An approach to the nutritional value for pigs of sweet potato vines (Ipomoea batatas (l.) lam)

P L Domínguez and J Ly

Instituto de Investigaciones Porcinas
Carretera del Guatao km 1, Punta Brava
La Habana, Cuba

Abstract

Sun-dried sweet potato vine (SPV) meal contained in dry basis (g/kg): 234 crude fibre, 403 NDF, 328 ADF, 160 detergent lignin, 29.2 N, 216 ash and 158 kj/10 g DM gross energy. The effect of feeding SPV on nutrient digestibility was studied in either ileorectostomized or intact 45 kg liveweight pigs fed graded levels of SPV (0, 100 and 200 g/kg in the dry diet). SPV significantly decreased both ileal and faecal digestibility of most nutrients. Estimated ileal and faecal crude protein digestibility of SPV meal was 22.1 and 54.1% whereas in vitro ileal crude protein digestibility of SPV meal accounted for 42.2% thus indicating a rather low availability of crude protein in this biomass. The contribution of the large intestine to the digestion of diets ranged from 19.7 to 20.8% of energy disappearance in the entire digestive tract. Daily ileal and faecal output of short chain fatty acids and ammonia showed a trend to be proportional to the level of SPV meal in the diet. A low level of inclusion of SPV meal in the diet is suggested in order to avoid a negative effect on nutrient digestibility. Methods to improve digestion of this biomass could be encouraged.

Keywords: pigs, sweet potato vine, digestibility, fermentation

Introduction

Sweet potato (Ipomoea batatas (L.) Lam) is a tropical crop of a relatively short vegetative cycle the tubers of which are usually employed for human and animal consumption (see Wolfe 1992). The aerial part mostly composed of vines, may be utilized as animal feed in traditional backyard animal production systems. In this connection the possibility of use of sweet potato vines (SPV) for pigs has been explored either alone or together with the tubers (see Dominguez 1992) which in turn have been extensively studied (Nwokolo 1990). Moreover, the use of pigs for biological harvesting of sweet potato crops has long been recognized (Bishop 1957).

There are few data available concerning the feeding value for pigs of the foliage from sweet potato. Ffoulkes et al (1978) and Ruiz et al (1980) have reported a high DM digestibility of SPV in ruminants. However a depressing effect of this type of biomass on animal digestibility has recently been reported for diets offered to growing pigs (González et al 1994; Ly and Diéguez 1995), thus supporting early findings of Zarate (1956).

The aim of the experiments described herein was to obtain information concerning digestion of SPV meal in the pig.

Materials and methods

Diets

Experimental diets were formulated to contain 0 (control), 100 and 200 g/kg SPV meal (Table 1). The aerial part of sweet potato plants was collected at the end of the vegetative period, during tuber harvesting. The sweet potato tubers were destined to human consumption. The foliage was sun-dried and milled to obtain the SPV meal. The analyzed composition of the resulting meal was (g/kg dry basis): 234 crude fibre, 403NDF, 328 ADF, 160 detergent lignin, 29.2 N, 216 ash, 75.5 hemicellulose, 167 cellulose and 158 kj/10 g DM gross energy.

Experiment 1

Three castrated male pigs weighing 45 kg liveweight were placed in metabolism cages for a 7 day adjustment period and were fed the control diet (Table 1) after which the animals were surgically prepared with an end-to-end ileo-rectal anastomosis (Green et al 1987). The pigs recovered their appetite seven days after surgery and were fed one of the three diets (Table 1) according to a 3 x 3 Latin square design for a 4-days adaptation period and a 3-days ileal digesta collection period. Pigs were fed two equal portions (9:00 am and 3:00 pm) of feed daily in meal form to total 0.08 kg DM per kg body weight^0.75. Drinking water was available ad libitum from nipple waterers.

Ileal digesta was collected as described elsewhere (Ly et al 1995) and pH, short chain fatty acids (SCFA) and ammonia were estimated in fresh samples as outlined by Ly et al (1995). Ileal digesta was dried 24 hr in a forced-draft oven (60^oC), weighed and ground to pass a 1-mm screen using a cyclone-type sample mill. The ground samples were pooled for each pig for each of three days of digesta collection and then subjected to analysis.

Dry matter and ash content were determined on feed and ileal digesta samples by drying at 105^oC for 24 hr and ashing at 550^oC for 24 hr respectively. Caloric content of feed and ileal digesta samples was determined by bomb calorimetry. Lignin, ADF and NDF were determined by the methods of Goering and Van Soest (1970). Hemicellulose and cellulose were calculated as the difference between NDF and ADF, and between ADF and lignin respectively. Nitrogen was determined on all samples by the macro-Kjeldahl method.

Experiment 2

Six castrated male pigs weighing 45 kg liveweight were placed in individual pens containing a stall feeder and a nipple-waterer, in an open front building with solid concrete floors. The animals were fed at random the same diets described in Experiment 1 according to a double 3 x 3 Latin square design for a 6-days adaptation period and one-day faeces collection period, by grab sampling. The level of feed intake was 0.08 kg DM per kg bodyweight^0.75 and drinking water was available ad libitum.

The analysis of feed and faeces were carried out as described in Experiment 1. In addition acid insoluble ash was determined by the Van Keulen and Young (1977) technique.

Experiment 3

The pepsin-pancreatin procedure as described by Dierick et al (1985) was employed to determine in vitro crude protein digestibility in SPV meal. In addition the in vivo ileal and faecal N digestibility of SPV meal was calculated by difference according to Crampton and Harris (1969).

Statistical analysis

The results were analyzed statistically following procedures described in Steel and Torrie (1981). Regression analysis was conducted in the required cases.

Results and discussion

Bulking characteristics

Both ileal and faecal bulk were influenced by the introduction of SPV meal in the diet (Table 2). In fact the foliage from sweet potato caused a trend (P<0.10) for ileal fresh digesta to increase from 2149 to 2640 g/day per kg DM intake. This effect was more evident on fresh faeces (P<0.05), which increased from 317 to 772 g/day per kg DM intake. On the other hand ileal DM concentration was increased slightly (P<0.05) by the inclusion of SPV meal in the diet. However, faecal DM concentration was not affected by treatment. Bulking characteristics of digesta and faeces are usually enhanced by dietary fibre in the pig (Bardon and Fioramonti 1983; Bach Knudsen and Hansen 1991) and other animal species (Stephen and Cumming 1980; Nyman and Asp 1982). In this connection, water holding capacity of fibre in SPV meal could at least partially account for the increase in bulking characteristics of digesta and faeces of pigs (Metz 1985).

Digestibility indices

The apparent ileal digestion coefficients for the diets (Table 3) showed that digestibility of DM and N significantly decreased (P<0.05) when graded levels of SPV meal were included in the feed. This same trend was found for ash, organic matter and energy digestibility (P<0.10). Nevertheless, the reverse was true for ileal crude fibre (P<0.05) and NDF digestibility. On the other hand the depression of total nutrient and energy digestibility was also evident (P<0.05), caused by the dietary SPV meal. In this connection it has been pointed out by Vervaeke et al (1991) and by Shi and Noblet (1993) that some fraction of dietary fibre can disappear anterior to the caecum in pigs. On the other hand, it is well known that the inclusion of fibrous feeds in the diet brings about a depression of several nutrients (see Close 1993).

Apparent energy digestibility (Y, %) correlated with apparent OM digestibility (X, %) in the ileum (r = 0.943; P<0.001) and faeces (r = 0.993; P<0.001) according to the respective regression equations:

Y = - 2.681 + 0.986 ( 0.126) X......(i)

Y = - 0.886 + 1.004 ( 0.040) X......(ii)

In vitro crude protein digestibility was low for SPV meal (Table 4). In turn this same value was considerably higher than that obtained for in vivo ileal crude protein digestibility from this type of biomass (22%). In vivo total crude protein digestibility was rather low too (54%). Brown and Chavalimu (1985) found that a substantial increase in the amount of unavailable nitrogenous substances could be a direct effect of drying sweet potato foliage. These findings can support the general trend observed in the present study, but do not provide explanation for the difference between the in vitro and in vivo results. In fact it is generally accepted that this difference accounts for endogenous protein losses (Boisen and Eggum 1991).

Digestion in the large intestine and fermentative indices

The addition of SPV meal to the control diet reduced the amount of nutrients and energy disappearing in the large intestine (Table 5) thus largely reflecting the pattern of digestion in the entire gastrointestinal tract. In this connection energy disappearance in the large intestine accounted for some 20% of total energy digestibility in the gastrointestinal tract. These values are in accordance with the data provided by Shi and Noblet (1993). Different fibre fractions were digested in caecum and colon, but the contribution of large intestine to total fibre disappearance in the gastrointestinal tract was largely influenced by the presence of SPV meal in the diet. In fact a decrease in large intestinal digestion of fibre was noteworthy. It was probably due to the nature of the cell wall content in SPV, since detergent lignin accounted for some 40% of the NDF fraction. Although lignin is partially digestible in the pig, low values are usually found for this entity (Low 1985, Close 1993) otherwise bound to beta-linked carbohydrate polymers.

Ileal SCFA concentration was significantly higher (P<0.05) with low levels of SPV in the diet (Table 6) and this same trend was observed in faecal SCFA concentration. However, ammonia concentration was negatively influenced by SPV in both sites of measurement. Ileal pH was significantly higher (P<0.001) as influenced by SPV. This same phenomenum was observed in faecal pH (P<0.01). On the other hand daily flow of SCFA was higher when graded levels of SPV were introduced in the diet either at the ileum (P<0.10) or faeces (P<0.05). The daily flow of ammonia was also higher (P<0.05) in faeces but was indifferent to treatments in the ileum.

A decrease in intestinal pH as a consequence of microbial activity on fibre constituents has been suggested to depress ammonia levels in the large intestine (Kauffman 1986). In fact, Varel et al (1984) have found that inclusion of fibre in the diet decreases large intestine ammonia levels. These circumstances would lead to a reduced urinary N output as a result of a decrease in non alpha-amino N absorption through the caecum and colon as it has been observed by Malmlof and Hakansson (1984). This hypothesis could not be proved in this study, since a trend was found of SCFA concentration to increase along with pH values in digesta entering the caecum or faeces. At the same time daily ammonia flow remained unaltered in the distal ileum or increased in faeces, while reducing N digestibility (Table 3) was an effect of the introduction of SPV in the diet.

Based on the experiments described above, methods to improve digestion of SPV meal could be encouraged, otherwise the low digestible energy and N availability of a biomass such as that used in this study does not suggest its inclusion in the diet of the pig.

Acknowledgments

The authors acknowledge Mrs. Martha Caron, Mrs. Maritza Castellanos for technical assistance, Mrs. Olga Martínez for assistance with the statistical analysis and Eng. Rosa María Martínez for typing and computing the manuscript.

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Received 1 May 1997

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