Increasing potato equivalent yield increases returns to investment under potato-legume intercropping systems

Abstract In order to enhance sustainable intensification of potato-based cropping systems, especially in sub-Saharan Africa (SSA), there is a need to investigate the economic viability of investing in this lucrative venture. This study evaluated the economic returns under legume intercropping systems using value/cost ratio (VCR) and benefit/cost ratio (BCR) under treatments comprising of potato intercropped with dolichos (Lablab purpureus L.) (P-D), climbing bean (Phaseolus vulgaris L.) (P-B) and garden pea (Pisum sativum L.) (P-G), and a potato pure stand control (P-S). Across the seasons, tuber yield was not significantly (p < 0.05) affected by intercropping with P-D, whereas under P-B and P-G, it decreased by 19% and 16%, respectively compared to P-S. P-G, P-B and P-D recorded 6, 7 and 12% higher potato equivalent yield (PEY) relative to P-S. P-D was the most profitable intercropping system with VCR of 35 and BCR of 5.1 as compared to values recorded in P-S of 31 and 5, respectively. Regression of VCR against PEY resulted in a stronger coefficient (0.98) compared to that of BCR against PEY (0.82) implying that VCR is a simple tool that could be adopted for economic returns to investment studies such as potato-legume inter-cropping systems.


Introduction
Low crop production is jeopardising food security, particularly in Africa, where the population is projected to increase exponentially from the current 1.2 billion to 1.7 and 2.5 billion by 2030 and 2050, respectively (UNDESA, 2017). As the population continues to increase, the traditional cultural practice of expanding croplands to increase food production is no longer tenable, especially in sub-Saharan Africa (SSA) (Lambin et al. 2013). Therefore, given that land is an inelastic resource, the only viable option is to improve production of the current croplands, which can be achieved by adopting sustainable intensification strategies such as intercropping (Garnett et  Through efficient utilization of resources like land, soil nutrients, light and water, intercropping systems have been proved to be yielding higher relative to mono culture systems (Singh et al. 2016; Gitari et al. 2018). This is particularly important given that potato is considered a heavy nutrient feeder crop (particularly for nitrogen) of which about 50% comes from inorganic fertilizers (Gitari 2018;Rens et al. 2018). In SSA, fertilizer use is often constrained as farmers have not only meagre financial capacity, but also they are inadequately informed, which make them sceptical about the viability of investing in this valuable input (Jansen et al. 2013;Dawi et al. 2017). Despite the high cost of fertilizers, especially in Africa, there is need to incentivise small-holder potato growers to invest in fertilizer use under the existing potato intercropping systems. Under intercropping systems with different types of yields, calculation of returns to fertilizer use can be complicated especially when detailed information regarding the total cost of production is not available. By converting all the yields from the intercropping systems into potato equivalent yield ), a value/cost ratio (VCR) can be applied to show the profitability of different cropping systems (Niyuhire et al. 2017). This is a nutrient/output ratio that indicates the economic value of the extra yield gained by unit investment in the usage of fertilizers.
As an indicator for assessing returns to fertilizer use, VCR has been applied on studies focusing on monocropping systems (Kaizzi, et al. 2012(Kaizzi, et al. , 2018Dawi et al. 2017;IFDC 2017;Niyuhire et al. 2017). The VCR differs from the benefit/cost ratio (BCR), which gives returns to the overall investment cost of production. There is a dearth of literature dedicated to the use of these ratios, particular under intercropping systems involving potato and legumes. Therefore, this study aimed to assess the economic viability of fertilizer use in potato-legume intercropping systems using VCR and BCR. This has a far-reaching effect of ending poverty and achieving food security as postulated in the first and second Sustainable Development Goals (SDGs), respectively (UNDESA, 2017).

Site Description
The study was conducted over four successive seasons (from 2014 to 2016) in a clayey Nitisol in a Research Farm based at the University of Nairobi, Kenya (1° 15' S, 36° 44' E, and 1860 m above sea level). The site receives rainfall twice per year from October to December, usually referred to as 'short rains' and March to June, referred as 'long rains'. During the study period, short rains recorded an average rainfall of 547 mm with a minimum and maximum temperature of 17 and 29°C, respectively. The long rains received 788 mm of rainfall with an average maximum temperature of 28°C and minimum temperature of 16°C. More detailed site description is available in Gitari et al. (2019).

Experimental design
In this experiment, we adopted a randomised complete block design with four replicates in 4 by 6 m plots. The treatments consisted of potato intercropped with legumes, namely dolichos (Lablab purpureus L.) (P-D), climbing bean (Phaseolus vulgaris L.) (P-B) and garden pea (Pisum sativum L.) (P-G) and a control (P-S) of pure potato stand. Planting was done at the beginning of each growing season with potato seed planted at an inter-row spacing of 90 cm and inter seed spacing of 30 cm whereas, for legumes, two seeds were planted per hill in a single row between potato rows.

Crop management
At planting, potato was supplied with 200 kg ha −1 of NPK (17:17:17) fertilizer and an equivalent quantity of calcium ammonium nitrate fertilizer 28 days after planting (DAP). Weeding, hilling for potato and staking for bean were carried out manually 28 DAP. Potato was sprayed against late blight with Ridomil Gold MZ 68 WG (Mefenoxam 40 g kg −1 + Mancozeb 640 g kg −1 ) alternated with Daconil 720 SC (Chlorothalonil 720 g L −1 ) after every 14 days starting at 28 DAP.

Data collection
Harvesting was carried out manually from 12 m2 central area per plot at 65 and 75 DAP for pea, 84 DAP for potato and bean, and 120 DAP for dolichos. The yield (tuber and grain) was recorded in kg ha-1 then converted into potato equivalent yield (PEY) using Eq. (1) Where PEY = potato equivalent yield from Eq. (1) and CP = total production costs as given in Table 1. Profitability of cultivating potato under legume intercropping systems was assessed using value/cost ratio (VCR) and benefit/ cost ratio (BCR) as shown in Eq. (4) and Eq. (5)

Data analysis
The economic viability of applying fertilizer to potato under legume intercropping systems was assessed by analysing tuber yield, potato equivalent yield (PEY) and value/cost ratio (VCR) using generalized linear models in R software version 2.2.3. Tukey's post hoc test was used to separate means at p ≤ 0.05. Regression analysis was performed to show relationships between PEY and VCR as well as between PEY and VCR.

Potato tuber yield, potato equivalent yield, gross and net income, value/cost ratio and benefit/cost ratio in response to integration of legumes into potato-based cropping systems
The tuber yield was significantly (p < 0.05) influenced by the type of legume integrated into potato-based cropping system, and the differences varied with season. For instance, the tuber yield recorded in P-D (35 t ha −1 ) was comparable to P-S with a negligible difference of 1 t ha −1 (Figure 1). However, intercropping potato with pea and bean resulted in a significantly (p < 0.05) yield decrease of 19 and 16%, respectively compared P-S.
Similarly, the type of potato-legume intercrop had significant (p < 0.05) effect on potato equivalent yield (PEY), gross and net income, value/cost ratio and benefit/ cost ratio, but these differences fluctuated from season to season (Table 2). With respect to seasons, 2015 long and short rains recorded significantly higher yield (42 and 43 t ha −1 ), respectively) compared to 2014 short rains (36 t ha −1 ) and 2016 long rains (37 t ha −1 ). Across the season, the significantly highest and lowest potato equivalent yield values were recorded in P-D (40 t ha −1 ) and P-S (35 t ha −1 ), respectively whereas an intermediate value of 38 t ha −1 was observed in P-G and P-B.
Integration of legumes into potato cropping systems had a positive impact of economic returns. For instance, across the seasons, P-D had the highest gross and net returns at US$ 10707 and, 8967, respectively compared to US$ 9596 and, 8007 recorded for P-S. Regardless of the cropping systems, significantly higher gross returns was recorded in 2015 long (US$ 7333) and short (7544) rains compared to values of US$ 6233 and 6576 for 2014 short

Relationship between potato equivalent yield (PEY) and value/cost ratio, and between PEY and benefit/cost ratio.
A regression analysis of VCR against potato equivalent yield indicated a very strong linear dependence of VCR on PEY with R 2 of 0.98 (Figure 2a). This was an indication that when other factors are held constant, a unit increase in VCR would be due to an increase in PEY by about 1.17 Mg ha -1 . Similarly, regression of BCR against PEY showed a positive and direct relationship though with a weaker coefficient of 0.82 (Figure 2b), implying that a one-ton increase in PEY would result in 0.14 unit increase in BCR given that all other confounding factors are fixed.

Discussion
This study revealed that the viability of using fertilizer under potato legume intercropping systems based on the promising higher returns compared to potato pure stand system. Assessing intercropping systems could be difficult without considering legume yield. However, given that there were different yield types involved, this complicated the issue even more. Gitari  Increase in PEY resulted in increase in value/cost ratio (VCR) values, as denoted by the regression analysis, which revealed a stronger and direct relationship (R 2 = 0.98) with PEY than that of benefit/cost ratio (BCR) (R 2 = 0.82). Generally, VCR is the most commonly employed indicator used to assess the financial incentives for growers to use fertilizer using non-economic means (Niyuhire et al. 2017;IFDC 2017). All the VCRs values reported in this study were > 1, inferring that fertilizer use was profitable. However, we only considered the values greater than 35 to provide satisfactory incentive. In this case, the observed VCRs suggested that fertilizer use is satisfactorily profitable only under potato-legume intercropping systems, especially P-D.
IFDC (2017) recommends that VCR should be at least four to accommodate price and climatic risks and still provide an incentive to the farmers. In the current study, the VCRs recorded in all intercropping systems surpassed this threshold by over eight times indicating adequate risk coverage against investment in use of the fertilizer under potato-legume intercropping. In sub-Saharan Africa, especially Kenya, potato farming is faced with various uncertainties, which include erratic rainfall patterns and fluctuation of ware tuber prices. Hence, covering such ). This can act as a good impetus to smallholder farmers who mainly practise intercropping given that they are assured of higher returns to investment in fertilizer application. Therefore, VCR proofs to be a reliable tool that can be used to show the economic viability of potato-based intercropping systems. It has an advantage over the BCR given that it does not require incorporation of all variable costs involved in crop production.

Conclusion
Intercropping is a viable practice for intensification of potato cropping system, particularly in Kenyan highlands where farmers have small landholdings of less than a hectare. This study was carried out with the objective of determining the economic viability of potato-legume intercropping systems under tropical field conditions value/cost ratio and benefit/cost ratio in relation to potato P-S, pure potato stand; P-D, potato intercropped with dolichos; P-G, potato intercropped with pea; P-B, potato intercropped with climbing bean. 2 Means followed by different letters within a column differ significantly at p ≤ 0.05. equivalent yield. The results of this study demonstrated that value/cost ratio (VCR) and benefit/cost ratio (BCR) are vital tools for determining the profitability of incorporating legumes into potato-based intercropping systems. The expectation is that smallholder potato farmers in sub-Saharan Africa will be incentivised to use fertilizers due to improved economic returns associated with fertilizer use. In this regard, dolichos was identified as the most qualified candidate for intensification of these cropping systems given that it not only resulted in the highest VCR but also BCR without a significant penalty on tuber yield.