Mechanization, fertilization and staking options for environmentally sound yam production

An on-station study at Fumesua and Ejura in Ghana with two yam varieties (Pona and Dente), seedbed option (ridge and mound) and NPK fertilizer rates (0, 45-45-60, 60-60-60 and 60-60-80 kg ha N-P205-K20) revealed significant (p < 0.05) increases in soil carbon and phosphorus with fertilizer application to yam. Fertilized yam had acceptable culinary qualities. Another study on staking options using a promising non-staked yam line TDR95/19177 showed that non-staked yam resulted in a high (32.5%) yield reduction, suggesting the need for further plant breeding work. On–farm studies showed significant (P  0.05) interaction between fertilizer rate and seedbed preparation method on continuously cropped lands, with mechanised and manual ridging having significantly (p < 0.05) higher yam plant population (5,5037,483 plants ha) than farmers’ mounding (4,219-4,579 plants ha), and a greater yield response to fertilizer. Benefit Cost Ratio (BCR) was highest (2.7:1) when 45-45-60 kg ha N-P205-K20 was applied to yam on ridges. On newly cleared fields, yam planted on ridges with trellis/minimum staking gave highest tuber yields and BCR of 3.8:1. Mechanised ridging for yam seedbed preparation fertilizer application and the significant reduction in the use of stakes will reduce drudgery and the contribution of yam production to deforestation and climate change.


INTRODUCTION
Yam is one of the two major root crops produced and consumed in Ghana and West Africa.It is currently the major non-traditional export crop of Ghana.For the past decade, Ghana since 2008 has ranked second in world yam production and contributes about 16% to the National Agricultural Gross Domestic Product (FAOSTAT, 2014).Ghana is also Africa's leading exporter of yam, exporting 94% of yam from West Africa (FAO, 2013).However, yam production is characterized by clearing of new areas on yearly basis in search for fertile lands leading to deforestation and soil degradation.
Yam is a heavy soil nutrient feeder; a ton of yam is reported to extract 3.8 to 4.0 kg/ha, 0.39 to 1.1 kg/ha, 4.2 to 5.9 kg/ha of N, P 2 O 5 and K 2 O, respectively (Ferguson and Haynes 1970;Le Buanec, 1972).The struggle for fertile lands coupled with the increasing human population has led to pressure on cropland and forestlands in the yam growing communities (Akwaag et al., 2000;Asante, 1996).There is therefore, reduced length and quality of fallowed land and available fertile lands, and distances to yam fields are increasing and becoming more difficult to access.Where pressure on land is high, farmers are increasingly compelled to grow yam on non-fallowed land.For example in Sekyere-West and Ejura-Sekyedumasi districts of Ghana, a major yam growing area, the forested land before 1983 was 782 km 2 , it was predicted that by 2010 it would reduce to 78.2 km 2 whiles the grasslands would increase from 1337 to 2247 km 2 in the same period (Akwaag et al., 2000).
Gradually, the transitional zones are giving way to grasslands resulting in shortage of staking material for the cultivation of yam especially in the Guinea savannah zones.Farmers have therefore reported declining yields of yam (Asante, 1996;Sagoe and Sally, 2006).Moreover, staking which is an integral part of yam production has been identified as a major contributor to deforestation and a major constraint for yam production especially in the Guinea savannah where there is unavailability or scarcity of stakes (Akwaag et al., 2000;Asante, 1996;Ndegwe et al., 1990).The search for stakes is laborious and forms about 20% of the labour requirement and major cost of yam production (Asante, 1996;Koli, 1973;Wholey and Haynes, 1971).There is therefore the need to focus research activities on breeding, agronomy, soil management and other management practices to provide farmers with technology to produce yam without stakes or with minimum number of stakes without a corresponding reduction in yields.This will increase yam production and enhance the potential of the crop to reduce household food insecurity and reduce poverty among producers, processors and traders (IITA, 2010) and mitigate This paper reports on-station and on-farm studies from 2009 to 2011 with the goal of investigating the contribution of chemical fertilizer, mechanized seedbed preparation and staking options to sedenterizaton of yam production and reduce the associated drudgery.The specific objectives were: i) To evaluate the effect of staking options on the growth and yields of a promising non-staked yam line TDR95/19177, ii) To evaluate effect of fertilizer and seedbed preparation options on yam varieties on-station, iii) To evaluate the effect of seedbed preparation options and staking options on performance of yam on newly cleared farmers' fields, iv) To evaluate the effect of fertilizer and seedbed options on the performance of yam on continuously cropped farmers' fields.

Seedbed preparation and fertilizer study
The study was conducted at Fumesua in the Forest, and also in Ejura in the Forest savannah Transition where there is a greater chance for mechanization.It was on-station researcher managed in years 1 (2009) and 2 (2010) and then moved on-farm in year 3 (2011) in Hiawoanwu, Aframso/Teacherkrom, and and Atebubu communities in the Forest-transition zone.The mean annual rainfall during the study period in Fumesua and Ejura were similar with 1,356 and 1,360 mm, respectively (Table 1).The on-station was conducted in a split-split plot design with three replications.The main plot treatments were yam varieties with two levels (Dioscorea rotundata) and subplot treatments with two levels of seedbed preparation (Ridges and Mounds) whiles the sub-subplot treatments were fertilizer rates (0, 45-45-60, 60-60-60 and 60-60-80 kg ha -1 N-P 2 0 5 -K 2 0).Ridges were constructed mechanically (Ejura) and manually (Fumesua).Fertilizer was 50% split applied at 4 and 12 to 16 weeks after planting.Yam was planted at a spacing of 1.2 × 1.2m with target poulation of 6,944 plants ha -1 .Planting was done with first rains in March and April.Data were collected on soil physico-chemical properties (0 to 30 cm depth) at planting and at harvest, stand establishment, rate of sprouting, shoot growth (leaves and vine), yields, yield components and tuber shape.
At on tuber, a randomized complete block experiment with each farmer as replication was esatblished in 2011 for 4 farmers on continuously cropped fields, traditionally not used for yam production.A 2 x 2 Factorial Randomized Complete Block Design with each farmer as a replicate was used.The factors were fertilizer rate (0,45-45-60 kg ha -1 N-P 2 0 5 -K 2 0) and seedbed preparation (farmers' mound, mechanized/manual ridge of 40 to 45 cm height).Farmers' variety Dioscorea alata and Dioscorea rotun data were used in Hiawoanwu and Aframso/Teacherkrom communities, respectively near Ejura.Fertilizer was 50% split and applied at 4 and 12 to 16 weeks after the planting).Data was collected on soil physico-chemical properties at 0 to 30 cm depth at planting and at harvest, fresh and dry weight of tuber and yield components.

Seedbed preparation and staking options study
The study was conducted in Fumesua in 2011 with TDR 95/19177, a yam line with potential for high yield under no-staking (Otoo et al., 2008).The experimental design was a Randomized Complete Block Design (RCBD) with three treatments (Vertical staking, Trellis/minimum staking (80% the number of stakes in vertical staking) and no-staking) and three replications.There were two plants within a row to one stake in the vertical staking.Stakes used were 1.5 m high for Trellis/minimum staking and 2 m for the vertical stakes.Yam was planted at a spacing of 1.2 x 1.2 m with target poulation of 6,944 plants ha -1 .Data were collected on number of stands at 31st day after planting till time of harvest, above ground biomass (leaf and vine weight), yield and its components.Following the on-station study, farmer managed on-farm verification and demonstrations were conducted on typical newly cleared yam fields in Hiawoanwu and Aframso/Teacherkrom farming communities in the Ejura/Sekyedumase district in 2011.The study was conducted in a 2 × 2 factorial Randomized Complete Block Design with 4 farmers.Each farmer was considered as a replicate the number of stakes in the trellis were further reduced to 50% of vertical staking.The two factors therefor were staking (trellis/minimum staking 50% of vertical, and farmers' staking) and seedbed preparation (mechanized/manual ridge and farmers' mound).Stakes ranged between 1.5 to 3 m high for trellis/minimum staking 1.5 to > 5 m (trees) for farmers's staking.Planting on ridges was at 1.2 × 1.2 m, set size ranged between 170 to 400 g and no fertilizer was applied.The total annual rainfall during the period of the study on-farm was 1186.7 mm (Table 1).Data in all the studies were subjected to analysis of variance at 5% significant level using the Statistical Analysis Software (SAS, 2007).

Changes in soil chemical properties
There were significant (p  0.05) increases in soil carbon, organic matter and phosphorus with fertilizer application to yam.However, there was no effect of seedbed preparation method on the soil chemical properties (Figures 1 and 2).

Yields
There were no significant difference between yam planted on ridges and mounds (Table 2).The similarity of yields on ridges and mounds and of fertilizer application (Table 3) at the station can be attributed mainly to similarity in plant population and poor rainfall pattern.At the on-farm, higher plant population density on ridges compared to farmers' mounds may have been a major contributor to the higher tuber yields on ridges (Figure 3).In farmers' fields, there was a significant (p  0.05) interaction between seedbed preparation method and fertilizer application with similar yields obtained for yam on ridges without fertilizer application and yam on mounds with 45-45-60 kgha -1 N-P 2 O 5 -K 2 O. Yield increase due to fertilizer application ranged from 22% on mound to 28% on ridges.Contrary to the perception by farmers and consumers, that fertilizer application to yam reduces its culinary features, sensory evaluation by farmers and consumers in the study found fertilized yam to have acceptable culinary qualities (Table 4).

Seedbed preparation and staking study
The on-station study showed significant (p0.05)differences in the fresh and dry weight of the treatments Fertilizer application rate (kg ha -1 N-P 2 0 5 -K 2 0)   (Table 5).Vertical staking option had highest fresh and dry leaf and vine weight which translated into highest fresh and shoot weights, tuber yields and numbers (Table 6).No significant (p  0.05) differences were found in the shoot growth, tuber yield and tuber numbers between the trellis/minimum and no-staking options.No-staking led to 32.5% reduction in yield as compared to vertical staking option.At on farm, a significant interaction (p  0.05) was observed between seed bed preparation method and staking option in tuber yield and its components.
Fertilizer application rate (kg ha -1 N-P 2 0 5 -K 2 0) Available P[ppm]    Irrespective of the staking option, yam planted on mechanized and manual ridges had significantly (p  0.05) more number of stands at harvest which translated into significantly (p  0.05) more number of tubers and total tuber fresh weight than yam on mounds (Figure 4).However, ridges with Trellis/minimum staking had significantly (p  0.05) higher number of stands at harvest, number of tubers and total fresh weight of yields as compared to the other treatments.

Partial budget and benefit cost ratio analysis of onfarm studies
Partial budget and benefit cost ratio for yam onfields which had been continuously cropped for 5 to 20 years showed that net benefit value for yam planted on ridges and fertilized gave the highest value of 4,815.5 per hectare (Table 7).It was even more profitable to plant yam on ridges without applying fertilizer than on mounds Fertilizer rate (kg ha -1 N-P 2 0 5 -K 2 0)  with fertilizer application.The benefit cost ratio of 2.7:1 was highest for this treatment.This implies that a farmer who invests ₵1.00 would gain an additional ₵1.70 when yam is planted on mechanized or manual ridging and fertilizer is applied.Partial budget and benefit cost ratio for yam on newly cleared lands with trellis/minimum staking and farmer staking options also revealed that the ridge option still gave the highest net benefit values compared to mounds with trellis/minimum staking resulting in the highest benefits (Table 8), with benefit cost ratio was 3.8:1.A farmer would gain an additional ₵2.8 for every ₵1.00 that is invested in planting yam on ridges with Trellis/minimum stakes than farmers' current practice.

DISCUSSION
The significant (p  0.05) high shoot growth of the vertical staking treatment (Table 5) suggests vertical staking enhanced more shoot development as compared to trellis/minimum stake and no staking.This translated into significantly higher tuber numbers and yields (Table 6).However, no staking treatment option resulted in a 32.5% yield reduction of TDR95/19177 yam.This reduction is high, and consistent with an earlier report by Otoo et al. (2008) which however in addition reported a significantly (p  0.05) higher yield reduction (46.5 to 53.5%) of nostaked D. rotundata and D. alata varieties compared to TDR95/19177.The use of stakes to support yam vines has been identified as one of the reasons why farmers clear new lands and major contributor to deforestation (MTADP, 1990).There is the need to breed either through varietal screening or hybridization for a yam variety which can produce higher yields with minimum or no staking than TDR95/19177.This variety may have among other characteristics, high biomass production and high resistance to diseases and pests and high weed suppression.This would reduce the pressure on forest and the labour associated with the search for stakes for staking.The insignificant (p > 0.05) differences in the yields of the yam under ridges and mounds (Table 3) at the on-station might be due to the similar number of stands.However, in the verification trials on the farmers fields (Figure 4), there was a significant (p  0.05) interaction between seedbed preparation method and fertilizer application with similar yields obtained for yam on ridges without fertilizer application and yam on mounds with 45-45-60 kgha -1 N-P2O5-K2O.Ridges appears to increase the efficiency of fertilizer use possibly through increased plant popultion over farmers practice of mounding (Figure 4), and also increased moisture retention (Ennin et al., 2009;Tetteh and Saakwa, 1994) Yield increase due to fertilizer application ranged from 22% on mound to 28% on ridges.It is recommended that emphasis be put on ensuring that ridgers are accessible to farmers in the major yam producing areas in Ghana and the sub region.In Ghana, the Government's policy of establishment of Agriculture Mechanization centres must be capitalized on, to promote access to ridgers.This would reduce the drudgery associated with yam production and expand the scale of yam production.The significant increases in soil carbon when yam was fertilized could be attributed to greater vegetative growth, and subsequent litter fall and decay in the soil (Figure 1).The increase in soil phosphorus availability to yam increased with increasing rate of fertilizer application (Figure 2) and could be attributed to reported Mycorrhyza association with root and tuber crops (Onwueme andCharles, 1994: Zaag et al., 1980).The response of yam to fertilizer application could be improved with supplementary irrigation due to intermittent drought spells that occur during the growing period of yam.Fallowed fields/virgin lands which have soil fertility restored are what farmers would normally use for yam production (Akwag et al., 2000;Asante, 1996).However, due to population increases and pressure on croplands, these lands are not available or farmers have to travel longer distances to grow yams compared to fields of other food crops (Young, 1997;Nair, 1984).Figures from the Ministry of Food and Agriculture indicate that the yam production in Ghana increased over 51.6% between 1998 and 2007 with a corresponding increase in area of cultivation of about 53.6% in the same period.It is therefore projected that growth rate for yam in Ghana would be 2.71% per annum (PPMED, 2007).This suggests that, a 1% increase in area under yam cultivation would lead to a corresponding increase in yam yields of about 1%.However, with the increased pressure on cropping land as a result of population increase and the concomitant shortening of fallow periods, it would not be able to support this trend to ensure food security (Quansah et al., 2001;Garrity, 2004).
The result of this study has demonstrated fertilizer application can be used as a technique to increase productivity on continuously cropped lands and still achieve appreciable yields, thereby reducing the rate of deforestation due to yam cultivation.Also, contrary to the view that the use of fertilizer in yam production affects the quality of yam, sensory evaluation showed that the culinary qualities of fertilized yam is good and could even be better than unfertilized yam (Table 4).Reducing the number of stakes from optimum (2 plants per stake) by 50% (trellis/minimum staking) did not reduce the yields.Therefore, it appears trellis/minimum staking with stakes with heights of 1.5 m to 3 m high would favour shoot growth and better display of leaves for capture of photosynthetic active radiation for plant growth and higher tuber yields.Quantity of stakes can be further reduced by the use of ropes and this is being verified/demonstrated in 2010 in farmers' fields.It is recommended that farmers reduce the number of stakes used in staking by 20 to 50% whiles breeding effort continues in search of a yam variety, which would give high yields without staking.

Conclusion
Ridging as a method of land preparation for yam production is agronomically feasible and has a positive Ennin et al. 2229 interaction with fertilizer application, resulting in higher yields compared to farmers' practice of manual mound construction.It has the potential to remove drudgery and increase scale of production through mechanization, and improve fertilizer use efficiency.It is recommended that, emphasis be put on ensuring that ridgers are accessible for purchase or ridging provided as a hired service by farmers.Trellis/minimum staking with stakes taller than 1.5 m high of yam planted on ridges has the potential for higher yam yields and profits than farmers' current staking practices and planting on mounds.No staking of TDR95/19177 a line with promise for high yields under no-staking resulted in 32% yield reduction, and further breeding work is also required to develop varieties which will have minimum yield reduction under no-staking.Continuously cropped fields hitherto not used for yam production can support yam crop with minimum fertilizer application and is profitable.Further studies are needed in water management and time of planting to increase the fertilizer use efficiency.This coupled with mechanised ridging for yam seedbed preparation and the significant reduction or elimination of the use of stakes will greatly reduce the contribution of yam production to deforestation and climate change in the face of the increasing importance of yam as a food security and export crop in the West African sub-region.

Figure 3 .
Figure 3.Effect of seedbed preparation and fertilizer application on yam plant stand and yields on-farm 2011.
same letter are not significantly different; ns-Means are not significantly different from each other at 5% significant level.

Figure 4 .
Figure 4. Effect of staking options and seedbed preparation on yam plant stand and yields on-farm, 2011.

Table 1 .
Locations and total monthly rainfall data for the period used in the study(2009 -2011).

monthly Rainfall (mm) Location Year/Months Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sept. Oct. Nov. Dec. Totals
Figure 1.Changes in soil pH, percentage organic carbon and percentage organic matter after 2 years of continuous cropping (2009 and 2010).

Table 2 .
Effect of seedbed preparation after 2 years continuous cropping: on-station.
ns-Means are not significantly different from each other at 5% significant level, * means are significant different from each other at 5% significant level.

Table 3 .
Effect of fertilizer application on yam after 2 years continuous cropping: on-station.

Table 5 .
Fresh and dry shoot weight of TDR95/19177 yam line, on-station 2011.

Table 6 .
Yield and components of Non-staking, vertical staking and trellis/minimum staking of TDR95/19177 Yam line, on-station 2011.Means with the same letter are not significantly different; ns -Means are not significantly different from each other at 5% significant level.

Table 7 .
Partial budgeting of continuously cropped farmers' fields with and without fertilizer application 2011.

Table 8 .
Partial budget with trellis/minimum and farmer staking on newly cleared yam fields, 2011.