Grain Legumes Contribute Immediate and Residual-Effects in Rain-fed Maize Production Systems 

By Esther Mugi-Ngenga and Shamie Zingore 

Within-season effects of intercropping are receiving increasing attention. However, the rotational performance is rarely investigated across seasons, even though the two are complementary strategies providing spatial-temporal crop diversification. 

Smallholder crop production systems in much of the East African highlands are dominated by maize (Zea mays L.) which is commonly intercropped with grain legumes, mainly pigeonpea (Cajanus cajan (L.) Millsp.), common bean (Phaseolus vulgaris L.), dolichos lablab (Lablab purpureus (L.) Sweet) and cowpea (Vigna unguiculata (L.) Walp) (Mugi-Ngenga et al., 2021). Beyond direct yield benefits, grain legumes can provide additional ecological benefits that may enhance the productivity of maize in the short and long-term. Within-season benefits of cereal-legume intercropping include greater ground cover and suppression of diseases and pests. Residual benefits that accrue in subsequent seasons include the supply of nitrogen (N) from N2- fixation, improved soil health, weed (Striga) suppression and increased maize yields (Giller, 2001; Rusinamhodzi et al., 2012). 

Productivity of intercrops depends on the balance between intra- and inter-specific competition. When the component crop species have complementary growth patterns (i.e., filling different temporal niches by utilizing different periods of the season, or spatial niches through different rooting depths or canopy sizes), inter-specific competition will tend to be weaker than intra-specific competition, and resources will be acquired more efficiently (Lithourgidis et al., 2011). This results in relatively greater yields in intercrops than in sole crops (Willey, 1979). 

Within-season effects of intercropping are receiving increased attention, but the rotational performance is rarely investigated across seasons. Further, farmer management practices are critical factors that influence productivity of maize-grain legume intercrops, and a better understanding of their influence is particularly pertinent in northern Tanzania, due to the wide diversity of climatic conditions and soils. This article outlines a recent study aimed at evaluating the growth and development of maize-pigeonpea and maize-lablab intercropping systems and their interaction with fertilizer, and the assessment of their residual effects on the yields of a succeeding maize crop. 

Field trial description 
The study was conducted in Babati district, Northern Tanzania. Trials were conducted for three consecutive cropping seasons (2017/2018-2019/2020), on nine farms. Each farm acted as a replicate (one farm-one replicate design). In each of the selected farms, plots measuring 10 × 5 m were delineated at planting. Paths measuring 1 m wide were left in between plots. Test crops included maize Seed Co. 513 hybrid variety, dolichos lablab “Selian-Rongai” variety and pigeonpea long (ICEAP 00040) and medium-duration (ICEAP 00557) varieties. Sole maize, pigeonpea and lablab were planted at a spacing of 0.90 m × 0.50 m inter- and intra-row, respectively. Cereal legume intercrops followed an additive design, with legumes planted in the maize rows, in-between maize hills. 

Maize intercropped with pigeonpea (left) and lablab (right) grain legumes at on-farm trial, in Babati district, northern Tanzania.

Three seeds were planted per hill for both maize and legumes, which were thinned to 2 seedlings post emergence to achieve the target planting density of approximately 44,444 plants ha-1 for each sole and intercrop. Pigeonpea were planted simultaneously with maize, whilst lablab was relay-planted one month later. 

Fertilizer was spot applied in planting holes at three levels: (i) no fertilizer, (ii) P fertilizer only and (iii) N+P fertilizer. Sole legumes did not receive the N+P fertilizer. The P fertilizer was applied at planting in the form of triple superphosphate (TSP) at the rate of 40 kg P ha-1 to both maize and legumes. The N fertilizer was spot-applied in the form of urea at the rate of 90 kg N ha-1 in three equal splits only on maize: one third at planting, one third at four weeks after planting, and one third at eight weeks after planting. The combination of cropping system (various sole and intercrops) and fertilizer (control, +P and +NP) were randomly assigned within each farm. Individual plots were maintained, and treatments (cropping systems and fertilizer) allocated to the same plots in the 2017/2018 and 2018/2019 seasons. Maize was harvested 3-4 months before the legumes. 

To evaluate the residual benefits of the grain legumes on the yield of a succeeding maize crop, a third season (2019/2020) was included, where a sole maize crop was planted on all plots following a spacing similar to the sole crops of the previous seasons. No fertilizer was applied to the maize crop in the third season. 

At physiological maturity of each crop, all plants within the net plot (9 m2) were harvested. Maize cobs were manually separated from the stover and hand threshed. Legumes were also threshed manually to separate grains and haulms. After threshing, total fresh weight of maize and legume grains were separately taken in the field. Moisture content of the grains (%) was determined using a moisture meter, and grain yields corrected to 12.5% and 13% moisture content for maize and legumes, respectively. 

Impacts of cropping system and fertilizer on maize and legume productivity 
In most cases, maize growth was not affected by the presence of legumes as it produced similar grain yield in sole and intercrops for both seasons (Fig. 1). This is consistent with previous research on maize-pigeonpea systems showing insignificant effects of pigeonpea on maize (Rusinamhodzi et al., 2012). This is attributed to the fact that the growth duration of pigeonpea was 3-4 months longer than that of maize. Consequently, the greatest demand for water and nutrients in pigeonpea occurred after maize was harvested, following Dalal (1974), which is a form of temporal niche differentiation (TND). Relay-planting of lablab one month after maize planting allowed the maize crop to establish well before the closure of the lablab canopy. 

Figure 1A – F. Maize grain yield in various cropping systems as affected by fertilizer from on-farm trials during the 2017/2018 (A-C) and 2018/2019 (D-F) seasons in Babati, Northern Tanzania. MZ= maize; ldP= long-duration pigeonpea; mdP= medium-duration pigeonpea; LB= lablab. Error bars indicate the standard error of means. Mean differences of fertilizer treatments at 5% significance level in the various box plots are indicated with different small letters on the upper side of the box plot.

Further, a significant main effect of fertilizer was found for maize grain in both seasons (Fig. 1). Addition of NP fertilizer significantly enhanced maize grain yield compared with the control and/or +P plots. In the 2017/2018 season, +NP plots produced more maize grain yield than +P (+0.8 t ha-1) and control plots (+1.2 t ha-1). In the 2018/2019 season, maize production in control plots was significantly less than +P (-0.7 t ha-1) and +NP plots (-0.9 t ha-1). The increase in maize yields in response to direct N fertilization indicates that although integration of legumes contributes N through atmospheric N2-fixation, it clearly did not contribute enough to preclude the need for applying N fertilizer to maize (cf. Jeranyama et al., 2000; Giller, 2001). 

Contrary to maize, which was hardly affected by presence of a legume, the productivity of legumes was affected by the presence of maize. Sole legumes produced significantly greater yield than in the corresponding intercrops (up to 0.6 t ha-1) under almost all cases in the 2017/ 2018 season (Fig. 2A) and consistently in the 2018/ 2019 season (Fig. 2B). The smaller yields of legumes in intercrops than sole crops can partly be attributed to the reduced radiation reaching the lower part of the intercrop canopy occupied by the legumes before maize harvest. Indeed, legumes (e.g., lablab) have a high demand for light (Cook et al., 2005).

Figure 2A – D. Legume grain yields as affected by cropping system and fertilizer from on-farm trials during the 2017/2018 (A&C) and 2018/2019 (B&D) seasons in Babati, Northern Tanzania. ldP= long-duration pigeonpea; mdP= medium-duration pigeonpea; LB= lablab, inter= intercropped. Mean differences at 5% significance level in the various box plots are indicated with different small letters on the upper side. Error bars indicate the standard error of means.

Further, significant fertilizer effects were observed in both seasons (Fig. 2C-D). Plots with +NP fertilizer produced significantly smaller legume grain yields (up to 0.4 t ha-1) than +P plots. We attribute this to the increased maize growth with application of N fertilizer, resulting in stronger competition with the legumes. In relation to this, application of N fertilizer in cereal-legume intercrops has been reported to increase the competitiveness of cereals, very likely leading to a competitive imbalance and a failure of legumes in mixtures (Yu et al., 2016). 

Residual effects of two seasons of legumes on productivity of a succeeding maize crop 
In the third season of experimentation a significant main effect of cropping system was found for grain yield of the succeeding maize crop (Fig. 3A). Grain yield following two seasons of continuous maize was smallest and significantly less (0.8-1.9 t ha-1 less) than in all other systems (Fig. 3A). The greater grain yield in plots where a legume was included during the preceding seasons can be attributed to benefits associated with both residual N and non-N effects (Franke et al., 2018). Since we did not retain maize and legume stover in the field, we attribute any residual N effects to the decomposition of the legumes’ roots, nodules, and fallen leaves (Ledgard and Giller, 1995). 

Figure 3. Grain yield of a succeeding maize crop as affected by preceding cropping system (A) and fertilizer (B) from on-farm trials during the 2019/ 2020 season in Babati, Northern Tanzania. MZ= maize; LB= lablab; ldP= long-duration pigeonpea; mdP= medium-duration pigeonpea. Mean differences at 5% significance level in the various box plots are indicated with different small letters on the upper side. Error bars indicate the standard error of means.

Further, a significant main effect of fertilizer was found for grain yield of the succeeding maize crop. Plots that had no fertilizer applied (control plots) in the preceding seasons yielded significantly less grain yield in the succeeding maize crop than where fertilizer was applied (0.9 t ha-1 less) (Fig. 3B). In the control plots, the lack of fertilizer addition for three consecutive seasons could have led to soil nutrient depletion, thus reducing productivity of maize crop planted in the third season. No difference was observed between the +P and +NP fertilizer, indicating that there was no residual effect of N fertilizer from the previous seasons.

Maize-legume intercropping systems were superior to sole maize crops, not only for the additional grain yield from legumes, but also due to their residual effect which resulted in greater productivity of the succeeding maize crop. Significant residual benefits on maize grain yields were observed after two consecutive seasons of sole and intercropped legume crops. This implies that inclusion of legumes in maize-based cropping systems presents potential advantages especially under low input systems, and in the current environment of fertilizer shortages and high prices. Additionally, P-fertilizer applied in the previous seasons also showed strong residual benefits. This is an indication that residual benefits of maize-legume intercropping are amplified by nutrient application. Assessment of the non-N effects of grain legumes to the associated or succeeding cereal crop, which was not covered in the current study, is highly recommended. Overall, our results confirm that when the component crop species in an intercrop have complementary growth patterns, temporal and spatial diversification provides a plausible pathway for ecological intensification of smallholders cropping systems. 

Dr. Mugi is a Post-Doctoral Researcher, African Plant Nutrition Institute (APNI), Nairobi, Kenya. e-mail: e.mugi@apni.net. Dr. Zingore is Director Research & Development, APNI, Benguérir, Morocco. 

The article is adapted from Mugi-Ngenga et al. 2022. Immediate and residual-effects of sole and intercropped grain legumes in maize production systems under rain-fed conditions of Northern Tanzania. Field Crop Res. 287, 108656 https://doi.org/10.1016/j.fcr.2022.108656. Funding for this work was provided by the African Plant Nutrition Institute and the United States Agency for International Development (USAID) through the Feed the Future Innovation Lab for Sustainable Intensification. 

Cite this article 
Mugi-Ngenga, E., Zingore, S. 2022. Grain Legumes Contribute Immediate and Residual Effects in Rain-fed Maize Production Systems. Growing Africa 1 (2) 27-31. https://doi.org/10.55693/ga12.BSVI1567 

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