Intercropping desmodium and maize improves nitrogen and phosphorus availability and performance of maize in Kenya

Highlights

• Desmodium increased available N and P compared to maize monocrop and other legumes.

• Desmodium increased maize shoot weight compared to maize monocrop and other legumes.

• Desmodium increased maize grain yield compared to maize monocrop and other legumes.

• The biomass of desmodium did not compromise maize grain yield.

Abstract

Low levels of nitrogen (N) and phosphorus (P) in soils in sub-Saharan Africa (SSA) limit maize (Zea mays) production in smallholder farms. We tested the hypothesis that desmodium (Desmodium intortum (Mill.) Urb.), a fodder legume, and food legumes commonly grown with maize in Western Kenya avail N and P and improve the performance of maize in intercrops. We measured available N and P in 15 cm topsoil at 4, 8 and 12 weeks after planting (WAP), and corresponding maize shoot weight in three seasons: 2017 long rains (LR), 2017 short rains (SR), and 2018 LR at Thomas Odhiambo Campus of icipe at Mbita Point in western Kenya. Treatments were additive 1:1 maize-legume intercrops; maize-bean (Phaseolus vulgaris L.), maize-cowpea (Vigna unguiculata (L.) Walp.), maize-crotalaria (Crotalaria ochroleuca G. Don), maize-desmodium, maize-green gram [Vigna radiata (L.) Wilczek] and maize-groundnut (Arachis hypogaea L.), and maize monocrop as a control, replicated four times in a complete randomized design. This experiment was established in 2003 and treatments were repeated every season on the same plot (for 31 seasons up to 2018 LR). At the beginning (year 2003), experimental plots were infested with 100 g of striga seeds per 250 g soil. Across the three last seasons corresponding to this study, we observed an increase in available N in maize-desmodium plots compared to maize monocrop plots. Available P measured at 4, 8 and 12 WAP in 2017 SR and at 4 WAP in 2018 LR was higher in maize-desmodium than in maize monocrop at the same sampling times (P < 0.05). The shoot weight for maize grown with desmodium was consistently higher than that for maize grown in monocrop. Maize grain yield for maize-desmodium was 5.0, 3.1 and 4.3 Mg ha⁻¹ in 2017 LR, 2017 SR and 2018 LR, respectively, compared to 0.5, 0.4 and 1.8 Mg ha⁻¹ for maize monocrop in the three respective seasons. Other intercrops were comparable with maize monocrop. Historic maize yields were constantly higher in maize-desmodium than maize monocrop and other maize-legume intercrops. We show that desmodium improves N and P availability, growth and yield of maize. We recommend studies on biological N fixation and changes in rhizosphere to understand mechanisms behind observed improvement in N and P availability.

Introduction

Maize (Zea mays) is a staple crop in sub-Saharan Africa (SSA) where it accounts for up to 30.0 % of the calorie intake of the population (Nuss and Tanumihardjo, 2010). Maize is also the most widely cultivated crop in the region, grown in 46 out of 53 countries in SSA (Badu-Apraku and Fakorede, 2017). Even though impressive gains have been achieved in other regions (e.g., 2.3-3.2 Mg ha⁻¹ in South and Southeast Asia, 2.9-11.8 Mg ha⁻¹ in Europe and 2.5 Mg ha⁻¹ in South America), the average yields in SSA are still below 2.0 Mg ha⁻¹ (Schils et al., 2018; Abate et al., 2017; Grassini et al., 2013; Wood et al., 2004), and projected to decline in many countries (Iizumi et al., 2014; Ray et al., 2013). Hundreds of locally adapted varieties with better yields have been developed (Abate et al., 2017). These include early and extra-early maturing, those resistant or tolerant to drought, striga, or those that use nutrients efficiently (Badu-Apraku and Fakorede, 2017). Despite these efforts and notable increase in area under maize, its production is still very low. For example, data from FAOSTAT (2020) indicate a dramatic increase in harvested area under maize in the last decade (2007 and 2017). During this period, the area on which maize is grown in SSA increased by 30.2 % while yield increased by 8.5 % in the same period (FAOSTAT, 2020). This highlights the need for innovative cropping systems to overcome constraints to maize production.

Nutrient limitation is a major constraint in crop production in SSA. In western Kenya, nitrogen (N) and phosphorus (P) are the major nutrients that limit maize productivity (Kihara and Njoroge, 2013; Boddey et al., 1997). This is attributed to continuous cropping without replenishing the depleted nutrients. In soils where these nutrients are adequate, maize develop robust roots and strong stalks, mature early and uniformly, and produce large quantities of grain. Generally, N limits growth of maize in all cases where P deficiency is overcome (Nziguheba et al., 2016); phosphorous deficiency is easily overcome by use of fertilizers because it is relatively stable and moves less in the soil. However, use of fertilizers is limited by high cost and limited access (Nziguheba et al., 2016). Farmers who cannot apply fertilizers therefore rely on residues and manure application to provide N and P for plant nutrition (Bouwman et al., 2009). A limitation to this approach is that crop residues and livestock manure have low P and N content (Nziguheba et al., 2016; Sharma et al., 2013) and hardly sustain maize production above 1.0 Mg ha⁻¹ (Peoples et al., 1995). Innovative ways of managing N and P in cropping systems is required to maintain productivity in maize based systems.

Intercropping maize with legumes is one of the options used by smallholder farmers to overcome constraints associated with maize monoculture. Intercropping maize and legumes is a common practice in western Kenya where famers plant maize in combination with common bean (Phaseolus vulgaris L.), cowpea (Vigna unguiculata (L.) Walp.), soybean (Glycine max (L.) Merr.), pigeon pea (Cajanus cajan (L.) Millsp.), groundnut (Arachis hypogaea L.), Bambara groundnut (Vigna subterranean (L.) Verdc.) or fodder legumes (Ndayisaba et al., 2020; Khan et al., 2009). Desmodium (Desmodium uncinatum (Mill.) Urb., – silverleaf or Desmodium intortum – Greenleaf) is a perennial fodder legume intercropped in a cereal crop in push-pull technology (Khan et al., 2006, 2002). Farmers mix maize with legumes to obtain higher yields, efficient use of growth resources, manage weeds and as an insurance against total crop failure (Sadeghi and Sasanfar, 2013; Woomer et al., 2004). Beneficial effects of legumes on soil health are well documented, and include addition of organic matter, biological nitrogen fixation (Peoples and Crasswell, 1992), and improving availability of N and P in soil solution through mineralization of N and P from root exudates, dead nodules and roots, and fallen leaves (Nziguheba et al., 2016; Sharma et al., 2013; Vanlauwe et al., 2000). Legumes, however, can compete with the cereal crop, leading to low yields (S. Kuyah, personal communication). Increasing yields through mixed cropping requires correct application of the technology, based on knowledge of the kind of constraints in the area and the mechanism by which different companion crops improve productivity of the system. For example, in push-pull strategy, desmodium boosts productivity of maize by improving soil fertility and controlling stem-borers, fall armyworm and striga (Mutyambai et al., 2019; Midega et al., 2018; Khan et al., 2006; 2000). It can fix 90 kg N ha⁻¹ from atmosphere (biologically) per season (Ojiem et al., 2007) and has a high N fertilizer equivalency which can be as high as 120 kg N ha⁻¹ (Midega et al., 2013). However, there is a general shortage of literature on relative effects of different legumes in improving availability of plant nutrients. The few studies on the subject e.g. Vanlauwe et al. (2008) are still inconclusive. This study aimed at assessing the availability of N and P due to desmodium and food legumes such as bean, cowpea, crotalaria (Crotalaria ochroleuca G. Don), green gram [Vigna radiata (L.) Wilczek] and groundnut commonly intercropped with cereals in western Kenya, and assessing the effect of these legumes on maize shoot weight and productivity. Because changes in soils need time (Vanlauwe et al., 2008; Peoples et al., 1995), we used a fifteen years old experiment in which the effect of desmodium, common bean, cowpea, crotalaria, green gram and groundnut on striga was assessed with reference to maize monocrop (Midega et al., 2014; Khan et al., 2007).