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| Livestock Research for Rural Development 11 (3) 1999 | Citation of this paper |
Abstract
The growth curve and nutritional characteristics of the tropical legume Lablab purpureus were determined to evaluate its potential as a forage in the American tropics. The study was conducted during the dry season at the Escuela Nacional de Agricultura in the Olancho region of Honduras (November 1996 to May 1997). Using a Completely Randomised Block Design the experimental field was divided into 4 blocks, with 10 parcels corresponding to the treatments (days post germination, dpg) within each block. Samples were taken approximately every 15 days from 13 dpg to 146 dpg. At each collection date dry matter (DM) yield data was estimated and individual plants were separated into leaf, petiole, stem and reproductive parts to estimate botanical composition and nutritional characteristics (crude protein, CP; neutral detergent fibre, NDF; acid detergent fibre, ADF; acid detergent lignin, ADL and in vitro dry matter digestibility IVDMD) of each plant fraction.
Cumulative rainfall during the trial was 212 mm, but there were two accidental floods at 85 and 112 dpg which appeared to have had an effect on the plant CP and fibre profiles. The lablab plants grew well throughout the experiment, yielding 4.7 tonnes of DM/ha at 117 dpg (DM yield = 0.168 - 0.014dpg + 0.00045dpg2; R2 = 0.75, n = 56). The leaf to stem (stem + petiole) ratio varied quadratically with maturity (Y = 4.55 - 0.069dpg + 0.0003dpg2; R2 = 0.78, n = 56), with maximum (2.55) and minimum (0.72) values at 28 dpg and 101 dpg respectively.
Average CP content (on a DM basis) in the whole lablab plant and its botanical fractions varied as follows: whole plant, 14.8% (101 dpg) to 21.0% (46 dpg); leaf, 21.4% (88 dpg) to 30.3% (135 dpg); petiole, 10.1% (88 dpg) to 12.6% (45 dpg) and stem, 8.0% (135 dpg) to 11.8% (45 dpg). As the plant matured the protein content decreased until approximately 100 dpg, after which increased until the end of the experiment. The fibre fractions (ADF, NDF, and ADL) followed an inverse pattern, increasing until 100 dpg and subsequently decreasing. The relationship between the IVDMD (of whole plant and botanical fractions) and dpg, followed patterns analogous to those of CP and inverse to those of fibre fractions. After reaching minimum values at 73 dpg, the IVDMD of plant fractions increased. Consequently, IVDMD values of the leaf and petiole fractions at 45 dpg did not differ from those at 146 dpg (P>0.05). Minimum and maximum IVDMD values were: whole plant, 59.0% (73 dpg) and 67.1% (45 dpg); leaf, 64.6% (73dpg) and 72.9% (117 dpg); petiole, 65.6% (73 dpg) and 71.6% (88 dpg) and stem, 51.6% (73 dpg) and 61.1% (45 dpg).
Based on the high protein content of the leaf fraction and the digestibility values of all botanical fractions, it may be concluded that lablab is a nutritionally valuable legume resource which should be employed more often in tropical agricultural production systems. Not only it can be used as a protein supplement and to provide maintenance requirements for ruminants, but also for achieving relatively high body weight gains and/or milk production. However, our results suggest that the nutritional characteristics observed were affected by the accidental flooding. Therefore, it would not be prudent to use the data determined for yield and nutritional characteristics after 85 dpg (first flooding) as predictive for lablab grown under completely dry conditions.
Introduction
Due to its potential for use as a vegetative cover, soil improvement qualities, ability to fix nitrogen and control weeds, the legume lablab (Lablab purpureus) is an important species in the American tropics. Lablab can be used in a pasture environment or can be fed as a supplement to animals on poor quality diets during the dry season (Hendricksen and Minson 1985). A summary of available literature reports indicates that Lablab purpureus contains an average of 17% protein, 46% Neutral Detergent Fibre, 41% Acid Detergent Fibre and an average Dry Matter Digestibility of 53% (Murphy and Colucci 1999). These nutritional characteristics coupled with the other environmental benefits make lablab a suitable fodder crop for the Tropics. At present, in-depth knowledge of the nutritional characteristics of this legume at different stages of growth and maturity is lacking. It is probable that the dearth of information about this species is preventing the use of lablab to its full potential. Recent work in Honduras comparing two forage systems during the dry season, has shown an improvement in milk production (per animal and per ha) and body condition of cows grazing a mixture of maize stover/lablab as compared with the traditional maize stover system (Sinclair 1996). Based on these findings the Unidad de Investigación y Extensión, Escuela Nacional de Agricultura, Honduras in conjunction with the Department of Animal & Poultry Science, University of Guelph, Canada, evaluated the growth and nutritional characteristics of Lablab purpureus for use as a forage.
Materials and Methods
The Rongai cultivar, used in this experiment, is most common in Honduras and is known as 'lablab', 'frijol dólico', 'caballero', or 'garbanzo' (Murphy and Colucci 1999).
Treatments and experimental design
Using a Completely Randomised Block Design (Cochran and Cox 1957), the field was divided into 4 blocks (24.5m x 6.5m). Within each block, 10 parcels were marked and divided in such a way that the legume could be harvested at ten different stages of maturity (13, 28, 46, 58, 73, 88-89, 101-102, 117-118, 135-136 and 146-147 days post germination, dpg). Each parcel was 3mx2m with 0.5m between the parcels and 1 m between the blocks. The field work for this trial was conducted during the Honduran dry season; it commenced on 15 November 1996 and was terminated 16 April 1997.
Growing conditions
The study was conducted at the experimental station in the Raúl René Valley at the Escuela Nacional de Agricultura in the Olancho region of Honduras. The average annual precipitation amounts to 1300 mm, with the rainy season commencing in May and terminating in mid-November. This climate corresponds to a semi-humid tropical forest agricultural zone. The average maximum temperature is 26oC, with a relative humidity of 75%. The soil type of the experimental plot was clay loam (Personal Communication - Ing P D Sanchez and Ing J Menjivar).
Cultivation
The soil was prepared with one pass of the tractor and two passes with a harrow until the soil was soft. Furrows, in which the seeds were planted, were 0.5m apart. Lablab (Lablab purpureus) seeds (20 kg/ha) for the experiment were provided by Escuela Nacional de Agricultura. Prior to sowing, the seeds were treated with Semevin insecticide (thiodicarb - 2.5 l/100 kg of seed). Within each parcel 2-3 seeds were planted manually every 0.2 m at a depth of approximately 4 cm. Formula 18/46/0 fertiliser (Fertica, USA) was applied, according to manufacturer's recommendations, at a rate of 130 kg/ha after the seeds were sown. During plant establishment (December 1996) DECIS EC 2,5 (deltametrina) insecticide was applied according to manufacturer recommendations. Weeds were removed manually until the plants were established at which point the plants out competed weeds.
Sampling procedure
For the first nine harvesting dates, 10 plants from one parcel, in each block were randomly selected (a total of 40 plants) for determination of nutritional characteristics. Logistics (i.e. plant size) dictated that for the last collection date only 5 plants from each parcel, in each block were harvested.
Total plant yield was determined by cutting 2 m2 at 5 cm above ground level from one parcel, within each block. Unfortunately, yield from the last two harvesting dates (135 and 146 dpg) could not be determined owing to involuntary damage of the material.
Dry matter and yield analysis
Individual fresh plant weights were recorded within 2-3 hours after harvesting. The 10 plants from each block were separated into two groups of five plants and hand-separated into different anatomical fractions (leaf, petiole, stem, flower, pod, and grain); thus giving a duplicate for the dry matter (DM) analysis. The fractions were weighed, dried in a forced draft oven at 65o C for 48 hours, and weighed again. The wet weight was determined on an individual plant basis, while the dry weight was determined by pooling the five plants (n=4 per treatment or one sample per block). Following dry weight data collection, the two groups of five plants per block were pooled and were stored in plastic bags for subsequent evaluation. Samples for total DM yield estimation were treated using the same procedures.
Analytical procedures
When a minimum of 15 g dry weight of each plant fraction per block (n=4 per treatment per plant fraction) was obtained, nutritional analysis was performed. Thus procedures were performed on the last eight of the ten treatment times . Samples analysed were ground in a Wiley mill to pass through 1 mm screen for fibre analysis and in vitro digestibility. A portion of the ground sample was ground through a 0.5 mm screen for micro-Kjeldahl analysis. After grinding, samples were allowed to moisture equilibrate at room temperature. Analytical DM and ash were measured according to the standard procedures of the AOAC (1980). Nitrogen (expressed as crude protein, CP) was determined using the standard micro-Kjeldahl procedure with CuSO4 as a catalyst (AOAC 1980; Labconco, Kansas City, USA). Acid detergent fibre (ADF), neutral detergent fibre (NDF) and 72% sulfuric acid lignin (ADL) were analysed as described by Van Soest et al (1991). This portion of the analysis was conducted in duplicate, at the Feed Evaluation Unit, Escuela Nacional de Agricultura, Honduras.
The in vitro DM digestibility (IVDMD) of different plant fractions was determined in duplicate using a modification of the two stage (48 h incubation with rumen fluid followed by 24 h incubation with pepsin-HCl) method of Tilley and Terry (1963) in the laboratory facilities at the University of Guelph. The procedure consisted of incubation of samples (0.25 g) with 5 ml of rumen fluid and 20 ml of artificial saliva (Goering and Van Soest 1970) in screw cap test tubes. The rumen fluid used was collected two hours post-feeding from a fistulated steer consuming a hay diet and maintained in accordance with the University of Guelph Animal Care and Use Committee protocol.
Statistical analysis
Using the Completely Randomised Block Design model, analysis of variance of growth and nutritional parameters was performed using the general linear models procedure of SAS (1996).
Yij = a + ßi + tj + eij
Where:
Yij = dry matter yield and nutritional parameters
a = general mean of the treatments
ß = block effects
i = I, II, III, IV (block)
t = treatment effects (days post germination)
j = 1,2,3,4,5,6,7,8,9,10 (sampling period)
e = experimental error
The primary result to report is that Lablab purpureus did grow and thrive during the dry season (December to April) at the Escuela Nacional de Agricultura, Catacamas, Honduras. During the experiment the natural conditions at the research facility were as would be expected for a semi-humid tropical forest region. The average temperature recorded from November 1996 to April 1997 was 25oC, the relative humidity was 85% and the cumulative rainfall was 212 mm.
On two occasions during the trial there was accidental flooding of the experimental plot in which the lablab plants were growing. The floods were caused by a manual error in the irrigation system supplying water to an adjacent experimental plot. The first and most severe flood occurred on 12 February 1997, 85 days post germination. The plot was inundated by water for several hours.
Once the error was recognised the irrigation system was shut off. As a result, the experimental plot was covered by approximately 10 cm of standing water, which drained within a period of approximately 24 hours. Due to a similar manual error, a second but less severe, flood occurred on 11 March 1997, 112 days post germination. Based on the yield and nutritional characteristics results, there appears to be an effect of the flooding.
Dry matter yield
The lablab plants grew well throughout the duration of the experiment and remained green despite dry season conditions (with the exception of the two accidental floods) . Figure 1 illustrates the dry matter yield of Lablab purpureus from 28 to 117 days post germination. At 117 dpg, the dry matter yield was determined to be 4.7 tonnes/ha. This result is comparable to work done by Schaaffhausen (1963) in Brazil and Milford and Minson (1968) in Australia where dry matter yield was reported to fall within a range of 3.1 and 5.9 tonnes/ha
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Proportion of plant fractions
Figure 2 is presented to visually describe the proportion of plant fractions. As would be expected the proportion of leaf decreased over time, while that of the stem fraction increased. At 28 dpg the leaf fraction accounted for approximately 72% of the total plant DM, declining until 101 dpg to a value of 34% and then slightly increasing until 146 dpg (39%). The stem fraction increased from 16%, at 28 dpg, to 44%, at 88 dpg, reaching a value of 35% at the end of the experiment. Flowering began at 73 days post germination, peaking at 88 dpg then declining until the end of the experiment. Evidence of the green pod began at the same time as flowering and peaked at 101 dpg when green pods accounted for 14% of the total plant dry matter. The grain and pod shell fractions appeared at 101 dpg and continued to increase until the end of the experiment at 146 dpg.
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Based on these results one could say that after 100 days post germination the nutritive value of lablab for cattle consumption is less than before this date due to reduced leaf to stem ratio. It is at this point when the animals would be forced to consume a greater proportion of the stem fraction, high in fibre and low in protein, thus reducing digestibility and the overall plane of nutrition. However, in order to determine optimal feeding value, one must consider not only botanical fractions but dry matter yield and nutrient content of fractions. In addition, feeding system utilised (i.e. grazing, cut and feed, cut and conserved as silage or hay, feeding allowance) and animal species (cattle, sheep, goats, pigs, rabbits, poultry) would contribute to the overall feeding value of the plant.
To add a human element to the observation, beyond 100 days post germination the pod and grain fractions, suitable for human consumption became mature. The green pod can be served in salads or with proper preparation, the lablab bean can be used in meat stews or mixed with rice, adding a much needed protein component to the human diet as well (Sinclair 1996).
With the information on total plant DM yield and the proportions of botanical fractions at the different sampling dates, the DM yield of individual plant fractions was determined mathematically. Table 1 presents the DM yield of plant fractions up to 117 dpg These results fall within the range of the literature review data reported by Murphy and Colucci (1999); 940 - 2230 kg/ha of leaf fraction and 836-4326 kg/h of stem fraction.
| Table 1. Dry matter yield (kg/ha) of whole plant and botanical fractions of Lablab purpureus | ||||||
Days post germination |
Whole plant |
Leaf |
Petiole |
Stem |
Other |
Leaf:stem ratio2 |
28 |
205 |
147 |
24 |
34 |
0 |
2.55 |
45 |
488 |
327 |
59 |
102 |
0 |
2.04 |
58 |
625 |
383 |
60 |
183 |
0 |
1.58 |
73 |
1530 |
736 |
97 |
654 |
44 |
0.98 |
88 |
2645 |
1135 |
217 |
1178 |
114 |
0.81 |
101 |
3399 |
1157 |
194 |
1404 |
644 |
0.72 |
117 |
4580 |
1678 |
310 |
1893 |
700 |
0.76 |
1 Includes flower, green pod, pod shell and grain; 2 Stem includes petiole and stem fractions |
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The leaf fraction of legumes often has a better nutritional quality in comparison to the more fibrous stems (Van Soest 1994). This coupled with the fact that cattle select for the highly nutritious leaf fraction (Hendricksen et al 1981; Wood 1983) makes the leaf to stem ratio an important parameter in determining the nutritional value of legumes. Legumes with high leaf to stem ratios would seem to be those of highest nutritional value (Norton and Poppi 1995). The leaf to stem (includes the petiole fraction) ratio values for lablab over time are given in Table 1. With maturity, lablab shows a decrease in leafiness resulting in a quadratic decrease in leaf to stem ratio (Y = 4.55 - 0.069dpg + 0.0003dpg2; R2 = 0.78, n = 56), as normally occurs in most forages.
Leaf to stem ratios are important in evaluating legumes, but the chemical composition of the plant fractions should also be considered, as it was mentioned before. Different legume species have different anatomical structures and subsequently different nutritional composition. For example, it could be speculated that the rigid stem configuration of the alfalfa plant would have a different fibre profile than the trailing, twining stem of lablab. However, the size and the total mass of each individual plant species should also be considered in such a comparison.
Nutrient composition
The chemical composition of plants and consequently their nutritive value is a result of the distribution of photosynthetic resources into the various plant tissues. This distribution into metabolic pools, reserves and structural parts is relevant in vegetative forages (Van Soest 1994). Tables 2 through 4 present summary statistics describing the nutrient composition (CP, NDF, ADF, ADL and IVDMD) of the leaf, petiole and stem fractions at different stages of maturity.
| Table 2. Chemical composition and In Vitro dry matter digestibility (IVDMD) of the leaf fraction of Lablab purpureus (dry matter basis) | ||||||||
Variable |
n |
Mean |
SD |
CV |
Min1 |
Min |
Max |
Max |
Crude protein |
32 |
24.7 |
3.92 |
15.9 |
21.4 |
88 |
30.3 |
135 |
NDF |
32 |
37.3 |
3.12 |
8.4 |
34.6 |
117 |
40.3 |
58 |
ADF |
32 |
23.4 |
2.39 |
10.2 |
20.2 |
146 |
26.4 |
58 |
ADL |
32 |
4.4 |
1.50 |
34.4 |
2.22 |
73 |
6.02 |
101 |
ADL/NDF |
32 |
11.6 |
3.76 |
32.3 |
5.95 |
73 |
15.6 |
101 |
IVDMD |
32 |
68.9 |
3.24 |
4.7 |
64.6 |
73 |
72.9 |
117 |
Min dpg =
day post germination on which the minimum value was recorded |
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| Table 3. Chemical composition and In Vitro dry matter digestibility (IVDMD) of the petiole fraction of Lablab purpureus (dry matter basis). For explanation of acronyms see Table 2 | ||||||||
Variable |
n |
Mean |
SD |
CV |
Min1 |
Min |
Max |
Max |
Crude protein |
23 |
11.4 |
1.28 |
11.2 |
10.1 |
88 |
12.6 |
45 |
NDF |
24 |
50.7 |
2.63 |
5.2 |
44.9 |
45 |
53.4 |
135 |
ADF |
24 |
42.3 |
2.80 |
6.6 |
38.3 |
58 |
45.3 |
101 |
ADL |
24 |
6.30 |
1.60 |
25.4 |
3.05 |
73 |
8.54 |
58 |
ADL/NDF |
24 |
12.4 |
2.94 |
23.7 |
6.15 |
73 |
17.5 |
58 |
IVDMD |
22 |
69.4 |
2.04 |
3.9 |
65.6 |
73 |
71.6 |
88 |
| Table 4. Chemical composition and In Vitro dry matter digestibility (IVDMD) of the stem fraction of Lablab purpureus (dry matter basis). For explanation of acronyms see Table 2 | ||||||||
Variable |
n |
Mean |
SD |
CV |
Min1 |
Min |
Max |
Max |
Crude protein |
26 |
10.16 |
1.17 |
12.5 |
8.04 |
135 |
11.79 |
45 |
NDF |
27 |
61.88 |
4.05 |
7.5 |
54.34 |
45 |
67.12 |
135 |
ADF |
27 |
49.41 |
3.95 |
8.0 |
44.92 |
73 |
54.14 |
117 |
ADL |
27 |
9.10 |
1.94 |
21.3 |
6.21 |
73 |
11.63 |
101 |
ADL/NDF |
27 |
14.61 |
2.43 |
16.62 |
10.27 |
73 |
17.36 |
101 |
IVDMD |
25 |
54.38 |
2.86 |
5.2 |
51.64 |
73 |
61.10 |
45 |
The days post germination on which minimum and maximum actual values were recorded are presented. This is necessary as the nutrient profiles were not as expected. Typically the protein content and digestibility of legumes decline with maturity while fibre fractions increase with maturity (Milford and Minson 1968; Minson 1990; Van Soest 1994)). This peculiarity will be discussed later in the paper.
Acid detergent lignin as a proportion of neutral detergent fibre is a theoretical calculation presented to quantify the ratio of lignin to total fibre (soluble fibre fractions + hemicellulose + cellulose + lignin). As the tables indicate the proportion of ADL to NDF does not differ noticeably between the leaf (11.6%), petiole (12.4%) and stem (14.6%) fractions. This result is related to the anatomical structure of the lablab plant. Being a trailing, twining plant the lablab stems are vine-like and climb other stable structures. Hence they do not have a high lignin to fibre ratio in the stem fraction as is found in other legumes (i.e. the erect structure of alfalfa). However, differences do exist among botanical fractions in the proportions of other cell wall (NDF) constituents. The cell wall of the leaf fraction exhibits higher proportions of hemicellulose (NDF - ADF) and lower proportions of cellulose (ADF - ADL) than the petiole and stem fractions. These differences are reflected in higher ratios of hemicellulose to cellulose in the leaf (0.75) than in the petiole (0.24) and stem (0.31) fractions.
The concentration of ADL as a proportion of DM, does change. Expressed as % ADL, one can see that the amount of ADL in the leaf fraction (4.4%) is less than that in the petiole and stem fractions (6.3% and 9.1%, respectively). Average total fibre (NDF) concentration values were 37.3%, 50.7% and 61.9% for the leaf, petiole and stem fractions respectively. This result concurs with the literature on tropical legumes, which indicates that the fibre content of the stem is significantly greater than that of the leaf fraction. Forage legumes typically retain a diminutive tree-like structure in which leaves have little structural matter and fibre resides mostly in supporting stems and branches (Van Soest 1994). Overall average IVDMD values were high for the leaf and petiole fractions (68.9% and 69.4%, respectively) and medium for the stem fraction (54.4%).
Nutrient composition related to maturity
It is well known that the chemical composition, and therefore the quality, of legumes changes as the plant matures (Norton and Poppi 1995). Plant development involves deposition of photosynthetic carbon into structural material. As forage plants mature, the accumulation of structural cell wall dilutes the metabolic pool as represented by cell contents (Van Soest 1994). This process may be further modified by environmental conditions such as soil fertility, season, temperature, shade and water stress during growth (Norton and Poppi 1995).
It is important to determine the nutritional characteristics of lablab on a botanical fraction basis because in a grazing situation cattle select for the leaf fraction of the plant (Hendricksen et al 1981; Wood 1983). Also nutritional data on individual botanical fractions is crucial for non-ruminant feeding. For instance, any evaluation of the potential of using the leaf fraction in non-ruminant diets requires data on the nutrient content. However, due to the use of lablab in a wide range of feeding schemes (Mayer et al 1986), it is also important to evaluate nutritional characteristics on a per plant basis.
In this trial data on the nutrient composition of botanical fractions was obtained throughout the experimental period (up to 145 dpg). The results presented in Figure 3 are a summary of all the information, showing the variations in the contents of CP, NDF, ADF and ADL in the whole plant. This summary represents well the trends observed in the concentration of chemical fractions in each botanical fraction.
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Figure 3: Variation in content of chemical constituents of Lablab (whole plant) with stage of growth
Variations in the CP content and IVDMD of the leaf, petiole, stem fractions and whole plant are shown in Figures 4 and 5 respectively.
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Figure 4: Variation in crude protein content of whole plant (leaf, petiole and stem only) and of constituent fractions with stage of growth
The average protein content in the leaf fraction of lablab ranged from 21.4% to 30.3% on a dry matter basis (Table 2 and Figure 4). This range agrees with results obtained by Hendricksen and Minson (1985) and Cameron (1988) who reported leaf protein in lablab to be 21.6% - 27.9% and 15% - 33%, respectively. It has been established that as legumes, including lablab, mature, the content of protein decreases (Milford and Minson 1968). This however, was not the case with the lablab samples in this experiment. Figure 4 illustrates that up to 88 dpg, the data follows an expected pattern of declining protein. However, beyond this point the protein content increased until 130 dpg, then it decreased again. The unexpected rise in protein content corresponds with the flood which occurred 85 days post germination. Thus it appears the flood had an effect on the protein profile of the leaf fraction. The average CP concentration at 135 dpg (30.3%) was higher (P<0.05) than at the other maturity stages. Differences among CP contents at the other growth stages are not significant (P>0.05).
The average protein content of the petiole fraction ranged from 10.1% to 12.6% (P>0.05). As with the leaf fraction, the protein content decreased until 88 dpg at which point started to increase (Figure 4). Average stem protein values ranged from 8.0% to 11.8% which falls within the range of 7.0% to 20.1% reported by Murphy and Colucci (1999). Crude protein content in the stem decreased with maturity until 101 dpg, after which remained stable.
Whole plant CP content followed the same pattern observed for the previously discussed botanical fractions (Figures 3 and 4). In Figure 3 CP values include all the botanical fractions separated in this experiment, while values in Figure 4 only include the leaf, petiole and stem fractions. The information contained in both graphs is important, since CP content of the plant material offered to animals would vary according to the management of the crop (harvesting of the grain) and feeding system utilised. Crude protein in the whole plant (Figure 4) ranged from 14.8% (101 dpg) to 21.0% (46 dpg). After 100 dpg the reproductive organs (flower, green pod, grain and pod shell) became more important, contributing 20% to 25% of the total CP content of the plant. Crude protein content values (dry matter basis) for reproductive organs were (mean ± SD): flower, 29.1% ± 1.5; green pod, 24.7% ± 1.5; grain, 25.3% ± 2.6; pod shell, 7.9% ± 0.9.
As the plant matured the content of fibre fractions (ADF, NDF and ADL) in the whole plant increased until approximately 100 dpg, at which point it decreased (Figure 3), reaching at 146 dpg values similar to the ones observed at earlier maturity stages (73 dpg). The fibre lines in Figure 3 depict the variations in the total fibre content from all botanical fractions, including reproductive organs, of the lablab plant. Corresponding fibre lines excluding reproductive organs (not shown in this paper) followed the same pattern as the lines shown in Figure 3. These results were not expected and corroborate that the first accidental flood, which occurred 85 dpg, had an effect on both the protein and fibre profiles of the plant. This can be clearly seen in Figure 3 which shows the inverse pattern of the fibre and protein lines
As the plants matured changes in the content of fibre fractions in the leaves, petioles and stems followed the same pattern observed for the whole plant. The only exception was ADF content in the leaf fraction which decreased linearly (P<0.001) with maturity.
Average IVDMD of botanical fractions (Tables 2 to 4) range as follows: leaf, from 64.6% (73 dpg) to 72.9% (117 dpg); petiole, from 65.6% (73 dpg) to 71.6% (88 dpg); stem, from 51.6% (73 dpg) to 61.1% (45 dpg). Literature information on digestibility of botanical fractions of lablab is limited (Murphy and Colucci 1999). Our results on IVDMD of the leaf fraction are higher that the DMD values obtained in in vivo trials with sheep and cattle by Hendricksen et al (1981). However, their results on DMD of the stem fraction (49.4% to 55.4) are more in line with our findings. No data on degradability of the petiole fraction could be found in the literature. Our results demonstrate that the IVDMD values of the petiole fraction are high and similar to those of the leaf fraction (Tables 2 and 3). This finding was consistent through all the stages of growth of the lablab plant (Figure 5). On a whole plant basis, the IVDMD values obtained in this experiment (59.0% to 67.1% at 73 dpg and 45 dpg respectively) fell within the range of 50.9% to 68.1% reported by Murphy and Colucci (1999).
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Figure 5: Variation in In vitro dry matter digestibility of whole plant (leaf, petiole and stem only) and of constituent fractions with stage of growth
Agronomic generalisation tends to associate decline in forage quality with plant maturity. Maturation is undeniably a major feature of the changing nutritional value of a feedstuff. However, the relationship can be greatly modified by environmental factors (Van Soest 1994). Wilson (1982) proposed that water stress may delay the onset of maturity and thereby maintain the high nutritive value characteristic of immature plants. As figure 2 demonstrates, despite the incidence of flooding, maturation or development of reproductive structures continued. Hence it is possible to say that the flood may have had a positive effect on the nutritional parameters.
When evaluating forage, plant yield during the dry season is an important element to consider. This work has demonstrated that Lablab purpureus has the tenacity to establish itself during the Honduran dry season and was able to prosper under flood conditions. Lablab had an adequate yield during the dry season and the proportion of plant fractions has the potential to support the selective instincts of cattle.
With the quantity element assured, the next and most crucial question becomes a matter of quality. From an animal nutrition perspective, chemical composition and digestibility are important characteristics in determining quality of feedstuffs. The protein content of the entire lablab plant varied between 14.8% and 21.0% (Figure 4). This coupled with the fact that cattle select for the protein rich leaf fraction (21.4% to 30.3% CP) indicates that lablab can be an excellent source of much needed protein during the dry season.
Based on the IVDMD values obtained, it may be concluded that lablab is a nutritionally valuable legume resource which should be employed more readily, especially in the agricultural systems found in Honduras. It is important to point out that the digestibility values of the botanical fractions and the whole lablab plant observed in this experiment, were higher than the digestible energy values necessary to provide maintenance requirements for adult ruminants (National Research Council 1981, 1985, 1989, 1996). Moreover, the high dry matter degradability combined with the high protein content of the lablab, makes this crop suitable for achieving relatively high production levels in ruminants (body weight gain and/or milk production).
The results obtained affirm that Lablab purpureus has potential not only as a forage but also as a protein supplement during the dry season in tropical areas. However, our results suggest that the nutritional characteristics observed were affected by the accidental flooding. In light of these circumstances it would not be prudent to use the data determined for yield, nutritional characteristics and digestibility parameters after 85 dpg (first flooding) as predictive for lablab grown under completely dry conditions.
In order to realise the potential of Lablab
purpureus as a feed source in the tropics, future studies are required. The following is a
list of topics which are believed to be worth of future exploratory work:
The authors wish to acknowledge the financial support from the Canadian International Development Agency. The assistance of M. Palacios, L. Tejada and C. Campbell during the laboratory phases at the ENA and at Guelph is gratefully acknowledged. We appreciate the review comments from L W Grovum, University of Guelph and D Formoso, Secretariado Uruguayo de la Lana.
AOAC 1980 Official Methods of Analysis (13th edition). Association of Official Analytical Chemists, Washington, D.C.
Cameron D G 1988 Tropical and subtropical pasture legumes. Queensland Agricultural Journal, March-April:110-113
Cochran W G and Cox G M 1957 Experimental Designs (2nd edition), John Wiley & Sons Inc., New York
Goering H K and Van Soest P J 1970 Forage fiber analysis. USDA ARS Agric. Handbook No. 379
Hendricksen R E and Minson D J 1985 Lablab purpureus - A Review. Herbage Abstracts 55:215-227
Hendricksen R E, Poppi D P and Minson D J 1981 The voluntary intake, digestibility and retention time by cattle and sheep of stem and leaf fractions of a tropical legume (Lablab purpureus). Australian Journal of Agricultural Research 32:389-398
Mayer L, Chandler D R and Taylor M S 1986 Lab-lab purpureus - A fodder crop for Botswana. Bulletin of Agricultural Research in Botswana No. 5:37-48
Milford R and Minson D J 1968 The effect of age and method of haymaking on the digestibility and voluntary intake of the forage legumes Dolichos lablab and Vigna sinensis. Australian Journal of Experimental Animal Husbandry 8:409-418
Minson D J 1990 Forage in Ruminant Nutrition. Academic Press Inc. Toronto, 483 pp
Murphy A M and Colucci P E 1999 A tropical forage solution to poor quality diets: A review of Lablab purpureus. Livestock Research for Rural Development 11(2): http://www.cipav.org.co/lrrd/lrrd11/2/colu112.htm
National Research Council 1981 Nutrient Requirements of Goats. National Academy Press. Washington DC
National Research Council 1985 Nutrient Requirements of Sheep (6th Ed). National Academy Press. Washington, DC
National Research Council 1989 Nutrient Requirements of Dairy Cattle (6th Ed). National Academy Press. Washington, DC
National Research Council 1996 Nutrient Requirements of Beef Cattle (7th Ed). National Academy Press. Washington, DC
Norton B W and Poppi D P 1995 Composition and Nutritional Attributes of Pasture Legumes. In. Tropical Legumes in Animal Nutrition; D'Mello, J P F. and C Devendra (Eds). CAB International, Wallingford, UK. pp 23-47
SAS 1996 The SAS System for Windows. Release 6.12. The SAS Institute, Cary, NC, USA
Schaaffhausen R V 1963 Economical methods for using the legume Dolichos lablab for soil improvement, food and feed. Turrialba 13:172-178
Sinclair R 1996 Dolichos lablab: una alternativa para la alimentación del ganado en épocas de verano. Centro Internacional de Información Sobre Cultivos de Cobertura (CIDICCO) Informe Técnico No. 15
Tilley J M A and Terry R A 1963 A two-stage technique for the in vitro digestion of forage crops. Journal of the British Grassland Society 18:104-111
Van Soest P J 1994 Nutritional Ecology of the Ruminant (2nd Ed). Comstock Publishing Association, Cornell University Press, Ithaca
Van Soest P J, Robertson J B and Lewis B A 1991 Methods for dietary fiber, neutral detergent fiber, and non-starch polysaccharides in relation to animal nutrition. Journal of Dairy Science 74:3583-3597
Wilson J R 1982 Environmental and nutritional factors affecting herbage quality. In: Nutritional Limits to Animal Production from Pastures; J B Hacker (Ed). Commonwealth Agricultural Bureau, Farnham Royal, UK; pp 111-131
Wood I M 1983 Lablab bean (Lablab purpureus) for grain and forage production in the Ord River Irrigation Area. Australian Journal of Experimental Agriculture and Animal Husbandry 23:162-171