Evaluation of Tropical Kudzu legume as a potential protein source for cattle in Vietnam

Nguyen Van Hiep

Faculty of Animal Science and Veterinary Medicine,

Nong Lam University, Ho Chi Minh City, Vietnam

nguyenvanhiep5@yahoo.com

Mixed crop-livestock farming systems in the tropics

In most developing countries, crop-livestock production systems constitute the backbone of agriculture. The systems involve crops, livestock, land, water etc., in which these sub systems and their synergistic interactions have a significant positive and greater total effect than the sum of their individual effects (Edwards et al., 1988). Mixed systems have been practiced by small scale farmers throughout the world and account for highest proportion of the meat production of the world (De Haan et al., 1997). Diversification, saving and recycling resources are their basic characteristics, which provide stability and a positive environmental impact of the systems. However mixed farming systems bring more complications to farm management, difficulties in isolating existing problems (Prawikrokosoumo, 1989) and generally lower yields compared to specialized and intensified production systems (Fick and Clack, 1998).

In the context of most tropical developing countries the production systems are changing from extensive to intensive, especially systems combining arable cropping. This situation increases with the decreasing availability of arable land and population growth and the mixed crop-livestock farming systems become more common and effective. In this sector, the principal aim should be improved feeding and nutrition, in which the objective is maximum use of the available feed resources, notably crop residues and low quality roughages, and also various leguminous forages as supplements (Devendra, 2000). Follow that strategy in the present study, better use of Kudzu legume inter-planted with rubber in biomass production improvement by cutting management and forage preservation was the aims of the study.

Forage biomass production and nutrition value

Forage is defined as plants and plant parts that are consumed by livestock. Forages are comprised of carbohydrate, protein, fats, minerals, vitamins and some unique compounds as important is lignin (Table 1: Van Soest et al., 1991). Concentration of cell wall generally is regarded as the most important factor affecting forage utilization because it comprises the major fraction of forage dry matter and is correlated with forage intake and digestibility. Cell wall, composed mostly of structural carbohydrates and lignin, account for 40 to 80% of the organic matter in forage crop. Stems of most forage have a higher concentration of cell walls than leaves (Buxton, 1990).

Production of large amounts of good quality forage per unit of area is an important pasture parameter because of its effect on carrying capacity and hence production per animal and per unit area (Whiteman, 1980). Optimum field management is aimed at getting the highest biomass production, which satisfies the animal’s needs, as usually determined by animal performance. The herbage cutting management strategies are mainly aimed to the dual purpose: the biomass production and the quality of the forage.

Factors causing the differential responses of individual forages are the height of the growth points of stems or tillers above the ground, and the proportion of vegetative to reproductive tillers (Osbourn, 1976). Leaf area index, the ratio of leaf area of the community to the area of land beneath that community, is a measure of the photosynthetic capacity of a plant community and used to be an indicator in cutting management. Vickery (1981) stated that there is an initial phase when the leaf area index of pasture increases together with growth rate, until 95% of all incidents light is intercepted. The yield of grasses producing mainly reproductive stems or tillers is highest when leaf area index comes to maximum (100%), with a marked reduction if defoliated early.

Quality of leaves and stems of most grasses and legumes are nearly equal when young, but quality of leaves decrease at a much slower rate. This is related to the mesophyll cell in leaves, which form a major part of leaf tissues, have a high CP content, and do not form secondary, highly lignified cell walls (Nelson and Moser, 1994). Mwangi et al. (2004) reported that longer cutting interval increased total forage yield but reduced quality when intercropping legumes and Napier grass in the central highlands of Kenya. Nadir et al. (2006) studied Moringa oleifera and reported the highest DM yield at 75-day cutting frequency, but the contents of CP and ADF were not affected significantly by cutting frequency. The effect of three cutting intervals: 4, 8 and 12 weeks on the yield of Desmodium intortum was reported by Jones (1973). The mean legume yields at 4, 8 and 12-week cutting interval were 4.23, 5.54 and 8.03 tons/ha/year, respectively. The effect of pruning frequency: 4, 8, 12 and 24 weeks on the DM production and nitrogen of Gliricidia sepium was shown by Isidro et al. (2005). The DM production increased from 0.50 up to 10.52 tons/ha in 4 and 24-week pruning frequency, respectively. The proportion of stem / biomass increased from 20 up to 53% in the 4 and 24-week pruning frequency, respectively. Nitrogen content in DM diminished from 3.19 to 2.64% upon reducing the pruning frequency from 4 to 24-week. In the present study similar reduction of leaf ratio to the increased cutting interval was found (Paper I). The N content in DM decreased with the longer cutting interval but with slow rate.

Legumes in the farming systems

According to Humphreys (1995) legumes have a significant role in many farming systems of the tropics and subtropics through their contribution to:

·  Enhanced nutritive value of the animal diet.

·  Biological nitrogen (N) fixation. The legume-rhizobial symbiosis converts atmospheric N to forms of N which can be taken up by plants and cycled within the plant – animal – soil system in ways which increase its productivity.

·  Landscape stability. Soil erosion and runoff are reduced by leguminous covers in plantations, by contour-planted trees, shrubs and hedgerows on sloping land and by the use of sod-forming legumes such as Arachis pintoi in perennial pastures.

Tropical legumes have the potential to produce large quantities of high protein for animal consumption. This is particularly important in areas where the majority of ruminants are currently fed forages and crop residues of low nutritive value (Preston and Murgueitio, 1992). In Vietnam, some previous reports on biomass yield and nutritive value of legumes have shown this potential. Biomass yield of leguminous shrubs and trees were reported by Man et al. (1995) after sixteen months planting with three harvests and Manh et al. (2003) after four months planting with four harvests (Table 2).

Legumes in ruminant feeding

Legumes can make a major contribution to improvement in the diet quality and productivity of large ruminants in the tropic and subtropics, because of desirable nutritional attributes of tropical legumes (Coates, 1995). Most legumes forage contains highly soluble protein which is easily fermented in the rumen (Leng et al., 1992). Supplementation with legumes results in an increased digestion and feed intake simulated largely by the provision of additional rumen degradable nitrogen (Norton and Poppi, 1995). According to Devendra (1992), Leucaena foliage supplementation increased total dry matter intake from 42.3 kg W^0.75 in the control group to 77.9 kg W^0.75 in Leucaena hay fed ad libitum. Mui et al. (2005) noted that foliages of Leucaena have high potential for replacing the concentrate in the diet or as supplement for ruminants in small scale farms. From a study on Stylosanthes guianensis, Caliandra calothyrus, Leucaena leucocephala, Gliricidia sepium, Huy et al. (2000) reported that DM and CP digestibility varied from 67 to 76% and from 75 to 80 %, respectively. Highest feed intake was obtained when 30% of Gliricidia leaves supplement were included in a goat diet (Hao and Ledin, 2000).

Effects of intake and digestibility of forage

Intake

Intake is the absolute amount of dry matter consumed per unit of time (Mertens, 1994). The feed intake depends on a number of factors, including live weight, milk production level, stage of lactation, animal behavior and environment, previous feeding history, body condition, type and quality of feed ingredients and management.

The fiber content of the forage plays often an important role. It can be measured in many ways and the relations between intake and fiber content have recently been reviewed (Mertens, 1994). The forage cell wall (NDF) contains the entire structural component of forage. It was reported that a critical level of NDF was 75% of DM in sole grass diets and that a higher concentration reduces intake and animal productivity (Buxton, 1996). Consumption of dietary NDF is limited by capacity of animal, it is estimated as 1.2% of the animal’s body weight and the equation describes this relationship: DM intake (% of BW) = 1.2/NDF (% of DM) (Merterns, 1987). NDF contains mainly hemicellulose, cellulose and lignin and the ratio of these components also affect the DM intake.

The protein concentration of the diet also influences on the intake of tropical forage, Intake of grass species declines rapidly when the CP concentration of the consumed forage falls below 7% (Milford and Minson, 1966). The effect on intake of adding a concentrate supplement to roughage depends on the digestibility of that roughage. If its digestibility is low total intake will be increased and if its digestibility is high then the concentrate will replace roughage, resulting in total intake lower (McDonald et al., 2002). Supplementing cassava tops silage to the grass diet tended to increase the feed intake (Aminah et al., 1999; Man and Wiktorsson, 2001) and supplementation of forage protein to low quality roughage diets improved feed intake (Merkel et al., 1999). The results in Paper II show that the sole grass intake was low and total DM intake increased when grass diet was supplemented by Kudzu foliage.

Low pH silage is often associated with low DM intake of the material (McDonald et al., 2002). That may explain the low DM intake of the diet including Kudzu silage supplement, as compared to the higher intake of the diet including Kudzu hay supplemented (Paper II).

Digestibility

Forage digestibility is a measure of the gross availability of nutrients (Norton and Poppi, 1995). Concentration of cell wall has a large influence on forage digestibility. Both grass leaf and stem increase in cell wall content with advancing maturity (Jung and Vogel, 1992). The digestibility of both leaves and stems of grasses declines with increasing forage maturity, although the rate of decrease is greater for stems than leaves (Mowat et al., 1965). In Legume forage, Albrecht et al. (1987) noted that alfalfa legumes stems increased in cell wall contents with increasing maturity, but alfalfa leaves maintained relatively constant composition across maturity levels. Thus, alfalfa legumes stem digestibility decreases considerably with increasing maturity, whereas digestibility of alfalfa legumes leaves is little affected. Low cell wall content is an indicator of high potential digestibility, immature growth has lower cell wall contents than mature growth and legume leaf is generally more digestible than stem (Minson, 1990). On the other hand, changes in leaf/stem ratio are less marked in legumes during progress to maturity, and nutritive value declines at a slower rate than grasses over a similar period (Norton and Poppi, 1995).

Lignin is an almost completely indigestible component of forage that increases in concentration as the forage mature. The fiber digestibility has been shown to be negatively correlated with the lignin and silica contents (Van Soest, 1994). The reason may be by forming complex bonds or association with cellulose and hemicellulose, lignin and silica limit the degradation of the structural carbohydrates by microbial enzymes.

Legumes usually contain higher concentrations of protein and other nutrients than grass. Low quality hays/straws, supplementation with legumes results in an increased digestion and feed intake stimulated largely by the provision of additional rumen degradable nitrogen (Norton and Poppi, 1995). The same result was found in the present study with the supplement Kudzu forage increased the feed intake and digestibility of the supplement diets (Paper II).

Tannin is phenolic polymers of relatively high molecular weigh which have the capacity to form complexes with carbohydrates and proteins. Tannin content in forage may be the factor affecting the feed intake. Reed et al., (1990) reported that increased tannin intake was associated with decreased dry matter intake. At high levels tannin may have detrimental effects on the nutritive value of forages by reducing palatability and digestibility (Kumar and D’Mello, 1995). In the present study, Kudzu tannin may not have any effect on feed intake and digestibility because its content is rather low as reported by Lowry et al. (1992). They found that the level of condensed tannins of Tropical Kudzu is less than 1%.

Forage conservation 

Hay making

The aim in haymaking is to reduce the moisture content of the green crop to a level low enough to inhibit the action of plant and microbial enzymes. Spreading the crop after cutting allows maximum exposure of the foliar surfaces to wind and solar radiation and consequent rapid evaporation of water. Green crop may be stored satisfactorily as the moisture content is reduced to 150 – 200 g/kg (McDonald et al., 2002). Mature grass may be safely stored at DM contents of 82 – 85%. By contrast, leafy immature grass harvested for hay must be dried to greater than 88% DM for safe storage without the risk of heating and molding (Merry et al., 2000). Kudzu hay used in the study was prepared by 3-day sun dry. The DM content was 92%, enough drying to store for feeding animal during the study time (Paper II)

Ensiling making

Ensiling is a natural process for preservation of herbage through an acidification process, caused by the production of organic acids, particularly lactic acid, by epiphytic bacteria fermenting released plant sugars. In the tropical humid countries like Vietnam, Laos, Cambodia, Thailand, etc., forages are usually available in the wet season, which is followed by a long dry season when much less high quality fresh feed is available. Thus, the research for finding a suitable method to preserve surplus forage in the wet season is the top priority in better use of feed resources. Silage making is much less weather-dependent than hay making and stored hay easily becomes moldy in the humid conditions. In many studies in the region ensiling were used for the conservation of protein rich foliages such as Cassava leaves (Phuc et al., 1996), Groundnut leaves (Thanh et al., 2000), Cassava top, Gliricidia top (Man and Wiktorsson, 2001), Sweet Potato (Giang et al., 2003), Taro leaves (Malavanh et al., 2006) and most of silage products were good in evaluations. In our study silage making of Kudzu legume was evaluated well with molasses additives (Paper II).

Ensiling process

Immediately after cutting the crop and during the early stages of ensiling, the plant enzymes continue activity; these are two processes of respiration and proteolysis, which reduce the nutrition value of the final product. Respiration results in the reduction of plant sugars, and their conversion to water, carbon dioxide and heat (McDonald et al., 1991). The products of proteolysis are amino acid and peptides of varying chain length (McDonald et al., 2002).

The aerobic phase in clamp silage is reduced considerably if the foliage is finely chopped and well compacted prior to rapid sealing of the silo. When anaerobic conditions are established, the fermentation process generally occurs during the first week of ensiling, enterobacteria and clostridia will be suppressed by homo-fermentative and hetero-fermentative lactic acid bacteria. The homo-fermentative lactic acid bacteria are of primary importance in the microbial succession, promoting rapid pH decline through the production of lactic acid as their main fermentation end product. The lactic acid grow actively for 2 to 4 weeks, lowering the crop pH usually to between 3.8 and 5.0, dependent upon crop moisture, buffering capacity and sugar content. The pH required for silage of excellent quality can vary from approximately 3.8 when herbage DM content is 15% to between 5.0 and 5.5 at 30 – 40% DM (Merry et al., 2000). In our study, the pH value of Kudzu silage with molasses additive was less than 4.5 (Paper II).

Clostridia grow best at pH 7.0 – 7.4. They cannot tolerate acid conditions and a pH of 4.2 is usually considered to be low enough to inhibit their growth. And optimum pH for the growth of enterobacteria is about 7.0, and they are usually active only in the early stages of fermentation, when the pH is favourable for their growth (McDonald et al., 2002). When the pH drops to 5, the enterobacteria decline rapidly, leaving the lactic acid bacteria as the principal microorganisms in the silage (Muck, 1991).

The importance of developing sufficient acidity in the silo has given rise to the concept of stable and unstable silage. Stable silage has a sufficiently high lactic acid concentration to prevent its deterioration by clostridia, and with a stable pH negligible amount of butyric acid accumulate (Merry et al., 2000).

Affecting silage quality during the storage period is the slow influx of oxygen through the silo walls, causing increases in yeast and mold population, losses of silage DM and heating of ensiled mass. The majority of moulds is strict aerobic and is active on the surface layers of silage. Many of them are capable of producing mycotoxins (McDonald et al., 2002). In the present study, thin mold layers presented on the top of silage product during storage time may be the case of oxygen influx (Paper II).

The nitrogenous components of well-preserved silages are mainly in soluble non-protein form in contrast to those present in fresh forage crops, where most of the total nitrogen is present as protein. Some deamination of amino acids may occur during fermentation, but this activity is likely to be low and consequently ammonia content of silage will also be low, usually less than 100 g NH₃-N/kg total nitrogen (McDonald et al., 2002). In this indicator the results in the present study (Paper II) showed a good ensiling of Kudzu legume.

Dry matter concentration

Wet crops are very difficult to ensile satisfactorily. Growth of clostridia is severely restricted if the dry matter of the ensiled material is above 300 g/kg, but complete inhibition may require considerably higher figures, perhaps as much as 400 g/kg (McDonald et al., 2002).

Thomas and Fisher (1991) have shown that certain critical levels of acidity must be reached, depending on the dry matter of the crop to prevent an undesirable fermentation. On the other hand, McDonald et al. (2002) stated grass crops having a dry matter content of about 200 g/kg and achieve a pH of about 4 will normally preserve the crop satisfactorily. In the present study, the DM content of Kudzu forage material was low (19.5%) the silage treatment without molasses additive lead to high pH value. But treatments with molasses additive resulted in satisfactory ensiling.

Water soluble carbohydrates (WSC)

The WSC is the basic substrate for most silage microbes. The main nonstructural carbohydrates occurring in temperate grasses are glucose, fructose, sucrose and fructans. Grasses of tropical and sub tropical origin accumulate starches instead of fructans in their vegetative tissue (McDonald et al., 1991). These carbohydrates are all soluble in cold water and are collectively known as the WSC (McDonald et al., 1991). The WSC are fermented by lactic acid bacteria. It is considered that glucose and fructose are immediately available to the fermenting LAB, but sucrose and fructans only become available during the first few days after ensiling through acid hydrolysis (Pettersson, 1988).

According to McDonald et al. (1991) factors influencing the WSC content of grasses are species, cultivar, stage of growth, diurnal variations, climate and fertilizer levels.

Legumes usually have lower WSC contents and higher CP content than grasses (McDonald et al., 2002), so the buffering capacity is high, thus they require more WSC additives for good fermentation (Paper II).

Molasses has a WSC content of about 700 g/kg DM and the additive has been shown to increase the dry matter and lactic acid contents, and to reduce the pH and ammonia levels in treated silages (McDonald et al., 2002). The results were found similar as increasing molasses additives on the Kudzu foliage resulted in well-preserved silage (Paper II).

Buffering capacity

Buffering capacity is expressed as milliequivalents (mE) of alkali required to change the pH of 1 kg DM from 4 to 6. The buffering capacity may vary depending on the species and cultivar and legumes are more highly buffered than are grass (McDonald et al., 1991). Thus, legumes are consequently more difficult to ensile satisfactorily. Wilting the herbage before ensiling reduces the buffering capacity (Playne and McDonald, 1966). The pH in the silo is related to the buffering capacity of herbage (Merry et al., 2000).

Conclusions

• Tropical Kudzu which is a cover crop in rubber plantation provides optimum of biomass production and nutritive value around 60 days cutting interval.

• Two traditional feed conservation methods: hay making and ensiling making with more than 3% (of fresh material) molasses additive can be applied to preserve Kudzu foliage as protein supplement source on low quality roughage diet for ruminant.

• Supplement Kudzu hay on grass diet increases total intake of diet, DE and DCP intake. Kudzu hay and Kudzu silage had high digestibility, DE and DCP, but the intake of Kudzu silage was low when fed to crossbred Red-Sindhi heifers.

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