THE EFFECT OF PLANT POPULATION DENSITY ON THE GROWTH AND YIELD OF CASSAVA (A CASE STUDY OF CASSAVA FARMERS IN IKWERRE LOCAL GOVERNMENT AREA, RIVERS STATE)

CHAPTER ONE

INTRODUCTION

1.1 Background of the Study

Cassava is one of the most widely grown food crops in Nigeria, and Nigeria is the largest producer of the crop in the world. A study prepared for the Food and Agriculture Organization and the International Fund for Agricultural Development put national output at about 34 million tonnes in 2002 (Phillips et al., 2004), and a recent modelling study reports that the country now harvests more than 59 million tonnes of fresh roots a year (Srivastava et al., 2023). Most of this comes from small-scale farmers, for whom cassava is both a staple and a source of cash income (Umunakwe et al., 2015). Any improvement in the yield these farmers obtain from each hectare therefore matters for household food supply, for rural incomes and for the industries that depend on cassava roots.

The yields that farmers actually obtain are, however, far below what the crop can give. Srivastava et al. (2023) assessed the cassava yield gap in Nigeria with a crop model and concluded that nutrient limitation and water limitation together account for the gap between actual and attainable yields, and that current yields could in principle be raised by a factor of five. It is worth noting what their benchmark assumes: the potential yields in the model are calculated for an optimum planting date, planting density and variety. Planting density is thus one of the management decisions on which attainable yield depends, and it is a decision that rests wholly with the farmer.

Work on smallholder farms in East Africa shows why density cannot be studied on its own. Fermont et al. (2009) combined farm surveys with 99 on-farm and six on-station trials in Uganda and western Kenya. They found large yield differences between farms, associated with differences in access to labour, soil quality and the timing of harvest. Even when improved crop establishment and genotypes were used, poor soil fertility, early water stress and sub-optimal weed management reduced yields by 6.7, 5.4 and 5.0 t/ha respectively, and many fields suffered several interacting constraints at once. Two lessons follow. Plant density is only one of several conditions that shape the yield of a cassava field, and a survey of farmers' fields can say something useful about density only if it also records the other conditions that influence yield.

Plant population density in cassava is the outcome of a set of choices: the distance between rows, the distance between stands along the row, the number of stem cuttings placed at each stand, and whether planting is done on the flat, on mounds or on ridges. Each choice carries a cost. Closer planting needs more stem cuttings per hectare, which the farmer must grow, buy or set aside from the previous harvest, and it adds to the labour of planting and weeding. Wider planting saves planting material but leaves the ground between plants uncovered for longer. These trade-offs explain why farmers in the same community plant at different densities, and why the best density cannot simply be read from a table of recommendations.

Trials confirm that these choices change yield. Onasanya et al. (2021) tested tillage, plant density, weed control and fertiliser in 230 farmers' fields in south-western Nigeria over two years and found that planting 12,500 plants per hectare instead of 10,000 raised root yield by 1.5 Mg/ha across tillage treatments, while fertiliser raised it by 4.2 Mg/ha. At Umudike in south-eastern Nigeria, Uko et al. (2026) compared spacings of 1 m × 0.6 m, 1 m × 0.8 m and 1 m × 1 m on a sparsely branching variety and a profusely branching one, and recorded the highest fresh root yields at the two closer spacings. They also found that the response of leaf area to spacing differed between the two varieties. The two studies agree in direction, since moderately closer planting paid, but they were run on specific fields with specific varieties, and neither describes the densities that farmers use when left to decide for themselves.

What farmers actually do is documented less well. Anyanwu (2018) surveyed 120 cassava farmers in Imo State with a structured questionnaire and found that planting spacing was among the least utilised cassava technologies, with a mean utilisation score of 2.4 against a grand mean of 2.6 for agronomic practices; high wage rates, lack of credit, inadequate land, and pests and diseases were the serious constraints. In Abia and Imo States, Nwaobiala (2018) found high overall adoption of cassava agronomic practices, with adoption indices of 86 and 80 per cent respectively, and reported that farmers got their information mainly from radio, extension agents and fellow farmers. Umunakwe et al. (2015), also in Imo State, found that most farmers (92.7 per cent) obtained information on improved cassava varieties from farmers' groups, and that inadequate credit, inadequate information and the land tenure system were the main constraints to adoption. Taken together, these surveys suggest that a farmer can be aware of and nominally follow recommended cassava practice in general while still planting at a density that departs from it, and that cost, land and information, rather than ignorance alone, shape what happens in the field.

Cassava in the south is also commonly grown in mixtures. Anyanwu (2018) records the adoption of cassava/maize/egusi and cassava/maize/telferia intercropping among farmers in Imo State. Intercropping changes the competition that the cassava plant faces, so a density that suits a sole crop may not suit a mixture, and any attempt to relate density to yield on farmers' fields must take the cropping system into account. Ikwerre Local Government Area lies in Rivers State, in the Niger Delta. The sources reviewed for this chapter contain no study of plant population density on the fields of cassava farmers in the area.

The survey will describe the full range of spacings and densities that farmers use, record the yields they say they obtain, capture the cropping system, variety, soil and labour conditions that surround each field, and ask farmers directly why they plant as they do. That information is the necessary starting point for sound advice on cassava planting in Ikwerre, and it is the purpose of this study to collect it.

1.2 Statement of the Problem

There is a large and well-documented gap between the cassava yields that Nigerian farmers obtain and the yields the crop can achieve, and planting density is one of the few factors in that gap that the farmer can change without buying anything but planting material (Srivastava et al., 2023). Trial evidence shows that density matters: moderately closer planting raised root yield in farmers' fields in the south-west (Onasanya et al., 2021) and the closer spacings gave the highest yields at Umudike (Uko et al., 2026). Yet the same evidence shows that the best density depends on the variety and the field, and it cannot be assumed to transfer to farmers in Ikwerre.

The available surveys indicate that spacing is one of the cassava practices that farmers in the south-east follow least consistently (Anyanwu, 2018). The reasons given are cost-related and practical: wages, credit, land and information. What is missing for Ikwerre is any description of the plant populations that farmers actually use, of how those populations vary with farm size, intercropping, variety and the cost of stem cuttings, and of the yields farmers report at different densities. Without such a description, it is impossible to say whether farmers in the area are planting too thinly and forgoing yield they could easily gain, or too thickly and spending on stem cuttings that return nothing.

The consequences are practical. Extension advice on cassava planting in Rivers State has no local baseline against which to be judged, so a single recommendation may be given to farmers whose circumstances differ widely. Farmers cannot compare their own densities and yields with those of others in their community. Researchers who design trials for the area have no farmer-level data on which to base their choice of treatments. Finally, any gain in cassava output that could come from better planting density remains unrealised at a time when Nigeria depends heavily on the crop for food and industrial raw material.

The problem to which this study responds is therefore the absence of survey evidence, for Ikwerre Local Government Area, on the plant population densities used by cassava farmers, on the growth and root yield they report at those densities, and on the factors that limit their choice of density. The study uses a questionnaire survey because it is the appropriate tool for describing farmers' practice and perceptions across the area.

1.3 Objectives of the Study

The main objective of the study is to assess the effect of plant population density on the growth and yield of cassava as reported by farmers in Ikwerre Local Government Area, Rivers State. The specific objectives are to:

(i) describe the socio-economic and farm characteristics of cassava farmers in the area;

(ii) determine the planting spacings and the resulting plant population densities used by the farmers;

(iii) determine the growth (plant vigour, height and branching) and the root yield that the farmers report under different plant population densities;

(iv) determine the relationship between plant population density and reported root yield, after taking account of cropping system, variety and farm size; and

(v) identify the factors that limit the farmers' choice of planting density.

1.4 Research Questions

1. What are the socio-economic and farm characteristics of cassava farmers in Ikwerre Local Government Area?

2. What planting spacings and plant population densities do the farmers use?

3. What growth and root yield do the farmers report under each density?

4. Is there a relationship between plant population density and the root yield that farmers report?

5. What factors limit the farmers' choice of planting density?

1.5 Research Hypotheses

The following null hypotheses will be tested at the 5% level of significance:

H0₁: There is no significant relationship between plant population density and the root yield reported by cassava farmers.

H0₂: There is no significant difference in the reported root yield of farmers who plant at the highest density and those who plant at the lowest density.

H0₃: The socio-economic and farm characteristics of farmers (age, sex, education, farm size, farming experience and cropping system) have no significant relationship with the plant population density they use.

1.6 Significance of the Study

For cassava farmers in Ikwerre, the study will show how the spacings in common use relate to the yields their neighbours report, which gives them a basis for comparison that does not require a trial of their own. Farmers who find that they plant much more thinly or thickly than others who report better yields will have a reason to reconsider.

For the Rivers State Agricultural Development Programme and other extension providers, the findings will indicate whether farmers' densities fall short of recommended levels, and whether the explanation lies in cost, labour, land or information. That distinction matters, because it determines whether the right response is more advice, cheaper planting material, or support with labour.

For researchers, the study supplies farmer-level survey data from the Niger Delta to set beside trial results from Umudike and south-western Nigeria. It also offers a documented starting point for any later on-farm trial in Ikwerre, since the densities that farmers use can be built into the choice of treatments.

1.7 Scope and Delimitation of the Study

The study is limited to cassava farmers in Ikwerre Local Government Area of Rivers State. It is a cross-sectional survey in which a structured questionnaire is administered to farmers who grew cassava in the most recent season. Plant population density is calculated from the row and plant distances and the number of cuttings per stand that farmers report, and growth and yield are the farmers' own reports, checked where possible by measuring a sample plot. The study covers the cropping system, variety and farm size as control variables. It does not cover fertiliser, weed control, pest and disease management, or the processing and marketing of cassava roots.

1.8 Limitations of the Study

a) Reported yields depend on farmers' recall and estimates, and cassava is often harvested piecemeal over several months, which makes the yield of a field difficult for a farmer to state precisely.

b) A survey can show association between density and reported yield but cannot prove that density caused the difference, because farmers who plant at different densities may also differ in soil, variety and management.

c) Fields differ in the age of the crop at harvest, and premature harvesting, which Fermont et al. (2009) found to be associated with lower yields, may affect the comparison.

d) Time and funds limit the number of communities and respondents that can be reached.

1.9 Definition of Terms

Plant population density: The number of cassava plants per hectare, calculated from the row and plant distances reported by the farmer.

Planting spacing: The distance between rows and between plants within a row.

Growth: The development of the cassava plant, described in this study by vigour, height and branching as reported by farmers.

Root yield: The weight of fresh storage roots harvested per hectare, as reported by the farmer.

Stem cutting: A length of cassava stem used as planting material.

Cropping system: Whether cassava is grown alone (sole cropping) or together with other crops (intercropping).

References

Anyanwu, C. G. (2018). Constraints to adoption and utilization of cassava production technologies among farmers in Imo State, Nigeria. Scientific Papers Series Management, Economic Engineering in Agriculture and Rural Development, 18(1).

Fermont, A. M., van Asten, P. J. A., Tittonell, P., van Wijk, M. T., & Giller, K. E. (2009). Closing the cassava yield gap: An analysis from smallholder farms in East Africa. Field Crops Research, 112(1), 24–36. https://doi.org/10.1016/j.fcr.2009.01.009

Nwaobiala, C. U. (2018). Farmers' adoption of cassava agronomic practices and intercrop technologies in Abia and Imo States, Nigeria. Journal of Agricultural Extension, 22(2).

Onasanya, O., et al. (2021). On-farm assessment of cassava root yield response to tillage, plant density, weed control and fertilizer application in southwestern Nigeria. Field Crops Research, 262, 108038. https://doi.org/10.1016/j.fcr.2020.108038

Phillips, T. P., Taylor, D. S., Sanni, L., & Akoroda, M. O. (2004). A cassava industrial revolution in Nigeria: The potential for a new industrial crop. International Fund for Agricultural Development & Food and Agriculture Organization of the United Nations.

Srivastava, A. K., Ewert, F., Akinwumiju, A. S., Zeng, W., Ceglar, A., Ezui, K. S., Adelodun, A., Adebayo, A., Sobamowo, J., Singh, M., Rahimi, J., & Gaiser, T. (2023). Cassava yield gap—A model-based assessment in Nigeria. Frontiers in Sustainable Food Systems, 6, Article 1058775. https://doi.org/10.3389/fsufs.2022.1058775

Uko, I., Ekeleme, F., Amadioha, A. C., Chukwu, L. I., & Uko, S. C. (2026). Effect of weed management methods and plant spacing on the growth and yield of cassava morphotypes. Agriculture, Food, and Natural Resources Journal, 5(1), 11–20. https://doi.org/10.5281/zenodo.20049591

Umunakwe, P. C., Nwakwasi, R. N., Ani, A. O., Ejiogu-Okereke, E. N., & Nnadi, F. N. (2015). Constraints to the adoption of improved cassava varieties among rural farmers in Imo State, Nigeria. Asian Journal of Agricultural Extension, Economics & Sociology, 6(1), 56–63.