Effect of Tillage Systems and Vine Orientation on Yield of Sweet Potato ( Ipomoea batatas L . )

In Zimbabwe sweet potato (Ipomoea batatas L.) production is practiced under different tillage systems with varying vine orientations, which contribute to variable yields. Limited research on yield loss due to these different practices is available despite that the majority of farmers are growing sweet potatoes to sustain their livelihoods. A field study was carried out at Midlands State University in the 2013/14 rainy season, to determine the effect of tillage systems and vine orientation on yield of sweet potato. A 2 × 3 factorial treatment structure in a randomized complete block design (RCBD), with three replications was used. Tillage systems had two factors (ridge and mound) and three vine orientations were used (horizontal, fold and loop). Data on storage root length, storage root diameter and storage root weight was subjected to Analysis of Variance (ANOVA) at 5% significance level. There was no interaction (p < 0.05) between tillage systems and vine orientations on storage root diameter, storage root length and storage root yield. The horizontal vine orientation gave statistically significant (p < 0.05) storage roots diameter of width 405 mm. Conversely the loop vine orientation had statistically the least (316 mm) storage root diameter. Horizontal vine orientation had significantly (p < 0.05) wider storage roots than the loop vine orientation. The ridge recorded longer (134.2 mm) mean storage root length, while those from mounds had shorter (115.9 mm) root length. The loop and the horizontal vine orientations recorded statistically the highest (35.5 t/ha and 34.8 t/ha respectively) sweet potato storage root yield. On the other hand, the fold vine orientation obtained significantly the lowest (28.7 t/ha) storage root yield. The research concluded that the horizontal and fold vine orientations had the widest storage root diameter and the ridge had longer storage root lengths. The loop and horizontal vine orientations are recommended in sweet potato production if high yields are to be achieved. Corresponding author.


Introduction
Sweet potato (Ipomea batatas L.) is a eudicotyledonous warm season crop, which belongs to the Convolvulaceae family [1].It is a herbaceous plant with creeping perennial vines and adventitious roots [2].The majority of the sweet potatoes grown in Zimbabwe are for human consumption, as a substitute for bread.Over the years, sweet potato consumption has been on the increase, with an estimated 1 -7 kg per capita, giving it an economic importance on both rural and urban markets [3].The relatively high per capita consumption has led to a growing number of farmers producing for commercial purposes.The crop's ability to adapt to marginal environments has made it popular with resource-poor farmers as yields of 15 and 50 tonnes per hectare has been obtained with minimum use of fertilizers and with proper fertilization and sufficient moisture respectively [4].When compared with other root and tuber crops, sweet potato yields considerably high amount of energy per unit area such that it comes third to the potato (Solanum tuberosum) and cassava (Manihot esculenta) in production across the world [5].With the fore mentioned characteristics rainfed sweet potato production has increased over the years, with major production areas stretching from Agro-Ecological Regions I, II and III of Zimbabwe [6].
Smallholder farmers in these regions of Zimbabwe produce sweet potato, using their own experience passed on from early generations.Depending on the experiences and location, farmers use various tillage methods and varying vine orientations for planting since they believe these to produce variable yields [4].The tillage systems, which include ridges and mounds, optimize infiltration and facilitate root expansion, vary in respective areas depending on environmental conditions and availability of draft animals [7].Vine placement at planting differs from horizontal, looped and folded vine orientations.
These methods make sweet potato a crop with varying yields across the country, creating a void as to the best tillage as well as vine orientation method to use.In view of this, there is need to establish the best method of planting and sweet potato vine orientation, to enhance reliable yields among farmers.The study, therefore, sought to evaluate the effect of different tillage systems and vine orientation method on yield and yield parameters of sweet potato.

Description of Study Site
The experiment was carried out at Midlands State University which is located 10 km southeast of Gweru Central Business District.The grid for the site is 19˚45'S and 29˚84'E and it is in Natural Region III (NR III) of Zimbabwe.The region receives an average annual rainfall of 674 mm which extends from November to February.The Natural Regions in Zimbabwe are a classification of the agricultural potential, from NR I, which represents the highest altitude and wettest area receiving more than 1000 mm of rainfall per year, to NR V that receives the lowest rainfall amounting to less than 450 mm per year and is dry.The average temperature for the site is 18˚C and is elevated 1428 m above sea level.The site has characteristic sandy loams belonging to the fersialitic group with kaolinite clay minerals dominant in the soil [8].The soil pH for the site is 6.5.

Experimental Design and Treatments
The experiment was carried out in a 2 × 3 factorial treatment structure arrangement, in a randomized complete block design (RCBD), with three replications.The experiment had two factors: tillage systems (ridge and mound) with three vine orientations: (horizontal, loop and fold) (Table 1).Slope was the blocking factor.

Research Procedure
The cultivar Shanga was selected for the experiment due to its growing popularity with smallholder farmers in Zimbabwe as well as its neighboring countries.It is a white skinned and white fleshed cultivar that is high yielding and has a maturation period of 120 -150 days.
The trial plot was disc ploughed to a depth of 45 cm in order to incorporate plant residues into the soil as well as improve water infiltration and root penetration into the soil.Nine ridges measuring 120 cm long, 50 cm wide and 40 cm high each and 18 mounds, measuring 55 cm long, 55 cm wide and 40 cm high each, were made.Mounds within each plot were spaced 20 cm between them.Blocks were spaced 100 cm apart with 50 cm spacing between plots.
Vine cuttings measuring 40 cm long with at least six nodes per vine were planted.On planting three to four nodes were buried into the soil for all the treatments.Horizontally planted vines (slide (c)) were done by creating a shallow furrow where the vine cuttings were horizontally placed and two thirds of the vine cutting was covered by soil.For the fold vine orientation (slide (b)), vine cuttings were folded half-way and the folded end was inserted into the soil leaving about a third of the cut ends protruding either sides.For the looped treatments (slide (a)), the vine cuttings were tied to form a loop that was placed in the soil with the tied ends left protruding on the surface.Two vine cuttings were planted at the top of each mound and were spaced 30 cm apart.On each ridge, four vine cuttings were planted at the crest of the ridge in a single row, with a plant spacing of 30 cm, leaving 15 cm spaces at both ends of the ridge.The trial plots were planted after the first effective rains, and were rain fed throughout the growing season.Fertilizer application was done by double banding 15 cm away from the crop.A basal fertilizer of compound D fertilizer (7 N :14 P :7 K ) was applied 15 days after planting to meet the 75 kg per hectare potassium requirement of the crop.

Data Collection and Analysis
Data on, growth parameters such as vine lengths were measured at 4, 7 and 10 weeks after planting using a tape measure.Yield parameters, namely: storage root length, storage root diameter and storage root weight were measured at harvesting 19 weeks after planting.Storage root lengths were measured using a meter ruler, the storage root diameter using a Vernier caliper and for storage root weight a digital scale was used.
The data on storage root length, diameter and weight was statistically analysed using Analysis of Variance (ANOVA) technique with GenStat 14 version software.Treatment means were compared by the Least Significance Difference (LSD), at 5% significance level.

Effect of Tillage System and Vine Orientation on Mean Storage Root Diameter
There was no interaction (p > 0.05) between the tillage systems and the vine orientations on storage roots diameter.There was no significant difference (p > 0.05) between different tillage systems on storage root diameter.However, there was significant difference (p < 0.05) between different vine orientations on storage root diameter (Table 2).The horizontal vine orientation recorded significantly (p < 0.05) the widest (405 mm) storage roots diameter.Conversely the loop vine orientation had statistically the least (316 mm) storage root diameter.

Effect of Tillage System and Vine Orientation on Mean Storage Root Length
There was no interaction (p < 0.05) between the tillage systems and the vine orientations on storage root length.There was no significant difference (p < 0.05) between different vine orientations on storage root length.The ridge and the mound had statistically (p < 0.05) different root length.The ridge recorded longer (134.2mm) mean storage root length, while those from mounds had shorter (115.9 mm) root length (Table 3).

Effect of Tillage System and Vine Orientation on Sweet Potato Yield
There was no interaction (p < 0.05) between the tillage systems and the vine orientations.There was no significant difference (p < 0.05) between the different tillage systems on sweet potato yield.The loop and the horizontal vine orientations recorded statistically the highest (35.5 t/ha and 34.8 t/ha respectively) sweet potato yield.On the other hand, the fold vine orientation obtained significantly the lowest (28.7 t/ha) storage root yield (Table 4).

Effect of Tillage Systems and Vine Orientation on Mean Storage Root Diameter
The wider storage root diameter recorded in the horizontal vine orientation than loop vine orientation could be attributed to the differences in spatial arrangement of the vine cuttings at planting.In horizontal vine orientation, the subterranean nodes were spaced further apart than those in the loop vine orientation.Formation of a loop design, automatically place subterranean nodes closer to each other.This increase competition as the nodes give rise to storage roots and therefore could have affected the water and nutrient uptake of the subterranean nodes in the loop than in the horizontal vine orientation.The subterranean nodes in horizontal vine orientation had more space to draw up nutrients and water which facilitated photosynthesis and production of photo-assimilates that were deposited in the vascular and anomalous cambia, which are responsible for storage root expansion as noted by Lewthwaite and Triggs (2009) [9].The structure of the loop vine orientation which resulted in subterranean nodes coming into close proximity of each other could have had detrimental effects on storage root expansion.Initiating storage roots experienced downward development, regardless of their position on the looped vine, due to positive geotropism.This could have contributed to increased competition for space and possibly affected storage root expansion.The horizontal vine orientation had subterranean nodes arranged linearly and had ample space for storage root development, resulting in significantly wider storage roots than those of loop vine orientation.This was consistent with the work of Parwada et al. (2011) [4] who also experienced thicker storage roots in horizontal vine orientation over vertical vine orientation.

Effect of Tillage Systems and Vine Orientation on Mean Storage Root Length
The reason as to why the ridge tillage system recorded higher mean root length as compared to the mound tillage system could be explained in three ways.Firstly, given the same area, ridges utilize gaps that are left between mounds such that ridges gets more soil than mounds, resulting in more space for lengthening of storage roots during development.This would facilitate lengthening of storage roots on ridges over mounds, which had limited space for lengthening before reaching the soil surface.
Secondly, soil moisture and temperature influence the sub-aerial soil microclimate around the sweet potato plant, which has an effect on storage root initiation, growth and development.Because of the protracted structure of ridges, they tend to have lower soil surface temperatures than mounds since they experience elongated heat transfer when compared to conical mounds, which experience circular heat movement [10].This is enhanced by tillage which results in soil inversion giving rise to increased pore spaces between soil particles, which facilitated penetration of heat through the soil resulting in soil within the first 10 cm below the soil surface having an elevated soil temperature than the next 10 cm [11].Lastly as Ennin et al. (2009) [12] also highlighted, mounds loose more moisture through evaporation than ridges due to the heightened soil surface temperatures, resulting in more soil moisture retention in ridges than in mounds under similar environmental conditions.This could have been the reason why greater root length was found on ridges than on mounds.In ridges, retained soil moisture facilitated nutrient uptake that was used in the process of photosynthesis in formation of assimilates which were partitioned as carbohydrate deposits during storage roots initiation.

Effect of Tillage System and Vine Orientation on Sweet Potato Yield
The higher yield of the horizontal than fold vine orientation could be because of formation of storage roots at the cut end of horizontal vine cuttings.Although all the vine cuttings had a uniform four subterranean nodes, the horizontal vine orientation had advantage over the fold vine orientation due to the development of adventitious roots at the callus tissue at the cut vine ends.These adventitious roots had a potential of undergoing storage roots initiation, secondary growth and lignification of stele; and develop into storage roots as observed by Rukundo et al. (2013) [13].This may have resulted in horizontal vine orientation being more yielding than fold vine orientation.This is also consistent with the findings by Pandey (2007) [14] who observed the horizontal vine orientation to be more yielding than the fold vine orientation.Differences in storage root yield due to different tillage systems and vine orientation could also be attributed to different stages of storage roots development at harvesting.Storage root initiation is not spontaneous and can take place over days due to the variability in cutting establishment, which in turn contributes to yield variability [15].Induction of fibrous roots into formation of storage root varies from plant to plant such that some plants yield with a high number of 4 to 6 storage roots per plant and sometimes less.In related research, Ravi and Indira (1999) [16] also highlighted how some sweet potato cultivars initiate storage roots from 7 -91 days after planting, which contradicts findings by Belehu (2003) [17] who suggested storage roots initiation takes place between 35 -60 days after planting.
Without considering the vine orientation, Villordon et al. (2009) [15] highlights how yield can vary by up to 50% due to inability to predict the potential number of storage roots that an individual plant of the same cultivar under similar field conditions may produce.In related findings, Firon, et al. (2009) [18] also noted that production of storage roots was inconsistent from plant to plant.There is however limited research on the plant intrinsic factors that determine the number of adventitious roots that may develop into storage roots [2].Different plants, regardless of the tillage system, produced adventitious roots that subsequently developed into storage roots.

Conclusion and Recommendations
The study demonstrates that, the horizontal and fold vine orientations produced the widest storage root diameters as compared to the loop vine orientation.The ridge tillage system had the longest storage root lengths when compared to mound tillage system.The loop and horizontal vine orientations were more yielding than the fold vine orientation.
Based on the results, farmers are recommended to use the horizontal and loop vine orientations in sweet potato production since they both are more yielding.Further research should be carried out to consider the plant part to use: growing tip, middle or base of the vine.This results in variation of subterranean buds in age as well as spacing due to their position on the vine.
Slides showing vine orientations at harvest time (a) loop (b) folded and (c) horizontal.

Table 1 .
Table of treatments.

Table 2 .
Effect of vine orientation on mean storage root diameter.
* Numbers with different letters (a, b) are significantly different.

Table 3 .
Effect of tillage system on mean storage root length.
* Numbers with different letters (a, b) are significantly different.

Table 4 .
Effect of vine orientation on mean storage root yield.
* Numbers with different letters (a, b) are significantly different.