AGRONOMIC ASSESSMENT OF SOME SWEET POTATO VARIETIES FOR INTERCROPPING WITH PIGEONPEA IN SOUTHERN GUINEA SAVANNA OF NIGERIA Egbe

Field experiments were conducted at the National Root Crops Research Institute sub-station, Otobi, in 2006 and 2007 to assess the suitability of improved sweet potato varieties for intercropping with pigeonpea and also to determine the planting pattern and the productivity of the intercropping system. Intercropping decreased total fresh root and saleable root yields of sweet potato when mixed or row-intercropped with pigeonpea. All intercropping combinations of sweet potato varieties and pigeonpea had land equivalent ratio above 1.0, except intercropping with WA Gabolige, signifying high intercrop advantages. TIS 87/0087 produced the highest total fresh root and saleable root yields in both cropping systems, irrespective of the planting pattern used. TIS 2532.O.P.1.13 and TIS 86/00356 sweet potato varieties had comparable yields with TIS 87/0087 in both cropping systems. Pigeonpea was the more competitive component of the intercrop. Farmers showed willingness to adopt the sweet potato + pigeonpea intercropping.


INTRODUCTION
Sweet potato (Ipomea batatas L. Lam) is a major food and industrial root crop in Nigeria with an estimated annual production of 2.52 million tons (FAO, 2004), and the mean yield on farmers' fields (3-7 t/ha) is considered low (Udealor et al., 2006).Sweet potato has a long history as a crop to stave off famine-especially as a cheap source of calories (Adam, 2005).Pigeonpea (Cajanus cajan (L.) Millsp.) is a multipurpose leguminous crop that can provide food, fuelwood and fodder for small-scale farmers in subsistence agriculture (Egbe and Kalu, 2006).Intercropping sweet potato with pigeonpea would ensure the supply of dietary carbohydrate, protein, fats, vitamins and minerals (calcium, magnesium, copper, iron, and zinc) for the rural household.Intercropping both sweet potato with pigeonpea will not only ensure better environmental resource utilization, but will also provide better yield stability, reduce pests and diseases and diversify rural income (Egbe, 2005;Njoku et al., 2007).Although presently unpopular, growing sweet potato with pigeonpea in mixtures or intercropping has enormous potentials in Southern Guinea Savanna agro-ecological zone of Nigeria, where poverty level is high, income generation opportunities are few and soil fertility status is low (Egbe, 2005;Egbe and Kalu, 2006).
The sweet potato variety commonly cultivated by farmers in the Southern Guinea Savanna Zone of Nigeria often results in low yields of 3-9.0 t ha -1 (BNARDA, 2007) and when intercropped with pigeonpea, the planting pattern is highly variable from one farm to the other.Though yield advantages occur in sweet potato intercropped with such other crops as maize and okra (Udealor et al., 2006;Njoku et al., 2007) and in pigeonpea intercropping with maize and sorghum (Egbe and Adeyemo, 2006;Egbe, 2005), documented information on yield advantages derivable from sweet potato + pigeonpea intercropping are lacking.The work reported here sought to determine the suitability of sweet potato varieties for intercropping with pigeonpea and to assess the productivity of the intercropping systems with the aim of enhancing food security in the region.The work also aimed at popularizing new sweet potato varieties suitable for intercropping in the Southern Guinea Savanna of Nigeria

MATERIALS AND METHODS
The study was conducted at the National Root Crops Research Institute Sub-Station, Otobi (07 0 10'N, 08 0 39'E, elevation 105.1m) in Benue State, located in the Southern Guinea Savanna of Nigeria (Kowal and Knabe, 1972).The texture of the top soil (0-30 cm) of the experimental site was sandy loam.The soil at the experimental site was classified as Typic Paleustalf (USDA).Eight core samples of soil were collected from different parts of the experimental field and bulked into a composite sample and used for the determination of the chemical and physical properties of the soil before planting.The level of total nitrogen (N) was 0.52%, phosphorus and potassium averaged 68.20 and 43.50 mg kg -1 soil, respectively.The rainfall amounts at the site were 1250.8mm and 1301. 1 mm in 2006 and 2007, respectively.The rains fell between the months of July and November of each year.The experimental field was ploughed, harrowed and ridged before planting in each year of the experimentation.The experiment was a 2 x 6 x 2 factorial set out in split-split plot in a randomized complete block design with three replications.The main-plot treatments were two cropping systems (i) sole cropping (sweet potato, pigeonpea) and (ii) intercropping (sweet potato + pigeonpea).
The sub-plot treatments comprised of five improved sweet potato varieties obtained from the National Root Crops Research Institute, Umudike, and a local check (TIS 87/0087, CIP Tanzania, WA Gabolige, TIS 2532.OP.I.13, TIS 86/00356 and Ogege-local check).The sub-sub-plot treatments were made up of two planting patterns of pigeonpea + sweet potato intercropping [mixture (top) and rowintercropping (side).The gross plot comprised of 5 ridges each of 4m long (20m 2 ), while the net plot had 3 ridges each of 3m long (9m 2 ).Plantings were done on the 20 th and 24 th July in 2006 and 2007, respectively.Sweet potato vine cuttings measuring 20 cm with four nodes were planted at the crest of ridges at a spacing of 1m x 0.3m (33,000 plants per hectare).Pigeonpea seeds obtained from the local market in Otobi were simultaneously planted at 1 m x0.3 m with two seeds per hole and at two different positions: (i) planting by side of ridge (row-intercropping) and (ii) at the top in between the sweet potato (mixture).The pigeonpea was thinned to one plant per stand at 10 days after planting to give a plant population of 33,000 plants per hectare (ha).Weeding was done at three weeks after planting (WAP).No fertilizer was applied.In addition to the on-station experiment, in 2007, ten farmers were randomly selected from five villages around Otobi for on-farm evaluation of TIS 87/0087 mixed or row-intercropped with pigeonpea.TIS 87/0087 was picked because it gave the highest yield in both cropping systems in 2006.The on-farm experiment consisted of four treatments, viz: (i) TIS 87/0087 intercropped with pigeonpea planted at ridge crest (ii)TIS 87/0087 intercropped with pigeonpea planted by the side of the ridge (iii) Sole TIS 87/0087 (iv) Sole pigeonpea.The spacing adopted for both sweet potato and pigeonpea were as in the onstation experiments.The gross plot size was 10 mx5 m(50 m 2 ) and the net plot measured 6 m x 3m (18 m 2 ).Harvesting of sweet potato was done as reported by Njoku et al. (2007) After harvest, the ten farmers involved in the on-farm experimentation were brought together and asked which of the experimental treatments they preferred to adopt.Questionnaires were also administered on the farmers to record their choices of the preferred experimental treatments.
All data collected were analysed using GENSTAT 4.23 (Copyright 2003, Lowes Agricultural Trust Rothamsted Experimental Station) following Standard analysis of variance procedures (Gomez and Gomez, 1984).Whenever difference between treatment means were significant, means were separated by F-LSD at P = 0.05 (Obi, 1990).The land equivalent ratio (LER) (Anders et al., 1996) and competitive ratio (CR) (Putnam et al., 1984), calculated from total fresh root yield of sweet potato and grain yield of pigeonpea, were used to determine the productivity of the intercropping systems.The LER, an accurate assessment of the biological efficiency of the intercropping situation, was calculated as LER = (Xab/Xaa) + (Xba/Xbb ) where Xaa and Xbb are yields as sole crops of a and b and Xab and Xba are yields as intercrop of a and b.
Values of LER greater than 1 are considered as advantageous.
Similarly, competitive ratio (CR) which simply represents the ratio of individual LERs of the two component crops, but takes into account the proportion of the crops in which there were initially sown, was calculated as CR = La/Lb X Zba/Zab Where La = LER of crop a Lb = LER of crop b Zba = Proportion of crop a in ab in intercrop Zab = Proportion of crop b in an ab intercrop CR could be useful in comparing the competitive ability of the different crops, measuring competitive changes in a given situation, identify which plant characters are associated with competitive ability and determining which competitive balance between component crops is most likely to give maximum yield advantages.

RESULTS
Tables 1-3 present the effects of cropping system x variety, cropping system x planting position, variety x position of planting, respectively and their interactions on the total fresh root yield of sweet potato intercropped with pigeonpea at Otobi.In 2006, intercropping reduced the fresh root yield of sweet potato in all the varieties, except CIP Tanzania, which had a reverse response.However, intercropped sweet potato varieties such as TIS 87/0087, TIS 2532.OP.1.13and TIS 86/00356 gave significantly higher fresh root yield than when grown in sole in 2007.The responses of the other varieties (Ogege(local check),WA Gabolige and CIP Tanzania) to intercropping were insignificant in 2007(Table 1).Under intercropping, sweet potato planted on top with pigeonpea gave higher fresh root yields than side planting in 2006(Tables2&3).
Tables 4,5 and 6 present results of the effects of cropping systems x variety, cropping systems x position of planting and variety x planting position respectively, and their interactions on the saleable root yield of sweet potato intercropped with pigeonpea in 2006 and 2007 at Otobi.Intercropping consistently reduced the saleable root yield of all sweet potato varieties intercropped with pigeonpea in both years of experimentation(Table 4).Under sole cropping, WA Gabolige gave the least saleable root yield in 2006,while CIP Tanzania did so in 2007.Intercropped sweet potato planted on top with pigeonpea gave significantly higher yields of saleable root than side planting in both years(Table 5).Similarly, TIS 87/0087,TIS 2532.OP.1.13and TIS 86/00356 produced significantly higher yields of saleable roots when planted on top of the ridge with pigeonpea than when planted by the side(Table 6).The other improved sweet potato varieties(CIP Tanzania and WA Gabolige) and the local check gave insignificant results.In both 2006 and 2007, sweet potato varieties (WA Gabolge,TIS 2532.OP.1.13and TIS 86/00356)gave significantly higher number of tuberous roots per plant under intercropping than in sole cropping, but not so with the local check(Ogege),TIS 87/0087 and CIP Tanzania, which gave insignificant responses (Table 7).Intercropping consistently reduced the number of tuberous roots per plant irrespective of the planting position adopted in both years(Table 8).WA Gabolige produced significantly higher number of tuberous roots per plant when planted with pigeonpea at the ridge crest than when planted with pigeonpea by the side,while TIS 86/00356 had the opposite response in both years of experimentation(Table 9).The responses of Ogege,TIS 87/0087,CIP Tanzania and TIS 2532.OP.1.13at both planting positions were inconsistent and mostly insignificant.
In 2006, sole pigeonpea planted on top of the ridge produced significantly higher number of pods than intercropped pigeonpea with TIS 87/0087 planted on top of ridge, which in turn gave higher number of pods than pigeonpea intercropped with TIS 86/00356 and planted by the side and sole pigeonpea planted also by side (Table 10).
All the other combinations gave lower number of pods per plant than the sole pigeonpea planted by the side of the ridge.The trend in 2007 was similar to that obtained in 2006 (Table 4).The results further revealed that intercropped pigeonpea gave lower number of pods per plant than sole planted pigeonpea in 2006, unlike in 2007, when there was no significant difference between both cropping systems.
Intercropped pigeonpea planted on top with TIS 87/0087, sole pigeonpea planted on top of ridge, intercropped pigeonpea planted by side of TIS 2532.OP.1.13and pigeonpea planted by the side of 87/0087 gave higher yields of grain than all the other treatments including sole pigeonpea planted by side of the ridge in both 2006 and 2007 (Table 10).Intercropped pigeonpea by side of WA Gabolige gave the least grain yield in both experimental years.The results further showed that there was no significant difference between sole and intercropped pigeonpea in grain yield in 2006, but sole pigeonpea gave a higher grain yield in 2007 than intercropped pigeonpea.All intercrop combinations had LER figures above 1.00, except the combination of pigeonpea and WA Gabolige with the pigeonpea by the side in both years (Table 11).The combination of CIP Tanzania planted at both positions gave the highest LER and this was closely followed by the combination of pigeonpea and TIS 87/0087 with the pigeonpea on top of the ridge.The combination of pigeonpea and WA Gabolige with the pigeonpea planted by the side gave the least LER.Table 11 further indicated that pigeonpea consistently had higher competitive ratio (CR) figures than sweet potato varieties in both years.
Results of yield of Sweet potato variety TIS 87/0087 intercropped with pigeonpea on ten farmers' fields in five villages around Otobi in 2007 are presented in Table 6.TIS 87/0087 planted on top with pigeonpea (mixed cropping) out yielded its planting with pigeonpea by its side (rowintercropping).There was no significant difference between the pigeonpea planted on top with sweet potato and by the side of sweet potato.Eighty percent (80%) of the farmers who participated in the research were willing to adopt the technology for various reasons; while twenty percent rejected the practice either because the practice was perceived as being strange or that the harvesting of pigeonpea component was tedious.

DISCUSSION
The reduction observed in the total fresh and saleable root yields of sweet potato varieties intercropped with pigeonpea in this study might have resulted from depression of photosynthesis due to decrease in solar radiation by shading of the sweet potato by the taller pigeonpea component.Fujita and Ofosu-Budu (1996) had indicated that when component legume is taller than non-legume, the legume can grow well due to high photosynthetic and high biological nitrogen fixation activities with adequate solar radiation and that the non-legume growth is severely suppressed due to depression of photosynthesis through decreases in irradiance.The superior performance of TIS 87/0087 under both cropping systems in total fresh and saleable root yields over the other varieties was similar to the reports of earlier studies (Njoku et al., 2007;Onunka, 2006;Okorie and Okpala, 2000).This implied that TIS 87/0087 might be more tolerant of shading and therefore more suitable for intercropping with pigeonpea in either of the planting positions tested.The differential performance of the varieties of intercropped sweet potato at the two planting positions of pigeonpea indicated that no single planting position could be recommended over the others for the varieties tested.However, such varieties as TIS 87/0087, Ogege and CIP Tanzania could be intercropped with pigeonpea, using either of the planting positions.The reasons for better performance of Sweet potato varieties at a particular planting position under intercropping with pigeonpea might need further investigation.
Although no consistent result was obtained for both years, the higher number of pods per plant and grain yield of sole pigeonpea over intercropped pigeonpea in 2006 might be due to interplant competition for natural growth resources such as soil nutrients and water by both intercrop components.It is known that competitive reactions reduce yields in intercropped crop species as compared with mono cropping (Egbe, 2007).
The non-significant result obtained in sole vs intercropping in 2007 for the number of pods per plant and grain yield of pigeonpea could be possible.Fujita and Ofosu-Budu (1996) had reported that when component legume was taller than nonlegume, biomass production of intercropping approached that of mono cropping of legume.
The LER of Sweet potato varieties intercropped with pigeonpea at the two planting positions were all above 1.0, except the combination of WA Gabolige and pigeonpea, indicating that greater productivity per unit land area was achieved by growing the two crops together than by growing them separately.These results showed that genotypic compatibility might exist between the sweet potato varieties and pigeonpea.Ali (1996) had stated that identification of suitable genotypes of the component crops was necessary for complementarity.Njoku et al. (2007) obtained similar results in sweet potato + Okro intercropping in Southeastern Nigeria.The results further revealed that WA Gabolige and pigeonpea would give higher productivity when grown separately than when intercropped and the two might therefore not be compatible for intercropping.The LER figures in this intercropping study were high depicting high yield advantages derived.This was probably because sweet potato serving as cover crop conserved soil moisture reduced soil temperature and added organic matter to the soil, while pigeonpea added organic matter through leaf litter production and biologically fixed nitrogen for the benefit of the intercropping systems.
Competitive ratio (CR) could be useful in comparing the competitive ability of the different crops and it may help clarify the nature of competition between component crops (Egbe, 2005).Pigeonpea proved more competitive than the sweet potato varieties in this study, probably because it was taller.Generally, biomass production of shorter component crops is reduced by depression of photosynthesis due to decreases in solar radiation.Snapp and Silim (2002) in their study on farmer preferences and legume intensification for low nutrient environment in Africa had indicated that research intended to translate benefit to smallholder farmers must keep farmer preferences and belief systems in the fore.The results obtained in this study pointed to the potential benefits and possible sustainability benefits of the Sweet potato + pigeonpea intercropping among farmers in Southern Guinea Savanna.