Smallholder farmers in the North eastern Tanzania practice cereal-legume mixed culture to manage risks of crop failure in sole crops. The production and productivity of cereal-legume intercrops may be influenced by the practice of cultivating two or more crops in the same space and time and the status of land races/cultivars involved as intercrops. Data on the effect of intercropping system on cowpea landraces with maize grown on Vertic Cambisols on yield and yield components are inadequate in the North eastern Tanzania although considerable knowledge has been accumulated on mixed culture system. A field experiment involving two cropping systems (sole crop and intercropping) was therefore conducted for two years (2017 and 2018) at Shirimatunda village, Kilimanjaro Region, North East Tanzania, to evaluate three local cowpea (Vigna unguiculata L.) landraces (i.e. landrace 1 (L1), landrace 2 (L2) and landrace 3 (L3)) grown in association with maize (Zea mays L). Intercropping significantly (P≤.05) decreased cowpea and maize yields. Performance of local cowpea landraces, however, did not show (P≤.05) any variation except for 100–seed weight where cowpea landrace 2 (L2) showed superiority compared with the other cowpea landraces. Although there was no significant variation in maize yield when intercropped with cowpea landraces, maize intercropped with cowpea landrace 1 gave numerically the highest grain yield. Regardless of cropping system and cowpea landraces, combined productivity of cowpea and maize increased in the intercropped plots as indicated by higher total land equivalent ratios (1.64 to 1.94). Highest LER_T value was observed in the ‘Cowpea landrace 1’+Maize mixed culture. The observed total Land equivalent ratio (LER) values correspond to 48.25, 38.54 and 43.66 % of lands saved which could be used for other agricultural purposes. In these cropping systems, both cowpea landrace 1 & 3 and maize crop components were significantly complementary and most suitable in mixed culture as shown by competitive ratio (CR) values of 1.85 and 1.45 respectively.

Keywords: mixed culture, Shirimatunda, landrace, cowpea, genotype, yield components

LER, land equivalent ratio; DAE, days after emergence; YICc, mass yields per unit area of cowpea; YICm, maize in mixed culture respectively; LERT, total land equivalent ratio; CR, Competitive ratio; LERm, partial ler for maize; LERc partial LER for cowpea; LS, percentage land saved

Intercropping, one of the cropping systems in developing countries,^1,2 is a practice by smallholder farmers of cultivating two or more crops in the same space and time,³ with a view to minimize risks of crop failure due to weather variability. With this type of cropping system all the environmental resources are utilized for the maximization of crop production and productivity.^2,4–10 In Tanzania and elsewhere in the tropics, intercropping is mostly practiced by small scale farmers characterized by subsistence agriculture, low agricultural mechanization, high labour and low - yield farming on small parcels of land.¹¹ Since intercropping has been reported to increase both the amount of biomass, crop yield per unit area of land harvested and crop failure mitigation,^12,13 farmers in the area practices mixed culture systems with a view that if one crop fails during the cropping season due to unequal distribution of rainfall and variability the other intercrop will be available for the household. According to Rusinamhodzi et al.¹⁴ intercropping can support increased aggregate yields per unit input, and thus ensure improved soil quality, food security and poverty alleviation through increased income. Several other benefits with regards to intercropping has been highlighted as increased land use efficiency; light capture and its use,¹⁵ soil moisture and nutrients; controlling weeds, insects, and diseases,¹⁶ increase in the length of production cycles; improved seed quality and better control of water quality through minimizing the use of inorganic N fertilizers, replacing them by the use of legumes.¹⁷ Typically this cropping system include maize (Zea mays L), millet (Pennisetum glaucum L) and sorghum (Sorghum bicolor L) as cereal crops components and beans (Phaseolus vulgaris), cowpea (Vigna unguiculata L), groundnut (Arachis hypogaea), pigeonpea (Cajanus cajan) and soybean (Glycine max) as legume crops components. However, in this study maize was intercropped with different types of cowpea local variety as used by local farmers. It has been shown that leguminous plants can benefit the intercrop cereals during the same season through N excretion and nodule decomposition,¹⁸ although according to Senaratne et al.¹⁹ each legume specie has different ability to supply N to its corresponding associated cereal neighbor. It is similarly assumed that intercropping cereals with legume crops, is that the legume, when associated with the specific Rhizobium, may satisfy most of its N requirement derived through N2 - fixation of atmospheric N, leaving the soil available N for the companion cereal. Legumes are therefore the major crop companion in many intercropping systems placed within the top ten (10) most frequently used intercrop species.²⁰ Therefore; the main target of this research was to study the yield and yield components of cowpea local varieties when intercropped with maize.

Maize (Zea mays L.) in Tanzania is a primary staple crop and it’s grown in nearly all agro-ecological zones.²¹ Maize production is characterized by smallholders farms with less than 10 hectares (i.e. an average of 2-3 ha each) and contributes about 85 % of total production. However, although around 45 % or over 4.9 million hectares are used for maize production,²² the national yield is pegged between 1.0 and 1.5 t ha-1, compared with the estimated potential yields of 4 - 5 t ha-1.^23,24 The main constraints of low yield and sporadic production are drought stress (shortage of rainfall), infestation by insects, molds, and other pests. Other factors include weeds and diseases, low agricultural inputs such as fertilizer and crop protection chemicals, low levels of technology and poor infrastructure and storage facilities.^25,26 Tanzania like other developing countries, maize is mainly used for human consumption. It’s a single most important staple food both in rural and urban areas.²⁷ Maize accounts for 31 % of the total food production and constitutes more than 75% of the cereal consumption in the country. It is estimated that the annual per capita consumption of maize is around 128 kg. According to Nyoro et al,²⁸ and Peter et al.²⁹ nearly 400 grams of maize are consumed per day per person in Tanzania; average national consumption is estimated to be over three (3) million metric tons per year.³⁰ Similarly, maize contributes about 34-36 % of the average daily calorie intake,^31–33 about 67 % starch, 10 % protein, 4.8% oil, 8.5% fiber, 3% sugar and 7% ash,³⁴ to both human (in the developing countries),³⁵ and it is recently used in production of biofuel. It is equally well accepted for animal feed ingredient with high feeding value and can contribute up to 30 % protein, 60 % energy, and 90 % starch in animal diet.³⁶

Cowpea (Vigna unguiculata L. Walp.) is a valuable crop in semiarid regions used by the resource poor farmers,^37–40 and is consumed as a source of high - quality plant protein in many parts of the world. Recently, cowpea has gained more attention from different stakeholders worldwide because of its exerted health beneficial properties, including anti-diabetic, anti - cancer, anti - hyperlipidemic, anti - inflammatory and anti - hypertensive properties. Its tolerance to moisture stress,⁴¹ along with numerous other advantages ranging from nutritional, soil fertility improvement, and weed control,⁴² make it a useful component of the cropping systems involving cereals.⁴³ Similarly, high protein and carbohydrate contents with a relatively low fat content and a complementary amino acid pattern to that of cereal grains make cowpea an important nutritional food in the human diet. Although considerable knowledge has been accumulated on intercropping system,⁴⁴ data on the effect of intercropping system of cowpea landraces with maize grown on Vertic Cambisols on yield components are inadequate. This study aim to assess the effect of cropping system on yield and yield components of three cowpea landraces grown in mixture with maize on Vertic Cambisols.

The study was conducted at the backyard garden in Osterbay Shirimatunda in the Northern Tanzania (3º22’S, 37º19E, and altitude 853.7m.a.m.s.l.) during the Masika rains growing seasons of 2017 and 2018. The study area receives mean annual rainfall of about 946mm (Table 1). The average mean annual low temperature is 15.5°C and maximum annual high temperature is about 33.3°C with possibility of rising to 40°C during the dry season. The mean annual potential evapotranspiration (ETo) as measured by Penman Monteith (Monteith, 1965) was 1.390 mm (Table 1). The soil is clay classified as Vertic Cambisol in the FAO soil classification system. Prior to planting each year, soil samples (0 to 20 cm soil depth) were collected from the experimental plots, pooled, and sub - samples taken for chemical analysis. The soil physico-chemical properties are shown in (Table 2).

The experimental variables included three local cowpea landraces (‘mixed colour’; ‘black’; and ‘light brown colour, Figure 1), and two cropping systems (sole cowpea/maize and cowpea+maize intercropping). A two factor completely randomized block design was used with three replicates. Plot size of 3mx2.6m (7.8m²) was used for maize and cowpea. Maize was sourced from Kibo Seed Company and was grown at a spacing of 75x60cm to maintain a uniform density of 22.222 plants•ha^-1. Cowpea seeds were obtained from Moshi Municipality local markets, and were grown at a spacing of 20 x 40cm with a population of 125,000 plants ha^-1. All seedlings were later thinned to two per hole. Weeding was done manually to all crops with a hoe four times until harvesting. Fertilizer was applied for both sole and intercropped maize using the recommended rate. DAP was applied at the rate of 100kg ha^-1 at planting. Urea at the rate of 100kg ha^-1 was applied as top dressing in two splits one - third at 20 days after emergence (DAE), and two third at 40 DAE. The benefit of intercropping was quantified by LER which is defined as the relative land area in pure stands that is required under monoculture to produce the yields of all products from the mixture.^45–47

Figure 1 Traditional landraces used in the experiment (C-TRAD1=Landrace 1, C-TRAD2= andrace 2, C-TRAD3=Landrace 3).

Intercropping efficiency

Intercropping efficiency was evaluated by using land equivalent ratio,⁴⁶ as given below:

$LE{R}_C=\frac{YI{C}_C}{YM{C}_C};LE{R}_M=\frac{YI{C}_M}{YM{C}_M};LE{R}_T=LE{R}_C+LE{R}_M$

Where LERc and LERm=Partial land equivalent ratio for cowpea and maize respectively; YICc and YICm=Mass yields per unit area of cowpea and maize in mixed culture respectively; YMCc and YMCm=Mass yields per unit area of cowpea and maize in monoculture respectively; LER_T=Total Land Equivalent Ratio. If LER_T is greater than one (LER_T>1), intercropping has a yield advantage while there is a yield disadvantage from intercropping if LER_T is less than one (LER_T<1).^48,49

Competition ratio and percentage land saved

Percentage (%) land saved was calculated as described in Ijoyah et al.⁵⁰ and Workayehu,⁵¹ and competitive ratio was calculated as described in Willey and Rao,⁵² as:

$CR=\frac{LE{R}_m}{LE{R}_c}\times 100$

Where: CR = Competitive ratio, LERm: Partial LER for maize; LERc: Partial LER for cowpea, and

$LS(\%)=100-\left(\frac{1}{LE{R}_T}\times 100\right)$

Where: LS=Percentage land saved, LER_T=Total Land Equivalent ratio

At physiological maturity, the plants were counted and harvested. Yield assessment was carried from the middle rows of each plot excluding the border rows. Sixteen (16) plants of cowpea were sampled from each plot to determine the numbers of pods plant–1 and seeds pod–1. From the maize plots, eight plants were sampled for determination of the weights of head plant–1 and seed plant–1. Both cowpea pods and maize heads were manually threshed and allowed to air dry (~ 13 % moisture content). Grain yields of cowpea and maize were determined from each plot and 100 - seed weight and shelling percentage recorded. Mean values of yield components were analyzed statistically using a factorial analysis of variance (ANOVA) using the software, STATISTICA 2007 (StatSoft Inc., Tulsa, OK, USA). Fisher’s least significant difference was used to compare treatment means at P<.05.⁵³

Intercropping cowpea (Vigna unguiculata L) with maize (Zea mays L) decreased (P≤.05) the weight of seeds plus pods plot-1, weight of seeds plot-1, weight of 100–seeds, grain yield and shelling per cent (Table 3). The decreased attributes as a result of mixed culture system is ascribed to the inter - and intra - specific competition for site resources typical of intercropping system treatments.⁵⁴ The observed cowpea seed yield was on the average of 2.7 t ha^-1 in the monoculture and 1.9 t ha^-1 in the mixed culture plots (Table 3). Previous studies by Alom et al.⁵⁵ showed that cowpea grain yield was (P≤.05) minimum when grown in mixed culture with maize compared with other treatment combinations. Intercropping cowpea with maize is characterized by a very low cowpea grain yields.^56–58 Studies have also indicated that intercropping cowpea with maize (P≤.05) reduced cowpea yield by 43 %.^54,59,60 The low productivity of cowpea in intercropping systems is due to among other reasons, to shading by cereals. Other studies by Nambiar et al.⁶¹ also reported similar negative effects for cowpea when intercropped with sorghum (Sorghum bicolor L).

Grain weight ear-1, number of grains•row-1 and 100-seed weight of maize were significantly (P≤.05) lower at intercropping compared with monocropping (Table 4). Likewise, intercropping maize with cowpea landraces reduced (P≤.05) the ear length, grains row-1, ear diameter, total ear weight and shelling per cent of maize similar to results observed in Egbe et al.⁶² Maize seed yield was on the average of 3.5±0.2 t ha-1 in the sole crop and 3.1±0.1 t ha^-1 in the intercropped plots. These results are similar to those reported by Randhawa et al,⁶³ and Alom et al.⁵⁵ that sole maize was significantly superior to maize + cowpea intercropping treatments. Likewise, according to Yilmaz et al.⁶⁴ the highest yield was achieved from sole maize compared with its combination with groundnut and soybean. Crop competition for site resources has been cited as probably the main reason for reduction in yields typical of mixed culture treatments.⁵⁴

Performance of cowpea landraces when intercropped with maize showed significant (P≤.05) difference between them only on the weight of 100 - seeds parameter (Table 3). The data showed that Landrace 2 (Black colour, Figure 1) was superior (P≤.05) compared with the other landraces. In maize however, with the exception of grain weight ear-1 and grain yield ha-1 which were not significantly (P≤.05) different, the other attributes of maize showed significant (P≤.05) differences when intercropped with cowpea landraces compared with sole maize (Table 4). The observed differences among maize grown in mixed culture with cowpea landraces is ascribed to crop competition and intrinsic differences in the ability of different cultivars to access growth resources and compete with associated crops. Similar cowpea landrace differences in yield have previously been reported in Krasilnikoff et al.⁶⁵ in cowpea – maize mixed cultures, and crop competition was reported as possibly the main reason for reduction in crop yields. In line with this finding, Kheroar and Patra,⁶⁶ reported that yield of cowpea during intercropping with maize was reduced due to lower amount of solar radiation received. This is because crops with C4 photosynthetic pathways such as maize have been known to be dominant when intercropped with C3 crops like cowpea. The reduction in seed yield by intercropping could be due to interspecific competition and depressive effect of maize, as C4 species on cowpea as C3 crop.

The results showed that there was significant (P≤.05) difference in the effect of intercropping system on land productivity potential or land equivalent ratio (LER) as indicated in Table 5. On the one hand, partial land equivalent ratio (LER_M) of maize ranged from 0.87 in maize grown in mixture with cowpea landrace 2 (L2) to 1.25 in maize grown with cowpea landrace 1 (L1). Maize LER_M was greater than cowpea LER_C, indicating that maize is the dominant species. This result is in line with most other field studies reported in Mukhala,⁶⁷; Tsubo,⁶⁸ and Ogindo.⁶⁹ On the other hand, partial land equivalent ratio (LER_C) of cowpea ranged from 0.69 in L1 to 0.77 in L2. Additionally, total land equivalent ratio (LER_T) ranged from 1.64 in L2 to 1.94 in L1. These findings are similar to those of Ijoyah et al,⁵⁰ and Workayehu.⁵¹ Regardless of cowpea landraces, LER_T values were all greater than 1 (i.e. 1.64 to 1.94), implying a significant yield advantage for the intercropping system. In other words, 64 to 94 % more land would have been required if the component crops were planted as sole crops to obtain the same yield as obtained in the intercropped plots. This corresponds to 48.3 as percent land saved with 1.85 competitive ratio. In this study, LER_T>1, an indication that intercropping is advantageous in terms of the improved use of available resources for plant growth and development.^70–74

In conclusion, intercropping system in the study area improved the overall profitability compared with monocropping even though the yield of the component crops was reduced when considered separately. Generally, although yields of the component crops were lower than yield from monocropping, the crop components taken together were more productive as shown by the total land equivalent ratio (LER_T), percentage land saved (% LS) and the competitive ratio (CR). It was also evident that although there was no significant difference between the cowpea landraces, yield of maize grown in mixed culture with cowpea landrace 1 was higher compared with maize grown with other cowpea landraces.

None.

None.

Author declares that there is no conflict of interest.

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