Yam ( Dioscorea spp.) is crucial for achieving food security ^{1 , 2} and economic stability in many tropical and subtropical regions, particularly in West Africa, where it serves as a primary source of carbohydrates and essential micronutrients for millions of people. Nigeria is the world’s leading yam producer, contributing 62 million metric tons annually, which accounts for 69.6% of its global output. ³ Yam plays a key role in rural economies and cultural traditions within producing communities. ⁴ Beyond its nutritional value, which provides approximately 200 calories daily to over 300 million people in West Africa, yam also offers a vital income source for smallholder farmers and serve as a symbol of cultural heritage in ceremonies and festivals. ² However, despite its importance, yam production faces increasing challenges that threaten its sustainability, including declining soil fertility, erratic weather patterns driven by climate change, ⁴ a lack of quality planting materials, and poor agronomic practices. Traditional methods of yam production, which often rely on whole tubers or tuber setts for propagation, further exacerbate these issues. These methods are resource-intensive, requiring large quantities of planting material, and are inefficient, with a high risk of transmitting pests and diseases through multiple cycles of vegetative propagation, ^{5 , 6} which limits the scope of scaling up the production of improved and released yam varieties. ² These challenges underscore the need for innovative cultivation strategies to enhance productivity and resilience, such as adopting more efficient propagation methods, like leaf-bud cuttings (LBCs). ⁷ LBCs have the potential to address significant issues, such as the unavailability of quality planting materials and high input costs, ultimately enhancing overall yields.

The LBC is an innovative vegetative propagation technique where a section of the yam vine, including a leaf and its associated bud with a piece of stem, is cut and planted directly into the soil or other substrates to grow a new plant. ⁸ This method differs substantially from traditional propagation techniques, which usually utilise whole tubers or cut setts weighing between 250 and 1000 g each. ^{9 , 10} To produce LBCs, minitubers weighing 1–10 g are used as planting material, reducing the economic burden on farmers and allowing more tubers to be reserved for food or sale. Additionally, LBCs enable faster multiplication rates, permitting rapid production of planting materials within a single growing season, with the potential to generate disease-free plants when cultivated in protected environments. ¹¹ Despite these advantages, the successful establishment and productivity of LBCs heavily depend on the optimisation of environmental conditions and management practices to maintain the delicate planting materials. Providing shade before the LBC begins to root is crucial.

Shade materials are vital in yam LBC cultivation because excessive sunlight can cause heat stress, increase evapotranspiration, ⁸ and hinder the survival of newly planted LBCs. Shade materials control light interception, temperature, and humidity, ⁹ thereby altering the microclimate around the plants. Artificial shading materials used in agriculture, such as coloured shade nets, provide a certain percentage of shade. ¹⁰ These are mostly imported and costly in the major yam-producing countries of West Africa. Nonetheless, natural alternatives like palm fronds can be utilised if available, offering different levels of light filtration and microclimate regulation. For freshly cut LBCs, which are fragile and more susceptible to environmental stress, the selection and use of shade materials are crucial for successful establishment and growth. ¹²

Equally important is the role of plant spacing, which directly affects resource competition and yield outcomes. ¹¹ In seed yam cultivation, plant population influences the availability of light, water, and nutrients among individual plants, shaping both growth dynamics and tuber production. ¹³ The production of seed yam at 40,000 plants/ha using minisett has been reported, ¹⁴ while higher planting densities of 500,000 and 1,000,000 using LBCs could maximise land use efficiency and increase total tuber yield per unit area. However, this often leads to smaller individual tubers due to increased competition. Conversely, wider spacing decreases competition, potentially producing larger tubers, but reduces overall productivity per hectare. ¹⁵ Seasonal variability further complicates the cultivation of LBCs, as yam is mainly grown under rain-fed conditions in tropical regions, ¹⁶ making it highly sensitive to fluctuations in rainfall, temperature, and humidity across cropping seasons.

Although the recognised importance of shade materials and plant populations in crop production is acknowledged, a notable knowledge gap remains regarding their individual and combined effects on directly planted LBCs in the field. The lack of research on LBCs under different shade and spacing regimes hinders the development of customised agronomic recommendations for this promising seed yam production system. This study aims to address this gap by investigating how directly planted LBCs respond to various shade materials at two different plant densities.

The study was carried out during the 2023 and 2024 cropping seasons at the research field of the International Institute of Tropical Agriculture (IITA), Kubwa Station, Abuja, Nigeria. This research site is situated at 9.164159° N latitude, 7.345470° E longitude, with an elevation of 423 metres above sea level. The soil is loamy and generally classified as Alfisols. ¹⁷ In 2023, rainfall was recorded from March to November, amounting to 1,587.8 mm. In 2024, the rainfall fell from February to November, with a total of 1,127.9 mm ( Figure 1). In both years, planting was undertaken after the onset of consistent rainfall. The average minimum air temperatures for 2023 and 2024 were 21.1°C and 21.4°C, respectively, while the average maximum temperatures were 32.0°C and 32.6°C.

The field was cleared of vegetation, harrowed, and ploughed before preparing the beds. Each bed measured 2 × 1.2 metres and represented a plot. Shade structures were erected by installing bamboo sticks firmly into the ground and securing them with ropes to form a skeletal framework. To minimise shading effects across treatments, a 2-metre buffer space was maintained between different shade materials. The shade materials ( Figure 2) used are as follows: Black nylon (T1), Green tarpaulin (T2), Fresh palm fronds ( Elaeis guineensis) (T3), Used polypropylene woven sacks sewn into a sheet (T4), Green shade net (T5) manufactured to provide 70.0% shading, ¹⁸ and Green taffeta cloth (T6). T1, T2, T3, T4, T5, and T6 provided 87.3, 88.4, 67.8, 74.6, 60.4, and 65.6% shading, respectively. These percentage shadings were calculated as recommended by Tabadkani et al. ¹⁹ equation (1). Percentage Shading = ( Unshaded Illuminance − Shaded Illuminance ) Unshaded Illuminance × 100 (1)

The T1, T2, T4 and T6 materials were purchased as whole pieces, then cut and sewn into the desired dimensions. The T5 was assembled using large pre-cut pieces fastened with binding wire, while palm fronds were sourced from palm trees within the experimental station. The cost of shade materials required to cover 100 m ² varied across the different types assessed. T3, which was freely available but incurred a gathering cost, were the least expensive, costing $8.1. This was followed by T4, which cost $19.8, and T6, priced at $28.1. These three materials were considered low-cost shade materials. The mid-cost category included T1, which cost $94.9, and green tarpaulin, which cost $158.2. The most expensive shade material was the T5, which fell into the high-cost category with a price of $845.0 for the same area coverage. The shade materials were then attached to the skeletal framework using binding wire. Two ventilation holes were made per square meter in the T1, T2, T4, and T6 shade materials to allow airflow.

Before planting, the beds were levelled and covered with a layer of about 5 cm of fermented rice husk (FRH). The FRH functions as mulch to control weeds and regulate soil temperature. Being loose, it causes minimal resistance to the root growth and sprout emergence of the LBCs. Each shade treatment or material was assigned to a main plot, and within each shade—consisting of six beds (subplots)—each bed represented a different LBC spacing treatment.

LBCs were obtained by cutting vines from 12-week-old mother plants grown from 3 to 5 g minitubers of the Kpamyo yam variety, maintained in a screen house. Kpamyo (TDr 95/19177) is an improved yam variety bred by the International Institute of Tropical Agriculture (IITA) and the National Root Crops Research Institute (NRCRI), and officially released by the National Centre for Genetic Resources and Biotechnology (NACGRAB). The variety is maintained at NRCRI, NACGRAB, and NRCRI are public institutions. ²⁰ Freshly harvested long vines were kept in clean water in a bowl. Then, a clean cut was made through the stem at an angle to produce LBCs. Each cutting included one node with a bud, one leaf, and approximately 1 cm of stem on either side of the node. The cutting was placed in a bowl containing a mixture of 2 g of broad-spectrum fungicide, Mancozeb (80% WP), per litre of water. After 5 to 10 minutes, the LBCs were removed and planted in beds at a spacing of 10 × 10 cm, resulting in a population of 1,000,000 plants/ha (S1), and at 10 × 20 cm (S2), supporting a population of 500,000 plants/ha, on June 30 ^th, 2023, and June 28 ^th, 2024, respectively, according to their different shade treatments. The entire stem portion and part of the leaf stalk were inserted into moist soil and gently pressed down. The planted LBCs were lightly watered after planting, and regular monitoring and watering continued for two weeks to prevent them from drying out. The LBCs sprouted and began to grow approximately two weeks later. Afterwards, the crop was managed as a rain-fed crop. In the seventh week after planting (WAP), all shade materials were removed, ²¹ and the crops were maintained using standard agronomic practices for yam production. The new shoots developed by the LBCs were staked using the trellis method. Due to the mulching effect of rice husks, minimal hand-weeding was performed by pulling a few weeds on the beds, while a handheld hoe was used to remove weeds between furrows. The NPK 15:15:15 fertiliser was applied at a rate of 400 kg/ha 10 WAP, using the side placement method.

The survival and eventual yield of plants largely depend on the climatic elements in their immediate surroundings. ²² Similarly, the successful establishment and seed yam yield of directly planted LBCs in the field were linked to the microclimatic conditions provided by the shade materials used. This study shows that using different shading materials creates distinct microenvironments in terms of temperature, relative humidity, and light intensity, which directly influence LBC survival, new shoot initiation, and ultimately seed yam production. According to Mignouna et al., ²³ the ideal temperature for yam growth is between 25°C and 30°C, with a relative humidity above 80%. These conditions are vital for supporting photosynthesis while preventing heat stress and photoinhibition, ensuring optimal establishment and development of yam LBCs. In this study, the high temperatures recorded within T1 and T2 shade materials, combined with decreased relative humidity, were less favourable for optimal LBC growth. These stressful microclimatic conditions resulted in low vine survival rates at S1 spacing, with very few new shoots emerging. This finding aligns with Pramanik et al., ²⁴ who observed that black plastic used as a soil cover increased soil temperature by absorbing higher levels of net radiation. Additionally, these black mulches raised heat flux in the atmosphere around the covered area. ²⁵ Although our study used the materials to provide overhead shade, the effect was similar to their use as mulch. In another study, it was reported that T1 reduced cucumber germination rates. ²⁶ When the spacing was widened to S2, vine survival improved, and more new shoots formed; however, this came at the expense of vine length and leaf production, leading to fewer tubers and a lower fresh tuber yield. This suggests that although T1 and T2 shade materials supported adequate vine establishment, they were not ideal for initiating new shoots. Nonetheless, they promoted tuber enlargement under lower plant density, which aligns with previous research emphasising the importance of optimal light penetration and microclimate regulation for seed yam production. ²⁷

T3, which is abundant in all tropical and sub-tropical regions of the world, ²⁸ has emerged as one of the most promising shade materials for yam leaf-bud cuttings. At S1 spacing, vine survival under this shade material was 91.2%, and 76.2% of the LBCs formed new shoots, representing a 257.8% increase in new shoot initiation compared to T1. Since plants established under T3 conditions had a good early start, they produced many leaves, resulting in a high LAI, which led to a high tuber number (859,722/ha) and a fresh tuber yield of 20 t/ha. Even when spacing was increased to S2, the reduction in LBC survival and new shoot formation was minimal, while the mean tuber weight was almost doubled. This flexibility in response demonstrates that T3 creates a favourable microenvironment by maintaining cooler temperatures (around 22.3°C) and higher relative humidity, which reduces evaporation, evapotranspiration, and heat stress, supporting both shoot proliferation and tuber development. Previous researchers ²⁹ have reported that T3 helps to reduce evaporation and withstand extremely hot meteorological conditions. In tree nursery management, Carsan and Munjuga ³⁰ recommended T3 for providing shade due to its excellent ability to create optimal microclimatic conditions for young seedlings.

The performance of T4 was fair, especially under denser spacing (S1), where LBCS achieved a good survival rate and above-average new shoot formation. However, when the spacing was increased to S2, vine survival declined significantly, even though vine length improved and the number of leaves increased; as a result, the fresh tuber yield of S1 was 13.4% higher than S2. This again demonstrates the trade-off between plant density and individual tuber development.

The shade net consistently proved to be the most effective shading material in promoting both vegetative growth and yield. Abd El-Naby et al. ³¹ also reported the effectiveness of synthetic shade nets in their experiment on orange trees, where covering the trees with shade nets for a specific period enhanced growth, increased yield, and preserved fruit quality. At S1 spacing, vine survival under the shade net was 91.51%, while the percentage of new shoots reached 82.0%. Other vegetative parameters measured under the shade net were, in most cases, superior to those of other shade materials; the vines were long and produced numerous leaves. Chlorophyll content was also higher, with readings reflecting optimal photosynthetic performance. As a result, the LBCs established under the shade net achieved the highest number of tubers per hectare and the highest fresh tuber yield (24.9 t/ha), despite a relatively lower mean tuber weight. When the spacing was increased (S2), vine survival decreased slightly, but the formation of new shoots remained high. Although the number of tubers per hectare dropped, the mean tuber weight increased by 76.7%. This indicates that the shade net can be optimised through plant spacing to either maximise total yield or enhance individual tuber size. These results align with a recent study, which revealed that shading nets can reduce solar radiation intensity, ³² thus increasing their benefit for optimising seed yam productivity.

Generally, the T6, which ranked as the second-best shade material in seed yam production, also created favourable conditions for high performance across all vegetative and yield parameters measured. This supports the findings of Ogbologwung et al., ³³ who observed that fresh shoot biomass and storage root yield increased significantly (p ≤ 0.05) at closer spacing compared to wider spacing. These results further emphasise that T6 can be a viable shading option, offering the dual benefits of high yield and improved tuber size through strategic spacing.

The results from these various shading materials highlight the vital importance of a suitable microclimate in seed yam production using field-planted LBCs. This study has demonstrated that improved microclimatic conditions—specifically, lower temperatures and higher relative humidity—can greatly boost yam establishment and yield when LBCs are used as planting material. T3, T5, and T6 proved particularly effective, as shown by their high vine survival rates, strong new shoot development, and superior yields. Plant spacing also influenced these shade effects, with closer spacing (S1) typically improving vine survival and tuber numbers due to increased plant density per unit area. In comparison, wider spacing (S2) tended to lessen competition, encouraging larger tuber development. Although T5 was the most effective shading material, creating ideal microclimatic conditions that enhanced vine survival, vegetative growth, and fresh tuber yield, it can be replaced with less costly shade materials such as T3 and T6, which cost $8.1 and $28.1, respectively.

The S1 promoted higher plant density and tuber production per unit area, while S2 favoured larger tuber sizes due to reduced competition for resources. The significant interaction effects between shading and spacing highlight the need for an integrated approach to optimise seed yam production using LBCs. Shade materials should be chosen based on their performance, availability, and cost-effectiveness to maximise fresh tuber yield. The T5 is the most effective option and should be prioritised by seed yam entrepreneurs, followed by the T6, which is less expensive and comparable to the T5 in all measured parameters. Although the yield from the T4 was good, if the seed yam entrepreneur cannot access other shade materials, the T4 should be utilised because it is always available in any locality and season. Following an integrated approach, T4 at S1 is recommended for producing whole minitubers, which are desirable for certified seed yam production.