Effect of feeding different proportions of sweet potato vines (Ipomoea batatas L. (Lam.))and Sesbania grandiflora foliage in the diet on feed intake and growth of goats

Vo Lam and Inger Ledin

Cantho University, Mekong Delta Farming Systems Research and Development Institute,
Campus II, 3/2 Street, Cantho city, Vietnam
vlam@agu.edu.vn
^* Swedish University of Agricultural Sciences, Department of Animal Nutrition and Management,
PO Box 7024,750 07, Uppsala, Sweden

Abstract

Twenty four weaned goats (Bach Thao*local) with initial weights of 9.7±0.32 kg and ages ranging between 3 to 4 months were used in an experiment to evaluate the use of fresh sweet potato vines (SPV) and Sesbania grandiflora foliage (SG) given as sole diets or as mixtures. Sesbania foliage was gradually replaced with fresh sweet potato vines at four levels: 0% (SPV0), 50% (SPV50), 75% (SPV75) and 100% (SPV100), based on DM content of the feeds.

The same amount of DM was offered to all treatment groups, but intake decreased linearly as Sesbania foliage was replaced by sweet potato vines. When given as the sole feed to growing goats, the foliage of Sesbania grandiflora was superior to fresh sweet potato vines, in terms of feed intake and live weight gain, the latter being 50% greater on Sesbania  (64 g/day versus 44 g/day). Replacement of Sebania with 50% fresh sweet potato vines on a dry matter basis resulted in acceptable live weight gains (60.6 g/day).

Farmer experience indicates that the constraint to the widespread use of Sesbania grandiflora  for feeding goats is the relatively low yield and slow rate of re-growth after pruning.

Key words:  Feed intake, goats, growth, Sesbania grandiflora, sweet potato vines.

Introduction

In the Mekong Delta of Vietnam, sweet potato (Ipomoea batatas) is one of the most widely cultivated crops grown on fertile soils in the rice-based farming system areas (Minh 2000). Traditionally, sweet potato has been grown exclusively for the purpose of tuber production, mainly for human consumption, and the forage (aerial part) has been considered as a waste material after harvest (Ruiz et al 1981). Alternative uses for the vines can be to return them to the field for mulching or using them as a feed for animals. The major proportion of vines is, however, not used in a planned way in the present farming systems. The constraints of using fresh sweet potato vines as a ruminant feed are low dry matter (DM) (15% DM) and the fact that the vines are only seasonally available. However, the crude protein (CP) content is similar to cassava foliage (Preston 2004).

Goats in the Mekong Delta of Vietnam are mainly produced on smallholder farms. These farms most often use cut-and-carry feeding systems with native grasses and native legumes such as Sesbania grandiflora, Leucaena leucocephala, Hibicus rosa-senensis and Ceiba pentadra as feed for their animals (Nguyen Thi Hong Nhan 1998a). Among these native legumes, Sesbania grandiflora is widely adapted to local conditions and has been fed to goats as a sole diet with excellent results (Nguyen Thi Hong Nhan 1998b).  There are, however, results showing that Sesbania leaf yields are low and decline rapidly with successive harvests (Mas'ud Panjaitan et al 1993). In practice, Sesbania foliage yields are maintained by lopping branches regularly while the main stem remains uncut (Chen et al 1994). The objective of this experiment was to evaluate the possibility of replacing Sesbania grandiflora foliage with fresh sweet potato vines in the diet of growing goats.

Materials and methods

Location

The experiment was conducted at the Research station of Cantho University, Cantho City, Vietnam located at 10^oN, 105^oE. In this area the climate is monsoon-tropical, with a wet season between May and November and a dry season between December and April. Annual rainfall ranges from 1400 to 2400 mm, and the mean annual temperature is 26.6^oC. The experiment was carried out from August to October, 2002.

Experimental animals and management

The animals used in the experiment were 24 weaned crossbred goats, 13 males and 11 females (Bach Thao x local female) bought from smallholder goat keepers in the area. The initial weight was 9.7±0.32 kg, and ages ranged from 3 to 4 months. The goats were vaccinated against foot and mouth disease and de-wormed with Ivermectine before commencement of the experiment. The goats were housed in individual pens on slatted floors and had access to clean water and a mineral lick ad libitum. The feed for each goat was weighed every morning and the animals were fed 50% of their daily ration at 8:00 h and 50% at 15:00 h. The quantity of feed offered at the start of the experiment was 3 % of the body weight in DM per day. The amount of feed offered during the experiment was adjusted weekly based on the weight of the treatment group with the highest average weight and was calculated to give 20% of refusals.

Experimental feeds

Fresh sweet potato vines and Sesbania grandiflora foliage were used in the experiment. One variety of sweet potato (Hshinchu)  was grown on 1,000 m² of land.  The sweet potato vines were cut at 20-day intervals to get vines of the same age. Sesbania foliage was collected from the areas surrounding Cantho City. The side branches were pruned at a length of about 50 cm and 1 cm in diameter. Only green twigs were used. Sweet potato vines and Sesbania foliage were cut the night before feeding, chopped into 30 to 50 cm pieces and mixed together before feeding.

A mineral lick containing 50% steamed bone meal, 10% commercial trace mineral mix, 5% molasses, 8% cement as a binding agent and 22% NaCl was available inside each individual pen during the experimental period.

Experimental design

The twenty-four goats were randomly assigned to four dietary treatments,  based on live weight and sex. The diets tested in the experiment were as follows (in % of DM offered).

• 0% sweet potato vines + 100% Sesbania grandiflora foliage (SE100)
• 50% sweet potato vines + 50% Sesbania grandiflora foliage (SP50SE50)
• 75% sweet potato vines + 25% Sesbania grandiflora foliage (SP75SE25)
• 100% sweet potato vines + 0% Sesbania grandiflora foliage (SP100)

The experiment was a Completely Randomised Design with 4 treatments, 6 replicates and 1 goat per unit. The experimental goats were gradually introduced to the new feeds during 14 days of adaptation before the experiment started.

Measurements

The DM content of the feedstuffs was determined twice per week with a microwave oven to balance the proportions of feedstuffs. Feed samples were taken during the experimental period and feed refusals were collected from individual animals every morning before feeding, weighed and analysed for DM twice per week. The samples were pooled per week and treatment group. The animals were weighed at the start of the experiment and then weekly, at the same day of the week and before feeding in the morning.

The samples of sweet potato vines, Sesbania foliage and refusals were analysed for DM, ash, CP, neutral detergent fibre (NDF) and acid detergent fibre (ADF). The DM, CP and ash were determined according to AOAC (1990). CP content was measured by the Kjeldahl method as N*6.25 and ash was assayed by incinerating samples at 600^oC. The content of NDF and ADF was determined according to Van Soest and Robertson (1985), using sodium sulphite and amylase and was expressed with residual ash.

Statistical analysis

The data were analysed statistically by an analysis of variance using the General Linear Model (GLM) procedure of Minitab Statistical Software Release version 12.21 (Minitab1998). When the differences in treatment means were significant at the probability level of P<0.05, the means were compared by using Tukey's pair-wise test. The statistical model used was

 Y_ij = m + T_i + bX_ij + eij

where Y_ij = the dependent variable, m =overall mean, T_i =effect of treatment, and e_ij =random error.

The initial weight X was used a covariate. The slope bX_ij describes the change in the response Y when the covariate X increases one unit. Effect of sex and the interaction between treatment and sex were tested but were not significant and therefore excluded from the model.

Results

The DM content of fresh sweet potato vines was 11.9%; crude protein was 19.8% of DM (Table 1). There was little variation in these data during the experiment. Sesbania had higher DM and CP contents (DM 23.8% and CP 22.7% of DM).  NDF and ADF contents of Sesbania foliage were higher than those in fresh sweet potato vines.

The same amount of DM was offered to all treatment groups, but intake decreased linearly as sesbania foliage was replaced by sweet potato vines (Table 2; Figure 1).  Crude protein intake declined in a similar way due to the combined effect of a lower content in SPV and the decreased DM intake.

Figure 1: Effect on DM intake of goats of replacing sesbania foliage with sweet potato vines (SPV)

The effects on growth rate and feed conversion of replacing Sesbania foliage with sweet potato vines mirrored the trends in feed intake, with declining performance as Sesbania foliage was replaced by sweet potato vines (Table 2; Figure 2).

Figure 2: Effect on growth rate and feed conversion of goats of replacing sesbania foliage with sweet potato vines (SPV)

Discussion

The DM and CP content of sweet potato vines did not show any great variation during the experiment due to the fact that the vines used were harvested at the same age of the plants. Brow and Chavalimu (1985) reported that sweet potato vine contained 18.1% DM and 17.2% CP at harvest. Also Man (1994), reported that sweet potato vines at 20 days age were low in DM, but with high protein content (9.25% and 23.6%, respectively). Dominguez (1992) reported a similar composition of sweet potato vines when using them in the diets of monogastric animals and ruminants. Sesbania foliage contained higher DM and CP than sweet potato vines and similar values as in the present experiment were reported by Thuy and Do (1992) and Hon (1998).

Literature reports on the feeding of Sesbania grandiflora to goats have all been positive. Ash and Petaia (1992) and Moog (1998)  found Sesbania leaves to be highly palatable and well digested by goats when fed as a major part of the diet. Sesbania fed as a sole diet for growing goats supported superior DM intake, DM digestibility and live weight gain than foliages of Leucaena leucocephala, Hibicus rosa-sinensis or Ceiba pentadra (Nguyen Thi Hong Nhan 1998). According to Dahlanuddin (1998), when Sesbania was fed as a single diet to goats, DM intake was 2.5 times higher than when they were fed only native grasses, resulting in almost 10 times faster growth rate.

Rumen ammonia levels in goats fed only Sesbania foliage were reported to be in the range of 300 to 400 mg/litre (Nguyen Thi Hong Nhan 1998) and on sweet potato vines similar levels could be expected in view of its relatively high protein content and the likelihood that the protein would be highly soluble. The digestibility of the sweet potato vines can be expected to be as high or higher than that of Sesbania, in view of the lower cell wall content, thus the rumen system should have been as efficient with sweet potato vines as with Sesbania.  The interpretation of higher intakes and faster growth on Sesbania is therefore that the foliage from thus shrub is a good source of escape (or bypass) protein, as is the case for most other edible shrub and tree foliages (Preston and Leng 1987)..

Generally, farmers grow sweet potato in the backyard on small plots, and the great majority of the vines come from seasonal cropping of the tubers. This means that normally the vines will be available only during a short period. A possible solution to this constraint can be to produce sweet potato vines as forage all year round or to conserve the vines in some way connected to the harvest of the tuber, such as silage or hay. Sesbania grandiflora is able to grow on many types of soil in the Mekong delta of Vietnam, but there are some constraints to using only Sesbania for feeding goats. The farmers usually grow a few trees at the edges of their plot and use them for fuel wood or timber. Sesbania trees are also used to shade the young citrus trees and the farmers prune the side branches for feeding goats. However, the young branches regrow slowly and it takes some months to get mature branches. Farmers keeping goats in the area have to collect or buy Sesbania grandiflora branches from their neighbours. Thus there is not a high potential for producing large amounts of Sesbania foliage on the farms.

Conclusions

• When given as the sole feed to growing goats, the foliage of Sesbania grandiflora was superior to fresh sweet potato vines, in terms of feed intake and live weight gain, the latter being 50% greater on Sesbania  (64 g/day versus 44 g/day).
  
  
• Replacement of Sebania with 50% fresh sweet potato vines on a dry matter basis resulted in acceptable live weight gains (60.6 g/day)
  
  
• Farmer experience indicates that the constraint to the widespread use of Sesbania grandiflora  for feeding goats is the relatively low yield and slow rate of re-growth after pruning.

Acknowledgements

The authors gratefully acknowledge the Swedish International Development Agency, and Department for Research Co-operation with Developing Countries (Sida-SAREC) for financial assistance, and the Mekong Delta Farming Systems Research and Development Institute (MFSRI), Cantho University, Vietnam and staff of the Sida-SAREC regional office in the University of Agriculture and Forestry, Thu Duc, Ho Chi Minh city, Vietnam for providing suitable conditions for the study.

This paper is based on research submitted by the Senior Author to the Swedish University of Agricultural Sciences in partial fulfillment of the requirements for the MSc degree in Tropical Livestock Systems.

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Received 6 August 2004; Accepted 11 August 2004

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