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The effect of seven different spacing treatments on weeding regimes, tuber yield, stem biomass and leaf biomass of cassava (Manihot esculenta L) was investigated at Akure in the rainforest zone of Nigeria.
The seven spacing treatments were 100x100cm (10,000 plants), 90x70cm (15,000 plants/hectare), 100cmx50cm (20,000 plants/ha), 70x50cm (28,600 plants/ha), 80x50cm (25,000 plants/ha), 65x50cm (30,500 plants/ha), 65x50cm (30,500 plants/ha) and 50x50cm (40,000 plants/ha) with four replications, arranged in a randomized complete design (RCB). A spacing treatment of 100cmx100cm which was conventionally used by farmers served as the control treatment.
The results showed that these spacing treatments significantly (P<0.05) influenced the cassava growth parameters, tuber yield, stem and leaf biomasses and weeding regimes. For the growth parameters, 100x100cm spacing treatment had the highest values of plant height, stem girth and leaf area followed by 100x50cm and 70x70cm. 100x100cm spacing increased the plant height, leaf area and stem girth by 13%, 4.5% and 12% compared to that of 100x50cm spacing.
For tuber yield, stem and leaf biomasses of cassava, 100x100cm spacing had second best value for tuber yield and least value of leaf biomass compared to 50x50cm. The weeding regime was the highest (four times) compared to others.
90x70cm spacing ranked third in term of tuber yield, tuber diameter and population. The weeding rate was three times compared to 100x100cm spacing while the leaf population and leaf litter values were better than that of 100x100cm spacing.
For 100x50cm spacing, it had the highest tuber weight (57.7t/ha) and second best tuber diameter compared to others. When compared to 100x100cm spacing, it had better values of leaf biomass and litter fall. Generally, 100x50cm spacing had good and balanced potentials for cassava root tubers, leaf biomass, stem biomass. It reduced weeding regimes from 4 to 3 compared to 100x100cm spacing.
For 70x50cm spacing, it ranked fifth in term of tuber weight and tuber diameter. It also had the highest weight of plantable stem cutting and produced good stem cuttings. The leaf biomass values were slightly higher than that 100x100cm spacing.
The 80x50cm spacing was fourth in performance in term of tuber weight and diameter. It reduced weeding regime by N12,000 compared to 100x100cm spacing. In term of leaf biomass, it had the second highest value compared to 50x50cm spacing.
The 65x50cm spacing was sixth in term of tuber yield performance but had good potentials for stem biomass and leaf biomass. It reduced cost of weeding by 50% compared to 100x100cm. The 50x50cm spacing had the highest values of leaf biomass, litter fall and reduced weeding rate by 50% compared to 100x100cm, but it was the least in term of tuber weight performance.
The 50x50cm spacing treatment had the highest gross income, net income gain and BC ratio of 6.343 with the highest contributions from sales of cassava stem cuttings and huge leaf biomass compared to BC of 456 in 4.69, 4.27 and 2.92 in 80x50cm, 65x50cm, 100x50cm and 100x100cm spacing treatments respectively. The BC ratio values were greater than 1 in all the spacing treatments which showed profitability of cassava farming business.
In these experiments, 100cmx50cm, 90x70cm and 80x50cm spacing treatments were recommended for farmers who are interested in commercial production of cassava for tuber yield preferably 100x50cm while those who are interested in production of cassava for leaf and stem biomasses as well as reducing cost of weeding should adopt 65x50cm and 50x50cm spacing treatment. This will ensure maximum income benefits from sales of tubers stem cuttings and leaf biomass of cassava.
However, 100x50cm spacing had good tuber yield, stem, leaf biomasses and lower weeding regimes.
Cassava (Manihot esculentum) is a major tuber crop grown by farmers for income and as staple foods. In recent times, it has become an important crop for making industrial starch, tapioca, cassava chips and other processed products in Nigeria which helped to enhance export drive for increased foreign exchange, provision of employment opportunities and for meeting local consumption Adeniyan (2008).
In spite of the above mentioned importance of the crop to economic development, the optimum yields of cassava to meet the needs of the society, have not been attained and the fundamental problems are that the present level cultivation of the crops is low and can not meet up with the local consumption talk less of the cassava processing industries and export drive, coupled with the continued decline in soil fertility and weed competition.
In order to sustain the demand for cassava to meet both local and export needs of the nation, there is need to increase the present cassava plant population per hectare from the present 10,000 plants to between 16,000 and 40,000 plants per hectare as well as maintaining soil fertility.
Interestingly, massive production of cassava at higher plant population will provide substantial leaf biomass for animal production as well as increasing the number of stem cutting available for sustainable cultivation of cassava. A farmer can make between N100,000 and N300,000 from sales of cassava stem cutting only, in addition to the sales of the tubers APAA (2007).
Having reviewed literature critically, there is scarcity of research reports on determining the best of optimum plant population of cassava that will give good performance of tuber yield, stem cutting and leaf biomass using appropriate spacing, thus, there is justification to look into this research for future development.
The objective of this research are (i) to determine the optimum plant population of cassava using seven spacing treatment that will give the highest tuber yield, leaf biomass and stem cuttings and best weeding regimes for cassava sole cropping in Akure Ondo State, Nigeria. (ii) to determine the economic benefit cost of cassava production under these spacing treatments.
Materials and Methods
The experiment was carried out at Akure in the rainforest zone of Nigeria between August 2004 and September 2005 and was repeated between March 2006 and April 2007 to validate the results. The soil is sandy clay loam, skeletal, kaolinitic, isohyperthermic oxic paleustalf (Alfisol). Soil sunny staff (1999). The annual rainfall is between 1100 and 1500mm while the average temperature is 24oC. The land had been under continuous cultivation for 5 years.
Soil sampling and analysis before panting
Thirty core samples were collected randomly from 0-15cm depth on the site using soil auger, mixed thoroughly and the bulk sample was taken to the laboratory, air dried and sieved to pass through a 2 mm screen for chemical analysis. The soil pH (1:1 soil/ water) and (1:2 soil/0.01M CaCl2) solution was determined using a glass calomel electrode system (Crockford and Nowell, 1956) while organic matter was determined by the wet oxidation chromic acid digestion method (Walkley and Black, 1934).
The total nitrogen was determined by the micro kjedahl method (AOAC, 1970) while available soil phosphorous was extracted by the Bray P1 extractant and measured by the Murphy blue colouration on spectronic 20 Murphy Riley (1962) while K, Ca, Na were determined with flame photometer while Mg was determined with an atomic absorption spectrophotometer (Jackson, 1958).
The exchangeable acidity (H+ and A13+) were determined using 0.01M HCl extracts and titrated with 0.1M NaOH (McLean,1965) while the micronutrient (Mn, Cu, Fe and Zn) were extracted with 0.1M HCl (Ogunwale and Udo, 1978) and read on Perkin Elmer atomic absorption spectrophotometer. The mechanical analysis of the soil was done by the hydrometer method (Bouycous,1951).
The land was cleared, ploughed, harrowed, ridged and laid out into plots. Each individual plot size is 12x17m2 (204m2) totaling 30 plots. There were discards of 1mx1m between and within plots for easy accessibility and for carrying out other cultural practices without trampling on the cassava plants. There were seven spacing treatments namely; 90cmx70cm, (15,000 plants/ ha), 100x50cm (20,000 plant/ha), 70x50cm (28,600 plant/ha), 80x50cm (25,000 plant/ha), 65cmx 50cm (30,800 plants/ha) and 50cmx50cm (40,000 plants/ha with four replications and arranged in a randomized complete block design (RCB). A spacing treatment of 100cmx100cm which was convectionally used by farmers served as the control treatment. Six tonnes/ha of poultry was applied basally to the field to supply initial nutrients and allowed 7days to decompose. Planting of viable cassava stem cuttings of 30572 variety into different plots based on the above mentioned spacing started on September 6 and ended on September 13, 2004. Specific labels sharing these spacing treatments were placed on the plots for easy identification and data collection.
The plots were sprayed with pre-emergence herbicides Atrazine and Gramozone to kill weed seeds. The stem cuttings sprouted and the first weeding took place after two months of sprouting. Another three weeding were done for 100cmx100cm while 65cmx50cm, 50cmx50cm and 80cmx50cm spacing treatments had only one extra weeding after the first weeding.
At 3 months after sprouting, Avesthrin (Cypermethrin 10% EC) was sprayed at 10ml per 10L of water to control white flies responsible for the widespread of cassava mosaic disease.
Field data collection on the growth parameters (plant height, leaf area and stem girth) were done every week starting from the 3rd week after sprouting till four months after sprouting. Another spraying of Avesthrin (a.l Cypermethrin 10% E.C) to control white flies was done at 6 months after sprouting. The bushes adjourning the cassava plantation was cut back to prevent rodent and grass cutter attack of cassava plants.
At 11 months after sprouting on the field, sampling harvest was done on August 22, 2005 using 3mx3m for each following parameters were determined which include tuber weight (kg), tuber diameter (cm), tuber population per plant, leaf biomass (weighed of 50 leaves (kg), and number of leaves per plant, weight of plantable stem cuttings (g), number of weeding regimes, estimate cost of weeding (N12,000 per hectare), weight of litter fall and number of good stem cuttings.
The average data for the 2 years on the growth parameters, tuber, stem and leaf yield parameters of the two experiments were analysed and subjected to ANOVA F-test and their means were separated using Duncan Multiple Range Test at 5% level of significance, Gomez and Gomez (1984).
Soil chemical composition before planting of cassava
The chemical properties of the soil before planting of cassava are presented in Table 1.
Based on the established critical levels for soils in South west Nigeria, the soil was slightly acidic with pH 5.6 and low in organic matter compared to the critical level of 3% O.M (Agboola and Corey, 1973). The total nitrogen is less than 0.15% which is considered optimal for most crops (Sobulo and Osiname, 1981).
The available P was less than 10mg/kg P considered as adequate for crop production (Agboola and Corey, 1973). The exchangeable K, Ca, Mg and Na were lower than 0.2mmol/kg critical levels (Folorunso et al. 2000) while the soil bulk density is high (1.60mgm-3).
Effect of different spacing treatment on the growth parameter of cassava.
There were significant increases (p<0.05) in the plant height, leaf area and stem girth of cassava under different spacing treatments. (Table 2).
The spacing 100x100cm treatment gave the highest values of cassava plant height, leaf area and stem girth followed by 100x50cm and 70x50cm respectively. For instance, 100x100cm spacing treatment increased the plant height, leaf area and stem girth of cassava by 13%, 4.5% and 12% respectively compared to 100x50cm spacing.
The 65x50cm and 50x50cm spacing treatments had the least values of cassava plant height, leaf area and stem girth.
Effect of different spacing treatments on the tuber, stem and leaf biomass yield of cassava.
There were significant increases (p<0.05) in the tuber, stem and leaf biomass yield of cassava under different spacing treatments (Table 3).
For 100x100cm spacing, the plant population is 10,000 plants. The tuber weight is 54.4t/ha while the tuber diameter and population values were 5.1cm and 7 respectively. The implication is that the convectional spacing employed by farmers would give an average of 54.4 t/ha tuber but the cost of weeding was at the highest at N48,000 which might not give much profit margin (Table 4).
The weight of 50 leaves sampled and leaf population were 0.11kg and 103 in 100x100cm compared to 0.25kg and 405 in 50x50cm spacing showing that the leaf biomass was the least of all the spading treatments.
Farmers employing this spacing might not make much money from the export of the leaf biomass, also, the litter fall on the ground was 50g thick compared to 250g of 50x50cm. The litter fall on the ground was the least signifying fastest rate of erosion and soil fertility decline than other treatments. In addition, good stem cuttings were obtained from the plots for the planting season.
For 90x70cm spacing treatment, it ranked third in term of tuber weight (52.2t/ha) tuber diameter (4.6cm) and tuber population (5). It has a population of 15,900 plants per hectare and the weeding rate was three time at N12,000 each (N36,000:00). The implication is that farmers adopting this spacing for cassava would save at least N12,000 from weeding cost compared to 100x100cm, thereby, enhancing higher profit margin.
In addition, the weight of 50 leaves 0.13kg and leaf population is 176 showing higher leaf biomass than the convectional spacing 100x100cm. The litter fall is 121.1g showing that the thickness might influence the rate of erosion and increased soil fertility maintenance. Nevertheless, it has lower weight of plant able stem cuttings than 100x100cm spacing.
The 100x50cm spacing treatment, it had the highest tuber weight (57.7t/ha) and second best tuber diameter compared to others. It also had plant population of 20,000 plants per hectare, hence increasing the plant population and also reduced the weeding regimes to three compared to four or more weeding regimes in 100x100cm spacing which led to reducing the cost of weeding by N12,000.
In term of leaf biomass, the weight of 50 leaves was 0.15kg and leaf population of 155 which were higher than that of the convectional spacing of 100x100cm used by farmers showing that they would make additional income from sales of leaves as source of feed to animals. The litter fall weight was 155.4g showing that the spacing had great potential in reducing soil erosion fertility decline.
In term of stem biomass, it had on average of 21 good stem cuttings per sampled plants showing that farmers would also make money from sales of stem cuttings (i.e. planting materials).
Generally, the 100x50cm had good potentials for root tuber, leaf biomass and stem biomass yield.
For 70x50cm spacing treatment, the plant population is 28,600 plants per hectare. It was fifth in performance in term of tuber weight (33.3t/ha) and tuber diameter (3.1cm). Thus, the increased cassava population also reduced the weeding cost by N12,000 for farmers.
The leaf biomass for 50leaves (0.12kg) and leaf population were slightly higher than that of 100x100cm spacing. Nevertheless, the weight of litter fall of 199.6g showed that it had great potential in reducing rate of erosion. In addition, this spacing treatment had the highest weight of plantable stem cuttings (22) per sampled plants.
For 80x50cm spacing treatment, the plant population is 25,000 per hectare and it was fourth in performance in term of tuber weight 50 tonnes/hectare and tuber diameter of 4.5cm compared to that of 100x100cm, 100x50cm and 90x70cm spacing. This showed great potentials in producing good tuber yield, interestingly, it had the highest number of tubers (9) per plant.
The spacing also reduced the cost of weed by N12,000 compared to the convectional spacing of 100x100cm used by the farmers.
In term of leaf biomass, the weight of 50 sampled leaves and leaf population were 0.17g and 366 respectively showing its potential in producing cassava leaves for export or as source of feed to animals or for food consumption.
The weight of litter fall was 180kg showing that the higher plant population reduced the rate of erosion. In term of stem biomass, it had on average 25 good stem cuttings per sampled plants and third best weight of plantable stem cutting implying that farmers using this spacing for cassava would receive additional income from sales of stem cuttings.
For 65cmx50cm spacing, the plant population is 30,800 plants per hectare and its performance was sixth in term of tuber weight 28.8 tonnes/hectare (26kg/9m2) and tuber diameter of 2.6cm. This implied that it might not be good for cassava tuber yield production.
However, in term of leaf biomass, it had second best weight of sampled 50 leaves (0.2kg) and an average leaf population of 244.
For the stem biomass, the number of good stem cuttings were 28 per sampled plants and farmers would make additional income from sales of stem cutting. However, the major problem was that the stems were thin in nature caused as a result of etiolation.
In term of soil fertility maintenance, the litter fall weight was 200.8g which effectively checked the rate of erosion and weed growth.
The weeding rate was done two times before total covering of the land by the plants. The implication is that this spacing had reduced the cost of weeding from N48,000 in 100x100cm spacing to N24,000 which is 50% reduction. This is a significant achievement considering the fact that the low cassava tuber yield would have been compensated for by cost of production, high returns on stem cuttings and leaf biomass.
For 50x50cm spacing treatment, the plant population is 40,000 plants per hectare and its performance was the least in term of tuber weight (26.6 tonnes/ha or 24kg/9m2) and tuber diameter of 1.8cm.
In term of leaf biomass, it had the highest weight of sampled 50 leaves (0.25kg) and leaf population of 405 implying that this spacing treatment would be the best for farmers wishing to produce cassava for leaves export or as source of feed to animals.
It also had the highest weight of cassava leaf litter fall (250g) showing that it reduced most the rate of erosion and soil fertility decline. This was evident on the field as it had little or no weed growth. This led to low weeding rate of two compared to four or more weeding rates in 100x100cm spacing and a benefit cost gain of N24,000 from weeding exercise alone.
In term of stem biomass, it had the least number of good stem cuttings of 19 and weight of plant able stem cuttings (0.58g) because of high population and etiolation. This spacing is also good for those who are interested in setting up cassava starch and tapioca production enterprise because the large number of small sized tubers available which are not lignified.
Economic cost benefit for tuber, stem and leaf biomass yield under different spacing treatments
Table 5 presents the yields of tuber, stem and leaf of cassava, the gross income derived from them, the variable cost, net income and benefit cost ratio under different spacing treatments.
100x100cm spacing treatment had the least net income gain of N567,800 and benefit cost (BC) ratio of 2.92 compared to net income gain of N1448,200 and BC ratio of 6.343 in 50x50cm spacing (40,000 plants).
The highest contribution to the BC ratio and net gain income in 50x50cm spacing came from the sales of stem cuttings and huge leaf biomass which are at present considered as negligible contributions financially by majority of farmers. For-instance, in 50x50cm spacing, leaf biomass/hectare gave about 10t/ha and 1 million naira from sales compared to N110,000 in 100x100cm spacing.
Therefore, the higher the BC ratio of an enterprise such as cassava farming, the more profitable it is to the farmers. Interestingly, the lower tuber yields in 50x50cm, 65x50cm spacing had been effectively compensated for by the high stem and leaf yields.
The 100x50cm (20,000 plants/hectare) had good financial returns from the tuber, stem and leaf biomass totaling N1192400.00, net income gain of N913,400 and BC ratio of 4.27 which ranked fourth after 50x50cm, 80x50sm, 65x50cm spacing treatments respectively.
The initial low soil fertility caused as a result of continuous cultivation of land for five years was solved by the general basal application of 6t/ha of poultry to the soil before cultivation of cassava. Therefore, the application of the poultry had enhanced growth of cassava in all the spacing treatments.
The low plant population of 10,000 in 100x100cm encouraged quick growth of weeds, thereby increasing the cost of production compared to 65x50cm and 100x50cm spacing. The implication is that the high cost of production would reduce the profit margin of farmers hence, the least value of BC ratio. This observation agreed with the views of Smith (2002) who reported faster emergence of weeds in Indian spinach plots with wider spacing because of the high isolation rate which encouraged weed competition and high weed density.
The low plant population in 100x100cm spacing also reflected in the least values of leaf biomass and liter fall. The implication is that the wider planting spacing between cassava plants exposed the soils to torrential rains and run-off which resulted in fastest rate of erosion and probably fertility decline than other spacing treatments. This observation agreed with Salako et al. (2008) who reported that low litter fall in yam/maize rotation resulted in low fertility status and increased the rate of erosion.
In-addition, the second best yield of cassava tubers, presence of good stem, cuttings and good growth parameters in 100x100cm could be attributed to the low plant population which encouraged maximum exploitation of nutrients by the cassava crops for big tubers (tuber girth or diameter), good leaf area and stem girth values.
This observation agreed with views of Osundare (2008) who reported that low plant population in either cassava sole or intercrop always resulted into good tuber yield because the tubers had enough nutrients to utilize for growth.
The increased plant population in 90x70cm spacing to 15,000 plants reduced the weeding rate from four in 100x100cm spacing to three. The implication is that farmers adopting this spacing for cassava would save N12,000 from weeding cost (25% reduction) and consequently, enhanced higher profit margin.
This observation agreed with the views of Nyen de et al 2001 who reported that reduced weed profiles and good management assessment occurred during increased finger millet production.
However, the increased plant population of cassava in 90x70cm, 100x50cm, 65x50cm, 80x50cm and 50x50cm can spacing was responsible for higher leaf biomass, litter fall weight, stem cuttings and leaf population compared to the 100x100cm. The implication is that farmers adopting these spacing treatments would make additional income from export of leaf biomass hence, they had the highest BC ratio values and there would be a better crop/livestock enterprise development because of the leaves serving as feeding materials. This observation agreed with the views of AIT (2007) who reported that varying cassava plants density helped to increased the number of good leaf biomass and stem cuttings of cassava.
However, the presence of some etiolated stems of cassava in 65x50cm, 50x50cm spacing could be as a result of intra specific competition among the plants for nutrients, water and air.
The highest tuber weight, moderate tuber diameter, good stem cuttings and leaf biomass in 100x50cm compared to 100x100cm, 80x50cm, 65x50cm could be due to its moderate plant population which reduced intra specific competition between them, thereby resulting into good tuber yields, leaf biomass and stem cutting yields and BC ratio of 4.27. This observation agreed with the work of Osundare (2008) who reported that 20,000 plants in maize/cassava intercrop gave significant tuber and grain yields of both crops compared to low yields in 30,000 plants/hectare, 40,000 plants per hectare and 50,000 plants per hectare respectively.
Therefore, the low tuber weight and diameter of cassava in 65x50cm, 50x50cm and 70x50cm could be due to the intra specific competition between the plants as a result of high plant population. This observation agreed with that of Osundare (2008) who reported that production of small sized cassava roots was associated with increasing population of maize or cassava in the intercrop, between 30,000 and 50,000 plants, thus there will be decline in quantities of commercially acceptable roots.
However, the production of many small sized tubes from these spacing treatments will help to serve as suitable raw material for starch and tapioca industries. This is because of low lignin content. For-instance, the presence of 40,000 or more small sized tubers will go a long way in meeting raw materials needs of cassava processing industries.
In-addition, the 50x50cm and 65x50cm spacing had the highest leaf biomass, and litter fall which would serve as additional source of income for farmers. This is against the backdrop that farmers only derived maximum benefits from cassava tubers alone and neglected the income to be derived from stem cuttings and leaf biomass. For-instance, many East African countries such as Democratic Republic of Congo, Tanzania, Kenya and Rwanda used cassava leaves for soup preparation and also to feed their livestock as observed by Cam (2007).
Furthermore, the weeding regimes in these spacing treatments reduced from four to two signifying a remarkable reduction of N24,000 (50%) in weeding cost. This would definitely reduce the cost of production and increase profit margin analyses for the farmers. This observation agreed with the views of Rye (2007) who reported that optimal and higher plant density of cassava/maize mixture covered the ground, thereby, reducing weed density, growth and competition as well as covering the soil against soil erosion.
Also, the highest values of litter fall in 50x50cm and 65x50cm spacing signified the usefulness in checking the rate of erosion and fertility decline. The higher value for litter fall will preserve the soil physical and chemical attributes for continuous productivity of cassava crop by farmers.
The 50x50cm spacing had the highest value of BC ratio, gross income, net income gain and this could be due to its highest contributions from sales of stem cuttings and leaf biomass which are at present considered as insignificant by majority of farmers. Adegeye (2004) reported that the higher the BC values, of an enterprise, the more profitable it is financially. This should be considered by stakeholders so that maximum financial benefits could be derived from cassava production.
Recommendation and Conclusion
From this experiment, the spacing treatments for cassava at 100x50cm (20,000 plants per hectare), 50x50cm (25,000 plants) and 90x70cm (15,900 plants per hectare) compared favourably with the convictional spacing 100x100cm (10,000 plants per hectare) in term of tuber yield, weeding rate reduction, low cost of production, leaf biomass and litter fall while 65x50cm (30,500 plants per hectare) and 50x50cm (40,000 plants per hectare) had great potentials for stem and leaf biomass yield of cassava.
These recommendations are that farmers who are interested in commercial cassava tuber yields should adopt 100x50cm, 90x70cm and 80x80cm spacing while those who are interested in leaf and stem biomass yields should adopt 65x50cm and 50x50cm spacing treatments. This will ensure maximum income benefits from the sales of tubers, stem cuttings and leaf biomass by the farmers compared to the present experience of getting income from cassava tubers alone.
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