Influence of Cultivation Practices on Yield and Spoilage of Kponan Yam (Dioscorea cayenensis-rotundata) During Storage ^†

Abstract

The Kponan yam (Dioscorea cayenensis-rotundata) is a culturally and economically important crop in Côte d’Ivoire but faces significant post-harvest losses, reducing farmers’ and traders’ incomes. This study aimed to identify strategies to minimize these losses during storage. To this end, Kponan yams from Bondoukou, Bouna, and Kouassi-Kouassikro, key production areas, were grown in experimental plots and monitored over three months in both field and warehouse storage. The results showed that yams harvested in Kouassi-Kouassikro (2.53 ± 0.06 kg) and Bondoukou (2.37 ± 0.09 kg) were heavier than those from Bouna (2.01 ± 0.11 kg). Storage conditions influenced spoilage: yams stored in pits had lower alteration rates (40.00% to 48.57%) compared to those stored under straw huts or trees (100%). In warehouses, alteration rates ranged from 72.29% (Bondoukou) to 100% (Kouassi-Kouassikro), with rot rates varying from 47.05% to 70.00%, respectively. Weight losses varied from 15.15% to 36.67% in warehouses versus 8.47% to 42.86% in field storage. These results underline the importance of storage methods, with pit storage significantly reducing deterioration.

1. Introduction

The genus Dioscorea, part of the Dioscoreaceae family, encompasses around 600 species primarily found in the subtropical and temperate regions of Africa, India, Southeast Asia, Australia, and the Americas [1]. Yam serves as a vital food crop for over 13 tropical countries, sustaining more than 500 million people [2,3]. Yam is a widely consumed staple food in West Africa [4,5]. Its value chain supports the livelihoods of approximately 5 million people, including rural farming households, traders, transporters, and processors [6]. Yam is a vital source of carbohydrates, vitamins, essential minerals, and fiber, and it has a low glycemic index [7,8]. Yam cultivation is used both for self-consumption and for generating income through the sale of surplus produce [9]. As a result, yam has become a cash crop that supplies urban markets [10,11]. Nigeria stands as the world’s leading yam producer, contributing more than 50 million tons, which represents over 67% of the global production, which was estimated at 75.14 million tons in 2021 [3].

In Côte d’Ivoire, approximately 60% of the population relies on yam as a staple food [12]. Accounting for over 10% of the global production (7.85 million tons in 2021), Côte d’Ivoire stands as the third-largest yam producer, following Nigeria (67%) and Ghana (11%) [3]. Yam takes the lead in food crop production [13], positioning Côte d’Ivoire at the forefront of consumption among major producing countries, with an average of 502 kcal per capita per day [3,14].

Although yam tubers play a significant role in the nutritional well-being and economy of populations, they are perishable and seasonal products. Notably, the losses are higher in early-harvested tubers [15]. These post-harvest losses are attributed to storage-related difficulties, resulting in weight losses ranging from 65% to 85% [16]. Moreover, losses of up to 50% of the harvest are attributable to factors such as dehydration, harvest injuries, pests, rats, and the use of chemicals [17].

Within the yam varieties cultivated in Côte d’Ivoire, those belonging to the Dioscorea cayenensis-rotundata complex exhibit a higher susceptibility to rot [18]. Regrettably, the Kponan variety, highly favored by consumers in the city of Abidjan [18,19,20], is particularly susceptible to rot during storage [18]. This particular yam variety, available in wholesale markets in Abidjan, originates from six different geographical areas, with the primary sources being Bondoukou (60%), Bouna (33.3%), and Kouassi-Kouassikro (6.7%) [20].

Several authors have emphasized that the cultivation, harvesting, and storage techniques used for Kponan yams, both in the field and in urban warehouses, play a crucial role in their shelf life [11,17,21,22]. This study aims to examine the impact of cultivation practices on the yield of Kponan yams, as well as the spoilage rate during storage both in warehouses and in the field.

2. Material and Methods

2.1. Material

The biological material used in this study consists of the Kponan yam variety (Dioscorea cayenensis-rotundata), cultivated and harvested in experimental fields located in Bondoukou, Bouna, and Kouassi-Kouassikro [21].

2.2. Methods

2.2.1. Implementation of the Experimental Design

The experimental field used a completely randomized block design with three blocks, each subdivided into three plots (one per yam origin: Bouna, Bondoukou, Kouassi-Kouassikro) as described in [21] (Figure 1). Within each zone, Kponan yams from Bouna, Bondoukou, and Kouassi-Kouassikro were cultivated in the same field, employing the cultivation techniques of selected local producers [21]. Land clearing in the fields was carried out using herbicides in Bouna, Bondoukou, and Kouassi-Kouassikro, as described by Kouadio et al. [17]. The height of the ridges varied between 70 and 100 cm across the three areas. The distance between the ridges varied between 30 and 50 cm in Kouassi-Kouassikro, and between 50 and 100 cm in Bondoukou and Bouna. Regarding staking, in Bouna and Kouassi-Kouassikro, the stakes were made up of shrubs left in the field. In Bondoukou, in addition to shrubs, corn stalks were used as stakes. Field maintenance, mainly consisting of weeding, was carried out manually with a daba in Bouna (2 weeding sessions: May and July) and in Bondoukou (4 weeding sessions: May to August). In Kouassi-Kouassikro, weeding was first carried out with herbicide (in May) and then a second time with a hoe just before the first harvest [17]. Regarding crop association, yams were grown alongside cashew trees in Bouna and Kouassi-Kouassikro, whereas in Bondoukou, they were intercropped with both cashew trees and maize. The first yam harvest was conducted when the tubers reached six months of physiological maturity. Tubers were harvested using dabas in Bondoukou and Bouna but a hoe in Kouassi-Kouassikro [17,22].

2.2.2. Tuber Weight

Tuber weight was determined using a pre-calibrated hand scale which displays the weight directly on screen. The tubers were weighed individually and their average weight (kg) was calculated [21].

2.2.3. Storage Conditions for Kponan Yams

Healthy yam tubers from each experimental plot, free from bruises and cuts, were sorted and divided into two lots. One of these lots was kept on-site in the experimental fields. The second lot was carefully transported to Abidjan to be stored in a warehouse.

In Bouna, the storage technique used was pit storage. Three pits were dug per block—one for each yam origin (Bouna, Bondoukou, Kouassi-Kouassikro)—resulting in a total of nine pits across the three blocks. Each pit contained the following: 38 yam tubers from Bondoukou, 38 yam tubers from Bouna, and 40 yam tubers from Kouassi-Kouassikro. Thus, in total, 114 tubers from Bondoukou, 114 tubers from Bouna, and 120 tubers from Kouassi-Kouassikro were stored in the pits.

In Bondoukou, the storage method employed was straw hut storage. One straw hut was constructed per block, totaling three straw huts. Each straw hut was divided into three sections corresponding to the different yam origins. The quantities stored per straw hut were as follows: 41 yam tubers from Bondoukou, 38 yam tubers from Bouna, and 44 yam tubers from Kouassi-Kouassikro. This amounted to 123 tubers from Bondoukou, 114 tubers from Bouna, and 132 tubers from Kouassi-Kouassikro stored in barns.

Finally, in Kouassi-Kouassikro, the yams were stored in open-air piles under the shade of a large tree, arranged into nine piles (one per origin and sub-plot). The quantities per sub-plot were as follows: 37 yam tubers from Bondoukou, 36 yam tubers from Bouna, and 35 yam tubers from Kouassi-Kouassikro. In total, 111 tubers from Bondoukou, 108 tubers from Bouna, and 105 tubers from Kouassi-Kouassikro were stored in this way.

The yams transported to Abidjan were stored in a facility set up at Nangui ABROGOUA University for a storage period of three months. The Kponan yam tubers were placed on stacked platforms and classified according to their origin and cultivation area. The storage facility had wall openings to facilitate air circulation. The average storage temperature was 27.98 °C, with an average relative humidity of 85.61% [17].

The number of healthy tubers stored was 115 for yams from Bondoukou cultivated in Bouna, 114 for those from Bouna cultivated in Bouna, 118 for those from Kouassi-Kouassikro cultivated in Bouna, 122 for those from Bondoukou cultivated in Bondoukou, 114 for those from Bouna cultivated in Bondoukou, 133 for those from Kouassi-Kouassikro cultivated in Bondoukou, 108 for those from Bondoukou cultivated in Kouassi-Kouassikro, 105 for those from Bouna cultivated in Kouassi-Kouassikro, and 100 for those from Kouassi-Kouassikro cultivated in Kouassi-Kouassikro.

Temperature and relative humidity were recorded three times a week and measured four times a day (at 8 a.m., 12 p.m., 4 p.m., and 8 p.m.) [23].

2.2.4. Determination of Tuber Damage During Storage

Each month, yams are checked for mold, wilt, tuber desiccation, and rot. The damage rate was calculated as a ratio of the number of damaged yams to the total number of yams in stock [22].

$$\% \mathrm{Damage}={\displaystyle \frac{\sum \mathrm{Damaged} \mathrm{yams}}{\mathrm{Total} \mathrm{yams} }}\times 100$$

(1)

The rate of weight loss (due to wilting or dehydration) was determined according to the following equation [21]:

$$\mathrm{Weight} \mathrm{loss} \left(\%\right)={\displaystyle \frac{{\mathrm{M}}_{\mathrm{i}}-\sum {\mathrm{M}}_{\mathrm{f}}}{{\mathrm{M}}_{\mathrm{i}}}}\times 100$$

(2)

M_i: Initial weight of tubers at harvest;

M_t: Final weight of tubers after storage.

2.2.5. Data Analysis

Analyses of variance (ANOVA) were performed with Stastica 7.1 software to compare means within the same production zone during storage. Differences were highlighted using Tukey’s test at the 5% significance level.

3. Results

3.1. Distribution of Tuber Weight at Harvest

Figure 2 illustrates the distribution of yam tuber weight at harvest according to the cultivation practice in Bouna, Bondoukou, and Kouassi-Kouassikro. The average weight of freshly harvested tubers varied significantly (ANOVA, p < 0.05), ranging from 2.01 ± 0.11 (Bouna) to 2.53 ± 0.06 kg (Kouassi-Kouassikro) (Figure 2a). Following the Gaussian distribution (Figure 2b), yam tubers with greater weight (weight > 2.30 kg) were harvested in Kouassi-Kouassikro (2.53 ± 0.06 kg) and in Bondoukou (2.37 ± 0.09 kg), unlike those in Bouna, which recorded a lower weight (2.01 ± 0.11 kg).

3.2. Damage Recorded During Field Storage

During the storage of yam tubers in the field (

Table 1. Impacts of cultural practices and the conservation method on Kponan deterioration.

	Rot (%)	Weight Loss	Crack (%)	Total Losses
Bouna*Open air	60.00 a	25.71 e	14.29 a	100.00 a
Bondoukou*Open air	57.14 ab	28.57 d	14.29 a	100.00 a
Kouassi-Kouassikro*Open air	57.14 ab	31.43 c	11.43 b	100.00 a
Bouna*Straw hut	48.58 c	42.86 a	8.57 c	100.00 a
Bondoukou*Straw hut	57.14 ab	31.43 c	11.43 b	100.00 a
Kouassi-Kouassikro*Straw hut	54.29 b	34.29 b	11.43 b	100.00 a
Bouna*Pit	22.86 e	8.57 g	8.58 c	40.00 d
Bondoukou*Pit	25.72 e	11.42 f	5.71 d	42.85 c
Kouassi-Kouassikro*Pit	31.43 d	11.43 f	5.71 d	48.57 b

Data are presented as mean ± standard deviation. Values in the same column that have different superscript letters differ significantly at the 5% level, based on Tukey’s test for variance analysis. (*) indicates an interaction between the factors.

), those kept in pits encountered fewer damages (40.00 to 48.57%) compared to those stored under thatched roofs and those stored in the open air under the shade of a tree (100%). The storage duration was one month in the open air, two months under straw huts, and three months in pits (Figure 3). Open-air-stored yams from Bouna had the highest rot incidence (60.00%), followed by Bondoukou and Kouassi-Kouassikro. Yams from Bondoukou stored under thatched roofs (Bondoukou Thatch) had a rot rate of 57.14%. Meanwhile, yams from Bouna, Bondoukou, and Kouassi-Kouassikro stored in pits exhibited the lowest rates of rot (22.86 to 31.43%). Regarding weight loss, yams stored under thatched roofs recorded the highest rates of tubers losing weight (31.43 to 42.86%), while yams stored in pits showed the lowest rate of tubers losing weight. As for internal tuber cracks, the rates ranged from 5.71 to 8.58% for yams stored in pits, from 8.57 to 11.43% for those stored under straw huts, and from 11.23 to 14.29% for those stored in the open air.

3.3. Damages Recorded During Storage in the Warehouse

During storage in the warehouse, the rates of deterioration of yams varied according to their origin. For yams cultivated in Bondoukou, the rates of deterioration ranged between 72.29% and 77.35%, respectively, for those from Kouassi-Kouassikro and from Bouna. For yams cultivated in Bouna, the rates of deterioration fluctuated between 79.40% and 84.81%. Regarding yams cultivated in Kouassi-Kouassikro, all were deteriorated by 100% after 2 months of storage in the warehouse. Yams cultivated in Bondoukou and Bouna could be preserved for 3 months (Figure 4). The storage duration was two months for yams cultivated in Kouassi-Kouassikro, and three months for those cultivated in Bouna and Bondoukou (Figure 5).

4. Discussion

The impacts of cultivation and conservation practices on the physical parameters of Kponan yam were assessed. At harvest, a significant variability in the physical parameters of yams was observed. In terms of weight, the results revealed a dispersion of the average weight of freshly harvested tubers, ranging from 2.01 kg to 2.53 kg. These findings corroborate those reported by Ola et al. [24], who also noted a similar average weight of Dioscorea cayenensis-rotundata tubers. According to the Gaussian distribution, tubers of a lower weight (weight < 2.30 kg) were mainly harvested in Bouna, while those of a greater weight (weight > 2.30 kg) were harvested in Kouassi-Kouassikro and Bondoukou.

These findings highlight that Kponan yam yield depends more on cultivation zone conditions (such as rainfall and soil) than cutting origin. Meteorological data from SODEXAM for the year 2021 underscored the significant impact of rainfall on yam yield, particularly in the Kouassi-Kouassikro region, which recorded higher precipitation compared to Bondoukou and Bouna. This impact of rainfall on the yield of Kponan yam in the Zanzan region (Bouna and Bondoukou) was also observed by Kouakou and Anoh [11], who found that although Kponan yam is not highly demanding in terms of precipitation, its yield is negatively affected by rainfall.

Regarding conservation, rot appeared as the main alteration of yams, whether in the field or in storage, regardless of the origin of the cuttings or the conservation method. These results are consistent with previous studies [22]. Kponan yam tubers stored in pits in Bouna showed lower rates of alteration compared to those stored under straw huts in Bondoukou or exposed to open air in Kouassi-Kouassikro. The conservation duration seems to be influenced by the location and method of storage, with more effective conservation in pits and in areas characterized by a dry season. These findings are consistent with previous research, emphasizing the importance of pit storage for optimal conservation duration [25]. According to previous studies [11,22], the use of chemical fertilizers and pesticides is inversely correlated with the shelf life of Kponan yam.

Furthermore, the use of herbicides was identified as a key factor affecting conservation duration, with an excessive use of herbicides in Kouassi-Kouassikro, which could explain the observed differences in yam conservation compared to Bondoukou and Bouna.

According to Kouadio et al. [17], herbicides are not directly responsible for tuber rot. However, excessive or improper use of herbicides, combined with other inadequate agricultural practices, can create conditions conducive to tuber deterioration [17]. Certain components of herbicides, such as nitrogen, can influence tuber quality. For instance, an excess of nitrogen may alter their chemical composition, making them more vulnerable to diseases and rot. Therefore, balanced fertilization is essential [26].

Furthermore, environmental conditions, particularly temperature (27.97–28.87 °C) and relative humidity (85.6–90.15%) which were recorded during the storage of the yam tubers, can accelerate their rotting. High humidity likely promotes fungal proliferation, explaining the accelerated tuber degradation observed in warehouse storage. Similar findings were reported by Osunde and Orhevba [23], who observed that tubers stored under ventilated conditions had a lower rot rate (1.85%) compared to those stored without ventilation (12.03%).

Other factors, such as the presence of internal cracks in the tubers or the shocks sustained during their transport from the field to the city, can also contribute to the decay of Kponan yam. Aidoo et al. [27] demonstrated that the rate of decay was higher in tubers visibly pierced by rhizomes compared to healthy, unpierced tubers. Furthermore, Assiri et al. [28] suggested that decay is caused by fungal agents that invade the tissues of tubers damaged by insects, nematodes, or mechanical actions during and after harvest.

Bosrotsi [29] highlighted that physical damage sustained during harvesting operations is a major contributor to post-harvest losses in roots and tubers. It is crucial to handle tubers carefully to prevent damage, as injured tubers have poor storage potential and deteriorate quickly. Kponan yam rot is mainly caused by improper storage practices, adverse environmental conditions, and physical damage during harvest and transportation. The proper management of herbicide use, effective pit storage, and sustainable agricultural practices are essential to reduce post-harvest losses and improve the shelf life of tubers.

5. Conclusions

The study demonstrates that Kponan yam deterioration is driven by storage conditions and cultivation practices. Tubers maintain better physical quality when grown with moderate herbicide use and stored in pits. Tubers can be stored for two to three months in storage and up to three months in pits. However, this storage period is reduced to one month when yams are cultivated and stored in the open air, as in Kouassi-Kouassikro.