On-farm management and participatory evaluation of pigeonpea (Cajanus cajan [L.] Millspaugh) diversity across the agro-ecological zones of Benin Republic

Background: Pigeonpea is a multipurpose food legume that contributes to food security in Benin. However, its production declined and some landraces are being threatened of disappearance. Previous investigations on pigeonpea in Benin have been restricted to South and Central Benin. Therefore, pigeonpea diversity in northern is still unknown. This study aimed to have a better knowledge of pigeonpea genetic diversity, for its promotion and valorization. Methods: 500 producers of pigeonpea belonging to thirteen sociolinguistic groups were selected through 50 villages. Data were collected using methods and tools of participatory research appraisal. Folk nomenclatures, taxonomy of pigeonpea and seed system were investigated. The distribution and extent of pigeonpea landraces were evaluated using Four Square Analysis method. A comparative analysis of pigeonpea uses categories, production systems, pigeonpea production constraints, famers’ preference criteria and participative evaluation for existing landraces across agro-ecological zones was done. Result: Folk nomenclature and taxonomy were mainly based on seed coat colour and size colour. Seven pigeonpea uses category were recorded including sacrifice, grain processing and fertilization. The results showed that pigeonpea seed system is informal. Based on seed characteristics, fifteen landraces were recorded with seven new landraces. A high rate of landraces threatened of disappearance was observed across the ecological zones. Ten constraints are known affecting pigeonpea production in Benin with pests and diseases as the most important in all agro-ecological zones. This study revealed that pigeonpea cultivation is increasing in the Sudanian zone. Landraces to be produced must be selected on the basis of 11 farm crop genetic

criteria among them precocity and resistance to pests and diseases, in the three ecological zones and adaptability to any type of soil in the Sudanian zone were the most important. The participatory evaluation revealed the existence of a few performing landraces. and Latin America [1]. Pigeonpea is an excellent source of protein (21.7 g/100 g), dietary fibres (15.5 g/100 g), soluble vitamins, minerals and essential amino acids [2,3]. Moreover it is also used in traditional medicines and leaves, flowers, roots; seeds are used for the cure of bronchitis, sores, respiratory ailments and also acts as an alexeritic, anthelmintic, expectorant, sedative, and vulnerary [4,3].

Conclusions
In Benin, pigeonpea is highly consumed in the South-East through the Adja cultural area and contribute to the improvement of household incomes [5]. The plant is used for soil conservation and weed management in the fields [6,7,5]. Despite the importance pigeonpea [5], very few research effort has been undertaken to improve the production of the specie. As a result, the potential yield of pigeonpea is estimated at 2,500 kg/ha, while the yields obtained on farmer's fields is estimated at 620 kg/ha in Benin [8]. This low yield could be due to the lack of improved varieties in Beninese agriculture [9]. Therefore, an exhaustive collection of pigeonpea diversity cultivated at the country level is the base for the development of any varietal improvement program and the implementation of conservation strategy.
Several studies were done on pigeonpea diversity in Benin [9, 10, 11, 12]. However, all previous investigations on pigeonpea in Benin have been restricted to South and Central Benin [9, 10, 11, 12]. Therefore the pigeonpea diversity in northern Benin is still unknown. In addition, no comparative studies on pigeonpea production constraints across different ecological zones in Benin are not yet documented, varietal diversity as well as farmers' varietal preference criteria and their variation throughout ecological zones and sociolinguistic groups have been very little documented. While it is known that understanding the genetic diversity, uses, and distribution of orphan crops is essential in determining what to conserve and where to conserve, for sustainable utilisation [13,14,15]. Thus, there is important to dispose a comprehensive collection of pigeonpea genetic resources of Benin and to document all associated ethnobotanical knowledge by extensive survey [9,11].
Seeds are the lifeblood and foundation of a successful farming and a crucial element in the lives of agricultural communities [16]. The procedures, through which a cultivar is bred, produced, certified, stored, marketed and used which includes all the channels through which farmers acquire genetic materials and in interaction with the commercial seed industry is known as seed system [17]. Thus, the success of crop varieties introduction is tightly linked to the uses, biophysical conditions, the cropping systems in which the crop is integrated which vary across growing areas [10].
In developing countries where agriculture is the spearheading of the economy, improved varieties must be developed or simply searched for within the existing diversity. In both cases, a good knowledge of the existing varietal diversity and the agronomic performances of varieties are necessary [18,19]. Thus, farmers' participation in varietal selection process is determinant of variety adoption [20].
Moreover, documentation and identification of high-performance landraces based on farmer's varietal preference criteria will provide strategies to overcome constraints affecting pigeonpea production in Benin. Hence, it is important to evaluate the performance of pigeonpea existing landraces under participatory approach to enhance pigeonpea production and productivity contributing thereby to attain food security and reduce poverty.
This study on pigeonpea aimed to: (1) document the different pigeonpea landraces in Beninese agriculture (2) compare seeds management and conservation systems of pigeonpea genetic resources and use categories across different ecological zones, (3) compare constraints associated with pigeonpea production and varietal preference criteria across different ecological zones and sociolinguistic groups and (4) evaluate in participatory way the performances of different landraces in relation to agronomic and culinary traits.

Study area
The study was carried out in Benin. With a population size of 10 008 749 habitants [21] the Benin is located in the inter tropical zone between parallels 6° 30 ' North and 12° 30' North latitude, and meridians 1° East and 30° 40' East longitude [22].
With an area of 114,763 km², Benin is limited to the north by the Niger River in the northwest by Burkina Faso, to the west by Togo, the south by the Atlantic Ocean and to the east by the Nigeria (Figure 1). The Republic of Benin is divided into three ecological zones: the Guinean zone in the South (6° 25' North latitude and 7° 30' North longitude), the Sudano-Guinean zone (7° 30' North latitude and 9° 45' North longitude) in the Central and the Sudanian zone (9° 45' North latitude and 12° 25 North longitude) in the north [23]. The Guinean and Sudano-Guinean zones are both located in moist agro ecological zone characterized by a subequatorial bimodal climate with two dry seasons and two rainy seasons. The Guinean zone is characterized by an annual rainfall varying between 1200 and 1500 mm/year. The temperature ranges from 24 to 30 °C. The Sudano-Guinean zone annual rainfall varies from 1100 to 1300 mm/year ( Table 1). The temperature in this zone varies between 25 and 34 °C. The Sudanian zone is located in the semi-moist agro ecological zone characterized by a unimodal climate pattern with one rainy season and one dry season. The annual rainfall varies between 900 and 1100 mm/year while the temperature ranges from 21 to 35 °C [23] (Table 1).
After exploratory study in agricultural research institutions, visits to local and urban markets, discussion with farmers and sellers, the villages surveyed were selected based on the pigeonpea production, their accessibility and the manner to cover all sociolinguistic groups. A total of 50 villages were prospected ( Figure 1).

Data collection
The surveys were done using methods (group discussions, individual interviews and field visits) and tools (questionnaires) of participatory research appraisal following Dansi et al. [24].

Focus groups
In each village, a group of 15 to 28 pigeonpea farmers has been identified and brought together with the help of administrative and/or local authorities (village chief, farmers' associations, etc.). Interviews were conducted with the help of local translators to facilitate discussions [25]. Prior to the meeting, farmers were requested in advance to bring samples of pigeonpea landraces they cultivate or knew about. After a brief presentation of the objectives to the farmers, they were asked to list all the pigeonpea landraces in cultivation, in the village. The distribution and extent of these landraces were evaluated using the participatory method of Four Square Analysis described by Brush [26]. This method permit to classify at village-level existing landraces into four groups (produced by many households on large areas, produced by many households on small areas, produced by small households on large areas and produced by few households on small areas). In agreement with the farmers, we agreed that a cultivar cultivated by few households is that grown by no more than 20% of farmers in the context of the village; and that cultivated on a small area is that cultivated on not more than 0.25 ha. The participatory evaluation of identified landraces for agronomic and culinary parameters was carried out according to Gbaguidi et al. [27]. The considered parameters were the productivity, vegetative cycle, cooking, sensitivity to pests and disease and sensitivity to storage insects. The two-level evaluation method described by Loko et al. [28] was used. In this approach and for a given trait, a landrace is scored 1 when it is performing and 0 when it is not. After that, local nomenclature, folk taxonomy and the vegetative cycle of landraces have been documented. According to Dansi et al. [29], farmers were asked to list all the constraints related to pigeonpea production. The identified constraints were prioritized in groups by identifying and gradually eliminating the most severe constraint. In a first step, farmers were asked to identify, among the constraints they have listed, the most critical one. The constraint thus identified is ranked first and is eliminated from the list. The same procedure was repeated until the last constraint was ranked. Secondly, farmers were asked to list all the traits that could interest them and motivate them to continue growing pigeonpea. Using the same approach (gradual elimination of the most important criterion), the identified criteria were then prioritized. The discussions were free, open-ended and without a time limit being set.

Household surveys
After group discussion, ten households were identified for individual interviews by village. In each household, the person interviewed was chosen based on the common agreement from the host couple according to Christinck et al. [30]. Socioeconomic data (gender, educational level, age, year of experience in pigeonpea cultivation and household size); the biophysical resources, cultural practices and seed system (number of cultivated landrace, sowing time, crop type, cropping system, perception about the evolution of pigeonpea cultivation, fertilization, sources of labour, level of intervention in the production chain, pests and diseases incidence and its management, pigeonpea cropping areas for 2015, 2016, 2017); the reasons for pigeonpea production; the different pigeonpea uses categories; pests incidence and its management have been documented. According to their incidence pattern, pests incidence was categorized by farmers as negligible (none), low, high and very high. Incidence was categorized as negligible when pest appeared in very low number while it categorized as low when infestation was responsible for growth retardation and high when infestation involves damage to flowers or pods and very high when infestation are responsible for death of plant.

Data analysis
Descriptive statistics were used to analyze data. To avoid overestimation of pigeonpea diversity in each ecological zone, correspondences between vernacular names were made following seed characteristics (seed color, color pattern, color pigmentation and the seed eyes color) according to Mohammed et al. Thus, the importance of the constraint is determined by the formula described by Dansi et al. [28]: IMC= (NTV+MAC+PCO)/3. The same approach was used to rank farmers' varietal preference criteria. To compare socio-economic data, biophysical resources, cultural practices and seed system from one ecological zone to another, the Analysis of variance (ANOVA) and Turkey test was used for quantitative variables using Minitab 16 Software while the bilateral Z test was used for qualitative variables, using Statistica 7.1 Software. Before ANOVA, data were logtransformed (log(x + 1)) for variances homogeneity. In order to determine a potential significant change in the cropping area from 2015 to 2017, analysis of variance was conducted.

Socio demographic characteristics of respondents
In total 500 pigeonpea producing households including 190 in the Guinean zone, 200 in Sudano-Guinean zone and 110 in the Sudanian zone were surveyed. The age of the surveyed pigeonpea farmers ranged from 21 to 76 years with an average of 45.9±9.2 years old. The majority (62.4%) of farmers were men. The majority of pigeonpea farmers were found to be illiterate (43.4%), while 31.6% and 25% were found to have primary and secondary levels of education, respectively. The average household size was 6.4±2.1 members (ranging from 3 to 11 members). The experience year old was 15±8 years, on average ( Table 2).
Significant differences in age of the surveyed pigeonpea farmers were observed across ecological zones. On average, farmers in the Guinean zone are older (48.7 years against 44 years) and more experienced than those in the Sudano-Guinean zone (18.4 years of experience against 16.5). The number of farmers with none, primary and secondary level of education varied between ecological zones.

Local nomenclature
Across the thirteen sociolinguistic groups surveyed in the study area, 50 different pigonpea local names were recorded in the local dialects. Referring to the various vernacular names identified, the generic name of pigeonpea varied according to sociolinguistic group and ecological zones (Table 3).
In the Guinean and Sudano-Guinean zones, pigeonpea is called Hounkoun, Kloué or Klouékoun referring to Cowpea by farmers belonging to Fon and Mahi sociolinguistic groups while in the Guinean and Sudanian zones, pigeonpea is called Otili in reference to a pod-producing tree by farmers belonging to Nago and Dendi sociolinguistic groups. However, Bariba and Peulh sociolinguistic groups designated pigeonpea by Wotiri in reference to a pod-producing erected tree. Moreover, in Guinean zone, farmers belonging to Holly and Yoruba sociolinguistic groups call pigeonpea Otini. Pigeonpea is called Ekloui or Kloui by Adja sociolinguistic group. In Sudano-Guinean zone, pigeonpea is called Colo (meaning is unknown to farmers) by Idaasha sociolinguistic group while pigeonpea is called Tissi Tounan and Itoun by Biali and Somba sociolinguistic groups respectively, referring to a cowpea.

Folk taxonomy
In the study area, 5 criteria were used by surveyed farmers to identify pigeonpea landraces. The great majority of names (90.7%) given to pigeonpea had a meaning.

Diversity of cultivated pigeonpea landraces
Based on seed characteristics, fifteen pigeonpea landraces were idesntified in the study area ( Figure 2). At village level, the number of pigeonpea landrace ranges from 1 to 5 with an average of 2.7 ± 1. The highest number of landrace (5)

Distribution and extent of pigeonpea landraces
Within each ecological zone, the production was limited to specific districts and departments. In the Guinean zone, the production was restricted to the districts of  (Table 12).

Reasons for pigeonpea production and uses category
Our study revealed that pigeonpea is produced for three main reasons depending on the ecological zones (Table 6).
In the Guinean and Sudano-Guinean zones, nutritional value is the main source of motivation while in the Sudanian zone land fertilizing power is the main source of motivation. The third reason is the market value. The different pigeonpea uses categories were mainly concentrated on grains. Based on their fidelity level, pigeonpea is more used in medicine in the Guinean (FL = 19.5%) and Sudanian (FL = 23.9%) zones. According to famers, boiled leaves are used by oral route to treat malaria. Also, the decoctate of the leaves is used in bath to treat measles and is also used as an antibiotic to treat mouth's sores or tooth decay. The roots, when chewed, prevent the rise of snake venom, in the case of snake bite. The use of pigeonpea grains as an offering for food or symbolic purposes and in sacrifice to divinity was specific to the Sudano-Guinean and restricted to Holly and Nago sociolinguistic groups. While grain processing into donuts is specific to Guinean (FL = 4.3%) and Sudano-Guinean (FL = 2%) zones and restricted to Holly and Adja sociolinguistic groups. In these zones, pigeonpea are roasted and reduced to flour to sprinkle sauces as nutritional supplement by the first one or to make donuts by the second one. Consumption, weed control and land fertilization are common to all three ecological zones (Table 6).

Cultural practices
Pigeonpea was considered as an annual plant by most of surveyed farmers (93.2%).
Only 6.2% of farmers considered this legume as a perennial plant. For the last one, plant is left in the field and is harvested the following year. The main pigeonpea farming activities included: ploughing, sowing, weed control, pod harvest, pod plugging and winnowing. Seeding and weed control were practiced by all the farmers. Pigeonpea is sown between April, May, June (73.6%) in intercropping with other seasonal crops (82.8%) or in pure stand (17.2%). Three sources of labour were observed. For farming activities, 13.2% of farmers used family labour, 73% combined family and friends labour while 13.8% used a combination of family, friends and jobber labour (Table 7).
Land fertilization was not reported while only 14% of farmers included in this study used pesticide. The average grain yield in farmers' fields was estimated at 553.4±36.3 kg/ha. According to farmers, during the three last years, Sudano-Guinean zone were the largest cropping area followed by the Guinean zone while farmers in the Sudanian zone produced pigeonpea on a small cropping area (Table   7). Sowing was more realized between April, May and June in the Guinean and Sudano-Guinean zones (97.9% and 91% respectively) whereas it was more realized between June, July and August in the Sudanian zone (68.2%). Intercropping with other seasonal crops such as maize and millet was specific to Guinean (100%) and Sudano-Guinean (98.5%) zones while pigeonpea was more cultivated in pure stand in Sudanian zone (75.4%). Family and the friends was the main source of labour for various farming activities in the Guinean and Sudano-Guinean zones (87.9% and 61.5% respectively) while it was family (49.3%) in the Sudanian zone. Our results revealed that the average pigeonpea yield in the Sudanian zone is lower (522.3 ± 44kg/ha) compared to the Guinean and Sudano-Guinean zones (557.5 ± 15.9 kg/ha and 566.6 ± 35.8 kg/ha respectively).

Seed system
Three sources of seeds were observed. Farmers used seeds from previous harvest (60.2%) or friends (22%) or local market (17.8%). After each harvest, 67.8% of farmers stored seeds until scarcity at market while 32.2% of them sell seeds in local markets. Comparing seed system between ecological zones, previous harvest was the main source of seed in the Guinean and Sudano-Guinean zones (70% and 62.9% respectively) while friends was the main source (50.4%) in Sudanian zone and after each harvest, farmers stored more grains in the Guinean and Sudano-Guinean zones (70% and 84% respectively) while they were more immediately sold in Sudanian zone (65.5%) ( Table 7).

Pigeonpea production constraints
In total, 10 constraints were identified as affecting pigeonpea production. The long vegetative cycle was ranked as the major constraint in Benin following by pests and diseases and rainfall irregularity (Table 8). According to farmers descriptions, pigeonpea production is faced by Low productivity is seventh among the constraints followed by the sensitivity to storage insects. All constraints have been reported in the three ecological zones. However, their relative importance varied from one zone to another. The most important constraint in the Guinean and Sudano-Guinean zones is the long vegetative cycle followed by the sensitivity to pests and diseases, while in the Sudanian zone pest and diseases followed by soil poverty were the most important constraints (Table 8).

Incidence of pests on pigeonpea yield and control methods
According to farmers, the impact of pests and diseases in pigeonpea production in farmers' fields varied from one zone to another (Table 9). The impact is low in the Guinean and Sudano-Guinean zones (52.6% and 42.5% respectively) while it is high in the Sudanian zone (81.8%). Pests control was only reported in the Sudanian zone (63.7%). Three reasons justified the non-control of pest reported by farmers: the high price of pesticides (49.6%), the risk of intoxication (29.6%) and the lack of sprayers (20.8%).

Evolution of pigeonpea production in Benin
Overall, the majority of farmers (69.4%) reported a decrease in pigeonpea production in Benin. According to farmers, this downward trend was the fact to the Guinean and Sudano-Guinean zones (75.79 % and 85.5% respectively). In these zones, the decrease in cropping area is highly significant (p ˂ 0.001). Indeed, the average of cropping area was 0.9 ±0.  (Table 7). According to farmers, the reasons for this increasing are the fertilizing power of the plant (89.1 %) and weeds control (10.9%).

Farmers' preference criteria of pigeonpea
Through the study area, 11 criteria depending on the ecological zones and different sociolinguistic groups underlined the choice of pigeonpea landraces to be cultivated by the farmers. The most important criteria were precocity, resistance to pests and diseases, rapid for cooking, adaptability to any types of soil, good taste and high productivity (Table 10).
In Guinean and Sudano-Guinean zones, precocity was the most important criterion followed by the resistance to pests and diseases while in the Sudanian zone, the resistance to pests and diseases was ranked first followed by the adaptability to any type of soil (Table 10). Precocity appeared at the front of the criteria of all sociolinguistic groups except for Nago sociolinguistic group for whom adaptability any types of soil was the first criterion. Moreover, in addition to precocity, resistance to pests and diseases, rapid for cooking, adaptability to any types of soil and good taste were the choice criteria for farmers belonging to Bariba sociolinguistic group (Table 11).
In addition to the choice criteria for Bariba sociolinguistic group, farmers belonging to Boo sociolinguistic group had strong tendency to varieties cultivable at any time of the year and resistant to storage insects while those belonging to Dendi sociolinguistic group preferred varieties with high productivity and cultivable at any time of the year and those belonging to Peuhl sociolinguistic group preferred high productive and resistant to storage insects varieties. Lastly, precocity, resistance to pests and diseases, rapidity for cooking and adaptability to any type of soil were farmers belonging to Yoruba sociolinguistic group criteria.

Participatory evaluation of pigeonpea landrace grown in Benin
The results revealed that for landraces identified simultaneously in the three ecological zones, none of them were performing for a given character simultaneously in the three ecological zones (Table 12). Moreover, none landrace were performing simultaneously for all 5 evaluated characters. Nevertheless, the Carder ekloui (Adja sociolinguistic group) only identified in Guinean zone combined 4 good performances (high productivity, rapid for cooking, resistant to pests and diseases, resistant to storage insects). Moreover, Carder ekloui (Adja sociolinguistic group) and Otili founfoun kékélé (Idaasha sociolinguistic group) showed high productivity in Guinean and Sudano-Guinean zone but showed low productivity in the Sudanian zone. These two landraces, however, showed resistance to pests and diseases. In addition, Klouékoun vôvô (Fon and Mahi sociolinguistic groups) showed high productivity, rapid for cooking, resistant to pests and diseases, resistant to storage insects and short vegetative cycle in Guinean and Sudano-Guinean zone, however, showed low productivity and susceptible to pests and diseases in the Sudanian zone (Table 12).

Discussion
Our study showed that pigeonpea generic names varied according to sociolinguistic group and ecological zones. Our findings are similar to those of Ayenan et al. [11] who distinguished respectively 2 and 3 infra-specific pigeonpea taxa. However, local names do not necessarily reflect the genetic history of landraces of crops because different names may be given to identical seeds of landraces or a single name may apply to heterogeneous crops [37]. Such a situation may contribute to under or over-estimate the diversity within a species [38,23,39,9]. So to avoid redundancies and optimizing the efficient conservation and sustainable use of pigeonpea, it is important to conduct morphological and molecular characterizations to avoid redundancies and establish equivalence between the local names [26,40,41].
Farmers use morphological aspect of seeds (coat colour, seed eyes colour, and seed size), plant type, seed origin and vegetative cycle for identification of folk varieties.
These criteria of pigeonpea classification and identification are among the descriptors of C. cajan recommended by IBPGR and ICRISAT [42] and used by many authors in morphological characterization of this legume. Our study revealed that morphological aspect of seeds (in particular the seed coat colour) was the predominant criterion used by farmers to classify and identify pigeonpea landraces.
The main reason is that seed coat colour is unique to each landrace while other traits may be commonly shared [15]. Our finding is contrary to those of Manyasa et Thus, given that the previous studies did not take into account the entire production area as insignificant as it may seem, a part of the existing pigeonpea landraces in Benin was left out. This finding suggests that extent of the study area affect varieties richness [45,15]. Thus a study that better reflects the existing diversity of cultivated species should not be restricted to the major production areas of the species.
Our results also revealed that the on-farm level diversity of pigeonpea was specific to ecological zones. In fact, the same landraces haven't the same distribution and The fertilizing power of pigeonpea as main reasons for producing this legume reported in present study is not surprising because pigeonpea has significant position in dry land farming systems especially adopted by small and marginal farmers in many parts of world by fixing nitrogen, flexibility for mixed cropping or inter crop [46,47]. The use of pigeonpea leaves to treat various diseases such as malaria corroborates the observations made by Ayenan et al. [9] and Zavinon et al.
[11] in Benin and those of Aiyeloja and Bello [48] and Oladunmoye et al. [49] in Nigeria. Also, the use of pigeonpea as weeds control has been reported by several authors in Benin [50,51,5]. However, the use of pigeonpea roots to prevent the rise of snake venom, in the case of snake bite reported in the current study has not been reported elsewhere. Also, grain processing into donuts identified in the current study has not yet reported by previous studies. Unfortunately, this technological ability of pigeonpea is weakened by its retention of oil. Thus, this possibility of transformation of pigeonpea must be explored and improved, like the soybean's transformation in cheese, in Benin. This action will help to reduce malnutrition in rural populations and improve in situ conservation of the existing pigeonpea diversity. Moreover, the use of pigeonpea grains as an offering for food or symbolic purposes and in sacrifice to divinity has not yet reported by previous work. All these findings are found to be sociolinguistic groups and ecological zones-dependent and suggest that pigeonpea farmers in Benin do not have the same knowledge on the use of pigeonpea. However, specific knowledge related to the plant part uses might be kept and transmitted within communities in some areas as a result of vertical knowledge transmission [52,15]. Knowing that integrating cultural practices of local communities permit an efficient on farm conservation [53,36], this specific use category of pigeonpea genetic resource show the potentiality of cultural approach for the conservation of this legume in Benin.
Our study reveals that in the Sudanian zone, pigeonpea cultivation is increasing while it is in decreasing in the Guinean and Sudano-Guinean zone. In fact, the productivity of the smallholder farming system in this zone is under threat due to soil fertility decline [54]. Research in many parts of Africa including Benin has shown that legumes have the potential to sustain soil fertility in smallholder farming systems [55,56,47]. Thus, thanks to the project "Protection and Rehabilitation of The results showed that pigeonpea seed system in Benin is informal. Similar observations were made on pigeonpea in Tanzania [57] and India [16]. This informal seed system has the advantage to facility seed exchanges among farmers and among villages [17]. However, marketed seeds must deserve attention. In fact, seed acquisition from market does not guarantee genetic purity [58]. It is so important to make available to farmers good quality seeds in order to increase productivity of pigeonpea [58,10]. The association of pigeonpea with other crops has been reported in others countries such as Uganda [43] and Kenya [59]. After each harvest, the great majority of farmers stored seeds until scarcity at market before selling them. However, farmers in Sudanian zone sold immediately their seeds that help resolving urgent problems such as children education. These findings justify the fact that pigeonpea is an essential source of household income which can contribute to poverty reduction in Benin as reported by Dansi et al. [5].
In Benin, many factors negatively affect pigeonpea production. Long vegetative cycle followed by pests and diseases were the main constraints affecting pigeonpea production. Indeed, african pigeonpea were characterized by the late maturity [60,12]. According to farmers theses genotypes cultivation in sole crop occupies land that should be used for other crops. The pests and diseases ranked first in the Sudanian zone are not surprising. In fact, in this zone pigeonpea is more cultivated on pure land, which facilitates pests' attraction. Our findings confirm the observations made by Sarkar et al. [47] which revealed that intercropping system minimizes the attack of pest and diseases as compared to intercropping system. Farmers in this zone have limited access to pesticides and are suffering most from this production loss. Although the impact of pests and diseases was found to be low in the study area, their presence is the key indicator of the urgent need to develop strategies against these pests. Instead of the use of pesticides, an integrated pest management is recommended, through the combination of biological control based on the use of natural enemies of these pests and genetic control based on the use of tolerant or resistant cultivars [61,62,27].
As in the majority of leguminous where the attack of storage insects is a major constraint [63], surveyed farmers reported that seeds are sometimes subject to attack by storage insects. Farmers use some toxic products to protect their seeds. A sensitization of farmers or consumers for a purely biological conservation, as the use of small peppers, is highly recommended, as it is the case of Kersting's groundnut [64]. Curiously, low productivity ranked seventh among constraints. This suggests that low productivity represents only a small portion of the constraints mentioned by famers. Thus, low productivity is the direct consequence of the negative effects of the other constraints [59]. In view of the observed, the lack of improved varieties as a challenge to pigeonpea production. The availability of improved varieties and their distribution across the different ecological zones according to their specific needs can alleviate the constraints affecting pigeonpea production in Benin. Therefore, government should encourage small-scale enterprises to provide farmers with improved seeds.
Farmer preference criteria take an important place in breeding program and facilitate the adoption of improved varieties [38,11]. Our study reveals that famers perceived precocity, resistance to pests and diseases, good taste and rapidity for cooking as the most important preferred traits. Similar observation was observed, on pigeonpea, by Mergeai et al. [59] in Kenya, Shiferaw et al. [65] in Tanzania, Changaya [66] in Malawi, Ogbe and Bamidele [67] in Nigeria and Ayenan et al. [9] in Southern Benin. All these preference criteria are correlated with identified constraints. This suggests a veritable link between these two parameters as reported by Odjo et al. [68] on Rice. The precocity as criterion is important for famers because short vegetative cycle varieties should certainly encourage them to produce pigeonpea. Indeed, in the global climate context where changes are noticeable, early varieties will provide producers the guarantee that pigeonpea plants attain a significant level of vegetative development before the cuts of rain.
The high productivity as criterion of varietal choice is also not surprising as it is for any breeders and famers the most desired criteria [69,28]. Our findings are however contrary to those of Zavinon et al. [11] for whom, high market value is the main famers' preference criterion. In fact, the high market value cannot appear at the first rank of preference criteria in the sense that this criterion is only the result of the adoption of an improved variety for one or the other of the criteria. Our study revealed that preference criteria varied across different sociolinguistic groups however, convergence in preference criteria between certain sociolinguistic groups was observed. This could be explained by the cultural links and the intensive knowledge's exchange between these sociolinguistic groups or due to the common origin of these sociolinguistic groups.
For a given character, the same landrace doesn't have the same performance, from one ecological zone to another. For instance, the landrace called Otili founfoun kékélé (Idaasha sociolinguistic group) which perceived by farmers as having high productive in Guinean and Sudano-Guinean zone has presented a low productivity in the Sudanian zone. This may be due to the variability in soil types, fertility and organic matter turn over, soil nutrient dynamics [70], water regime [71] across these ecological zones. In addition, the landrace called Klouékoun vôvô (Fon and Mahi sociolinguistic groups) which showed high productivity in Guinean and Sudano-Guinean zone but low productivity in the Sudanian zone reinforce the fact that variability in soil types, fertility and organic matter turn over, soil nutrient dynamics or water regime justify these agronomical difference. The Carder ekloui (Adja sociolinguistic group) only identified in the Guinean zone must deserve attention.
This landrace combine four good performances (high productivity, rapid for cooking, resistant to pests and diseases, resistant to storage insects), according to famers and is appear as a promising landrace. Unfortunately, this landrace is threatened of disappearance. There is urgent need to process to an ex-situ as well in situ conservation to preserve this landrace as well as all those threatened of disappearance in Benin. All identified landraces in the current study must however be tested for their identified performance according to farmers. Thus, morphological and molecular characterization is highly recommended to help select suitable varieties for breeding programmes. Thereafter, association mapping of candidates' genes/QTLs for desirables traits can be done and used in future marker-assisted breeding program. Otherwise breeding of adapted pigeonpea to any type of soil and resistant to pests and disease will be of dual interest to famers in the Sudanian zone. It will enhance the chain value this legume and will also help to restore the fertilizing power of impoverished lands. Waiting, taking into account farmer's preference criteria, the few performing landraces identified can be used in a varietal exchange programs to enhance pigeonpea production in Benin.

Conclusions
Our study area showed a great varietal diversity of pigeonpea with fifteen landraces identified based on seed characteristics. Seven new landraces were found and some were specific to an agro-ecological zone. A highly significant decrease in cropping areas occurred in the Guinean and Sudano-Guinean zones. Some of the landraces are threatened of disappearance due to several factors, which constraint pigeonpea production and need to be considered under specific conservation strategy to avoid diversity loss. The few performing landraces identified through participatory evaluation can be used in varietal exchange programs in order to mitigate the effects of these constraints. The development of new varieties based on farmers' criteria is important to enhance pigeonpea production in Benin. To develop new varieties, morphological and molecular characterizations of identified landrace are highly recommended to help select suitable varieties for breeding programs. In situ and ex situ conservation strategies on the one hand and on the other hand the preservation of traditional knowledge associated to pigeonpea is important to preserve landraces threatened of disappearance and to conserve pigeonpea diversity in Benin.

Funding
Not applicable.

Acknowledgments
We express our sincere gratitude to all farmers, chiefs of village, and leaders of farmer groups for their contributions to the success of this study. We would like to acknowledge the technical support of Falil BANI OROU KOUMA during the prospection and collection of landraces.

Availability of data and materials
Raw and treated data generated during study are available from the corresponding author on reasonable request.

Authors' contributions
GK designed the study, collected and analyzed data and drafted the manuscript.
AGF participated in the interview work. GD, LEYL, AD, CA and AD supervised data analysis and revised the manuscript. All authors read and approved the final manuscript.

Consent for publication
Not applicable.

Competing interests
The authors declare that they have no competing interests.         Table 8 Comparative table of   have no common letters are statistically different (p<0.05); ***p˂0.001; ns: nonsignificant difference at the 5% level GZ: Guinean zone; SGZ: Sudano-Guinean Zone; SZ: Sudanian Zone; TNV: Total Number of Villages in which the criterion is cited; MCR: Number of villages where the criterion is the major one or ranked first; PCr: number of villages in which the criterion was classified among the principal criterion i.e. among the first five; Imp: Importance   Different pigeonpea landraces cultivated across ecological zones of Benin.