Livestock Research for Rural Development

Volume 8, Number 4, November 1996

Ileal and in vitro digestibility in the pig of three floating aquatic macrophytes

P L Domínguez, Yamilet Molinet and J Ly

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


Growing water plant in ponds fertilized with pig manure is one way of recycling the N given in the original feed. Either water hyacinth (Eichhornia crassipes (Mart, EC), duckweed (Lemna minor, L) or azolla (Azolla spp, A) grown on liquid pig slurry after an anaerobic treatment were sun dried, milled and included as an alternative source of N at 0, 100 or 200 g/kg in sugar cane molasses and soybean meal diets given to pigs. N and OM content (g/kg in dry basis respectively) of macrophytes were EC, 26.3 and 810; L, 45.6 and 643; A, 50,9 and 809. Nine 45 kg pigs prepared by an ileo-rectal anastomosis were distributed at random to diets in 3 digestion trials according to a 3 x 3 Latin square design. Ileal OM but not N digestibility of the basal diet was high as expected (OM, 811 - 842; N, 696 - 719 g/kg respectively). In vitro and in vivo ileal protein digestibility were lowest in water hyacinth and highest in azolla (EC, 412 and 162; L, 674 and 560; A, 701 and 646 g/kg respectively). Ileal OM digestibility was less influenced by azolla than by water hyacinth at 200 g/kg level of inclusion in the diet (EC, 739; L, 769; A, 778 g/kg) thus indicating that the efficiency of recycling the excreted N by the pig may largely depend on the type of macrophyte to be used for this purpose.

Key words: pigs, digestibility, N recycling, cane molasses, water hyacinth, duckweed, azolla


Cultivation of free-floating aquatic macrophytes on wastewater from pigs may be a means of producing feedstuffs in either a nitrogen recycling system (Lincoln et al 1986; Salomoni et al 1991) or in crop-pigs integrated systems (Gavina 1987). In the case of the pig, macrophytes such as water hyacinth (Eichhornia crassipes Mart.) have not revealed any particular advantage when included in diets for the growing pig (Combs 1970; Berto et al 1988), whereas azolla (Azolla spp) has been suggested to be a promising N source in pig feeding (Querubin et al 1988; Becerra et al 1990). On the other hand, little is known about the nutritive value of duckweed (Lemna minor), another macrophyte which can be grow in effluents from swine manure treatment.

The free-floating aquatic macrophytes are characterized by high content of N, crude fibre and ash (Bonomi et al 1981; Alcantara and Querubin 1989; Domínguez and Ly 1996, 1997). Since protein sources are usually the most expensive component of feeds, interest related to protein utilization by pigs fed on free-floating aquatic macrophytes is of high concern. On the other hand, the ability of the pig to utilize high fibre feeds is dependent on several factors, such as the site within the gastrointestinal tract where cell wall degradation does occur. In this connection Graham (1988) has suggested that some dietary fibres can be degraded in the small intestine thus facilitating a more complete pre-caecal digestion of these nutrients.

In fact, little is known about the nutritive value of this type of aquatic biomass for pigs. In this connection a high total digestibility of nutrients has been reported for azolla (Alcantara and Querubin 1989; Dominguez and Ly 1997) while the reverse seems to hold true for water hyacinth, as has been found by Dominguez and Ly (1996).

The present experiments were conducted to compare ileal digestibility of nutrients by pigs fed a non-conventional, low fibre sugar cane molasses-soybean meal diet containing supplements of high fibre and N content. Three fibre sources were used which differed in their susceptibility to degradation within the gastrointestinal tract of the pig: water hyacinth, duckweed and azolla. In vitro N digestibility was included for a further comparison.

Materials and methods

Diet characteristics

A control diet composed of sugar cane molasses type "B" and soya bean meal was formulated to contain 130 g/kg crude protein (Table 1) and was progressively substituted by 100 and 200 g/kg of free-floating aquatic macrophytes on dry basis.

Table 1: Composition of the diets
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Macrophytes in diet DM, g/kg

0 100 200
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Ingredients, g/kg DM
Sugar cane molasses
type "B" 712 636 568
Soya bean meal 258 234 202
Macrophyte meal - 100 200
CaPO4H.2H2O 15 15 15
NaCl 5 5 5
Premix1 10 10 10
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1 Contents per kg: 600 IU vitamin A, 160 IU vitamin D3, 10 mg vitamin E, 2 mg vitamin B1, 3 mg vitamin B2, 15 mg vitamin B6, 0.025 mg vitamin B12, 5 mg panthotenic acid, 300 mg choline chloride, 2 mg menadione sodium bisulphite, 0.5 mg folic acid, 0.4 mg cobalt, 10 mg iron, 0.5 mg iodine


Diets were supplemented with minerals and vitamins as described in Table 1. The control diet contained (g/kg dry matter) ash 107, organic matter 893, N 21.1, crude fibre 24.8, NDF 43.0, gross energy 14.8 MJ.

The macrophytes were either a mixed culture of azolla, where Azolla microphylla was predominant, duckweed or lemna (Lemna minor) or water hyacinths (Eichhornia crassipes Mart.). These plants were collected from a secondary lagoon containing effluents from pig stables of the Swine Research Institute at Havana. A batch of each plant type for the entire experiment was collected and sun-dried on a concrete floor prepared ad hoc, then milled through a hammer mill. The analysed composition of the three resulting meals is listed in Table 2.

Table 2: Composition or the free-floating aquatic macrophytes
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Water hyacinth Lemna Azolla
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Dry matter, g/kg 934 941 932
Composition, g/kg DM
Ash 190 356 291
Organic matter 810 644 809
N 26.3 45.6 50.9
Crude fibre 370 200 195
NDF 555 382 423
ADF 356 289 266
Energy, MJ 15.4 16.2 16.1
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The amounts of macrophytes used varied to some extent the composition of the diets, thus increasing the neutral detergent fibre (NDF) and ash values above the level present in the control diet. As was to be expected, the control diet formulated with sugar cane molasses type "B" and soya bean meal had a relatively low NDF content but a high ash value. Nevertheless gross energy differences among the diets were practically irrelevant.

In vivo digestibility trials

Three digestibility trials were conducted using nine castrated male pigs averaging 45 kg liveweight at the start of the project in June 1993. The pigs were accustomed to metabolism cages and fed a standard diet of sugar cane molasses type "B" and soya bean meal (Table 1) for one week before being surgically prepared with an end-to-end ileo-rectal anastomosis (Green et al 1987). The animals recovered appetite one week after surgery and then were randomly assigned to one of the three dietary treatment for every macrophyte experiment according to a 3 x 3 Latin square design. The average feed supply was 0.08 kg DM per kg body weight0.75 per day. The diets were offered twice daily in two equal meals at 9:00 and 15:00 hours. Drinking water was given ad libitum.

Ileal digesta was collected for two consecutive days preceded by five days of adaptation to the experimental diet. The procedure for digesta sampling and preparation for analysis was as outlined by Ly et al (1995). Ileal protein (N x 6.25) digestibility of the studied macrophytes was estimated by difference (Crampton and Harris 1969). Ileal protein digestibility of cane molasses was assumed to be zero therefore the same index was calculated in the content of diets for protein digestibility in soya bean meal from the three in vivo trials (nine animals in total).

In vitro digestibility trials

The pepsin-pancreatin procedure of Dierick et al (1985) was employed for estimating in vitro protein digestibility in the macrophytes. The sample size was reduced to pass 1 mm mesh by grinding in a cyclone-type mill. In vitro protein digestibility was conducted in quadruplicate and casein (analytical grade) was used as reference protein. The calculation of in vitro undigested dry matter was made as recommended by Boisen and Fernandez (1995). The endogenous protein losses of every feed used in these studies were calculated as outlined by Boisen and Fernandez (1995), but assuming a constant protein intake of 130 g/kg DM.

Chemical analyses

Dry matter and ash content were estimated on feed and ileal digesta samples by drying at 105 °C and ashing at 550 °C for 24 hr respectively. Energy was determined with an adiabatic calorimeter, and nitrogen was determined on all samples by the macro-Kjeldhal technique. ADF and NDF were measured by the methods of Goering and Van Soest (1970).

Statistical analyses

Data were subjected to analysis of variance using standard procedures as outlined by Steel and Torrie (1980), with means compared by Duncan's new multiple range test. In addition regression analysis was conducted in the appropriate cases.


In vivo digestion of macrophytes

The determined composition of diets and ileal digestibility indices in pigs fed water hyacinth are set out in Table 3. The increased ash and fibre content of the diets was noteworthy, whereas gross energy contents were similar in all treatments. Water hyacinth meal was associated with a significant decrease (P<0.05) in ileal DM, ash, organic matter, energy and N digestibility in pigs fed graded levels of this macrophyte. In this connection, ileal N digestibility was markedly depressed from 703 g/kg in the control diet to 507 g/kg in the diet with 200 g/kg of water hyacinth meal. This same effect was absent for ileal crude fibre and NDF digestibility.

Table 3: Composition of diets and ileal digestibility of nutrients and energy in pigs fed water hyacinth

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Water hyacinth, g/kg





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Composition, g/kg











Crude fibre









Energy, MJ




Digestibility, g/kg











Organic matter










Crude fibre










Energy, J/KJ





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abc Values within a row with the same superscript were not significantly different (P<0.05)


The composition of diets and ileal digestibility of nutrients and energy in pigs fed duckweed are presented in Table 4. The duckweed meal included in the diet increased the ash and fibre fractions. The gross energy concentration was not influenced by the introduction of the macrophyte in the feed but the N content of diets increased concomitantly for this reason, due to the high crude protein concentration found in duckweed (Table 2). On the other hand, DM and energy digestibility up to the terminal ileum were significantly depressed (P<0.05) when graded levels of duckweed meal were included in the diet. However, the digestibilities of organic matter, N, crude fibre and NDF were not influenced by the use of duckweed in the diet of the pigs.

Table 4:. Composition of diets and ileal digestibility of nutrients and energy in pigs fed duckweed

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Duckweed, g/kg





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Composition, g/kg DM









Crude fibre








Energy, MJ/kg




Digestibility, g/kg











Organic matter










Crude fibre










Energy, J/KJ





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ab Values within a row with the same superscript were not significantly different (P<0.05)


Table 5 shows the data on the composition of diets formulated to contain azolla meal, and ileal digestibility indices. In this case, as observed with the other studied macrophytes, as azolla meal was added to the diet, the concentration of dietary ash and fibre increased and no difference in gross energy concentration was found. A sharp increase in N content with increasing levels of azolla meal in the diet was in accordance with the high N content of this macrophyte (Table 2). No evident treatment effect on ileal digestibility of nutrients and energy was found although ileal digestibility of ash and organic matter seemed to decrease (P<0.10) in treatments where azolla meal was included.

Table 5: Composition of diets and ileal digestibility of nutrients and energy in pigs fed azolla

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Azolla, g/kg





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Composition, g/kg









Crude fibre








Energy, MJ




Digestibility, g/kg











Organic matter










Crude fibre










Energy, J/KJ





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The regression analysis indicated that energy digestibility (Y, in %) could be calculated from the OM digestibility (X, in %) in the macrophyte-based diets, this relationship being more pronounced in water hyacinth and azolla (P<0.001) than in duckweed (P<0.01). The equations were:

Y = 3.078 + 0.972 X (SEb = ± 0.14; r = 0.93) for water hyacinth

Y = -15.536 + 1.19 X (SEb = ± 0.38; r = 0.76) for duckweed

Y = -16.412 + 1.16 X (SEb = ± 0.053; r = 0.99) for azolla

In vitro digestion of macrophytes

The estimates of protein digestibility in macrophytes for pigs are in Table 6.

Table 6: Protein digestibility in macrophytes for pigs

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Protein digestibility

In vitro

In vivo1




---g/kg DM---

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Soybean meal





Water hyacinth















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1 Calculated by difference (see text)
2 Endogenous protein losses
3 In vitro undigested dry matter
4 Mean and SE of four determinations
5 Mean and SE of nine animals (see text)


The values for in vitro protein digestibility were always higher than the in vivo ileal digestibility values obtained for soybean meal and every macrophyte, when estimated difference in the case of in vivo ileal protein digestibility. On the other hand either in vitro or in vivo protein digestibility data from macrophytes were lower than the same indices for soybean meal. A rather wide range of protein digestibility among the macrophytes was observed, the lowest value being for water hyacinth either in vitro or in vivo (411.8 and 161.8 g/kg respectively) and the highest for azolla (700.5 and 646.4 g/kg respectively). The reversal trend was found for endogenous protein losses and in vitro undigested dry matter.


Digestion of fibre

Either by the Weende or by the Van Soest approach of fibre estimation, pre-caecal fibre disappearance was apparent in this study, thus supporting previous findings in the pig (Graham 1988; Bach Knudsen and Hansen 1991). It could be thought that sugar cane molasses did favour fibre degradation in the stomach and small intestine of the pigs, perhaps through an osmotic shock on the network of beta-polysaccharide linkages in these free-floating aquatic macrophytes. Nevertheless the magnitude of fibre disappearance was dependent on the type of macrophyte. In this respect, crude fibre and NDF digestibility were equal to 177 and 304 g/kg respectively when water hyacinth was included in the diet. These same indices were outstanding when 200 g/kg DM of azolla were used, 440 and 499g/kg, respectively. In this connection it could be assumed that these results are explained by the architecture of the cell wall of the aquatic plants involved in the present investigation, more or less susceptible to microbial attack in these circumstances (Chesson 1993). In this connection, Uchida et al (1993) have found a very low crude fibre digestibility of water hyacinth leaf blades (20.5%) when compared to alfalfa leaf meal (44.0%) even in herbivorous animals such as guinea pigs.

The well known depressive effect of fibre on the digestibility of other nutrients (see Close 1993) could be responsible for the results obtained in this experiment. This was more evident in the water hyacinth containing diets, if compared with those formulated with either azolla or duckweed.

Digestion of protein

In this study a trend was found of ileal N digestibility to decrease with increasing graded levels of water plants in the diet, which in turn could be adscribed to the well recognized influence of dietary fibre on protein digestion (Eggum 1995). Nevertheless this effect was very variable, due to the fact that digestion of protein up to the terminal ileum was revealed to be very low in water hyacinth based diets as compared with those formulated with duckweed or azolla. Moreover, when in vivo ileal protein digestibility was calculated, the data indicated a very low value for water hyacinth (163 g/kg), whereas the values for duckweed and azolla were 560 and 646 g/kg, respectively. In accordance with the results obtained in the in vivo trials, the values observed in vitro followed the same trend (412,, 674 and 701 g/kg, respectively). In this connection, in vitro protein digestibility data from duckweed and azolla are within the range of previous reported values for some fibrous sources. Boisen and Fernandez (1995) obtained a value of 79% for grass meal and Dierick et al (1985) found values of 66 and 56%, respectively for alfalfa and sugar beet pulp.

Endogenous protein losses calculated for a constant protein content in feed (130 g/kg DM) were lower in this study (14.9 g/kg DM intake) than reported by Boisen and Fernandez (1995) for soya bean meal (27.0 g/kg DM intake). The reason for this difference is difficult to explain, unless the suggestion of Boisen and Fernandez (1995) could be accepted, in the sense that considerable variation for endogenous protein losses could be found within the same feedstuff. On the other hand, endogenous protein losses determined for the macrophytes appeared to increase inversely with in vitro undigested dry matter as observed by Boisen and Fernandez (1995).

Concluding remarks

Digestibility data arising from the present study have revealed a considerable difference in the pattern of nutrient utilization among different free-floating aquatic macrophytes. This clearly indicates that the efficiency of recycling the excreted N by the pig may largely depend on the type of macrophyte used for this purpose.


Financial assistance for this project was provided in part by the UNDP Project Cuba 91-011. The technical assistance of Maritza Castellanos and Martha Caron was greatly appreciated.


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Received 10 December 1996