl Millet          Pearl Millet  Pearl Millet  Pearl Millet   
Continuous Cowpea                Cowpea        Cowpea        Cowpea   
Continuous Intercrop             Intercrop     Intercrop     Intercrop    
Intercrop-Pearl Millet Rotation  Intercrop     Pearl Millet  Intercrop 
Pearl Millet-Intercrop Rotation  Pearl Millet  Intercrop     Pearl Millet 
Millet-Cowpea Rotation           Pearl Millet  Cowpea        Pearl Millet 
Cowpea-Millet Rotation           Cowpea        Pearl Millet  Cowpea 
Fallow                           Fallow        Fallow        Fallow
--------------------------------------------------------------------------  

The improved local pearl millet variety Toroniou Cl was planted in sole crop treatments in hills spaced 0.8 x 0.8 m apart. The indeterminate local cowpea variety Amarisho was planted in sole crop treatments in 1990, 1991, and 1994 in hills spaced 0.8 x 0.5 m apart. Due to concern about potential striga [Striga gesnerioides (Willd.) Vatke] infestation, the semi-determinante, tolerant variety Gorom-Gorom was planted in 1992, 1993 and 1995. Intercrop treatments were similar except alternate rows of pearl millet and cowpea were planted. To accommodate this, pearl millet and cowpea were planted in hills spaced 1.6 x 0.4 m, and 1.6 x 0.25 m respectively. Both pearl millet and cowpea were thinned to two plants per hill, thus giving plant populations of 31,250 plants ha^-1 pearl millet and 50,000 plants ha^-1 cowpea for all plots. Pearl millet and cowpea were planted on the same day following a 20 mm or higher rainfall event. Actual planting dates were 16 July 1991, 12 July 1992, 8 July 1993, 15 July 1994, and 18 July 1995.

All plots received a basal application of 300 kg ha^-1 Tilemsi rock phosphate (equivalent to 82 kg P2O5) in 1990 and 1993, and an annual application of 50 kg ha^-1 Tilemsi rock phosphate (equivalent to 82 kg ha^-1 P2O5) in 1990 and 1993, and an annual application of 50 kg ha-1 K2) as potassium sulphate. Weed control was done by hand hoeing. Cowpea was sprayed with deltamethrin three times at 15 day intervals starting at 50% flowering, to control thrips (Megalurothrips sjostedti), pod borers (Maruca testulalis), and Aphis craccivora.

At maturity, the number of pearl millet panicles per plot was counted, and then both grain and stover were hand harvested and weighed. The sole crop harvested area was 15.36 m^2 and the intercropped area was 16.0 m^2. Yields were recorded on a dry matter basis.

Soil samples were taken at 0-20 cm depth in 1990 prior to the establishment of the study, and again in 1994 after completing two cycles of the crop rotation treatments. The soil was analysed for pH, Bray-2 P, total nitrogen, exchangeable K, Ca, Mg, and for the cation exchange capacity at the Institut d'Economie Rurlae Soil Testing laboratory in Sotuba, Mali.

Grain stover and plant population for pearl millet and cowpea were analysed separately by year using standard analysis of variance procedures to determine individual responses. Single degree of freedom contrasts on significant effects (P<0.05) were determined to assist in understanding the cropping system and nitrogen application effects. Intercropping responses were expressed as Land Equivalent Ratios (LER) following guidelines of Mead and Willey (1980). Since LERs for the continuous intercrop and intercrop following pearl millet were not significantly different, the average LER of these two cropping systems is presented.

RESULTS AND DISCUSSION

Rainfall.

Annual rainfall at the experimental site in 1991 and 1995 was similar to the long-term average of 650 mm, while rainfall was above average in 1992 and 1994, and below average in 1993 (Table 2). The amount of rainfall across years varied by 158 mm for July and 167 mm for September, and over 32% of the annual rainfall occurred in the month of August, except in 1992 when high rainfall amounts occurred in the month of July and September and in 1995 with high rainfall in September.

TABLE 2. Monthly rainfall distribution during the growing season at Cinzana, Mali in 1991 through 1995

--------------------------------------------
Month       1991   1992   1993   1994  1995
          ----------------------------------
                           mm
--------------------------------------------
May          29     14     15     60     73 
June         57    123     25    138     22 
July        174    259     99    184    158 
August      279    155    205    274    173 
September    79    164     25    120    192 
October      22      5     11     71     18

Total       640    719    479    856    637
--------------------------------------------
Pearl millet grain and stover yields.

Pearl millet grain yield was not influenced by the Cropping System x Nitrogen interaction in any year (Table 3), while the number of heads produced m^-2 was affected by this interaction in 1992 and 1995 (data not shown), and stover yield in 1992, when the analysis was more sensitive due to lower coefficients of variation (Table 4). The lack of interaction indicated that the response to nitrogen application was similar across cropping systems. Harvest index (data not shown) was not affected by either cropping systems or nitrogen rate. Compared to pearl millet sole crop, intercropping (contrast comparison 2 + 3 vs 4 + 5) reduced grain and stover yields, and number of heads m^-2 likely due to increased interspecific competition with the cowpea intercrop, and intraspecific competition due to the altered plant arrangement to accommodate the intercrop (Tables 3 and 4). Previous crop had no effect on intercropped pearl millet grain and stover yields (contrast comparison 2 vs 3) and cowpea-pearl millet sole crop rotation (contrast comparison 1 vs 3) increased pearl millet grain and stover yields in 1993, 1994 and 1995. On a relative percentage basis, a previous cowpea crop increased pearl millet yields by 19, 17, 31,27 and 30% over continuous pearl millet in 1991, 1992, 1993, 1994 and 1995, respectively, while the previous pearl millet-cowpea intercrop increased yields by 8% or less. The effects of previous cowpea crop on pearl millet yield confirms previous reports from Niger (Reddy et al., 1994; Bationo et al. 1996), Burkina Faso (Stoop and Van Starveren, 1981), and Senegal (Nicon, 1978). All of these studies showed that rotation with a legume crop such as cowpea increases pearl millet grain yield with low use of external inputs and thus should be an attractive alternative for small farmers. However, none of these studies clearly elucidated the reason (s) for these yield increases, suggesting the need for additional research. Previous pearl millet-cowpea intercrop effects in this study were similar to the small yield increases reported by Reddy et al. (1994) with a "traditional intercrop" having a low plant population of an indeterminate cowpea, and for cowpea planted one week after pearl millet (16). Nitrogen application increased pearl millet grain and stover yields (Tables 3 and 4), and the number of heads m^-2 (data not presented) linearly in most years. This response across years with large differences in rainfall (Table 2), and the lack of interaction with cropping systems (Tables 3 and 4), suggested that application of up to 40 kg ha^-1 nitrogen fertilizer usually would increase pearl millet grain and stover yields as reported by Bationo et al. (1996).

TABLE 3. Pearl millet grain yields as influenced by cropping system and nitrogen application rate, and analysis of variance summary, 1991 through 1995

---------------------------------------------------------------------------
Cropping System (CS)                 1991    1992    1993    1994    1995 
                                                   kg ha^-1
---------------------------------------------------------------------------
Cowpea-Pearl Millet Rotation (1)     1367    2350    1439    1633    2107 
Intercrop-Pearl Millet Rotation (2)  1161    2184    1178    1317    1635 
Continuous Pearl Millet (3)          1142    2014    1099    1289    1625 
Continuous intercrop (4)              837    1547     925    1021    1391 
Pearl Millet-Intercrop Rotation (5    760    1327     792     859    1304 
Nitrogen Rate (N)

 0 kg ha^-1                           970    1699     967    1042    1494 
20 kg ha^-1                          1144    1922    1112    1218    1754
40 kg ha^-1                          1048    2032    1117    1411    1589
--------------------------------------------------------------------------- 
Mean                                 1054    1882    1085    1224    1612
---------------------------------------------------------------------------
Source of Variation    Degrees              P>F 
                     of Freedom -------------------------------------------
Replication (Block) CS   3           0.05    0.36    0.04    0.15    0.21 
                         4          <0.01   <0.01   <0.01   <0.01   <0.03
(1) vs (3)               1           0.11    0.13   <0.01    0.02    0.05
(2) vs (3)               1           0.14    0.44   <0.01    0.03    0.06
(4) vs (5)               1           0.60    0.31    0.06    0.25    0.70
(2)+(3) vs (4)+(5)       1          <0.01   <0.01   <0.01   <0.01    0.10
Error A                  12 
C.V. (%)                 30            26      15      26      34 
                          
N                        2           0.38   <0.01    0.03   <0.01    0.18 
Linear                   1           0.54   <0.01    0.01   <0.01    0.50 
Non-Linear               1           0.22    0.42    0.56    0.02    0.10 
CSxN                     8           0.25    0.13    0.71    0.25    0.43
Error B                  30              
C.V. (%)                               24      14      22      23      27
--------------------------------------------------------------------------- 

Cowpea grain and stover yields.

Cowpea grain and stover yields were not influenced by nitrogen application (Tables 5 and 6) as would be expected for a legume crop (contrast comparison 2+3 vs 4+5). In general, intercropping reduced cowpea grain and stover yields as compared to sole cropped cowpea, largely due to the shading effect of the taller pearl millet plants. Rotation with pearl millet (contrast comparison 1 vs 2) only increased cowpea grain yield in 1991 (16%) and stover yield in 1992 (54%), and actually produced lower grain yield in 1995. Coefficients of variation were high due to striga infestation and further difficulty in controlling insects, but the rotational benefit to cowpea was small in this study.

TABLE 4. Pearl millet stover yields as influenced by cropping system and nitrogen rate, and analysis of variance summary, 1991 through 1995

--------------------------------------------------------------------------
Cropping System (CS)                 1991   1992    1993    1994    1995 
                                                  kg ha^-1
                                    --------------------------------------
Cowpea-Pearl Millet Rotation (1)     5233    5603    4735    5121    6022 
Intercrop-Pearl Millet Rotation (2)  3820    5250    3620    3861    5073 
Continuos Intercrop (4)              2811    3665    2753    2870    3689 
Pearl Millet-Intercrop Rotation (5)  2577    2897    2085    2550    3445

Nitrogen Rate (N) 
 0 kg ha^-1                          3289    3960    2850    3045    3979 
20 kg ha^-1                          3943    4415    3551    3741    4704 
40 kg ha^-1                          4171    4969    3659    4327    4997
---------------------------------------------------------------------------
Mean                                 3768    4448    3353    3704    4560
---------------------------------------------------------------------------
Source of Variation    Degrees                       P>F
                     of Freedom ------------------------------------------- 
Replication (Block)       3          0.31    0.16    0.21    0.69     0.10
CS                        4          0.02   <0.01   <0.01   <0.01    <0.01
(1) vs (3)                1          0.29    0.26    0.01    0.12     0.02 
(2) vs (3)                1          0.09    0.60    0.02    0.06     0.10 
(4) vs (5)                1          0.77    0.26    0.12    0.16     0.65'

(2)+(3) vs (4)+(5)        1          0.02   <0.01   <0.01    0.01    <0.01

Error A 
C.V. (%)                              49      36      29      40        9

N                         2          0.01   <0.01   <0.01   <0.01    <0.01 
Linear                    1         <0.01   <0.01   <0.01   <0.01    <0.01 
Non-Linear                1          0.36    0.78    0.02    0.87     0.39 
CS x N                    8          0.61   <0.01    0.18    0.57     0.42 
Error B                  30 
C.V. (%)                              27      15        14    27       20
---------------------------------------------------------------------------

Intercrop efficiency.

On the average, intercropping increased the LER by 14% over the sole crops (Table 7), although considerable variation occurred across years. These LER's were lower than those reported by Reddy et al. (Reddy et al.,1994) under similar conditions. Intercropping increased LER in all years except 1995, with the largest LER occurring in the intermediate rainfall year of 1991 and the lowest in the intermediate rainfall year of 1995. Land equivalent rations for intercrops would be expected to be higher in the low rainfall year of 1993 and greatest in the high rainfall year of 1994 (Fukai and Trenbath, 1993).

Soil nutrient levels.

No significant soil nutrient level differences were present when the experiment was initiated in 1990, but high coefficients of variation indicated the great microvariability present (Table 8). In 1994, after two cycles of rotation treatments, all cropping systems had lower pH, K and Mg levels in comparison to the fallow check. Coefficients of variation were also reduced for the variability of soil nutrient levels. The basal

application of Tilemsi rock phosphate increased the available P in all plots to very high levels for West African production situations. In 1994, the available p level was lower in the intercrop treatments and highest in the continuous cowpea treatment, perhaps the result of solubilization of P in the rhizosphere of cowpea plants (Gardner et al., 1981). Total nitrogen declined in all cropping systems between 1990 and 1994, while the CEC remained constant. Continuous pearl millet and pearl millet-cowpea rotation had similar soil nutrient levels after four years of cropping, in contrast to the earlier results of Bationo et al. (1996) who found that rotation lowered soil pH, cation exchange capacity, and base saturation. However, continuous pearl millet, pearl millet-cowpea rotation and pearl millet-cowpea intercrops had lower soil pH, K, Ca, Mg and cation exchange capacity than the fallow treatment, suggesting that all the cropping systems studied were mining the soil of nutrients as also reported on a regional basis by Van der Pol (1992).

TABLE 5. Cowpea grain yields as influenced by cropping system and nitrogen application rate, and analysis of variance summary, 1991 through 1995

    
--------------------------------------------------------------------------- 
Cropping System (CS)                 1991    1992    1993    1994    1995
                                                   kg ha^-1 

--------------------------------------------------------------------------- Pearl Millet-Cowpea Rotation (1) 1949 939 388 105 994 Continuous Cowpea (2) 1680 334 234 128 201 Pearl Millet-Intercrop Rotation (3) 1105 868 464 319 1241 Continuous Intercrop (4) 1133 345 49 128 196 Nitrogen Rate (N) 0 kg ha^-1 1433 645 264 106 613 20 kg ha^-1 1518 684 331 137 679 40 kg ha^-1 1450 611 257 155 682 --------------------------------------------------------------------------- Mean 1467 647 284 133 658 --------------------------------------------------------------------------- Source of Variation Degrees P>F of Freedom ----------------------------------------- Replication (Block) 3 0.29 0.94 0.31 0.50 0.20 CS 3 <0.01 <0.01 <0.01 0.47 <0.01 (1) vs (2) 1 0.05 0.63 0.22 0.24 0.02 (1)+(2) vs (3)+(4) 1 <0.01 <0.01 <0.01 0.38 <0.01 (3) vs (4) 1 0.84 0.55 0.01 0.60 0.95 Error A 9 c.v. (%) 20 53 50 100 30 N 2 0.50 0.52 0.41 0.27 0.19 CS x N 6 0.87 0.61 0.57 0.33 0.04 Error B 24 C.V. (%) 14 28 59 64 17 ---------------------------------------------------------------------------

TABLE 6. Cowpea stover yields as influenced by cropping system and nitrogen application rate, and analysis of variance summary, 1991 through 1995

---------------------------------------------------------------------------
Cropping System (CS)                 1991    1992    1993    1994    1995
                                                   kg ha^-1
---------------------------------------------------------------------------
Pearl Millet-Cowpea Rotation (1)     1803    1429     816     422     742 
Continuos Cowpea (2)                 2069     925     823     549     944 
Pearl Millet-Intercrop Rotation (3)  1190     666     483     635     384 
Continuos Intercrop (4)              1033     425     263     358     456
Nitrogen Rate (N) 
 0 kg ha^-1                          1455     760     496     493     630 
20 kg ha^-1                          1540     968     678     504     605 
40 kg ha^-1                          1576     856     615     477     659
---------------------------------------------------------------------------

Mean                                 1524     861     596     491     632
---------------------------------------------------------------------------
Source of variation     Degrees                      P>F 
                      of Freedom     
Replication (Block)       3          0.07    0.61    0.04    0.76    0.42   

CS                        3         <0.01   <0.01   <0.01    0.61   <0.01   

(1) vs (2)                1          0.14   <0.01    0.95    0.58    0.08   
(1)+(2) vs (3)+(4)        1         <0.01   <0.01   <0.01    0.94   <0.01  

(3) vs (4)                1          0.36    0.13    0.08    0.24    0.50 
                                     
Error A                   9          
                                     
C.V. (%)                 26           41      46     100      40 
N                         2          0.55    0.14    0.06    0.99    0.57 
CS x N                    6          0.54    0.72    0.72    0.47    0.05 
Error B                              24 

C.V. (%)                              21      33       35     99      23    
--------------------------------------------------------------------------- 

TABLE 7. Average land equivalent ratios (LER) and partial LERs for pearl millet-cowpea intercrop, 1991 through 1995

-------------------------------------------------------------
                1991    1992    1993    1994    1995    Mean 
-------------------------------------------------------------
Pearl Millet    0.70    0.72    0.78    0.73    0.83    0.75 
Cowpea          0.67    0.39    0.31    0.40    0.16    0.39 
Total           1.37    1.11    1.09    1.13    0.99    1.14
-------------------------------------------------------------

TABLE 8. The influence of pearl millet-cowpea cropping systems on soil properties in 1994 as compared to the initial conditions in 1990

---------------------------------------------------------------------------
Cropping System        pH (Water)     pH (KCL)     P (Bray 2)     Total N   
                       -----------   -----------   -----------   ---------- 
                       1990   1994   1990   1994   1990   1994   1990  1994 
                                                ppm                         
--------------------------------------------------------------------------- 
Pearl Millet-Cowpea 
     Rotation #         5.0    5.2    4.3    4.4     10    36    2.3   0.8 
Pearl Millet-Intercrop 
    Rotation#           5.0    5.2    4.3    4.3     10    24    2.0   0.8  
Continuous Pearl Millet 5.2    5.2    4.5    4.2     11    94    2.8   0.6 
Continuous Intercrop    5.1    5.2    4.4    9       23    3.4   0.8   0.21 
Continuous Cowpea       5.0    5.4    4.3    4.4     12    39    3.4   0.8  
Fallow                  5.1    5.5    4.3    4.7      9    34    1.9   1.0  
L.S.D. (0.05)            NS    0.18    NS    0.25    NS    9.1    NS  0.004 
P>F                    0.98    0.04   0.97   0.02   0.15   0.02  0.74  0.31 
C.V. (%)                  3    0.5     10     6     100     35    100   86  
---------------------------------------------------------------------------
Cropping System               K             Ca           Mg         CEC
                          ----------   ----------   ----------   ----------
                          1990  1994   1990  1994   1990  1994   1990  1994 
                                              cmol/kg
---------------------------------------------------------------------------
Pearl Millet-Cowpea 
    Rotation #           0.22   0.11   0.53  0.53   0.24  0.16   2.3    1.7
Pearl Millet-Intercrop                                           
    Rotation #           0.20   0.10   0.94  0.60   0.25  0.15   2.2    1.9
Continuous Pearl Millet  0.25   0.12   0.52  0.55   0.24  0.15   2.1    1.9
Continuous Intercrop     0.10   0.53   0.62  0.18   2.1   1.8    
Continuous Cowpea        0.21   0.14   0.43  0.54   0.54  0.17   2.0    1.9
Fallow                   0.20   0.30   0.65  0.83   0.83  0.28   1.9    2.1
L.S.D. (0.05)             NS    0.06    NS   0.323   NS   0.048  NS   0.228
P>F                      0.77  <0.01   0.94  0.42   0.42  <0.01  0.61  0.11
C.V. (%)                  14     47     97    61     44    32     35    14

# Average of two phases of the rotation
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The cowpea-pearl millet rotation system produced approximately 25 % greater pearl millet grain and stover yields than the continuous pearl millet system during 1991 to 1995 (Tables 3 and 4) with similar soil nutrient levels (Table 8). This suggests that the yield enhancement due to crop rotation in comparison to continuous pearl millet was roughly balanced by increased nutrient availability and/or increased ability to take up nutrients. Since soil P levels were high in this study, and the apparent lack of differential response to nitrogen application among cropping systems, we speculate that the basis for rotational enhancement of pearl millet grain and stover yield in this study was not due to nutrient availability. Research to better understand the basis of the rotational enhancement of yield in Mali merits further attention.

CONCLUSION

Nitrogen fertilizer application increased pearl millet grain and stover yields linearly up to the highest rate of 40 kg ha^-1. Rotation with cowpea enhanced grain and stover yield of pearl millet, although the basis for this yield increase was not clear and likely not due to increased nutrient availability. Rotation with pearl millet had little effect on cowpea grain and stover yield. Intercropping reduced the grain and stover yields of both crops likely due to inter-and intraspecific competition. However, the system productivity measured by LER was greater for the intercropping system than the sole crops. After 4 years of the different cropping systems, soil nutrient levels declined and were similar except for P which was very high due to fertilizer application. All cropping systems had lower pH, K, Ca, Mg and cation exchange capacity than the fallow, indicating that all the cropping systems were mining the soil of nutrients. The basis of cropping system yield response and the consequent effects on soil nutrient levels merit further study. The research indicates that nitrogen fertilizer application up to 40 kg ha^-1 and crop rotation with cowpea are ways to increase grain and stover yield of pearl millet.

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Copyright 1996 The African Crop Science Society

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