African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 8, Num. 4, 2000, pp. 411-418

African Crop Science Journal, Vol. 8. No. 4, pp. 411-418

EFFECT OF CROP RESIDUE MANAGEMENT AND CROPPING SYSTEM ON PEARL MILLET AND COWPEA YIELD

Adama Coulibaly, Minamba Bagayoko, Samba Traore and S.C. Mason¹

Institut d’Economie Rurale, Bamako, Mali
¹University of Nebraska, Lincoln, NE 68583-0915 U.S.A.

(Received 6 January, 1999; accepted 30 July, 2000)

Code Number: CS00043

INTRODUCTION

Pearl millet [Pennisetum glaucum (L.) Br.] is the most common crop produced and consumed in the Sudano-Sahelian zones of Mali. Over 1.7 million hactares are produced annually in Mali with low grain yield levels of 450 to 735 kg ha^-1. Despite increasing population growth the area under millet production is expanding with shorter traditional fallow periods, stagnant crop yields, and the soil nutrient status is declining (Bagayoko et al., 1996; van der Pol, 1992).

Leaving crop residues in the field and using a pearl millet-legume rotational system have been suggested as ways to simultaneously increase yields and replenish soil nutrient levels.

Crop residue application to fields in West Africa has resulted in increased pearl millet grain yields (Muehlig-Versen et al., 1997; Marschner et al., 1995; Bationo and Mokwunye, 1991, Verma et al., 1992), reduced crusting, enhanced seedling growth and improved N, P, and K nutrition of seedlings (Muehlig-Verson et al., 1997), reduced wind erosion losses and improved crop stands (Michels et al., 1995a; 1995b), and enhanced root growth (Bababe, 1997; Hafner et al., 1993; Kretzschmar et al, 1991; Rebfaka et al., 1994). Crop residues trapped wind-blown dust with high nutrient levels (Bationo et al., 1993; ), and higher soil pH and lower Al and Mn levels (Bationo et al., 1993; Hafner et al., 1993; Kretzschmar et al., 1991). In addition, crop residue application increased N fixation and dry matter of groundnut (Arachis hypogaea L.)(Rebafka et al., 1993), but legumes have generally been affected less than cereal crops (Buerkert et al., 1997). Although retaining crop residues on the field reduces the export of nutrients (Bationo et al., 1993), a net loss of nutrients commonly occurs (Buerkert, 1995). Crop response to residue application is greater in the lower rainfall Sahelian zone than in the Sudanian zone (Buerkert et al., 1997), and is greater on unfertile degraded soils than on more fertile soils (Buerkert, 1995). Crop residue application is commonly used by producers to regenerate soils with wind and water-eroded surfaces (Taylor-Powell et al., 1991).

Crop residues are used for many other purposes in West Africa, reducing the amount of residues available for soil replenishment. Lamers and Bruentrup (1996) studied the use of crop residue and found that the highest gross marginal returns for land was mulching with crop residues, but the highest gross marginal returns for total labour was using residues for livestock feeding and for weeding labour was burning residues. Linear programing indicated that the multiple uses of crop residues for soil improvement, livestock feeding and weed control, as already practiced, was an economically sound approach for crop residue utilisation.

Cropping systems research in West Africa indicates that crop rotation of pearl millet with cowpea [Vigna unguiculata (L.) Walp] or groundnut enhances pearl millet grain yields (Bagayoko et al., 1996; Nicou, 1978; Bationo et al., 1996; Reddy et al., 1994). Buerkert et al. (1997) indicated that yield increases from rotation cropping systems with cowpea varied from 4 to 37% and was site specific. Bagayoko et al. (1996) reported varied grain yield responses across years from 17 to 31% at one site in Mali, and that the pearl millet-cowpea rotation and continuous pearl millet both removed soil nutrients at rates greater than they were being naturally replenished.

The objective of this study was to investigate the influence of two cropping systems and three crop residue management practices on the grain and stover yields of pearl millet and cowpea. In addition, the paper also discusses changes in soil nutrient status resulting from the experimental treatments used in the study.

MATERIALS AND METHODS

A long-term pearl millet crop residue management and cropping system study was initiated in 1990 at the Cinzana Agricultural Research Station near Segou, Mali and was conducted over a period of 7 years. The area is characterised by an average annual rainfall of 650 mm and has low soil organic matter and nutrient levels on an acidic, sandy, leached ferriginous (Paleustalf) soil. The experimental site had been kept under fallow for approximately 10 years, and we assumed that the soils possessed great microvariability for N, P, K, Ca, Mg and cation exchange capacity as is commonly found in the region (Bagayoko et al., 1996).

The experiment was conducted in a randomised complete block design with four replicates. The treatments consisted of a factorial combination of three crop residue management treatments and two cropping systems on fixed plots over time. Crop residue management treatments included total removal of residues by hand gathering, incorporation of pearl millet residues produced the previous year using animal traction, and retention of pearl millet residues produced the previous year on the soil surface. Cowpea residues were constantly removed from the plots by hand gathering. The cropping system treatments were continuous pearl millet, continuous cowpea, and pearl millet-cowpea rotation. Only one phase of the rotation was included in the study; thus, in the crop rotation treatment pearl millet was present in even-numbered years and cowpea in odd-numbered years.

The local pearl millet variety ‘Boboni’ and the indeterminate cowpea variety ‘Suvita 2’ were used throughout the duration of the experiment. Pearl millet was planted in hills spaced 0.8 x 0.8 m apart, and thinned to two plants per hill two weeks after emergence giving a seedling population of 31,250 plants ha^-1. Cowpea was planted in hills at a spacing of 0.8 x 0.5 m and thinned to two plants per hill giving a plant population of 50,000 plants ha^-1. Pearl millet and cowpea were planted on the same day for each year planting was done in July.

All plots received a basal broadcast application of 300 kg ha^-1 Tilemsi rock phosphate (equivalent to 36 kg ha^-1 P) prior to planting in 1990, 1993 and 1996. Rock phosphate particle size distribution was 20% by weight less than 0.5 mm diameter, and 85% less than 3 mm. Annual application of 21 kg ha^-1 N as urea was done as a side-dress applied near the pearl millet hills at the early tillering growth stage. Soil samples were taken at 0-20 cm depth in 1990 prior to the establishment of the study, and again before planting in 1996 when three cycles of the crop rotation were completed. The soil samples were analysed in 1990 and 1996 for pH, Bray-2 P, organic carbon, cation exchange capacity (CEC), and exchangeable K, Ca and Mg at the Institut d’Economie Rurale Soil Testing Laboratory in Sotuba, Mali. The same soil analysis methods were used in both years.

Plots consisted of 6 rows, 8 m long (38.4 m² ) and the center 5 m of the middle four rows (16 m²) was harvested for grain and stover. Yields were reported on a dry matter basis. Grain and stover yields were analysed separately for each year and across years using analysis of variance (ANOVA). Where the F statistics indicate significance between treatments, mean separations were done using the LSD test at the 5% level of significance.

RESULTS AND DISCUSSION

Rainfall. Annual rainfall at the experimental site in 1991, 1995, 1996 and 1997 was similar to the long-term average of 650 mm (Table 1). Seasonal rainfall was below average in 1993, and above average in 1992 and 1994. Rainfall distribution was variable across the growing season with June rainfall ranging from 22 to 138 mm, July from 99 to 259 mm, August from 155 to 279 mm, and September from 22 to 195 mm. August rainfall was the most uniform, and over 32% of the annual rainfall occurred in August, except in 1992 when high rainfall amounts occurred in July and September, and in 1995 and 1997 with the highest rainfall received in September.

Residue management. Crop residue management treatments had no influence on pearl millet grain or stover yields from 1991 to 1997 (Tables 2 and 3), except for grain yield in 1993 when plots with incorporated crop residues yielded more than plots with residues retained on the soil surface. This year had the lowest seasonal rainfall of the seven years of the study (Table 1). However, crop residue incorporation consistently increased grain and stover yields (Tables 2 and 3). The seven-year mean pearl millet grain yield for plots with incorporated crop residues was 180 kg^-1 ha^-1 year^-1 (12%) greater than plots with residues removed (P = 0.07). The mean pearl millet stover yield in plots with crop residue incorporated was 211 to 247 kg^-1 ha^-1 year^-1 (15 to 18%) greater than plots with residues removed or retained on the surface (P=0.19). Although the short-term benefit of retaining crop residues in the field for this non-degraded soil was small, as reported by Buerkert et al. (1997), the long-term benefits of crop residue incorporation appeared to be important. In this study, the beneficial effect on pearl millet yield of retaining crop residues in the field were optimised by residue incorporation treatments. This is in agreement with the report of Bababe (1997).

Crop residue management treatments did not over the years, influence cowpea grain and stover yields (Table 4). However, the mean yields over 1991, 1993, 1995 and 1997 indicated that, relative to residue removal, surface retention of residues increased cowpea grain yields by 240 kg^-1 ha^-1 year^-1 (25%) and residue incorportion increased grain yield by 186 kg^-1 ha^-1 year^-1 (19%). Cowpea stover yield over the years increased by an average of 316 kg^-1 ha^-1 year^-1 (41%) due surface retention of crop residues, and by 350 kg^-1 ha^-1 year^-1 with residue incorporation, as previously found by Rebafka et al. (1993) for groundnut. In this study, the grain and stover yields increases from retaining crop residues in the field were slightly greater for cowpea than for pearl millet, contrasting with results of Buerkert et al. (1997).

Cropping systems. Rotation of cowpea with pearl millet increased pearl millet grain yields in 1996, as well as mean yields over 1992, 1994 and 1996 (Table 5). Over the years, pearl millet grain yields increased by an average of 228 kg^-1 ha^-1 year^-1 (15%) and stover yields increased by 628 kg^-1 ha^-1 year^-1 (19%) in response to rotation with cowpea. This is less than the 17 to 31% yield increase reported by Bagayoko et al. (1996), but well within the range of the 4 to 37% yield increase reported by Buerkert et al. (1997). Cowpea grain and stover yields were not significantly (P=0.05) increased by rotation with pearl millet (data not presented) similar to results found by Bagayoko et al. (1996).

Soil nutrient levels. Regardless of crop residue treatment, all cropping systems over the six-year period increased soil pH, carbon and phosphorus concentration, and decreased potassium and cation exchange capacity (Table 6). Phosphorus concentration increase was certainly due to basal rock phosphate application in 1990 (after soil sampling) and in 1993 prior to planting. The 1996 rock phosphate application was applied after the soil samples were collected. Crop residue treatments had little influence on soil pH, carbon and cation exchange capacity after six years. However, leaving crop residues on the soil surface resulted in higher soil P concentrations than other residue treatments, possibly due to entrapment of wind blown dust with higher phosphorus concentrations previously observed by Bationo et al. (1993). Both treatments of incorporating and retaining crop residues on the surface of plots resulted in higher soil K concentrations than for crop residue removal treatment.

CONCLUSIONS

Crop residue management had apparent short-term effects on the productivity of pearl millet and cowpea. Incorporation of crop residues increased pearl millet grain and stover yields over seven years of production. Also both incorporation or surface retention of residues on the surface increased cowpea yields over the same time period. Crop residues retained on the surface helped maintain soil P and K concentrations, suggesting that this treatment reduced the rate of soil degradation. These results suggest leaving crop residues in the production fields is an important practice that would help reduce nutrient mining and thus ensure sustainable production of these two important crops. Producers need to weigh the long-term benefits to grain and stover production, and soil maintenance against the short-term economic benefits associated with burning crop residue, using crop residues for livestock feed or construction, and other alternate uses.

ACKNOWLEDGEMENTS

Contribution of the IER (Institut d’Economie Rurale), B.P. 258, Bamako, Mali and Dept. of Agronomy, University of Nebraska, Lincoln, NE 68583-0915 U.S.A. Paper No. 12490 of the Journal Series of the Nebraska Agricultural Research Division are highly appreciated. This research was supported by USAID Grant No. DAN 1254-G-0021 through INTSORMIL, the International Sorghum and Millet Collaborative Research Program.

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