A comparative study on the digestibility of cassava, maize, sorghum and barley in various segments of the digestive tract of growing pigs^*

Basilisa Pascual-Reas

Department of Animal Medicine and Production, Faculty of Veterinary Science,
University of Queensland, Australia
*Submitted December 1996 to the Department of Animal Medicine and Production, Faculty of Veterinary Science, University of Queensland as a requirement for the degree of Master in Veterinary Studies

Abstract

The apparent DM, OM and energy digestibilities of diets based on cassava, maize, sorghum and barley were determined in the different segments and over the total digestive tract of 16 growing pigs. The test materials were finely ground and fed at a 90% inclusion rate. The relative digestibilities at the level of the stomach, small intestine, caecum, large intestine and anus (faeces) were determined using TiO₂ as inert marker.

The cassava diet differed markedly from the cereal diets in that most of the digestion took place in the stomach, whereas for the cereals the major site of digestion was the small intestine. Digestion in the hind gut was least on the barley diet. Overall digestive tract digestibility was highest for cassava, followed by maize, sorghum and barley in that order. The marker (Titanium oxide) and the total collection method ranked the diets in the same order although absolute values were higher for the former.

Cassava can be considered as an adequate energy substitute for cereals in pig diets.

Key words: Pigs, maize, sorghum, barley, cassava, digestibility

Introduction

Carbohydrates, in the form of starch and non-starch polysaccharides from cereals are the main sources of energy in pig diets (Drochner 1991). In most industrial countries like Australia, the most important grain cereals used in intensive pig production are sorghum, barley, wheat and wheat-by-products (Kopinski and Willis 1996). In Asia and the US, maize is more commonly used whilst cassava is used as an alternative especially in the tropical countries.

The feeding value of carbohydrates as a major nutrient in pig diets can be evaluated in terms of digestibility. Pig responses to various components of diets are related to the actual amount of material digested, where the loss of nutrients between the two sites of the digestive tract is taken to be digestion and absorption (McDonald et al 1995). Several reports show that the digestibility of starch from the major grains used for livestock feeding is almost complete (96% to 98%) in the small intestine (Bach Knudsen and Hansen 1991). The rest is digested by the microorganisms present in the caecum and the large intestines leading to bacterial growth in the lower gut. Research conducted by Siba et al (1993) suggests that when the digestibility of the diet in the small intestine is high, the amount of nutrients entering the lower gut will be minimal, hence bacterial multiplication will be lower. Moreover, as the small intestine is the main site of nutrient absorption, diets with higher digestibility rates occuring before the large intestine tended to give higher nutrient absorption and reduction in the amount of nutrients available for bacterial fermentation in the hind gut. This lowering of fermentation suggests reduced risk of bacterial scouring that may occur in the lower tract as a result of hind gut fermentation.

Most of these carbohydrates are supplied by cereal grains such as corn, sorghum, wheat and barley. However, these grains are not always available locally prompting the feedmillers and pig producers to use imported materials which are relatively more expensive. Thus, it is always economically advantageous to consider and compare them with other cheaper alternative energy sources such as cassava. Cassava roots contain highly digestible energy and are capable of providing high yields of energy per hectare, just like maize (Hahn et al 1992). Moreover, cassava contains the highest digestible energy (DE) among the rootcrops with an average of 14.7 MJ/kg (Bradbury and Holloway 1988; Walker 1985) consisting of about 70 to 80% starch (Gomez 1991). However, its practical use and value has to be evaluated in different situations taking into account the low protein content, presence of hydrocyanic acid and any further processing requirements that could either limit or justify its utilisation. The potential of cassava as an alternative source of energy in pig diets can be assessed on the basis of its chemical composition and digestibility (Just 1980). Reports on feeding cassava to pigs have been reviewed by several authors (Wu 1991).

The present experiment was conducted to assess the DM, OM and energy digestibilities of cassava in comparison with maize, sorghum and barley in various intestinal segments and over the whole length of the digestive tract of growing pigs.

Materials and methods

The present experiment was conducted at the Pig Research Centre of the Department of Primary Industries (DPI) at Wacol, Queensland. The pigs were kept in a temperature-controlled room (27^oC) during the whole duration of the study.

Experimental diets and animals

Diets

Four mixed diets based on cassava, maize, sorghum and barley (milled using a laboratory mill with a 1mm screen) were used in this experiment. Supplements (casein, synthetic lysine, vegetable oil, mineral sources and vitamins/minerals premixes) were added to satisfy the requirement for growing pigs (Table 1). Titanium dioxide (TiO₂, 0.1g/kg) inert marker was incorporated in the diet to enable the determination of digestibility of the diets in the different parts of the digestive tract.

The cassava variety was MAUS 7, and was grown at the University of Queensland Horticulture Station, Redlands. Shortly after harvesting, the roots were chipped (unpeeled) and sundried to reduce the cyanide content and for storage purposes.

Table 1: Feed formulation

^a Cassava, maize, sorghum, barley.

Animals

Sixteen (two batches of 8) entire male pigs ranging from 26-40 kg body weight were assigned with four pigs on each diet. The average initial weights of the pigs were 30.4 and 36.9 kg for the first and second batch respectively (Table 2). The pigs were kept in individual metabolism cages during the whole duration of the trial. A 5-day pre-feeding period using the same diet was done for acclimatisation and to clear the digestive tract of the pigs from any other feed residues. This was followed by 5 day actual feeding trial and collection of faeces.

Table 2: Mean liveweights of pigs and feed intakes on each treatment

Faecal collection

A colour indicator (ferric oxide) was used to identify the start and conclusion of the total collection of the faeces. Pigs were fed twice daily a constant amount of approximately 70%-80% of their maximum feed intake or 1.6 to 1.9% of their body weight during the collection period which lasted for 5 days.

All faeces were collected and weighed every day, frozen and totalled after the collection period. At the end of the collection period, the frozen faeces were chilled, mixed, oven dried (95^oC for at least 48 hours) and milled using a 1mm screen.

Digesta collection

After the 10 days experimental period (5 days pre-feeding and 5 days faecal collection), all the pigs were killed and the whole length of the digestive tract was removed for intestinal sampling. The digesta were collected from the stomach, the terminal part of the small intestines (ileum), caecum, and the middle part of the large intestine.

Laboratory analysis

The diets, the faeces and the digesta were analysed for dry matter, ash, and gross energy (Tables 3 and 4). The analysis of energy was done in duplicate using a bomb calorimeter (DPI, Biochemistry Lab.). Analysis of ash was done in a furnace at 600^oC for at least 6 hours, and the indicator (TiO₂) determined via Inductive Coupled Plasma Mass Spectrocopy (ICPMS, DPI) (Table 4).

Table 3: Mean values for the DM and GE in feed and faeces

Table 4: Mean values for content of dry matter in the digesta and the organic matter and gross energy of the digesta dry matter

NB. Small intestine samples were taken from the most caudal part, while the large intestine samples were taken just before the colon.

Results and discussion

The digestibility coefficients were analysed using the procedure of the General Linear Model (GLIM) of the Statistical Analysis System, Inc. (SAS). The flows of DM, OM and energy are summarised in Table 5. The digestibility coefficients of the DM, OM and energy in the different segments of the digestive tract are presented in Table 6, and over the whole length of the digestive tract (faeces) in Table 7.

The daily DM intake was lowest on the cassava diet (Table 1) and highest on the diets of sorghum and barley. This may have been due to the very fine texture of the cassava root meal and the fact that it was a new "feed". By contrast, the pigs had been fed with sorghum and barley-based diets prior to the start of the pre-feeding trial.

Nutrient flow in the different parts of the digestive tract

The flows of DM, OM and energy showed a similar pattern of constant decline starting from the stomach to the caecum and then a smaller rate of decline down to the large intestine. The flows of OM and energy from the stomach were less on the cassava than on the other diets which did not differ from each other (Table 5). At the end of the small intestine, the only significant difference was for the flow of energy which again was less for the cassava diet than for maize, sorghum or barley.

In the caecum, the flows of DM, OM and energy were not significantly different between cassava and maize and between sorghum and barley. But maize and cassava were significantly different from sorghum and barley. Finally, at the middle of the large intestine, the flow of DM, OM and energy tended to be less for cassava than for sorghum and barley.

Table 5: Rates of nutrient flow in the different parts of the digestive tract

^abc In this and subsequent tables, means in columns with the same superscript are not significantly different (P<0.05).

DM, OM and energy digestibility coefficients from the different parts of the digestive tract.

The relatively high disappearance of cassava in the stomach may be a result of the rapid hydrolysis of the starch to glucose by gastric secretions (Kvasnitski 1951). In addition, Cunningham et al (1963) have detected volatile fatty acids and lactic acid in the stomach suggesting a microbial fermentation resulting from starch digestion. Furthermore, the lower dry matter intake of the cassava diet (Table 2) may have caused a longer retention time in the stomach of the pigs causing a slower rate of passage and thereby exposing the nutrients to greater gastric secretions and bacterial digestion (Keys and DeBarthe 1974a).

The coefficients of digestibility of OM and energy in the stomach for the three cereals(Table 6) were low and far below those in the report of Keys and DeBarthe (1974b) where ingested starch from different cereals gave values of between 45 and 75% for digestibility cranial to the duodenum. In the present experiment, the pigs were fed approximately 2 hours prior to slaughter and may not had enough time for gastric secretion or microbial fermentation to affect those ingredients with higher crude fibre content such as maize, sorghum and barley. The negative result for sorghum is difficult to explain. A specific effect of the TiO₂ is unlikely to be the cause as three other markers used to test the same samples gave a similar pattern of negative responses in the stomach for the sorghum diet (Kopinski, Personal communication). Rooney and Pflugfelder (1986) reported that, among the cereals, sorghum has the lowest starch digestibility due to the resistance to digestive enzymes of the hard peripheral endosperm layer. There appears to be variation among sorghum cultivars as in a study conducted by Cousins et al (1981) a low-tannin sorghum had the same digestibility as maize.

At the level of the small intestine, the digestibility coefficients were similar for all diets. Thus the digestive action in this part of the tract was sufficiently efficient to allow the cereals to compensate for the low digestibility in the stomach.

Digestibility in the caecum was higher for cassava and maize than for sorghum and barley. However, between the caecum and the middle of the large intestine, the disappearance of organic matter was greater for sorghum and barley, although the cumulative values still favoured the cassava and the maize diets. Digestibility in the whole digestive tract was highest for cassava, followed by maize, then sorghum with the lowest value for the barley diet.

Table 6: Cumulative values for digestibility¹ along the digestive tract

¹ All values determined using the Titanium dioxide marker

Comparison between the total collection and marker methods

For all the parameters (DM, OM and energy), the digestibility coefficients tended to be higher for the marker method than for the total collection (Table 7). However, the ranking of the diets was similar for both methods.

Table 7: Comparison of marker and total collection method for determination of digestibility

¹ Mean of 4 values, except for maize which had only 2 values in the total collection; ² Analysed using T tests; ³ Significance level: * P <0.05 ** P <0.01 *** P <0.001

Conclusions

• The cassava diet differed markedly from the cereal diets in that most of the digestion took place in the stomach, whereas for the cereals the major site of digestion was the small intestine
• Digestion in the hind gut was least on the barley diet
• The marker (Titanium oxide) and the total collection method ranked the diets in the same order although absolute values were higher for the former. intestine.
• Overall digestive tract digestibility was highest for cassava, followed by maize, sorghum and barley in that order.
• Cassava can be considered as an adequate energy substitute for cereals in pig diets.
  

References

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Received 14 August 1997

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