A cost-benefit analysis of sweet potato production for sheep feeding in the southwest of Nigeria

O A Olorunnisomo

Department of Animal Production and Health Sciences, University of Ado-Ekiti, PMB 5363, Ado-Ekiti, Nigeria
sholanisomo@yahoo.com

Abstract

This study examined the costs and benefits of sweet potato (SP) production (with or without tillage and fertilizer application) for livestock feeding in the southwest of Nigeria. Feed cost and live weight gain from West African dwarf sheep fed different mixtures of SP forage and roots were also examined.

Although tillage and fertilizer application raised the cost of producing SP forage and root, the increased yield obtained when these treatments were applied more than compensated for the increased cost. A combination of both practices maximized returns from SP cultivation than when either was applied alone. When the forage or roots were used for livestock feeding, production costs increased, however when both components were incorporated into livestock feeds, production costs reduced drastically.

Mixing SP forage and root in equal proportions (DM basis) improved nutrient utilization, reduced cost of live weight gain and maximized economic returns from sweet potato cultivation for sheep in the southwest of Nigeria.

Keywords: cultivation, forage, roots, sheep, sweet potato

Introduction

Under improved cultivation practices, sweet potato is capable of very high dry matter (DM) yield per unit area of land (Moat and Dryden 1993; Rashid et al 2000). In terms of chemical composition and digestibility, sweet potato forage is superior to most grass forages. The crude protein content of SP forage ranged from 18 to 30% in DM among different varieties (An et al 2003) while the crude fibre content is about 18%. With a DM digestibility of 70% (Ffoulkes et al 1978), sweet potato vine is an ideal forage for ruminants. The root DM consists mainly of starch and is considered to be a good source of energy in livestock diets (An et al 2003).

The scarcity of forage during the dry season in the southwest of Nigeria, and the poor quality of silage and hay prepared from tropical grasses (Crowder and Chedda 1982; Gallaher and Pitman 2001) makes it imperative to look for alternative crops that can be preserved for dry season feeding of ruminant stock in this part of Nigeria. The high yield and chemical composition of sweet potato make it a crop of great potential for preservation for dry season feeding of ruminants. The concept of using whole crop biomass as the basis for feeding livestock species in the tropics (Ruiz 1982) is applicable in Nigeria. Whole crops of sweet potato (forage and root) can be ensiled or dried, and used as the basis for feeding ruminant animals during periods of feed scarcity.

In many African communities sweet potato root is a major staple hence its use as livestock feed is limited by competition with humans. In Nigeria however, where sweetness in tuber crops is largely unacceptable, yam is preferred to sweet potato root as a staple hence sweet potato is usually consumed as a snack. During periods of peak production a high proportion of the sweet potato root produced is wasted because supply exceeds demand at these periods. The vine which is also valuable as forage for livestock is usually left on the field as a residue after root harvest. This current tendency provides an opportunity for sweet potato to be used as a commercial livestock feed in the southwest of Nigeria.

This study seeks to evaluate the costs and benefits derivable from cultivating sweet potato solely for ruminant feeding.
 

Materials and methods

The data used in this study were derived from agronomic and animal-feeding trials conducted at the Teaching and Research Farm, University of Ibadan, Nigeria.(These trials were part of a PhD research conducted by the author in the Department of Animal Science, University of Ibadan, Nigeria). A dual-purpose variety of sweet potato (TIS-Ex-Igbariam) collected from the National Root Crops Research Institute (NRCRI), Umudike, Nigeria was used in this study. Animals used in the feeding trial were young, growing sheep of the West African dwarf type.

Treatments in the agronomic trial consisted of 4 combinations of tillage and fertilizer application on sweet potato plots of 24 m². The treatments were; tilled, fertilized (TF); tilled, not fertilized (TNF); not tilled but fertilized (NTF); and not tilled, not fertilized plots (NTNF, control).A split plot design was used with tillage as the main plot and fertilizer application as sub plot factor. In the feeding trial, the treatments consisted of 5 combinations of sweet potato forage and root in the diets of growing sheep. The proportions of forage: root in the diets (DM basis) were, 0:100, 25:75, 50:50, 75:25 and 100:0. The experimental design adopted was the randomized complete block design.

Untilled plots were cleared with a cutlass to remove weeds before planting while tilled plots were ridged using a hoe. Planting, weeding, fertilizer application and harvesting were done manually using a hoe or cutlass. Harvested materials (forage and roots) were chopped using an automated chopper; chopped materials were sun dried on polythene sheets for 5-6 days. The costs of production per plot for each treatment were computed as a summation of cost of individual operation and inputs while the cost of producing 1 kg of sweet potato forage, root or total biomass were each derived by dividing the cost of production per plot with the DM yield per plot. In calculating the cost/kg of sweet potato forage, the root yield was not taken into consideration because, as in this case, it was assumed to be a by-product. A similar assumption was made to calculate the cost/kg of sweet potato root. Since the forage or root cannot be produced in isolation from any sweet potato crop, this assumption becomes necessary in order to calculate the cost/kg of individual components of the crop. The cost per kg of feed was derived from the cost/kg of yield for sweet potato forage and root in tilled and fertilized plots (TF), and the proportion of these components in the mixed diet. Feed cost per kg of weight gain was calculated by multiplying the feed conversion ratio (FCR) by the cost/kg of feed.

Agronomic and animal performance data obtained were subjected to analysis of variance and significant means were separated by Duncan's multiple range tests using the SAS (1995) procedures. Economic parameters were explained using simple descriptive analysis.
 

Results and discussion

The costs of production for sweet potato crop receiving different tillage and fertilizer treatments on plots of 4 x 6m are given in Table 1.

The cost of production increased with additional input of tillage and fertilizer application on sweet potato plots. Plots receiving no tillage or fertilizer application (NTNF) had the least cost of production while plots receiving a combination of tillage and fertilizer application had the highest cost of production. Tillage and fertilizer application increased cost of production by 12 and 41% respectively. When tillage and fertilizer application were combined, cost of production increased by 52%. Fertilizer application contributed more to the increased cost of producing sweet potato than tillage.

The cost-benefit analysis of sweet potato production with additional agronomic input is given in Table 2.

Yield of sweet potato forage and root increased when tillage or fertilizer was applied to the plots. There was a 20% increase in yield of the forage when the plots were only tilled (TNF) and 39% increase when the plots were only fertilized (NTF). When both tillage and fertilizer application were combined (TF), yield of sweet potato forage increased by 76%. Root yield improved by 31, 31 and 133% while biomass production improved by 25, 35 and 101% for TNF, NTF and TF respectively. This suggests that tillage and fertilizer application had similar effects on root yield of sweet potato while fertilizer application had a greater influence than tillage on biomass production in sweet potato.

The cost of producing 1 kg of sweet potato forage was reduced by 7% when the plots were only tilled but when they were only fertilized, cost of 1kg of the forage increased by 1%. When tillage and fertilizer application were combined, cost per kg of forage reduced by 14%. Production cost per kg of sweet potato root and total biomass followed a similar trend with sweet potato forage. Tillage reduced cost per kg of product by 15 and 11% for sweet potato root and total biomass respectively. Fertilizer application however, increased the cost per kg of product by 8 and 4% for sweet potato root and total biomass. When tillage and fertilizer application were combined, cost per kg of product reduced by 35 and 24% for the root and total biomass respectively. These results show that the best economic returns from application of fertilizer to sweet potato plots are realized when fertilizer is combined with tillage. This agrees with the reports of Lal (1979) and Ndaeyo (2000), that tillage practices enhanced better utilization of fertilizer and soil nutrients in arable crops.

The chemical composition of sheep diets and feed cost per kg of live weight gain in WAD sheep fed mixtures of sweet potato forage and root are presented in Tables 3 and 4.

Total intake in 140 days varied from 83.4 to 92.1 kg with animals on the 50:50 diets showing the highest intake and the 100:0 diet the least. (Table 4). Animals on the 0:100 diets (100% root) lost weight, scoured persistently, and went off feed by the 4th week, hence they were excluded from the computation of results. This observation is thought to be due to the very low level of fibre (3.3% CF) and protein (4.9% CP) in the diet. The CP content was below the minimum of 6-7% recommended for ruminant diets by Milford and Haydock (1965). Also, when ruminant diets are devoid or low in fibre, digestive disorders may occur (Preston and Leng 1987). Feed intake by the animals reached a peak when sweet potato forage and root were mixed in equal proportions. Beyond this level of mixture, additional sweet potato forage in the diet did not improve intake by the animals. The 50:50 diets probably represent the optimum protein-energy intake by the animals, consequently promoting optimum microbial activity in the rumen.

Weight gain followed a similar trend with feed intake, with 50:50 diets having the highest gain. The 50:50 diets also had the best feed conversion ratio, showing that feed was more efficiently utilized at equal levels of sweet potato forage and root in the diet. 

The cost/kg of feed consumed increased as the level of sweet potato forage in the diet increased. This was a reflection of the higher production cost of the forage compared to the roots. When tillage and fertilizer were applied to sweet potato plots, the yield of the root was higher than that of the forage, leading to a lower production cost for the root than the forage. The feed cost relative to live weight gain followed the same trend as feed conversion ratio (FCR), indicating that the FCR may be used as an effective measure of economic gains in WAD sheep. Animals on the 50:50 diets had the best feed conversion ratio and least feed cost per kg of live weight produced. Animals on the 100% root diet (0:100) showed a negative weight change, hence their feed conversion ratio and feed cost per kg of live weight gain were not computed.

The feed costs per kg of live weight gain obtained in this study (N 122 - N 170) were slightly lower than the range of N 130 - N 246 reported by Arigbede et al (2005) for sheep fed pigeon pea based diets. This suggests that under the present socio-economic conditions, sweet potato may be a more economical feedstuff for sheep in Nigeria.
 

Conclusions

• Additional agronomic input of tillage and fertilizer application raised the cost of producing sweet potato forage and root, however, the increased yield when tillage and fertilizer were applied compensated for the increased cost. Cost of production per unit of biomass was reduced when tillage and fertilizer were applied together in sweet potato production. Combining fertilizer application with tillage practice was beneficial to the yield of sweet potato and maximized returns on investment than when either was applied alone.

• The cost of producing sweet potato forage was higher than that of the root when tillage and fertilizer were applied to the crop. This is a reflection of the higher yield of the root compared to the forage when tillage and fertilizer were both applied to the plots. When sweet potato was cultivated only for the forage or root, production cost was high, however, when the forage and root were both harvested and utilized, production cost dropped drastically.

• When sweet potato forage and root were mixed in the diet of sheep, nutrient utilization improved and cost per kg of live weight gain was reduced. Mixing the forage and root in equal proportions (sun-dried basis) in sheep diets maximized economic returns from cultivating sweet potato for sheep feeding.
   

References

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Received 22 February 2007; Accepted 24 March 2007; Published 4 June 2007

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