Review of the nutritive value and effects of inclusion of forages in diets for pigs

Bui Huy Nhu Phuc

Nong Lam University , Ho Chi Minh City,  Vietnam
bphuc.ldanh@viettel.vn

Abstract

The tropics have a huge natural potential for the production of forage plants, many of which are valuable sources of nutrients for monogastric animals. This article attempts to review the work that has focused on the chemical composition of some of the most important tropical forages, including amino acid balance, and on evaluating the effects of preservation method and the effects of including these forages on the intake, diet digestibility and performance of growing pigs and pregnant sows. The major green forages covered are cassava leaves (Manihot esculanta Crantz), groundnut foliage (Arachis hypogaea), sweet potato vines (Ipomoea batatas), leucaena leaves (Leuceana leucocephala), mungbean foliage (Phaseolus aureus), water spinach (Ipomoea aquatica Forsk), duckweed (Lemna minor) and Tricantera leaves (Trichantera gigantea). There is some evidence that green forages generally have an acceptable amino acid profile, but that for a variety of reasons, in particular the presence of anti-nutritional factors, including them in pig diets at high levels depresses digestibility and feed intake. Sun-drying and ensiling can be good preservation methods under tropical conditions that reduce the content of some anti-nutritional factors. There are differences in response to forage inclusion in diets for growing pigs, depending amongst other things, on the material that the forages replace. However, pregnant sows can utilize high fiber diets, and green forages can be a useful ingredient in diets for sows during pregnancy, as they not only are relatively high in protein but also in vitamins and minerals, and can give good reproductive performance.

Key words: Forages, digestibility, pigs, preservation method, reproduction, sows.

Introduction

In view of the worldwide demand for additional sources of food, the exploitation of plants of low economic importance would be a step towards better resource utilization (Telek and Martin 1983). The tropical zones contain the biggest genetic diversity of species and particularly of vascular plants of interest in this context. The non-conventional forage plants are an important example of this huge natural potential (Rosales et al 1993). To be useful these plant species must be locally and readily available, cheap, and acceptable to animals. The green part of biomass is potentially the most abundant protein source (D'Mello 1995). However, the major problems, which limit the use of these plants as protein sources for monogastrics, are their low palatability and high levels of fibre, which may decrease the feed intake and availability of nutrients (Rosales et al 1993). Further, the occurrence of anti-nutritive and toxic substances (e.g. tannins, saponins, phenols, alkaloids and steroids) may also inhibit the exploitation of these materials. High water content in some plants may also be a limiting factor.

An important question is the preservation of these biomass products. The most common way of preserving feedstuffs in tropical countries is to sun-dry. However, in the rainy season it is difficult to sun-dry, and extending the drying period diminishes the nutritional quality of the product. Thus, ensiling or high temperature drying have an advantage over sun-drying in this respect (Brown and Chavalimu 1985). At present only a limited proportion of potential plant species are used because of the failure to appreciate their potential value, and also the seasonality of supply and cost of preservation. In addition, the lack of knowledge of the nutritional characteristics of the materials, the perceived reduced nutritional value, and the lack of appropriate guidelines and recommendations for their use are also constraining factors (Close 1993).

The aims of this paper are to review studies on the chemical composition and amino acid pattern of some tropical biomass products, including leaves and vines, including the effects of preservation methods such as sun-drying and ensiling on the chemical composition, and on the nutrient digestibility and the amino acid availability at the ileal and faecal level in pigs, and to summarize data on the nutritional value of potentially useful tropical biomass products.

Influence of biomass species on chemical composition

The major green forages covered in this review are cassava leaves (CL; Manihot esculanta Crantz), groundnut foliage (GF; Arachis hypogaea), sweet potato vines (SP; Ipomoea batatas), leucaena leaves (LL; Leuceana leucocephala), mungbean foliage (Mb; Haseolus aureus), water spinach (WS; Ipomoea aquatica Forsk), duckweed (DW; Lemna minor) and Trichanthera leaves (Tric; Trichanthera gigantea). (Table 1).

Chemical analyses have indicated marked differences in chemical composition among plant species and varieties of plants. The content of organic matter (OM) varies from 73.6 to 92.8 % of dry matter (DM). This is the result of anatomical differences between plant species and depends on the effect of plant development as well as on the ratio of leaf/stem. The content of OM in the aquatic species is lower than that of the terrestrial species, due to their higher content of minerals (Phuc et al. 2001). Among the terrestrial plant species the leaf products are characterized by a higher crude protein (CP) content as compared with the vine products (averaging 28.2 and 21.8 % of DM, respectively), while the neutral detergent fiber (NDF) contents of the leaves are correspondingly lower (28.4 versus 33.6 % of DM, respectively). Therefore a further separation of the leaves from the structural plant elements would probably result in an upgrading of their nutritive value.

A large variation in the content of fibrous constituents is found among species. The content of NDF between the species varies from 18.5 to 38.7 % of DM (average 29.5 %). However, a considerable variation of crude fiber (CF) content is found between varieties of cassava leaves (CL) (i.e. 9.7 to 14.6 % of DM). The biomass products investigated in the study of Phuc et al (2001) were found to be high in CP, on average 27 % of DM (from 20.6 to 33.2 % of DM). However, on a DM basis the CP contents may change with the varieties, as in the six CL varieties studied, concentrations ranged between 24 and 35 % of DM (Phuc et al 2000), and of the biomass products studied CL are known to be characterized by a high content of CP. The chemical composition values of biomass products are similar to those of earlier studies on WS (Bruemmer and Roe 1979; Jain et al 1987), LL (D'Mello and Acamovic 1989; Rosales et al 1993; Garcia et al 1996), DW (Dudley and Culley 1978; Leng et al.1995; Journey et al 1991), GF (Göhl 1981), Mb (Göhl 1999) and other studies on CL (Eggum 1970; Ravindran 1990, 1993).

Green forages generally have an acceptable amino acid (AA) profile in comparison to e.g. alfalfa and soybean meal (Figure 1). The sum of essential AA (EAA) as percentage of CP varies between 38.6 and 48.9 %. According to Phuc et al (2000) (Table 2) the average lysine (Lys) content of the biomass products was 4.3 % of CP. An inter-species comparison revealed an inferior content of Lys in the CP of GF and Tric compared with other species. For threonine (Thr) the variation between species was limited, with an average of 4.1 % of CP. The contents of Thr in DW, Tric, Indicago, Mb, and CL were quite acceptable compared with the ideal AA pattern for pigs, while the concentration of the S-containing AA, Met and Cys (average 1.5 % of CP), would be limiting (NRC 1998). The AA composition of these forages is therefore of great importance when the feeds are used as protein sources for monogastric animal species.

Figure 1. Amino acid contents (g/16 g N) of various forage biomass products and alfalfa and soybean meal

Effect of inclusion of forage biomass on feed intake

Increased levels of protein replacement by forage biomass CP in pig diets at the expense of soybean, leads to increased fibre contents of the diets. The variation of inclusion level of biomass products has to be addressed as an important factor for the acceptability of the diets by the animals. Reductions of feed intake have been noted at high levels of inclusion of forages, especially for LL, Tric, Ind and CL (Phuc et al 2001). This was partly due to the increased content of dietary fibre but also possibly to anti-nutritional factors present in the biomass products. Most soluble polyphenolics have a bitter or astringent taste (Van Soest 1994; Kumar and D'Mello 1995), which would also tend to reduce intakes. The bitter taste of LL has been attributed partly to the presence of tannins as well as saponins (D'Mello and Fraser 1981). High HCN levels in CL were also seen in the studies by Lee and Hutagalung (1972) and Mahendranathan (1971).

Effect of inclusion of forage biomass on digestibility and N-retention

The studies of Phuc and Lindberg (2000a) have clearly demonstrated that increasing levels of inclusion of biomass products in diets for growing pigs, lowered the ileal and total tract digestibility of OM and CP (Table 3), in accordance with earlier reports (Kornegay 1978; Kennelly and Aherne 1980; Sarwat et al. 1988). Figure 2 illustrates the relationship between the dietary NDF content and dOM for rats and pigs. This can be explained by differences in chemical composition between the basal diet and the biomass products, and the depressive effect of fibrous constituents on the apparent digestibility of the non-fibrous components of the diet (Fernandez and Jørgensen 1986).

The negative effects of dietary fibre are partly a result of a reduced transit time of the ingesta in the small intestine caused by fibrous components, limiting the time for nutrient digestion and absorption (Low 1982; Den Hartog et al 1985). The extent of the reduction in digestibility has been shown to vary with the level of fibre and the feeding level (Dierick et al 1989; Ravindran 1990). Furthermore, the rate of diffusion of solubilized nutrients towards the mucosal surface may be slowed down, and solubilized nutrients may also be adsorbed by fibrous constituents, due to water binding (Bergner 1982). In this context the observation that the process of nutrient digestion is greatly influenced by differences in the chemical and physical structures of fibrous components has to be considered (Chen et al 1982; Agarwall and Chauhan 1989). Furthermore, anti-nutritional factors in the biomass products may have interfered with the process of digestion (Makkar 1993). Tannins are known to occur in cassava leaves (Ravindran 1993; Oke 1994) as well as in Tric and LL (Rosales et al 1993), which may adversely affect dCP. In addition, part of the dietary CP disappears in the hindgut and it is reasonable to assume that this is the result of microbial fermentation and breakdown (Mason and Palmer 1973). A large variation in the disappearance of the nutrients in the hindgut demonstrates that the extent of this process differs among individual animals (Table 3).

Table 3.  Ileal and total tract apparent digestibility coefficients of nutrients and energy in the experimental diets† (Phuc and Lindberg 2000a)
	Control	Cassava leaves		Groundnut foliage	Leucaena	SEM	P-value
		Sun-dried	Ensiled		leaves
Ileal
Organic matter	0.84 a	0.76 b	0.77 b	0.79 c	0.77 b	0.01	***
Crude protein	0.73 a	0.63 b	0.64 b	0.67 c	0.64 b	0.01	***
Crude fat	0.57	0.47	0.52	0.52	0.56	0.03
NFE	0.90 a	0.85 b	0.86 b	0.86 b	0.85 b	0.01	***
Crude fibre	0.14 a	0.23 b	0.25 b	0.31 c	0.19 a	0.02	***
NDF	0.29 a	0. 28 a	0.27 a	0.39 b	0.28 a	0.02	*
ADF	0.32 a	0.26 b	0.27 b	0.35 a	0.27 b	0.03	**
Energy	0.84 a	0.76 b	0.76 b	0.79 c	0.75 b	0.01	***
Total tract
Organic matter	0.91 a	0.85 b	0.87 c	0.87 c	0.85 b	0.01	***
Crude protein	0.83 a	0.73 b	0.73 b	0.75 c	0.70 b	0.02	***
Crude fat	0.64 a	0.52 b	0.55 b	0.57 b	0.58 b	0.02	**
NFE	0.97 a	0.94 b	0.94 b	0.95 b	0.94 b	0.01	***
Crude fibre	0.55 a	0.52 b	0.51 b	0.52 b	0.54 a	0.02	***
NDF	0.67 a	0.52 b	0.52 b	0.63 a	0.49 b	0.02	***
ADF	0.61 a	0.43 b	0.46 b	0.56 c	0.47 b	0.02	***
Energy	0.90 a	0.83 b	0.84 b	0.85 c	0.83 b	0.01	**
a,b,c,d Means with different superscripts within rows are different (P<0.05) † Ileal flow of sugars and starch was found to be zero

Comparison of digestibility of tropical and temperate biomass products

The results of the studies of Phuc et al (2001) (Table 4) on tropical herbages show that the digestibility coefficients were lower than those of temperate herbage reported by Lindberg and Andersson (1998). For dOM a regression coefficient of -1.1 per percentage unit increase in NDF versus -0.8 was found by the latter authors. The reason for this is the differences in composition between the materials studied, in which the character of temperate and tropical biomass products play an important role. Tropical herbages mature more rapidly, and their protein content falls to very low levels, and fibre content increases with maturity. In the wet tropics the herbage available is usually fibrous but lush (i.e. high in water content), whereas in drier areas the mature herbage becomes desiccated. In both cases digestibility may be reduced. Another factor of nutritional importance is that tropical herbages have higher tensile strength than temperate plants due to their having more vascular bundles, thick-walled bundle sheaths and hence more lignin. Also the mesophyl cells are more densely packed than those in temperate forages. The consequence of this is lower digestibility of tropical compared with temperate forages. However, the difference is much less for tropical legumes compared to temperate legumes, because they are more similar anatomically (Van Soest 1994).

Table 4. Regression equations of the crude protein digestibility (dCP) and nitrogen utilization (Nu ; N retention as a percentage of N intake) and overall on protein replacement level (x, i.e. 0, 25 or 50 %) and organic matter digestibility coefficients (dOM) of the diets and overall on dietary NDF content ( % of DM) (Phuc et al  2001)
	WS	LL	DW	GF	Tric	Ind	Mb	CL	Overall
dCP
Intercept	92.5	92.5	93.2	92.3	93.0	93.0	92.5	93.0	92.1
Slope	-0.42	-0.42	-0.30	-0.36	-0.60	-0.28	-0.34	-0.32	-0.36
R2	0.98	0.98	0.99	0.98	0.96	0.98	0.99	0.98	0.80
Nu
Intercept	70.5	71.0	71.3	70.5	72.7	71.2	71.0	71.5	71.8
Slope	-0.42	-0.52	-0.40	-0.50	-0.68	-0.62	-0.40	-0.30	-0.49
R2	0.98	0.98	0.99	0.99	0.97	0.98	0.99	0.98	0.77
dOM
Intercept	95.6	95.3	97.6	96.6	97	97.0	96.4	96.9	97.7
Slope	-0.77	-0.80	-0.90	-0.94	-1.37	0.96	-0.91	-0.88	-1.10
R2	0.74	0.94	0.87	0.83	0.43	0.98	0.57	0.86	0.80
† For abbreviations see footnote Table 2 a,b,c,d Means with different superscripts within rows are different (P<0.05)

Digestibility of different fibre constituents

Studies with pigs have shown that part of the dietary fibre had been digested at the terminal ileum and that this slightly increased with the incorporation of biomass products in the diets (Table 3). This suggests that pigs are able to digest a substantial part of plant fibre pre-caecally, as has been documented in reports on lucerne, red clover, white clover and perennial ryegrass (Lindberg et al.1995; Andersson and Lindberg 1997a; 1997b), coastal Bermuda grass (Dierick 1989), malt culms, dark grains and wheatings (Zoiopoulos et al 1983) and wheat bran and sugar beet pulp (Graham et al 1986). Due to a solubilization of fibre in the upper gastrointestinal tract (Graham et al. 1986), the ileal digestibility of NDF is generally higher than that of crude fibre. The digestibility of ADF, containing more resistant components, could also be expected to be much lower at both the ileal and faecal level (Table 3).

Digestibility of AA at the terminal ileum

Most AA are digested to a greater extent than the CP (Table 5). The reduction in EAA apparent ileal digestibility when biomass products are included in the diet is due to an increase in the ileal flow of AA with increasing fibre content in the diets (Phuc and Lindberg 2000b; Reverter and Lindberg 1998; Reverter et al 1999). Furthermore, an increase of endogenous AA secretions could also be expected (Boisen and Moughan 1996; Jondreville et al 2000). The magnitude of this effect will depend on the digestibility of the basal diet, the level and type of the fibre and the contribution of the fibre source to the dietary amino acid supply (Sauer et al 1980; Lenis et al 1996). In addition to fibre level and quality, the presence of tannins (Eggum and Christensen 1975; Göhl and Thomke 1976) and enzyme inhibitors (Kidder and Manner 1978) might also influence the digestibility of protein and AA.

However, most AA are digested to a greater extent than CP, which may be due to the effect of NPN content and other factors in the products. The apparent ileal digestibilities of dietary Arg, His and Lys have been found to be highest, while that of Thr was the lowest (Phuc et al 2000b). There was a significant reduction (P<0.05) found in the apparent ileal digestibility of AA (dAA) in diets with inclusion of CL and LL compared to a control diet (Phuc et al 2000b) (Table 5).

Table 5.  Apparent ileal digestibility of crude protein and of essential and non-essential amino acids in experimental diets † (Phuc and Lindberg 2000b)
	Diet
	Control	CLM	CLE	GF	LLM	SEM
Crude protein	0.73a	0.63b	0.64b	0.72a	0.65b	0.01
Essential amino acids
Arginine	0.90a	0.80b	0.82 b	0.87 a	0.78 b	.007
Histidine	0.81	0.77	0.79	0.80	0.78	.017
Isoleucine	0.79 a	0.70 b	0.69 b	0.77 a	0.71 b	.007
Leucine	0.84 a	0.74 b	0.76 b	0.82 a	0.75 b	.005
Lysine	0.85 a	0.79 b	0.79 b	0.82 a	0.77 b	.007
Methionine	0.77 a	0.70 b	0.71 b	0.75 a	0.73 b	.009
Phenylalanine	0.87 a	0.76 b	0.78 b	0.84 a	0.78 b	.009
Threonine	0.73 a	0.66 b	0.68 b	0.72 a	0.66 b	.011
Tyrosine	0.80 a	0.75 b	0.74 b	0.77 a	0.73 b	.008
Valine	0.75 a	0.67 b	0.66 b	0.74 a	0.67 b	.008
Non-essential amino acids
Alanine	0.77 a	0.70 b	0.72 b	0.76 a	0.69 b	.005
Aspartic acid	0.83 a	0.73 b	0.75 b	0.81 a	0.75 b	.006
Glutamic acid	0.86 a	0.79 b	0.80 b	0.83 a	0.80 b	.006
Glycine	0.78 a	0.70 b	0.71 b	0.76 a	0.69 b	.012
Proline	0.82	0.77	0.76	0.80	0.77	.023
Serine	0.81 a	0.73 b	0.75 b	0.80 a	0.72 b	.008
† CLM = cassava leaf meal; CLE = ensiled cassava leaves; GF = groundnut foliage; LLM = leucaena leaves ab,c,d Means with different superscripts within rows are different (P<0.05)

Effect of preservation on chemical composition

In some studies sun-drying has been found to have only a small effect on the CP content compared to 60 ^oC oven- drying of cassava leaf meal (CLM) (Phuc et al 2001b). However, after drying at 105 ^oC,  Maillard reactions occur and Lys content has been found to decrease, most likely due to the formation of lignin-like polymers, which also resulted in a higher fibre content (Van Soest and Mason 1991). Ensiling has been found to have only a limited effect on the content of EAA as compared with drying (Phuc et al 2001) (Table 6). The high content of lactic acid and low content of ammonia as a percentage of total N confirmed the ideal ensiling conditions (Van Soest 1994) resulting from cane molasses addition (Table 7).

Table 6.  Chemical composition (% of DM), and essential amino acid (EAA) and non-essential amino acid (NEAA) content (g per 16 g N)  in the biomass products investigated  (Phuc et al 2001b)
	Cassava leaves			Cassava leaves		Sweet potato vines
	Batch A			Batch B
	Sun dried	Dried   60 oC	Dried 105 oC	Sun dried	Ensiled	Sun dried	Ensiled
Chemical composition
Organic matter	92.8	92.4	92.8	91.4	92.8	89.9	86.6
Crude protein	32.4	32.7	32.2	33.3	31.7	20.6	20.1
Ether extract	6.4	7.5	8.2	7.2	8.1	2.5	3.3
NDF	27.5	25.3	37.6	24.4	24.8	28.4	29.2
Essential amino acids
Arginine	6.3	6.4	5.8	6.5	5.6	6.0	5.9
Histidine	2.2	2.0	2.0	1.8	1.7	2.0	2.2
Isoleucine	4.1	4.4	4.5	4.2	4.2	4.2	4.2
Leucine	8.7	8.9	9.1	8.3	8.3	8.2	8.0
Lysine	5.1	5.1	4.2	5.5	5.4	4.8	4.9
Methionine	1.6	1.4	1.5	1.6	1.4	1.4	1.2
Phenylalanine	6.3	6.2	6.2	6.2	5.6	5.7	5.6
Threonine	4.4	4.2	4.4	4.1	3.9	4.4	4.2
Tyrosine	4.3	4.6	4.6	4.4	4.4	4.1	3.8
Valine	5.9	5.6	5.7	5.6	5.3	5.4	5.5
S EAA	48.9	48.8	48.0	48.2	45.8	46.2	45.5
S AA	87.6	87.3	87.3	85.5	83.3	84.0	82.9

Table 7.  pH and content of organic acids (% of DM) and ammonia (g per 100g N) in ensiled cassava  leaves (CLE) and ensiled sweet potato vines (SPV)
	CLE	SPV
pH	3.8	4.0
Succinic acid	0.4	0.6
Lactic acid	8.5	9.6
Acetic acid	1.0	1.0
Propionic acid	0.2	-
2,3 – butane-diol	0.1	0.3
Butyric acid	0.0	0.0
Ethanol	1.7	5.3
Ammonia	3.5	4.2

Anti-nutritional constituents

Anti-nutritional factors in livestock feedstuffs are widespread. Consumption of foods containing these constituents may lower feed intake, nutrient utilization, food conversion efficiency and hence animal performance as well as economy. At high levels of dietary intake toxicity ensues and sometimes even animals will die. An acute shortage of conventional foodstuffs for the feeding of livestock in developing countries has forced planners and nutritionists to look for unconventional feed resources, wherein there is no competition with humans. These unconventional feed resources may contain anti-nutritional constituents limiting their biological value. Nevertheless, an unconventional food today could be a conventional food of the future (Makkar 1993). Some of the biomass products under investigation have been reported to contain anti-nutritional constituents such as tannins, cyanogenic glucosides and mimosine.

Tannins

Most plant leaves contain tannins, which are a diverse group of polyphenolic substances. Tannins can be defined as any phenolic compound of moderately high molecular weight containing sufficient phenolic hydroxyls and other suitable groups to effectively form strong complexes with proteins and other macromolecules (Van Soest et al 1987). According to these authors conventional classification recognises two types of tannins, (i) the condensed tannins, which are polymeric forms of flavonols, and (ii) the hydrolysable tannins which are esters of sugar and polyhydroxyphenolic acids. These compounds have adverse effects on growth and have the capability to lower the protein digestibility and amino acid availability, either by forming indigestible complexes with dietary proteins or by inactivation of proteolytic enzymes (Kumar and Singh 1984). Most soluble polyphenolics have a bitter or astringent taste (Van Soest 1994). Reduced feed intake may occur due to the slowdown in the digestion of the feed or due to low palatibility (Kumar and D'Mello 1995). Tannin contents are reported to increase with maturity and vary between cultivars (Ravindran 1993), and in cassava leaves vary from 30 to 50 g/kg DM (Ravindran 1993). LL contain tannins, with levels ranging from 12 to 44 g/kg DM (D'Mello and Acamovic 1989, Rosales et al. 1993). Tric also has been reported to contain around 50 g/kg DM tannins (Rosales et al 1993).

Cyanogenic glucosides

The cyanogenic glucosides are toxic to animals when hydrocyanide acid (HCN) is generated (Van Soest 1994). The release of free HCN is brought about by the action of either the endogenous enzyme linamarase in damaged plant tissues or by β-glucosidases within the digestive tract of animals. Animals can detoxify cyanide via several pathways, but primarily by reaction with thiosulphate to form thiocyanate. This conversion represents a 200 fold reduction in toxicity. Thiocyanate, however, is a potent goitrogen and has been implicated in the aetiology of goitre in animals (Langer 1966; Shihombing et al 1971) and humans (Ekpechi 1973). In animals, while acute cases of cyanide toxicity usually result in sudden death, less severe cases may lead to gastrointestinal disorders and growth depression (Hill 1973).

Since the sulphur needed for the detoxification of cyanide is obtained from dietary methionine, the presence of cyanogenic glucosides could lead to a deficiency of this essential amino acid at poor or marginal supply of methionine, resulting in reduced animal performance (Oke 1978). Bitterness associated with high cyanogenic glucoside content in cassava has been reported in a number of studies (Lee and Hutagalung 1972; Mahendranathan 1971; Sundaresan et al. 1987). Cyanide is the main anti-nutritional factor in CL that reduces the nutritional quality of the leaf meal. Generally, the cyanide content ranges from 200 to 800 mg HCN/kg fresh leaf but values as low as 80 mg/kg (Wood 1965) and as high as over 4,000 mg/kg (Ravindran and Ravindran 1988) have been reported. Per kg DM the practical range of HCN content would correspond to between 800 to 3,200 mg. The glucoside concentration in cassava leaves decreases with the age (Lutaladio 1984; Ravindran and Ravindran 1988) and nutritional status of the plant, and is increased by e.g. N-fertilization (De Bruilin 1973). The elimination of cyanogens by heating will depend on the temperature, the stage of development of plant, and the type of heat. Simple sun-drying or oven drying has been reported to eliminate almost 90 % (Phuc et al 2000), and sun-drying reduces the cyanogen content of CL more effectively than ensiling (Table 8) (Phuc et al 2000, 2001b) because of the stability of the linamarase at low pH values (Oke 1994). Despite its high content of HCN, documented cases of poisoning due to the ingestion of CL are rare (Ravindran 1993).

Table 8. Contents of total HCN, intermediary products (cyanohydrine), free HCN and glucoside (linamarin) (mg/kg DM) of cassava leaves
	Cassava leaves  A			Cassava leaves B
	Sun dried	Dried 60 oC	Dried 105 oC	Sun dried	Ensiled dried
Total HCN	59	86	28	255	250
Intermediary  (cyanohydrine)	13	47	1	152	215
Free HCN	13	33	9	62	5
Glucoside  (Linamarin)	33	6	18	42	30
HCN content of ensiled cassava leaves (not dried): total HCN: 762; Cyanohydrine: 673;  Free HCN: 17 and Linamarin: 73. Hydrogen cyanide (HCN)

Mimosine

Mimosine is an alkaloid [beta-N-(3-hydroxy-4-pyridone)] found in Leucaena leucocephala (LL), a legume widely distributed in tropical areas as a fodder tree. Mimosine has been shown to be responsible for some animal disorders. Crounse et al (1962) suggested that mimosine may interfere with tyrosine metabolism by preventing iodination of tyrosine, the first step in the synthesis of thyroxine, resulting in goitre and loss of appetite. Mimosine poisoning also causes loss of hair (Jones et al 1976) and poor reproductive performance. The mimosine content of fresh LL can be decreased by heating to temperatures > 70 ^oC, or by addition of iron salts (i.e.ferrous sulphate) (National Academy of Sciences 1977, Meulen et al 1979; D'Mello and Acamovic 1989; Kumar and D'Mello 1995; Laswai et al 1997). The mimosine content can also be reduced by soaking in water and drying. Concentrations of mimosine in the leaf range from 10 to 25 g/kg DM, with even higher quantities of up to 145 g/kg DM in the seeds (D'Mello 1991). For a time Australian agronomists selected low mimosine strains in leucaena (Van Soest 1994). However, Brewbaker and Hutton (1979), Chen et al. (1982) and Phuc and Lindberg (2000) observed no harmful effects when including 10 % and 16 % LL in diets for growing pigs, although Wayman et al (1970) showed that diets containing 15 % LL reduced the ability of gilts to conceive and reduced average litter size and weight.

Saponins

Saponins are glycosides containing a polycyclic aglycone moiety of either C27 steroid or C30 triterpenoid attached to the carbohydrate. They are widely distributed in the plant kingdom and have a characteristic bitter taste (Kumar and D'Mello 1995). According to Basu and Rastogi (1967) and Oakenfull (1981) LL also contain saponins which have adverse effects on monogastric animal growth and may also affect cholesterol metabolism (Oakenfull 1981; Cheeke 1976). The bitter taste of LL has been attributed partly to the presence of tannins (D'Mello and Fraser 1981) and partly to saponins.

Other anti-nutritional factors

D'Mello and Acavomic (1989) claim that other anti-nutritional factors, such as protease inhibitors and galactomannan gums, are present in biomass products and may also reduce performance. There is evidence of relative low in vitro digestion of NDF and organic matter in Indicago hirsusta (Ind) (Brown and Pitman 1991) and early references (Bailey 1906; cited by Göhl 1981) refer to suspected poisoning of stock by Ind, but these suspicions apparently have not been confirmed.

Estimations of the nutritive value of forages in pigs

The chemical composition of forage biomass products tested with pigs and their nutritive value estimated by the difference method and by the regression method are shown in Tables 9 and 10 (Phuc et al 2001a; Phuc and Lindberg 2000b).

Table 9. Estimated digestible energy (DE, MJ/kg DM), digestibility (%) of organic matter (dOM) and crude protein (dCP) of  biomass products investigated † (Phuc et al., 2001b)
	WS	LL	DW	GF	Tric	Ind	Mb	CL
DE ‡	10.8	9.6	8.2	7.8	5.1	12.7	8.6	11.5
dOM	57	43	50	50	33	65	54	59
dCP	51	46	64	57	34	64	59	61
†For abbreviations see footnote Table 1 ‡ DE (MJ/kg DM) calculated from digestibility of dry matter and gross energy

 

Table 10. Gross energy (GE, MJ per kg DM), and organic matter (OM), crude protein (CP), NDF content (% of DM) and estimated digestible energy (DE, MJ/kg DM), digestibility (%) of organic matter (dOM) and crude protein (dCP) of  biomass products investigated in pigs  (Phuc et al 2000a, 2001b)
		LL	GF	CL			ECL
Chemical composition
GE		21.5	18.1	19.2	21.4	-	21.2	-
OM		91.4	90.6	91.4	91.3	89.1	89.5	89.7
CP		28.3	17.5	33.3	26.4	26.0	24.5	27.6
NDF		37.5	41.9	24.4	32.1	33.5	32.6	33.5
Digestibility
DE		10.9	10.8	11.3	11.0	-	12.0	-
dOM		53	64	59	54	51	59	52
dCP		42	47	59	45	44	46	59

The coefficients of digestibility of amino acids of some biomass products are shown in Table 11 (Phuc and Lindberg 2000b).

Table 11.  Apparent ileal digestibility of crude protein, essential and non-essential amino acids in sun-dried cassava leaves, ensiled cassava leaves, groundnut foliage and leucaena leaves
	Cassava leaves				Groundnut		Leucaena
	sun-dried		ensiled		foliage		leaves
	Mean	SD	Mean	SD	Mean	SD	Mean	SD
Crude protein	0.37	0.02	0.37	0.03	0.43	0.07	0.39	0.05
Essential amino acids
Arginine	0.50a	0.07	0.56 a	0.05	0.77 b	0.04	0.48 a	0.03
Histidine	0.61	0.05	0.68	0.03	0.73	0.04	0.67	0.05
Isoleucine	0.48 a	0.02	0.45 a	0.04	0.71 b	0.05	0.52 a	0.05
Leucine	0.50 a	0.03	0.57 a	0.02	0.72 b	0.09	0.52 a	0.03
Lysine	0.64 a	0.04	0.64 a	0.04	0.73 b	0.06	0.61 a	0.07
Methionine	0.56 a	0.03	0.55 a	0.04	0.73 b	0.05	0.57 a	0.12
Phenylalanine	0.55 a	0.05	0.52 a	0.06	0.68 b	0.07	0.55 a	0.05
Threonine	0.52 a	0.07	0.54 a	0.02	0.69 b	0.05	0.52 a	0.04
Tyrosine	0.64	0.06	0.61	0.03	0.65	0.06	0.60 a	0.06
Valine	0.60 a	0.05	0.62 a	0.03	0.72 b	0.04	0.61 a	0.03
Non-essential amino acids
Alanine	0.56 a	0.02	0.57 a	0.04	0.73 b	0.09	0.54 a	0.02
Aspartic acid	0.60 a	0.04	0.62 a	0.03	0.76 b	0.06	0.60 a	0.05
Glutamic acid	0.55 a	0.08	0.53 a	0.04	0.67 b	0.06	0.57 a	0.07
Glycine	0.50 a	0.06	0.54 a	0.08	0.69 b	0.07	0.49 a	0.09
Proline	0.62 a	0.04	0.57 a	0.07	0.74 b	0.05	0.63 a	0.04
Serine	0.62 a	0.03	0.61 a	0.04	0.78 b	0.04	0.64 a	0.03
a, b, c, d Means with different superscripts within rows are significantly different (P<0.05)

Effect of including forages in diets for growing pigs

It is necessary to select forages suitable for pigs, and to determine the extent to which they can contribute to the supply of nutrients for different categories of animal. This presupposes knowledge of the chemical characteristics of the biomass products, and of their nutritional limitations, especially anti-nutritional factors. If these are known then combinations of locally available products may be suggested as suitable alternatives to provide an adequate supply of both energy and protein to meet the animals' nutritional requirements.

The differences in response to forages depend on the material that forages replace in the diet. For example, when CLM replaced rice bran in diets for growing pigs it gave higher daily weight gain, probably because of the slightly inferior composition of the rice bran compared with the CLM. However, an inverse effect was found when the CLM replaced a basal diet, which was of higher nutritive value than the CLM (Phuc et al 2003). This was also found in other studies on growing pigs (Fernandez and Jørgensen 1986).

A number of studies have shown that the inclusion level of CLM can be up to 12-15 %, and of ensiled cassava leaves up to 9-13% in the diet of fattening pigs without strong negative effects on growth rate.(Du Thanh Hang et al 1999; Ly 2002; Nguyen Thi Loc1997, 1999; Phuc et al 2003).

To use CLM for pigs efficiently, it is necessary to identify the diet. Up to 15 % of CLM can be used in the diet without any problem. However, the optimum level is usually from 4 to 10 %, depending on factors such as the basal diet, prices and the availability of the leaves (Table 12).

Table 12. Effects of level of inclusion of CLM (% of DM) on the performance of growing-finishing pigs  (Phuc et al 2003)
Level of inclusion	0 %	4%	8%	12%
Growth performance
Initial weight	23.1	23.1	23.1	23.1
Final weight	84.1	87.2	84.7	86.7
Daily weight gain (g)	545	572	548	570
Relative to control (%)	100	105	100	104
FCR	3.24	3.24	3.27	3.33
Relative to control (%)	100	100	101	103
Carcass and meat quality
No of slaughtered pigs	2	2	2	2
Live weight (kg)	92.0	87.5	93.5	89.0
Carcass weight	71.3	72.0	73.5	73.5
Percentage (%)	77.5	82.3	78.6	82.6
Back fat (mm)	17.6	15.5	8.0	7.5
Chemical composition of meat (%)
Protein	21.9	23.3	22.6	22.6
Lipid	5.65	3.35	3.7	3.09

Effect of including forages in diets for pregnant sows

Pregnant sows can utilize high fiber diets, and so forage can be a useful feed ingredient for sows during pregnancy. It is not only high in protein but also in vitamins and minerals, and can give good reproductive performance. The limited energy requirement of pregnant sows means that sows can tolerate high fibre diets (up to 10% of fibre). An experiment where 10%, 20% and 30% of CLM replaced rice bran (Phuc 20003) showed that the number of piglets born was higher in the groups fed CLM than in the control group (Table13), and due to greater litter size, the total weaning weights per litter were higher in the CLM fed groups.

The better performance of the CLM fed groups was possibly due to the higher content of vitamins and minerals and the better amino acid profile of CLM than of rice bran. Especially beta-carotene can increase the production of uterine specific proteins which support embryo survival. It also contains a basic glycoprotein with iron-binding capacity and a group of acidic proteins with immuno-suppressive capabilities. Beta-carotene also increases the production of progesterone during the initial formation of the corpora lutea.

Table 13. Effects of level of inclusion of CLM in the diet on the performances of pregnant sows (Phuc et al  2003)
	0%	10%	20%	30%
Number of sows	10	9	10	10
No.of piglets born/litter	9.9	12.7	11.2	11.7
Difference to control (pigs)		+2.8	+1.3	+1.8
No. of weaned pigs/litter	8.5	10.1	9.2	9.3
Difference to control  (pigs)		+ 1.6	+ 0.68	+0.83
Birth weight (kg/litter)	15.5	17.6	16.9	16.6
Birth weight  (kg/piglet)	1.6	1.4	1.7	1.4
Weaning weight/litter (kg)	54.0	61.6	56.8	61.4
Difference to control   (kg/litter)		+ 7.6	+ 2.8	+7.4
Weaning weight (kg/piglet)	6.3	6.1	6.2	6.6
Diarrhea rate (%)	12.4	9.3	11.2	11.5
Feed intake/day/sow (kg)	2.3	2.3	2.3	2.3
Feed intake/day/sow in lactation (kg)	4.9	5.0	5.0	5.1

There were no indications of cyanide toxicity on any of the diets in the studies. It can be concluded that sun drying is a good processing method for eliminating the deleterious effects of HCN and to make CLM safe for animals. This result is in accordance with results reported by Phuc et al (2000; 2000a; 2001a, 2001b).

Conclusions

• Marked differences in nutritive properties have been found in tropical biomass products that are potentially useful as feed resources for pigs, especially in their content of protein. However, methionine deficiency in many tropical forages may impose nutritional limitations.

• Fiber content is known to be an important factor determining organic matter digestibility in different forages. The importance of harvesting at an early stage of development is more critical for pigs.

• Sun-drying and drying at 60 ^oC, as well as ensiling, have been found to be suitable techniques for preserving green forages under tropical conditions.

• Forages can be a source of benefit to pig farmers, especially if replacing a high fiber feed such as rice bran or coconut meal. Farmers have to make a decision as to what should be replaced. The determination of optimum levels of inclusion in pig diets, and also the effects of constraints such as anti-nutritional factors, need to be further investigated.

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