Chemical composition, gas production and degradation characteristics of haulms of improved dual purpose Cowpea (Vigna unguiculata l. walp) cultivars

C Antwi, E L K Osafo, A Donkoh and H Adu-Dapaah¹

Department of Animal Science, Kwame Nkrumah University of Science and Technology (KNUST), Kumasi, Ghana
cantwi.agric@knust.edu.gh
¹Council for Scientific and Industrial Research (CSIR), Crops Research Institute (CRI), Kumasi, Ghana

Abstract

The study investigated the chemical composition and the nutritive value of four improved cultivars of cowpea (Vigna unguiculata L. Walp) haulm using both in situ disappearance (ISD) and in vitro gas production (IVGP) techniques. The DM and CP disappearance were examined using a split-plot design to compare the effects of cultivar and haulm fraction as well as their interactions.

The highest gas production was observed in the cultivar IT93K-2045-93. All the cultivars had similar pools of digestible fibre and disappeared at the same rates. In a test that included separated leaves and stems, IT93K-2045-93 had a larger soluble fraction and leaves had larger pools of soluble and digestible fiber that disappeared at greater rates than stems. Assessment of CP degradation showed that cultivar IT93K-2045-93 had a greater percent soluble material than IT93K-2309. The amount of ruminal digestible protein (RDP) was greater in IT93K-2309 than in IT93K-2045-93, however, the latter disappeared at greater rates than the former. Cultivars IT93K- 2045-93 and IT93K-2309 may prove to be desirable supplements for small ruminants receiving a poor quality basal diet such as maize stover especially during the lean season when feed is limiting.

Keywords: in situ, in vitro, morphological fraction, ruminal digestible protein

Introduction

The major constraint to small-holder ruminant livestock production in Ghana is availability and quality of feed all year round. Ruminants survive on crop residues and unimproved swards deficient in nitrogen, energy, and minerals. This affects feed intake, feed utilization and animal productivity. The deficiencies in these roughages can be overcome partly by nitrogen supplementation. Leguminous fodders are promising and cheap source of nitrogen for use by smallholder farmers; among these is cowpea haulm. Cowpea haulm has been shown to increase microbial nitrogen supply in calves when used as supplement to teff straw (Abule et al 1995), and to promote intake of maize stover and improve rumen ammonia concentration and degradation of maize stover (Chakeredza et al 2002).

In evaluating feedstuffs, it is useful to know the chemical composition of such feeds. However, chemical assessments of feed for its nutrient content fail to provide information on feed degradation characteristics which determine its utilization and in turn the performance of animals (Blümmel et al 1997). A series of studies have shown that degradation characteristics of feed in the rumen offers a means of evaluating in more detail the nutritive value of feedstuffs. In situ nylon bag (Ørskov et al 1980) and in vitro gas production (Menke and Steingass 1988) techniques have been used extensively for measuring ruminal degradation, screening of feedstuffs and predicting digestible organic matter intakes because of a high degree of correlation with in vivo digestibility values (AOAC 1990).

Antwi et al (2012) in an earlier paper reported the haulm yields of four improved cowpea cultivars to be as high as 11.0 tonnes/ha which could be used as cheap source of nitrogen supplement. It would be useful to assess the chemical composition as well as the nutritional value of these four improved cowpea haulms namely, SORONKO, IT93K-2309, IT86D-716 and IT93K-2045-93. The objective of this study was to assess the chemical composition, gas production profiles and in situ degradation characteristics of haulms of four improved dual - purpose cowpea cultivars.

Materials and methods

Location of experiment

The chemical composition (with the exception of acid detergent lignin [ADL]) was assayed in the Nutrition laboratory of the Department of Animal Science (DAS), KNUST. The ADL contents, in situ dry matter and crude protein disappearance, and in vitro gas production studies were carried out in the Nutrition laboratories of the Animal Production and Research Institute (APRI), Noubaria, Egypt and Alexandra University, Egypt, respectively. The rams used for the fistulation were approved by the ethics committee of APRI. The laboratory assays lasted for two months.

Source of cowpea haulm

Seeds of the four improved dual-purpose cowpea cultivars (SORONKO, IT93K-2309, IT86D-716, IT93K-2045-93) were obtained from the Council of Scientific and Industrial Research – Crop Research Institute, Fumesua, Kumasi, Ghana. They were selected based on their biomass yield (Antwi et al 2004) and were sown on the arable field at the Department of Animal Science, KNUST.

Morphological fractionation of cowpea haulm

Leaves were manually detached from the cowpea plants after harvesting, and the stems were chopped and pooled to constitute the leaf and stem fractions, respectively.

Animals and feeding

Three rumen-cannulated Barki rams (45±2kg average live weight) were used. Each ram had ad libitum access to water and berseem hay ( Trifolium alexandrinum) provided at 08: 30 h every day.

Experimental design

The design used for the gas production was completely randomized with three replications per treatment. The DM and CP disappearance rates were examined using a split-plot design to compare the effects of cultivar and plant fraction as well as their interactions. The complete haulm formed the main plot structure; while the leaf and stem fractions represented the sub plot factor, and the cultivars were the blocks.

In situ degradation

The DM and CP disappearance in situ were assessed using Dacron sample bags (Ankom, Macedon, NY, USA; 42µm porosity) as described by Ørskov et al (1980). Duplicate samples of ground cowpea haulms (5g) were incubated in the three fistulated Barki rams for 0, 3, 6, 12, 24, 48, 72 and 96 h. The nylon bags were rinsed with tap water on removal at the end of each incubation period and stored in a freezer at –20^oC pending analysis. The frozen samples were thawed and washed to eliminate microorganisms associated with the residues. The bags were then dried at 55⁰C in a forced-air oven for 48 h and then residual material was weighed. The DM and CP disappearance from the bags of each cultivar at each incubation time were estimated as nutrient concentration in the original samples less nutrient concentration of residues after incubation and was used to calculate the kinetics of ruminal fermentation according to the formula of Ørskov and McDonald (1979). i.e. P = a +b (1-e^-ct), where p = cumulative amount of DM and CP degraded by time, t; a = readily soluble fraction; b = potential degradable fraction; c = rate constant for the degradation of b.

In vitro gas production (IVGP)

For the IVGP, 0.21g triplicate samples of each of the haulms were placed in 100 ml graduated glass syringe filled with 10ml of rumen fluid, and 20ml of buffer. The rumen fluids were sampled, before feeding berseem hay, from the rumens of sheep with permanent rumen fistulae. The rumen digesta was squeezed through four layers of cheesecloth, homogenized and kept at 39ºC in a water bath under continuous flushing with CO₂ before use. It was diluted with a culture medium containing bicarbonate buffer, macro- and micro-minerals, resazurine and a reducing solution. The buffered rumen fluid (30ml) was pipetted into each syringe and syringes were immediately placed in a water bath at 39ºC (Blümmel and Ørskov 1993). Syringes were shaken at hourly intervals and gas volumes were recorded at 0, 3, 6, 12, 24, 48, 72 and 96 h of incubation and corrected for blank syringes incubated in each run. The model used for the calculation of gas production (GP) described by Siaw et al (1993) was GP = b (1-Exp (^-ct)), where b = potential gas production, c = rate of gas production.

Laboratory analysis

Samples of cowpea haulms and in situ digestibility residues grouped by haulm type, replication and time in situ were analyzed for DM by oven drying at 100 ºC in an aluminum pan for 24h (AOAC 1990). The OM concentration was determined by burning in a muffle furnace at 540ºC for 4h (AOAC 1990). The N concentrations were determined using the Kjeldahl procedure (AOAC 1990) where samples were weighed and transferred into the Kjeldahl digestion flask containing 15g of catalyst (96% K₂SO₄/ 4% CuSO₄.5H₂0) and 25ml of concentrated H₂SO ₄. After 2 of digestion in a unit with electrical heat and fume removal and cooling to room temperature, 400ml of distilled water, 100ml of 50%NaOH and 0.25g Zn were added to each flask. By distillation, ammonium hydroxide was trapped as ammonium borate in a 4% boric acid solution (4g of boric acid in 100ml deionized water (w/v)). Total nitrogen was determined by titration with standardized HCl to a mixed indicator endpoint. The procedures of Goering and Van Soest (1970) were followed to determine neutral detergent acid (NDF) and acid detergent fiber (ADF), acid detergent lignin (ADL) and neutral detergent insoluble nitrogen (NDIN). Neutral detergent fiber assays were undertaken without sodium sulfite or alpha amylase and are expressed without residual ash.

Statistical analysis

The data obtained from the gas production technique were subjected to GLM PROC of SAS (1999). The in situ disappearance data were analyzed with a split plot analysis using the GLM and PROC MIXED procedures of SAS (1999). The degradation constants were determined by a curve fitting procedure of PROC NLIN, available in Version 8 of SAS (SAS 1999). When differences (i.e. P < 0.05) occurred, treatment means were compared by least square means.

Results

Chemical composition

The CP content in the whole haulm of the cultivars was highly variable ranging from 148g/kg DM (SORONKO) to 230g/kg DM (IT93K-2309); compared with 241 g/kg DM (SORONKO) to 342 g/kg DM (IT93K-2045-93) for leaves; and 57.6 g/kg DM (IT86D-716) to 98.4 g/kg DM (IT93K-2045-93) for stem (Table 1).

A significant amount of nitrogen was found to be associated with NDF in the haulm of the cultivars. Concentration of NDIN was comparatively higher in leaves than in whole haulm or stems. Lower levels of NDIN were recorded in the whole haulms of IT93K-2045-93 and SORONKO than in IT93K-2309 and IT86D-716. The NDIN level in the leaves of IT86D-716 was 2.2, 1.9 and 1.7 times higher than in SORONKO, IT93K-2309 and IT93K-2045-93, respectively. The difference in the concentration of NDIN in IT93K- 2309 and IT93K-2045-93 tended to approach significance (P = 0.086). On the contrary, NDIN levels obtained in the stem of all the cultivars were similar.

Gas production

 The highest gas production was observed in cultivar IT93K-2045-93 followed by 1T93K-2309, SORONKO and IT86D-716 (Table 2). 

Degradation Characteristics

The results of the disappearance studies involving the four cowpea cultivars is shown in Table 3. The cultivars differed (P = 0.0009) in the quantity of readily soluble material but no significant differences were found in the potentially digestible fraction (P = 0.9887) and the rate of degradation (P = 0.7783). The cultivar IT93K-2045-93 had the highest percent soluble material (24.3%) while cultivars 1T93K2309, IT86D-716 and SORONKO recorded 20.7, 18.6 and 18.3% readily soluble materials respectively. Similar pools (P = 0.9887) of digestible fibre (represented as “b”) that disappeared at the same rates (P = 0.7783) were observed among the cultivars.

Further investigations of the plant fractions of IT93K-2045-93 and IT93K-2309 (the two cultivars with highest “a” values) showed that rumen disappearance parameters for DM and CP were significantly influenced (P < 0.001) by the morphological fractions of the two cultivars (Table 4 and 5). It was realized that IT93K-2045-93 had a larger soluble fraction (P = 0.0396) and that the leaves had larger pools of soluble (P = 0.0023) and digestible fiber (P = 0.0638) that disappeared at faster rates (P = 0.0238) than stems. The only significant interaction (P = 0.0489) between cultivar and fractions of cowpea occurred for digestion rate. The rates of digestion for leaves and stems in IT93K-2309 were not as divergent as the rates of digestion for leaves and stems in IT93K-2045-93 (P = 0.0489). It is to be noted however, that otherwise the rates did not differ significantly between the two cultivars (P = 0.5121).

Analysis of the CP degradation (Table 5) in the whole plant showed that significant differences (P < 0.05) existed in all the parameters estimated. Cultivar IT93K-2045-93 had a greater percent readily digestible protein than IT93K-2309 (P = 0.0003). The amount of ruminal degradable protein (RDP) was higher (P = 0.0241) in IT93K-2309 than IT93K-2045-93. However, the latter disappeared at greater rates (P = 0.0002) than the former.

 An assessment that combined separated leaves and stems (Table 5) from the two cultivars led to the observation that the amount of CP readily degradable in the rumen was particularly higher for IT93K-2045-93 (P = 0.0087) and that leaves had larger pools of soluble (P = 0.0012) and potentially degradable protein (P = 0.0774) that also disappeared at faster rates (P = 0.0030) than stems. Significant interaction (P = 0.0399) existed between cultivar and fractions of cowpea haulm for potentially degradable protein.

Discussion

Chemical composition

Chemical evaluations of cowpea haulms have been established in studies by Savadogo et al (2000) and Kaasschieter et al (1998). This current study however, sought to undertake a more comprehensive chemical analysis of selected cultivars from a series of breeding programmes in Ghana. The results obtained in this study showed that the chemical composition of the cultivars differed from other published data (Chakeredza et al 2002 and Koralagama et al 2008). The CP content is an important indication of nutritional quality since the cultivars are intended to be used as supplements for poor quality crop residues. The CP content among the cultivars was variable, with cultivar IT93K- 2045-93 recording the highest. The reported CP values are higher compared to those reported by Savadogo et al (2000). The differences in CP values from the reported data, and even among the cultivars, may be as a result of genetic improvement of the cultivars and inherent genetic characteristics (Badve et al 1994; Singh and Schiere 1995; Subba Rao et al 1994); environmental factors such as soil characteristics and crop management (level of fertilizer application, plant density, stage of maturity at harvest, methods of harvesting, and storage) (Harika and Sharma 1994; Walli et al 1994). The relatively higher CP in the leaves and the stem of the same cultivar (IT93K- 2045-93) may explain the extensive in situ degradation and high gas production of that cultivar. Cultivars IT93K- 2045-93 and IT93K-2309 had similar CP contents but differed in the quantity of soluble CP. This could be related to the faster rate of nitrogen release from IT93K- 2045-93 given its low levels of NDIN. The whole and the fractionated parts of IT93K- 2045-93 recorded the least crude fibre and lignin contents. This may also help to explain the high DM and CP degradabilities, as well as higher gas production levels of this cultivar.

The higher nitrogen-bound protein to fibre in the leaves fraction of the cultivars relative to the stems may be an effective nutritional strategy in manipulating excess RDP in an attempt at reducing nitrogen losses in the form of ammonia nitrogen. Thus NDIN helps protect protein from complete degradation by proteolytic bacteria and provides greater protein supply to the small intestines. In this vein, cultivar IT86D-716 would have been the cultivar of choice, however, because of its high lignin and fibre, as well as low CP levels, its use as supplement would not be viable.

In sacco DM and CP disappearance

The disappearance parameters measured in this study are of paramount importance as they influence rumen fill and hence feed intake (Ørskov et al 1988). Differences therefore in the parameters estimated are suggestive that the effect of offering different cultivars of cowpea haulm as nitrogenous supplement on intake and animal performance may vary substantially.

The variations in the initially digestible dry matter among the cultivars may be related to differences in the chemical composition (Åman and Nordkvist, 1983) or variations in physical structure, such as the distribution of lignified cells within the tissues (Ramanzin et al 1991). Thus the high DM disappearance of IT93K- 2045-93 (Table 3) among the cultivars is a reflection of its low contents of lignin and crude fibre.

The larger pools of digestible fibre in the leaves (Table 4) that disappeared at faster rates (10.7% h^-1) than stems (6.0% h^-1) of the cultivars are indicative of the higher lignin content in the stem. This is in consonance with the assertion by Reed and Van Soest (1984) that, stems of dicotyledonous crop residues are characterised by high fibre, lignin and low nitrogen content, hence low digestibility.

The rapid ruminal protein degradation of cultivar IT93K- 2045-93 (Table 5) may result in the production of more peptides and amino acids (Broderick et al 1991). These end-products are incorporated into microbial protein during practical ruminant feeding. Among the microbial consortium that are partially dependent on the supply of amino acids and peptides are the cellulolytic bacteria. Therefore, degradable fractions of protein sources that provide suitable substrates would induce a stronger bacterial response (McAllan and Smith 1983) in ruminal feed fermentation. The results for the protein disappearance of leaf and stem fractions reported in this work revealed that, ruminal protein disappearance in the leaf was comparatively higher. This could be due to greater proteolytic activities of the ruminal microflora thereby evoking a higher bacterial degradation.

In vitro gas production

The highest fermentative gas production was observed in IT93K-2045-93 and was followed by, IT93K-2309, SORONKO and IT86D-716 in decreasing order (Table 2). The least gas accumulation, which was from cultivar IT86D-716, may be as a result of high cell wall content (crude fibre and lignin). Lignin content is reported to be negatively correlated with gas production (Jung and Deetz 1993). According to the authors, lignification of cell wall limits the functions of rumen microbial flora such as fermentation or enzymatic breakdown of forage polysaccharides. Since gas production is associated positively with feed fermentation, cultivar IT86D-716 could be described as having low feeding value owing to its low gas production.

Conclusions

• Nutritional value is a complex interplay of many factors, however, the assessment tools used (i.e. chemical composition, in vitro gas production and in situ disappearance) in this experiment indicated higher CP, readily soluble DM and CP, lower NDF, ADF and NDIN of cultivars IT93K- 2045-93 and IT93K-2309. These cultivars would therefore serve as a source of nitrogenous supplement and improve the productivity of ruminants fed poor quality diet.

Acknowledgment

This study was completed at the Nutrition laboratories of the Animal Production and Research Institute (APRI), Noubaria, Egypt and Alexandra University, Egypt. The authors thank Prof. Broharmi and Ashraf for their input to the study. The study was made possible by support from the Animal Production and Research Institute (APRI), Noubaria, Egypt.

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Received 12 July 2014; Accepted 29 October 2014; Published 3 November 2014

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